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Operator Generic Fundamentals
Basic Electricity - Part 2
© Copyright 2016 – Rev 2
Operator Generic Fundamentals
2
Basic Electricity 2 - TLOs
At the completion of this training session, the trainee will demonstrate
mastery of this topic by passing a written exam with a grade of ≥ 80
percent on the following topics (TLOs):
1. Describe the theory of operation and operating characteristics of
an AC generator.
2. Describe the construction and theory of operation of inductors
and capacitors, their effects on AC electrical circuits, and
relationship to power factor.
3. Describe the construction, operation, and applications of
transformers.
4. Describe basic industrial electrical distribution, including typical
wiring schemes used and the advantages of three-phase
systems.
5. Given an electrical measuring device or piece of test equipment,
describe the use of that equipment including the electrical
parameter measured.
© Copyright 2016 – Rev 2
INTRO
Operator Generic Fundamentals
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AC Generator
TLO 1 - Describe the theory of operation and operating characteristics of
an AC generator.
1.1 Describe the operation of a simple AC generator.
1.2 Describe the development of a sine-wave output in an AC
generator.
1.3 Define common terms in relation to AC generation.
1.4 Describe the relationship between peak, average and RMS values
of voltage in an AC power source.
1.5 Given a diagram of two sine waves, describe the phase
relationship between the two waves.
© Copyright 2016 – Rev 2
TLO 1
Operator Generic Fundamentals
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AC Generator Operation
ELO 1.1 - Describe the operation of a simple AC generator.
• A simple AC generator consists of a conductor or loop of wire in a
magnetic field
– The two ends of the loop connect to slip rings, that are in contact
with brushes
– When the loop rotates, it cuts magnetic lines of force
• As the conductor passes through the magnetic field, a voltage is
induced in the conductor and transferred through the slip rings as
voltage output
Figure: Simple AC Generator
© Copyright 2016 – Rev 2
ELO 1.1
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AC Generator Operation
Magnitude of Generated Voltage
• Dependent on field strength and speed of rotor
• Most generators at constant speed; generated voltage depends on
field excitation, or field strength
© Copyright 2016 – Rev 2
ELO 1.1
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AC Generator Operation
Knowledge Check
An AC generator has all of the following except:
A. A commutator
B. A magnetic field
C. Slip rings
D. A conductor in relative motion with the magnetic field
Correct answer is A.
© Copyright 2016 – Rev 2
ELO 1.1
Operator Generic Fundamentals
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Generator Sine Wave Output
ELO 1.2 - Describe the development of a sine-wave output in an AC
generator.
• As the generator windings are rotated through the magnetic field, a
voltage is induced in the conductors
• The magnitude and polarity of the induced voltage varies with
– The strength of the magnetic field
– The location and direction of travel of the conductors
© Copyright 2016 – Rev 2
ELO 1.2
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Generator Sine Wave Output
• When the loop is in the vertical
position, at 0°, the coils are
moving parallel to the magnetic
field and do not cut magnetic lines
of force
– At that instant, there is no
voltage induced
• As the coil rotates in a clockwise
direction, each side of coil cuts
the magnetic lines of force in
opposite directions
• The polarity of the induced
voltages depends on the direction
of movement of the coil
• The induced voltages are additive,
making slip ring X positive (+) and
slip ring Y negative (-)
© Copyright 2016 – Rev 2
ELO 1.2
Figure: Developing an AC Sine Wave Voltage
Operator Generic Fundamentals
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Generator Sine Wave Output
• This current increases until it
reaches a maximum value
when the coil is 90°
– At that instant, the horizontal
coil is cutting the greatest
number of magnetic lines
• As the coil continues to turn, the
induced voltage and current
decrease until both reach zero
– When the coil is again in the
vertical position (180°)
• The next half revolution
produces an equal voltage, with
reversed polarity (270° and
360°)
© Copyright 2016 – Rev 2
Figure: Developing an AC Sine Wave Voltage
ELO 1.2
Operator Generic Fundamentals
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Three-phase AC,
voltage induced
from rotating
magnetic field
C
C
Phase B
Time 1
T2
T3
Phase A
T4
T5
T6
T7
Phase C
© Copyright 2016 – Rev 2
ELO 1.2
Operator Generic Fundamentals
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Generator Sine Wave Output
Knowledge Check
In a simple AC generator, _______ causes the AC sine wave output.
A. the changes in relative motion of the conductor and the
magnetic field
B. commutation
C. changing speed of the rotating element
D. the pre-programmed oscillation of the field
Correct answer is A.
© Copyright 2016 – Rev 2
ELO 1.2
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AC Generator Common Terms
ELO 1.3 - Define common terms in relation to AC generation.
Period and Frequency
• Period – the time required for the generator to complete one cycle
• Frequency – the number of cycles completed per second, measured
in hertz
• One complete cycle is when the generator coil rotates 360°
– In one cycle, the voltage increases from zero to Emax in one
direction, decreases to zero, increases to Emax in the opposite
direction (negative Emax), and then decreases to zero again
Peak Voltage and Current
• Peak means the maximum appearing on an AC sine wave
– Peak voltage – Ep or Emax, occurs at 90°
– Peak current – Ip
• One way to quantify AC voltage or current is by peak value
© Copyright 2016 – Rev 2
ELO 1.3
Operator Generic Fundamentals
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AC Generator Common Terms
Peak to Peak Voltage and
Current
• Another commonly used term
associated with AC is peak-topeak value (Ep-p or Ip-p)
• Peak to peak refers to the
magnitude of voltage, or current
range, spanned by the sine
wave
Figure: AC Sine Wave Voltage
© Copyright 2016 – Rev 2
ELO 1.3
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AC Generator Common Terms
Effective Value of AC
• Amount of AC that produces the same heating effect as an equal
amount of DC
• Heating effect of an AC current is proportional to the square of the
current
• Calculate the effective value of AC, by squaring all the amplitudes of
the sine wave over one period, taking the average of these values,
and then taking the square root of the average
• The effective value, because it is the root of the mean (average)
square of the currents, is the root-mean-square, or RMS value
Figure: AC Voltage Sine Wave
© Copyright 2016 – Rev 2
ELO 1.3
Operator Generic Fundamentals
AC Generator Common Terms
Knowledge Check
Match the terms with their appropriate definitions.
A. The time required for the generator
to complete one cycle
1. Period
B. The number of cycles completed per
second
2. Peak-topeak
C. The magnitude of voltage or current
range spanned by the sine wave
3. Frequency
D. The root of the mean (average)
square of the currents or voltages
4. RMS
Correct answers are: 1-A, 2-C, 3-B, 4-D.
© Copyright 2016 – Rev 2
ELO 1.3
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Peak, Average and RMS Voltage
ELO 1.4 - Describe the relationship between peak, average and RMS
values of voltage in an AC power source.
Effective Value of AC
• Effective value (current or voltage)
of an AC signal is equal to the
(RMS) of the signal
• Calculated by squaring the
average amplitudes of the sine
wave over one period, and then
taking the square root
• The upper curve shows a plot of
the values of I over time and the
effective value of I
• Lower curve shows a plot of the
values of I2 over time, and the
average current
© Copyright 2016 – Rev 2
ELO 1.4
Figure: Effective Value of AC Current
Operator Generic Fundamentals
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Peak, Average and RMS Voltage
• The dashed line is the average of
the I2 values, and the square root
of that value is the RMS, or
effective value
• The average value is ½ Imax2
• The RMS value is √2/2 Imax which
is equal to 0.707 Imax
• The effective value of voltage or
current can be found using:
𝐸𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑣𝑎𝑙𝑢𝑒 (𝑅𝑀𝑆) = 𝑝𝑒𝑎𝑘 𝑣𝑎𝑙𝑢𝑒
× 0.707
• Normal convention is that stated
values of AC current and voltage
are RMS values
– No subscript is normally used
© Copyright 2016 – Rev 2
ELO 1.4
Figure: Effective Value of AC Current
Operator Generic Fundamentals
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Peak, Average and RMS Voltage
Knowledge Check
The peak value of voltage in an AC circuit is 250 volts. Calculate the
effective voltage.
A. 120 volts
B. 125 volts
C. 177 volts
D. 353 volts
Correct answer is C.
© Copyright 2016 – Rev 2
ELO 1.4
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Phase Relationship
ELO 1.5 - Given a diagram of two sine waves, describe the phase
relationship between the two waves.
Phase Angle Guidelines
• Phase angle is the fraction of a cycle that has gone by since a
voltage or current has passed through a given value
• Phase difference is another common term for phase angle
– It describes two different voltages with the same frequency, which
pass through zero at different times
© Copyright 2016 – Rev 2
ELO 1.5
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Phase Relationship
Phase Angle Guidelines Example
• Take point 1 on the sine wave as the starting point or zero phase.
The phase angle at point 2 is 30°, point 3 is 60°, point 4 is 90°, and
so on until point 13 where the phase angle is 360°, or zero once
again.
Figure: AC Voltage Sine Wave
© Copyright 2016 – Rev 2
ELO 1.5
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Phase Relationship
Phase Example
• In the figure below, the angles along the axis indicate the phases of
voltages e1 and e2. At 120°, e1 passes through the zero value, which
is 60° ahead of e2 (e2 equals zero at 180°)
• Voltage e1 leads e2 by 60 electrical degrees, or voltage e2 lags e1 by
60 electrical degrees
Figure: Phase Relationship
© Copyright 2016 – Rev 2
ELO 1.5
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Phase Relationship
Knowledge Check
When two AC voltages reach their peak voltage at the same time, the
voltages are said to be __________________.
A. leading
B. lagging
C. in phase
D. out of phase
Correct answer is C.
© Copyright 2016 – Rev 2
ELO 1.5
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Inductors and Capacitors
TLO 2 - Describe the construction and theory of operation of inductors
and capacitors, their effects on AC electrical circuits, and relationship to
power factor.
2.1 Describe how current flow, magnetic field, and stored energy in an
inductor relate to one another, and how an inductor opposes a
change in current flow.
2.2 Describe the construction of a capacitor, how it stores energy, and
opposes a change in voltage.
2.3 Describe inductive reactance (XL), and the phase relationship
between current and voltage in an inductive circuit.
2.4 Define capacitive reactance (XC), and the phase relationship
between current and voltage in a capacitive circuit.
2.5 Define impedance (Z).
2.6 Define apparent, true, and reactive power using a power triangle.
2.7 Define power factor as it relates to true power and apparent power,
and define leading and lagging power factors.
© Copyright 2016 – Rev 2
TLO 2
Operator Generic Fundamentals
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Inductor Theory
ELO 2.1 - Describe how current flow, magnetic field, and stored energy in
an inductor relate to one another, and how an inductor opposes a change
in current flow.
• An inductor is a circuit element that will store electrical energy in the
form of a magnetic field
• It is usually a coil of wire wrapped around a core of permeable
material
• Circuits containing inductors will behave differently from a purely
resistive circuit
© Copyright 2016 – Rev 2
ELO 2.1
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Inductor Theory
Induced and CounterElectromotive Force (EMF)
• In the figure, when current is
flowing through Wire A
– It generates a magnetic field
around Wire A
– There is no electromotive
force (EMF) induced into
Wire B because there is no
relative motion between the
magnetic field and Wire B
(DC circuit)
Figure: Induced EMF
© Copyright 2016 – Rev 2
ELO 2.1
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Inductor Theory
Induced and CounterElectromotive Force (EMF)
• If we now open the switch
– Current stops flowing in
Wire A
– Magnetic field collapses
– As the field collapses, it
moves relative to Wire B
and induces an EMF in Wire
B
Figure: Induced EMF
© Copyright 2016 – Rev 2
ELO 2.1
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Inductor Theory
Inducing Electromotive Force
• The three requirements for inducing an EMF are: a conductor, a
magnetic field, and relative motion between the two
• The faster the movement between the two, or the faster the magnetic
field collapses or expands, the greater the induced EMF
• Coiling the wire in either Circuit A or Circuit B, or both, as shown in
the figure below, increases the induction
• The EMF induced in Wire B causes a current to flow whose magnetic
field opposes the change in the magnetic field that produced it
• For this reason, an induced EMF is termed Counter-Electromotive
Force or CEMF
Figure: Induced EMF in Coils
© Copyright 2016 – Rev 2
ELO 2.1
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Inductor Theory
Self-Induced Electromotive Force
(EMF)
• Self-induced EMF is another
phenomenon of induction
• The circuit shown in the figure
below contains a coil of wire called
an inductor (L). As current flows
through the circuit, a large
magnetic field sets up around the
coil
• Since the current is not changing,
there is no EMF produced
• If we open the switch, current flow
stops and the field around the
inductor collapses
– Produces a voltage
– This is a self-induced EMF
© Copyright 2016 – Rev 2
Figure: Self-Induced EMF
ELO 2.1
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Inductor Theory
• Lenz’s Law gives the polarity of self-induced EMF
– The polarity is in the direction that opposes the change in the
magnetic field that induced the EMF
– The result is that the current caused by induced EMF tends to
maintain the same amount of current that existed in the circuit
before opening switch
– Inductor maintains current flow until magnetic field has collapsed
entirely
– For this reason, an inductor tends to oppose a change in current
flow
• This is an example of Lenz’s Law, which states that the induced EMF
opposes the EMF that caused it
© Copyright 2016 – Rev 2
ELO 2.1
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Inductor Theory
Inductance
• Measure of an inductor’s ability to induce CEMF
• Measured in henries (H)
• The induced or counter EMF, is proportional to the time rate of
change of current. The proportionality constant is the inductance (L)
∆𝐼
𝐶𝐸𝑀𝐹 = −𝐿
∆𝑡
• Where:
𝐶𝐸𝑀𝐹 = Induced voltage (volts)
𝐿 = Inductance (henries)
∆𝐼
∆𝑡
= Time rate of change of current (amp/sec)
• The minus sign shows that the CEMF is opposite in polarity to the
applied voltage
© Copyright 2016 – Rev 2
ELO 2.1
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Inductor Theory
Inductors in Series Example
• To calculate the equivalence of
inductors in series, add the
inductance values
• Equivalent inductance (Leq) is:
𝐿𝑒𝑞 = 𝐿1 + 𝐿2 + . . . 𝐿𝑛
Inductors in Parallel Example
• To calculate the equivalence of
inductors in parallel, combine
the values like resistors in
parallel as shown below
1
1
1
1
= + + …
𝐿𝑒𝑞 𝐿1 𝐿2
𝐿𝑁
© Copyright 2016 – Rev 2
Figure: Inductors in Series
Figure: Inductors in Parallel
ELO 2.1
Operator Generic Fundamentals
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Inductor Theory
Knowledge Check
Select all of the statements about inductors that are true.
A. An inductor is a circuit element that will store electrical energy in
the form of a magnetic field.
B. An inductor stores energy as a stored charge between two
plates.
C. An inductor is usually a coil of wire wrapped around a core of
permeable material.
D. Inductors oppose a change in voltage.
Correct answers are A and C.
© Copyright 2016 – Rev 2
ELO 2.1
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Capacitor Theory
ELO 2.2 - Describe the construction of a capacitor, and how it stores
energy, and opposes a change in voltage.
Capacitors
• Capacitors are electrical devices that are constructed of two metal
plates separated by an insulating material, called a dielectric (shown
below)
– The schematic symbols shown below apply to all capacitors
Figure: Capacitor Construction and Symbols
© Copyright 2016 – Rev 2
ELO 2.2
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Capacitor Theory
Charging a Capacitor
• Two conductor plates of the
capacitor are electrically neutral
– There are as many positive
as negative charges on
plates
– The capacitor has no charge
• When switch is closed
– Negative charges on Plate A
are attracted to the positive
side of battery
– Positive charges on Plate B
are attracted to the negative
side of battery
© Copyright 2016 – Rev 2
Figure: Charging a Capacitor
ELO 2.2
Operator Generic Fundamentals
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Capacitor Theory
Charging a Capacitor
• Movement of charges continues
until the difference in charge
between Plate A and Plate B is
equal to voltage of the battery
• Capacitor remains charged after
battery is disconnected
– opposite charges on
opposing plates attract each
other
– tend to oppose any changes
in charge
© Copyright 2016 – Rev 2
Figure: Charging a Capacitor
ELO 2.2
Operator Generic Fundamentals
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Capacitor Theory
Discharging a Capacitor
• When a conductor is placed
across the plates, electrons will
find a path back to Plate A, and
the charges will be neutralized
• This is now a discharged
capacitor
Types of Capacitors
• Dielectric material serves to
classify all commercial
capacitors
• Most common dielectrics are
air, mica, paper, and ceramic
capacitors, plus the electrolytic
type
© Copyright 2016 – Rev 2
Figure: Discharging a Capacitor
ELO 2.2
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Capacitor Theory
Capacitance
• The ability to store an electrical charge
𝑄
𝐶 =
𝑉
• Where:
𝐶 = Capacitance (F)
𝑄 = Amount of charge (C)
𝑉 = Voltage (V)
• Unit is farad (F), which is the capacitance that will store one coulomb
of charge when one volt acts across the plates of the capacitor.
• The dielectric constant (K, unitless) describes the ability to store
electrical energy
• The capacitance of a capacitor depends on three things:
– Area of conductor plates
– Separation between the plates
– Dielectric constant of insulation material
© Copyright 2016 – Rev 2
ELO 2.2
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Capacitor Theory
• The equation below illustrates the formula to find the capacitance of a
capacitor with two parallel plates
𝐴
𝐶 = 𝐾 8.85 × 10−12
𝑑
• Where:
𝐶 = Capacitance
𝐾 = Dielectric constant
𝐴 = Area
𝑑 = Distance between the plates
8.85 x 10-12 = Constant of proportionality
© Copyright 2016 – Rev 2
ELO 2.2
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Capacitor Theory
Capacitors in Series
• Capacitors in series are combined like resistors in parallel
1
1
1
1
1
= + + + …
𝐶
𝐶1 𝐶2 𝐶3
𝐶𝑁
Figure: Capacitors Connected in Series
𝑇
• When only two capacitors are in series, the equation simplifies as
shown below
𝐶1 𝐶2
𝐶𝑇 =
𝐶1 + 𝐶2
• When all the capacitors in series are the same value, compute the
total capacitance by dividing the capacitor’s value by the number of
capacitors in series as shown below
𝐶
𝐶𝑇 =
𝑁
• Where:
𝐶 = Value of any capacitor in series
𝑁 = The number of capacitors in series with the same value
© Copyright 2016 – Rev 2
ELO 2.2
Operator Generic Fundamentals
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Capacitor Theory
Capacitors in Parallel Example
• Capacitors in parallel are combined like resistors in series
• When connected in parallel, the total capacitance, CT, is the sum of
the individual capacitances as given below
𝐶𝑇 = 𝐶1 + 𝐶2 + 𝐶3 + … + 𝐶𝑁
Figure: Capacitors Connected in Parallel
© Copyright 2016 – Rev 2
ELO 2.2
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Capacitor Theory
Knowledge Check
A capacitor is __________________________________.
A. two metal plates separated by an insulating material that
opposes a change in current flow
B. two metal plates separated by an insulating material that
opposes a change in voltage
C. a coil of wire around a magnetic core that opposes a change in
voltage
D. a coil of wire around a magnetic core that opposes a change in
current flow
Correct answer is B.
© Copyright 2016 – Rev 2
ELO 2.2
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Inductive Reactance
ELO 2.3 - Describe inductive reactance (XL) and the phase relationship
between current and voltage in an inductive circuit.
• In AC circuits, inductors present a resistance to current flow that is
termed inductive reactance
• Any device relying on magnetism or magnetic fields to operate is a
form of inductor
– Motors, generators, transformers, and coils are all inductors
• In an inductive AC circuit, the current is continually changing and is
continuously inducing an EMF
– Effect is measured in ohms
• This opposition of the inductance to the flow of an alternating current
is inductive reactance (XL)
© Copyright 2016 – Rev 2
ELO 2.3
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Inductive Reactance
• Mathematical representation of the current flowing in a circuit that
contains only inductive reactance is:
𝐼=
𝐸
𝑋𝐿
• Where:
𝐼 = Effective current (A)
𝑋𝐿 = Inductive reactance (Ω)
𝐸 = Effective voltage across the reactance (V)
© Copyright 2016 – Rev 2
ELO 2.3
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Inductive Reactance
• The value of XL is dependent on
– Inductance of the circuit
– Rate the current is changing in the circuit
– This rate of change depends on frequency of the applied voltage
𝑋𝐿 = 2𝜋𝑓𝐿
• Where:
𝜋 ≈ 3.14
𝑓 = Frequency (Hertz)
𝐿 = Inductance (Henries)
© Copyright 2016 – Rev 2
ELO 2.3
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Inductive Reactance
• The magnitude of induced EMF depends on how fast flux is changing
• For self-induced EMF (such as in a coil), a CEMF is induced in coil
– This CEMF opposes any change in current
– Inductors in AC circuits expand and collapse magnetic fields
attempting to keep current in circuit constant
• In a purely inductive circuit, the resistance is negligible in comparison
to the inductive reactance
© Copyright 2016 – Rev 2
ELO 2.3
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Inductive Reactance
Voltage and Current
Relationship in an Inductive
Circuit
• A change in current in a coil
causes a change of magnetic
flux around the coil
• Changes at maximum rate when
going through its zero value at
– 90° (point b on figure)
– 270° (point d)
– The flux change is also the
greatest at those times
– The self-induced EMF in coil
is at maximum value at these
points
© Copyright 2016 – Rev 2
ELO 2.3
Figure: Current, Self-Induced EMF, and Voltage
in an Inductive Circuit
Operator Generic Fundamentals
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Inductive Reactance
Voltage and Current
Relationship in an Inductive
Circuit
• Current is not changing when it is
going through its peak value at
– 0° (point a)
– 180° (point c)
– 360° (point e)
– Flux change is also zero
– The self-induced EMF in the
coil is at zero value at these
points
• When the current is at its
maximum positive value, the
induced EMF is at a zero value
and rising
© Copyright 2016 – Rev 2
ELO 2.3
Figure: Current, Self-Induced EMF, and Voltage
in an Inductive Circuit
Operator Generic Fundamentals
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Inductive Reactance
Voltage and Current
Relationship in an Inductive
Circuit
• When current reaches a zero
value, the induced EMF is at its
maximum positive value
• When the current is increasing
from zero to its maximum
negative value at 360° (point d to
point e)
– the induced voltage is of the
opposite polarity as the
current and tends to keep the
current from increasing in the
negative direction
• The current lags the applied
voltage by 90° in a purely
inductive AC circuit
© Copyright 2016 – Rev 2
ELO 2.3
Figure: Current, Self-Induced EMF, and Voltage
in an Inductive Circuit
Operator Generic Fundamentals
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Inductive Reactance
Knowledge Check
Inductive reactance is caused by__________________________.
A. the induced EMF in inductors
B. stored electrical charge in circuit components
C. hysteresis losses
D. resistance in the conductors
Correct answer is A.
© Copyright 2016 – Rev 2
ELO 2.3
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Capacitive Reactance
ELO 2.4 - Define capacitive reactance (XC) and the phase relationship
between current and voltage in a capacitive circuit.
• Capacitors in a circuit present a resistance to current flow known as
capacitive reactance
• There are many natural forms of capacitance in AC power circuits,
such as transmission lines, fluorescent lighting, and computer
monitors
• Normally, the inductors counteract the effects of capacitance in an
electrical distribution system
• However, where capacitors outnumber inductive devices, capacitive
reactance will affect the amount of current flowing in an AC electrical
circuit
© Copyright 2016 – Rev 2
ELO 2.4
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Capacitive Reactance
• Capacitive reactance is the opposition by a capacitor (or a capacitive
circuit) to the flow of AC current
– Capacitors charge and discharge in an attempt to keep voltage
constant
• Frequency of the voltage supply determines the rate at which the
applied voltage is changing
– If the supply voltage frequency or the capacitance of a given
circuit is increased, the current flow will increase
• Capacitive reactance is inversely proportional to frequency and
capacitance
© Copyright 2016 – Rev 2
ELO 2.4
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Capacitive Reactance
• The units of capacitive reactance XC are ohms, just like inductive
reactance
1
𝑋𝐶 =
2𝜋𝑓𝐶
• Where:
– 𝑓 = Frequency (Hz)
– 𝜋 ≈ 3.14
– 𝐶 = Capacitance (farads)
• A mathematical representation for the current that flows in a circuit with
only capacitive reactance is:
𝐸
𝐼=
𝑋𝐶
• Where:
– 𝐼 = Effective current (A)
– 𝐸 = Effective voltage across the capacitive reactance (V)
– 𝑋𝐶 = Capacitive reactance (Ω)
© Copyright 2016 – Rev 2
ELO 2.4
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Capacitive Reactance
Voltage and Current Relationships in
a Capacitive Circuit
• Current flow is greatest at points a, c,
and e
– Voltage is changing at maximum
rate
• From point a and point b
– Voltage and charge are increasing
– Current flow is into the capacitor,
but decreasing in value
– At point b, capacitor fully charged
• From b to point c
– Voltage and charge are
decreasing as the capacitor
discharges
– Current flows in direction opposite
to the voltage
© Copyright 2016 – Rev 2
ELO 2.4
Figure: Voltage, Charge, and
Current in a Capacitive Circuit
NOTE: current flow
depends on the rate at
which the voltage changes.
Operator Generic Fundamentals
54
Capacitive Reactance
Voltage and Current Relationships
in a Capacitive Circuit
• From c to d
– Capacitor begins to charge in
the opposite direction
– Voltage and current are again
in the same direction
• At d
– Capacitor is charged
– Current flow again zero
• From d to e
– Capacitor discharges
– Flow of current is opposite to
voltage
• In any purely capacitive AC circuit,
current leads applied voltage by
90°
© Copyright 2016 – Rev 2
Figure: Voltage, Charge, and
Current in a Capacitive Circuit
NOTE: current flow
depends on the rate at
which the voltage changes.
ELO 2.4
Operator Generic Fundamentals
55
Capacitive Reactance
Knowledge Check
Capacitive reactance is dependent on all of the following except
____________________.
A. applied voltage
B. frequency
C. area of the conducting plates
D. dielectric constant
Correct answer is D.
© Copyright 2016 – Rev 2
ELO 2.4
Operator Generic Fundamentals
56
Impedance Theory
ELO 2.5 - Define impedance (z).
• Both resistive and reactive components in an AC circuit oppose
current flow
• The total opposition to current flow in an AC circuit depends on its
resistance, its reactance, and the phase relationships between them
Impedance
• Impedance is the total opposition to current flow in an AC circuit
• The mathematical representation for the magnitude of impedance in
an AC circuit is: 𝑍 = 𝑅2 + 𝑋 2
• Where:
𝑍 = impedance (Ω)
𝑅 = resistance (Ω)
𝑋 = net reactance (Ω)
© Copyright 2016 – Rev 2
ELO 2.5
Operator Generic Fundamentals
57
Impedance Theory
Resistance, Reactance and Impedance
• The figure below shows the relationship between resistance,
reactance, and impedance in an AC circuit
• The current through a resistance is always in phase with the applied
voltage. Resistance plots on the zero axis
• The current through an inductor lags applied voltage by 90°; inductive
reactance plots along the 90° axis
• Current through a capacitor leads applied voltage by 90°; capacitive
reactance plots along the -90° axis
Figure: Relationship Between Resistance, Reactance, and Impedance
© Copyright 2016 – Rev 2
ELO 2.5
Operator Generic Fundamentals
58
Impedance Theory
Knowledge Check
Adding a capacitor to an inductive circuit will ____________________.
A. reduce the impedance, because the capacitive reactance
counteracts some of the inductive reactance
B. increase the impedance, because all reactance adds to the
impedance
C. reduce the resistance of the circuit
D. cause no change to the circuit at all, since it is primarily
inductive
Correct answer is B.
© Copyright 2016 – Rev 2
ELO 2.5
Operator Generic Fundamentals
59
Apparent, True & Reactive Power
ELO 2.6 - Define apparent, true, and reactive power using a power
triangle.
• The power triangle equates AC
power to DC power, relates
– Generator output (Apparent
Power)
– Usable power (True Power)
– Wasted or stored power
(Reactive Power)
• The phase angle (θ) represents the
inefficiency of the AC circuit
– Corresponds to the total reactive
impedance (Z) to current flow in
the circuit
• Can be used to find the efficiency
level of generated power to usable
power
© Copyright 2016 – Rev 2
ELO 2.6
Figure: Power Triangle
Operator Generic Fundamentals
60
Apparent, True & Reactive Power
Apparent Power (S)
• The power delivered to an electrical circuit: 𝑆 = 𝐼2 𝑍 = 𝐼𝐸
• Where:
𝑆 = Apparent power (VA)
𝐼 = RMS current (A)
𝐸 = RMS voltage (V)
𝑍 = Impedance (Ω)
True Power
• The power consumed by the resistive loads in an electrical circuit:
𝑃 = 𝐼2 𝑅 = 𝐸𝐼 cos 𝜃
• Where:
𝑃 = True power (watts)
𝐼 = RMS current (A)
𝐸 = RMS voltage (V)
𝑅 = Resistance (Ω)
𝜃 = Angle between E and I sine waves
© Copyright 2016 – Rev 2
ELO 2.6
Operator Generic Fundamentals
61
Apparent, True & Reactive Power
Reactive Power
• The power component necessary for the expansion and collapse of
magnetic (inductive) and electrostatic (capacitive) fields
𝑄 = 𝐼2 𝑋 = 𝐸𝐼 sin 𝜃
• Where:
𝑄 = Reactive power (VAR)
𝐼 = RMS current (A)
𝑋 = Net reactance (Ω)
𝐸 = RMS voltage (V)
𝜃 = Angle between the E and I sine waves
• Unlike true power, reactive power is unusable because it is stored in
the circuit
– By inductors as they expand/collapse their magnetic fields in an
attempt to keep current constant
– By capacitors because they charge/discharge in an attempt to
keep voltage constant
© Copyright 2016 – Rev 2
ELO 2.6
Operator Generic Fundamentals
62
Apparent, True & Reactive Power
• Reactive Power is function of a system’s amperage
– The power delivered to the inductance is stored in the magnetic
field during field expansion, and returned to the source when the
field collapses
– The power delivered to the capacitance is stored in the
electrostatic field when the capacitor is charging, and returned to
the source when the capacitor discharges
• The circuit conserves reactive power, since none of the reactive
power delivered to the circuit by the source is consumed, but it is all
returned to the source
– These reactive loads consume no True Power in order to maintain
their magnetic and electrostatic fields
– Alternating current constantly changes; thus, the cycle of
expansion and collapse of the magnetic and electrostatic fields
constantly occurs
© Copyright 2016 – Rev 2
ELO 2.6
Operator Generic Fundamentals
63
Apparent, True & Reactive Power
• Circulating current is the term for the current that is constantly flowing
between the source and the inductive and capacitive loads in an AC
circuit in order to maintain magnetic fields
– Circulating currents account for no real work in the circuit
Total Power
• Delivered by the source
• The same as apparent power
– Part of this apparent power, called true power, dissipates by the
circuit resistance in the form of heat
– The rest of the apparent power returns to the source by the circuit
inductance and capacitance (reactive power)
© Copyright 2016 – Rev 2
ELO 2.6
Operator Generic Fundamentals
61
Apparent, True & Reactive Power
Knowledge Check
Match the power terms with their appropriate location on the power
triangle.
A. Apparent Power
1
B. Reactive Power
3
C. True Power
2
Correct answers are A-1, B-3, and C-2.
© Copyright 2016 – Rev 2
ELO 2.6
Operator Generic Fundamentals
65
Power Factor
ELO 2.7 - Define power factor as it relates to true power and apparent
power, and define leading and lagging power factors.
Power Factor
• Power factor (pf) is the ratio between True Power and Apparent
Power
– True Power is the power consumed by an AC circuit
– Apparent Power is a representation of the total power delivered to
an AC circuit
• Reactive Power accounts for a portion of the Apparent Power, which
is power that is stored in an AC circuit and accomplishes no real work
in the circuit
© Copyright 2016 – Rev 2
ELO 2.7
Operator Generic Fundamentals
66
Power Factor
• Power factor is represented by cos
θ in an AC circuit
• It is the ratio of True Power to
Apparent Power
– where θ is the phase angle
between the applied voltage
and current sine waves and is
the angle between P and S on
a power triangle
• The equation for power factor is:
𝑃
cos 𝜃 =
𝑆
• Where:
cos 𝜃 = Power factor (pf)
𝑃 = True Power (watts)
𝑆 = Apparent power (VA)
© Copyright 2016 – Rev 2
Figure: Power Triangle
ELO 2.7
Operator Generic Fundamentals
67
Power Factor
Lagging Power Factor
• Power factor also determines what part of the Apparent Power is True
Power
• It can vary from 1, when the phase angle is 0°, to 0, when the phase
angle is 90°
• In an inductive circuit, the current lags the voltage. This type of
circuit has a lagging power factor, as shown in the figure below
Figure: Lagging Power Factor
© Copyright 2016 – Rev 2
ELO 2.7
Operator Generic Fundamentals
68
Power Factor
Leading Power Factor
• In a capacitive circuit, the current leads voltage
• An electrical circuit that powers loads such as motors, will exhibit a
lagging power factor. An electrical circuit that powers loads such as
fluorescent lighting will exhibit a leading power factor
• Most industrial electrical distribution systems exhibit a lagging power
factor because inductive loads normally account for a larger
percentage of the reactance seen in these types of circuits
Figure: Leading Power Factor
© Copyright 2016 – Rev 2
ELO 2.7
Operator Generic Fundamentals
69
Power Factor
Knowledge Check
Select all of the statements about power factor (pf) that are true.
A. Power factor is the ratio of true power and reactive power.
B. Power factor cannot be greater than one.
C. Power factor is the sine of the power triangle.
D. Power factor (pf) is the ratio between True Power and Apparent
Power.
Correct answers are B and D.
© Copyright 2016 – Rev 2
ELO 2.7
Operator Generic Fundamentals
70
Transformer Construction & Operation
TLO 3 – Describe the construction, operation and applications of
transformers.
3.1 Define the common terms as they pertain to transformers: mutual
induction, turns ratio, impedance ratio, and efficiency.
3.2 Describe the construction of the following components of a
transformer: primary coil, secondary coil, and iron core.
3.3 Describe the voltage, current and power relationships between the
primary and secondary windings of transformers.
3.4 State the applications of each of the types of transformers:
distribution transformer, power transformers, control transformers,
auto transformers, isolation transformers, instrument potential
transformers, instrument current transformers.
© Copyright 2016 – Rev 2
TLO 3
Operator Generic Fundamentals
71
Transformer Terms
ELO 3.1 - Define the following terms as they pertain to transformers:
mutual induction, turns ratio, impedance ratio, and efficiency.
• A transformer is a device that transfers electrical energy from one
circuit to another by electromagnetic induction
– This energy always transfers without a change in frequency, but
usually with changes in current and voltage
Mutual Induction
• If flux lines from the expanding and contracting magnetic field of one
coil cut the windings of another nearby coil, a voltage will be induced
in that coil
• Inducing an EMF in a coil by magnetic lines of flux generated in
another coil is mutual induction
• The amount of electromotive force (EMF) induced by this method
depends on the relative positions of the two coils
© Copyright 2016 – Rev 2
ELO 3.1
Operator Generic Fundamentals
72
Transformer Terms
Turns Ratio
• The turns ratio is the ratio of the number of turns of wire in the
primary winding to the number of turns of wire in the secondary
winding, represented as:
𝑁𝑃
𝑇𝑢𝑟𝑛𝑠 𝑅𝑎𝑡𝑖𝑜 =
𝑁𝑆
• Where:
𝑁𝑃 = number of turns on the primary coil
𝑁𝑆 = number of turns on the secondary coil
© Copyright 2016 – Rev 2
ELO 3.1
Operator Generic Fundamentals
73
Transformer Terms
Impedance Ratio
• Maximum power transfers through a transformer when the
impedances are equal, or matched
– A transformer winding is constructed with a specific turns ratio
• The turns ratio establishes the proper relationship between the
primary and secondary winding impedances
2
𝑍𝑝
𝑁𝑃
=
𝑁𝑆
𝑍𝑆
Efficiency
• Efficiency of a transformer is the ratio of the power output to the
power input, as illustrated by the equation below
𝑃𝑜𝑤𝑒𝑟 𝑂𝑢𝑡𝑝𝑢𝑡
𝑃
𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
= 𝑆 × 100
𝑃𝑜𝑤𝑒𝑟 𝐼𝑛𝑝𝑢𝑡
• Where:
PS = power of secondary
PP = power of primary
© Copyright 2016 – Rev 2
𝑃𝑃
ELO 3.1
Operator Generic Fundamentals
Transformer Terms
Knowledge Check
Match the terms with their appropriate definitions.
A. Inducing an EMF in a coil by magnetic
lines of flux generated in another coil
1. Efficiency
B. The ratio of the number of turns of wire
in the primary winding to the number of
turns of wire in the secondary winding
2. Mutual
induction
C. The ratio between the two impedances
3. Impedance
ratio
D. The ratio of the power output to the
power input
4. Turns ratio
Correct answers are 1-B, 2-D, 3-C, and 4-A.
© Copyright 2016 – Rev 2
ELO 3.1
Operator Generic Fundamentals
75
Transformer Components
ELO 3.2 - Describe the construction of the following basic components of
a transformer: primary coil, secondary coil, and iron core.
• Every transformer has a primary winding and one or more secondary
windings
– Primary winding receives electrical power from an AC source and
induces electrical energy into the secondary winding(s)
– The energy appears as an electromotive force (EMF) across the
secondary winding, and if a load connects to the secondary,
energy in the form of current transfers to load
• A transformer works on the principle that varying magnetic flux
transfers energy by magnetic induction from one set of coils to
another
– An AC source produces this varying magnetic flux
© Copyright 2016 – Rev 2
ELO 3.2
Operator Generic Fundamentals
76
Transformer Components
• Transformers provide means of transferring electrical energy from
one circuit to another, with no direct electrical connections
• Used extensively for:
– AC power transmission
– Various control and indication functions
– Isolating electrical circuits
• An important application of a transformer is for raising (stepping-up)
or lowering (stepping down) the source of voltage
– The coil of a transformer energized from an AC source is called
the primary winding (coil)
– The coil that delivers the induced AC to the load is called the
secondary winding (coil)
© Copyright 2016 – Rev 2
ELO 3.2
Operator Generic Fundamentals
77
Transformer Components
• In actual construction:
– Half of the primary and
secondary coils wind on each
of the two legs
– Sufficient insulation between
the two coils and the core to
properly insulate the windings
– Reduces magnetic leakage
• Magnetic leakage is the magnetic
flux that passes through either
coil, but not through both
Figure: Basic Core Type Transformer
• As the distance between primary
and secondary windings
increases, the magnetic circuit
lengthens, and leakage increases
© Copyright 2016 – Rev 2
ELO 3.2
Operator Generic Fundamentals
78
Transformer Components
• When (AC) voltage is applied to primary winding, an alternating
current flows through the primary winding that magnetizes the
magnetic core, first in one direction and then in the other direction
• This alternating flux flowing around the entire length of the magnetic
circuit induces a voltage in both the primary and secondary windings
– The induced voltage will be at the same frequency as that of the
AC source
• Since the same flux links both windings, the voltage induced per turn
of the primary and secondary windings must be the same value and
same direction
• In the primary winding, this voltage opposes the voltage applied to
the primary winding and is counter-electromotive force (CEMF)
© Copyright 2016 – Rev 2
ELO 3.2
Operator Generic Fundamentals
79
Transformer Components
Knowledge Check
Match the transformer-related terms to their appropriate definitions.
A. The coil that is energized by the AC
source
1. Leakage
B. The coil that is connected to the load
2. Primary
C. Magnetic material that coils are wound
around
3. Secondary
D. Magnetic flux that passes though only
one of the coils
4. Core
Correct answers are: 1-B, 2-C, 3-D, and 4-A.
© Copyright 2016 – Rev 2
ELO 3.2
Operator Generic Fundamentals
80
Transformer Windings
ELO 3.3 - Describe the voltage, current and power relationships between
the primary and secondary windings of transformers.
• One of the most important functions associated with transformers is
their ability to step-up or step-down voltage
– Transmission lines are high voltages
o Limits current flow for a given power, limiting heating losses
• The voltage induced in the secondary windings of a transformer is
dependent on the ratio of turns of the primary winding to turns of the
secondary winding
© Copyright 2016 – Rev 2
ELO 3.3
Operator Generic Fundamentals
81
Transformer Windings
Primary to Secondary Voltage Relationship
• The voltages induced in the windings of a transformer are directly
proportional to the number of turns of the coils in the transformer
𝑉𝑃 𝑁𝑃
=
𝑉𝑆 𝑁𝑆
• Where:
𝑉𝑃 = voltage on primary coil
𝑉𝑆 = voltage on secondary coil
𝑁𝑃 = number of turns on the primary coil
𝑁𝑆 = number of turns on the secondary coil
𝑉𝑅 = 𝑇𝑅
• A voltage ratio of 1:5 means that for each volt on the primary, there
will be 5 volts on the secondary.
– Step-up transformer: Secondary voltage > primary voltage
– Step-down transformer: Secondary voltage < primary voltage
© Copyright 2016 – Rev 2
ELO 3.3
Operator Generic Fundamentals
82
Transformer Windings
Primary to Secondary Current Relationship
• The current in the windings of a transformer is inversely proportional
to the voltage in the windings. The equation to express this
relationship is:
𝑉𝑃 𝐼𝑆
=
𝑉𝑆 𝐼𝑃
• Where:
𝐼𝑃 = primary coil current
𝐼𝑆 = secondary coil current
• Since the voltage ratio is equal to the turns ratio, we can express the
𝑁
𝐼
current ratio in terms of the turns ratio as well by 𝑃 = 𝑆 , or we can
use: CR =
© Copyright 2016 – Rev 2
1
𝑉𝑅
=
𝑁𝑆
1
𝑇𝑅
ELO 3.3
𝐼𝑃
Operator Generic Fundamentals
83
Transformer Windings
Primary to Secondary Power Relationship
• Regardless transformer is a step-up or step-down device, the input
power of the transformer remains equal to the output power of the
transformer (minus any internal losses). The equation below
demonstrates this relationship.
𝑃𝑃𝑟𝑖𝑚𝑎𝑟𝑦 = 𝑃𝑆𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 and 𝑉𝑃 𝐼𝑃 = 𝑉𝑆 𝐼𝑆
• Where:
𝑃𝑃𝑟𝑖𝑚𝑎𝑟𝑦 = Input power
𝑃𝑆𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 = Output power
𝑉𝑃 = Primary voltage
𝑉𝑆 = Secondary voltage
𝐼𝑃 = Primary current
𝐼𝑆 = Secondary current
© Copyright 2016 – Rev 2
ELO 3.3
Operator Generic Fundamentals
84
Transformer Windings
Knowledge Check
Match the terms to their appropriate definitions.
A. Voltage on the primary side divided by
voltage on the secondary side
1. Step-up
transformer
B. Ratio of primary turns to secondary
turns
2. Step-down
transformer
C. A transformer with higher secondary
voltage than primary voltage
3. Voltage ratio
D. A transformer with lower secondary
voltage than primary voltage
4. Turns ratio
Correct answers are:1-C, 2-D, 3-A, and 4-B.
© Copyright 2016 – Rev 2
ELO 3.3
Operator Generic Fundamentals
85
Transformer Application
ELO 3.4 - State the applications of each of the common types of
transformers: distribution transformer, power transformers, control
transformers, auto transformers, isolation transformers, instrument
potential transformers, and instrument current transformers.
• Transformers construction matches a transformer’s characteristics to
its intended application
• Differences in construction:
– May involve the size of the windings
– Relationship between the primary and secondary windings
• Transformer types are also designated by function the transformer
serves in a circuit
– an isolation transformer
© Copyright 2016 – Rev 2
ELO 3.4
Operator Generic Fundamentals
86
Transformer Application
Distribution Transformer
• Extensively used in AC electrical distribution
• Has the highest power, or volt-ampere ratings, and the highest
continuous voltage rating
• Cooling method determines the power rating
• Some transformers use oil or other heat-conducting material to
remove heat
• Others use forced air cooling (fans)
• Ampere rating is increased by increasing the size of the primary and
secondary windings
• Voltage ratings are increased by increasing the voltage rating of the
insulation
© Copyright 2016 – Rev 2
ELO 3.4
Operator Generic Fundamentals
87
Transformer Application
Power Transformers
• Used in electronic circuits
• Generally have a rating of 300 volt-amperes and below
• Normally provide power to power supply circuit of an electronic
device
• Power amplifier in an audio receiver is an example of this type of
transformer
Control Transformers
• Used on electronic circuits that require constant voltage or constant
current with a low power or volt-amp rating
• Various filtering devices, such as capacitors, minimize variations in
the output of these types of transformers, resulting in a more constant
voltage or current
© Copyright 2016 – Rev 2
ELO 3.4
Operator Generic Fundamentals
88
Transformer Application
Auto Transformers
• Used in low power applications where a variable voltage is required
• Special type of power transformer, because it consists of only one
winding
• Tapping or connecting at differing points on the winding yield different
voltages
Figure: Auto Transformer Schematic
© Copyright 2016 – Rev 2
ELO 3.4
Operator Generic Fundamentals
89
Transformer Application
Isolation Transformers
• Low power transformers used to isolate noise from or to ground
• Any DC voltage present in the circuit (such as noise) will not pass
through; therefore, the transformer acts to isolate this noise
• Application in electrical circuits to ensure that a fault developed in
one portion of the circuit will not affect another portion
– no direct electrical connection between the primary and
secondary windings
Instrument Potential Transformers (PT)
• PT steps down the voltage of an electrical circuit to a low value
• Can effectively and safely be used for the operation of instruments
such as ammeters
© Copyright 2016 – Rev 2
ELO 3.4
Operator Generic Fundamentals
90
Transformer Application
Instrument Current Transformers (CT)
• Steps down the current of a circuit to a lower value
• Same types of equipment as the potential transformer use this type of
transformer
• Secondary winding is a coil consisting of many turns of wire, wound
around the primary coil, which contains only a few turns of wire
• Allows measurements of high values of current
© Copyright 2016 – Rev 2
ELO 3.4
Operator Generic Fundamentals
91
Transformer Application
Instrument Current Transformers (CT)
• Because of design, it is necessary to follow a special procedure when
operated at no load
• Current transformer should always be short-circuited when not
connected to an external load
• The magnetic circuit design of a current transformer is for low
magnetizing current when under load
• A large increase in magnetizing current will result in the build up a
large flux in the magnetic circuit
• This will cause the transformer to act as a step-up transformer,
inducing an excessively high voltage in the secondary when under no
load
© Copyright 2016 – Rev 2
ELO 3.4
Operator Generic Fundamentals
92
Transformer Application
Knowledge Check
Match the terms to their appropriate definitions.
A. Used in electrical power distribution and
transmission systems
1.
Isolation
transformer
B. Sometimes used in electrical circuits to
ensure that a fault developed in one portion
of the circuit will not affect another portion of
the circuit
2.
Auto
transformer
C. Consists of only one winding
3.
Distribution
transformer
D. Steps down the voltage of an electrical circuit 4.
to a low value that can be effectively and
safely used for the operation of instruments
Instrument
potential
transformer
Correct answers are 1-C, 2-A, 3- B, and 4-D.
© Copyright 2016 – Rev 2
ELO 3.4
Operator Generic Fundamentals
93
Electrical Distribution & 3-Phase
Systems
TLO 4 - Describe basic industrial electrical distribution, including typical
wiring schemes used and the advantages of three-phase systems.
4.1 Define common terms associated with electrical distribution
systems and wiring schemes used in these systems.
4.2 Describe the design of a basic industrial electrical distribution
system.
4.3 Describe the two methods of connecting single-phase loads to a
three-phase power source and the advantages of three-phase
systems.
4.4 Given a diagram of a wye or delta-connected three-phase system,
describe the voltage/current relationships of the circuit.
4.5 State the indications of an unbalanced load in a three-phase power
system.
4.6 Describe the purpose of common power distribution schemes.
© Copyright 2016 – Rev 2
TLO 4
Operator Generic Fundamentals
94
Electrical Distribution Terminology
ELO 4.1 - Define common terms associated with electrical distribution
systems and wiring schemes used in these systems.
Electrical Distribution Terminology
• Electrical distribution systems include numerous types of devices that
perform a specific function within the distribution system
• The following is a list of common devices found in electrical
distribution systems and their definitions
– Area Substation – receives power from the site distribution
system for use in a particular facility. A substation typically
contains a switch, a transformer and a circuit breaker
– Distribution Substation – a substation that receives high voltage
transforms it down to a lower voltage and distributes it to electrical
loads via circuit breakers
© Copyright 2016 – Rev 2
ELO 4.1
Operator Generic Fundamentals
95
Electrical Distribution Terminology
• Load Center – receives power from a substation and supplies power
to facility motor control centers and larger electrical loads
• Motor Control Center – receives 480 VAC power and distributes it to
individual process loads and other electrical panels via circuit
breakers, controllers, fuses, etc.
• Contactor – an electro-mechanical device that controls power to a
piece of equipment, normally associated with motor controllers
• Panel boards – small distribution panels that contain numerous
molded-case circuit breakers, usually provides power to 208 VAC or
120 VAC loads
• Tie-Breaker – circuit breaker used to tie two electrical busses
together
• Main Breaker – circuit breaker used to connect bus bars of
switchgear assemblies to the output of transformers
© Copyright 2016 – Rev 2
ELO 4.1
Operator Generic Fundamentals
96
Electrical Distribution Terminology
• Feeder Breaker – circuit breaker used to receive power from a
switchgear bus bar, and direct the power to downstream electrical
loads
• Switchgear – an assembly of circuit breakers electrically connected to
a system of electrical busses (solid copper conductors)
• Normal Power – power received from the normal supply source at an
industrial facility; normal power may be generated on-site or
purchased from a commercial utility
• Standby Power – power source that comes on line upon a loss of
normal power; a diesel generator or by battery backup power via a
UPS may supply standby power
• Vital/Essential Loads – loads requiring constant power to ensure that
a facility can operate safely
© Copyright 2016 – Rev 2
ELO 4.1
Operator Generic Fundamentals
97
Electrical Distribution Terminology
Wiring Scheme Terminology
To understand wiring schemes used in power distribution systems,
familiarity with the following terms is required
• Ampacity – the maximum sustained current (in amperes) that a
conductor can carry while remaining within its temperature rating
• Bond – the permanent joining of metallic parts or circuits assuring
electrical continuity, and safe current conductance for any expected
current
• Conductor – any wire, cable, or substance that is capable of carrying
an electrical current
• Ground – a conducting connection, whether intentional or accidental,
between a circuit or piece of equipment and the earth, or a body
serving as earth, which has zero electrical potential
© Copyright 2016 – Rev 2
ELO 4.1
Operator Generic Fundamentals
98
Electrical Distribution Terminology
Wiring Scheme Terminology (continued)
• Leg – a current-carrying conductor intended to deliver power to or
from a load normally at an electrical potential other than ground
• Neutral – a current-carrying conductor normally tied to ground so that
the electrical potential is zero
• Phase voltage – the greatest root mean square (effective) difference
of potential between any two legs of the circuit
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Electrical Distribution Terminology
Knowledge Check
Match the terms with their appropriate definition.
A.
An electro-mechanical device that controls power to 1.
a piece of equipment.
Contactor
B.
Receives power from a substation and supplies
power to facility motor control centers and larger
electrical loads.
2.
Feed breaker
C.
Small distribution panels which contain numerous
molded-case circuit breakers. Usually provide
power to 208 VAC or 120 VAC loads.
3.
Load center
D.
Circuit breaker, which receives power from a
switchgear buss bar and directs the power to
downstream electrical loads.
4.
Panel
Correct answers are: 1-A, 2-C, 3-D, and 4-B.
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Basic Electrical Distribution System
ELO 4.2 - Describe the design of a basic industrial electrical distribution
system.
Electrical Distribution System
• Provides the power for process loads such as pumps, fans
compressors, lighting, instrumentation, heating ventilation and air
conditioning, and operating and control circuits
• It also provides the means to connect and disconnect electrical power
to these loads via cables, wires and various types of protective
devices
• Sources of backup power such as emergency diesel generators,
batteries, and uninterruptible power supplies (UPS) help ensure
constant power to vital components
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Basic Electrical Distribution System
A basic electrical distribution
system consists of three parts:
• A generating system
• A transmission system
• A distribution system
Figure: Typical Industrial Electrical Distribution System
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Basic Electrical Distribution System
Generating System
• Typical electrical systems consist of sources of power, and
associated step-up, and step-down transformers
• Sources of power include local commercial (purchased) power
system or on-site electrical generation
• Power falls into two categories: normal and standby power
– Normal power is the power normally supplied to the facility’s
electrical equipment
– Standby power sources are generally from an on-site diesel
generator or from backup batteries
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Basic Electrical Distribution System
Transmission System
• Includes high-voltage cables, power poles, and switching stations
– Connect the generating system to the distribution system
– Forms a grid that extends across the industrial facility
• Voltage can be very high (115 kVAC is common) to limit power losses
• Limits power lost to the resistance of the electrical cables
Distribution System
• Consists of one or more electrical substations
• The incoming high voltage from the transmission system goes
through transformers down to a lower voltage for use by the facility’s
electrical loads
– 13.8 kVAC or 480 VAC
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Basic Electrical Distribution System
Substations
• Transform incoming high voltage down to usable value for the facility
• Distribute it to electrical equipment throughout the facility
• Transformation takes two steps
– Substation distribution transformers transform voltage from 115
kVAC to an intermediate value of voltage such as 13.8 kVAC
– This intermediate voltage may power some large facility loads
directly
– In order to obtain the lower voltage (480 VAC) required by most
facility electrical loads, a second substation transforms the 13.8
kVAC again, to 480 VAC
– From the 480 VAC substation, a switchgear lineup distributes
electrical power to various facility load centers, motor control
centers, and individual electrical loads
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Basic Electrical Distribution System
Load Centers
• Switchgear is a term used to describe a group of circuit breakers tied
to a common electrical bus
– Electrical buss is a copper bar (single phase) or set of copper
bars (three-phase) used to connect the switchgear to the
distribution transformer and to its associated breakers
• This type of switchgear is a load center
– typically equipped with a main supply breaker to receive power
from a distribution transformer
– several feeder breakers to supply power to various motor control
centers in the facility and larger electrical loads
• Electrical switchgear is often provided with meters and indicating
devices which can be used by facility operators to monitor distribution
system performance
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Basic Electrical Distribution System
Motor Control Centers (MCCs)
• Act as centralized distribution and control points for various 480 VAC
components and loads
• Receive power from a load center and distribute this power to
individual electrical loads via breakers, fuses, motor controllers, etc.
• May also feed lighting and instrumentation loads via transformers
(480 VAC to 208/120 VAC)
• Like load centers, MCCs are equipped with monitoring and
instrumentation devices such as voltmeters, ammeters, etc.
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Basic Electrical Distribution
Knowledge Check
Match the sub-system of the electrical distribution system with the
appropriate definition.
A.
One or more electrical substations, where the incoming
high voltage is transformed down to a lower voltage
1.
Substations
B.
Includes the high-voltage cables, power poles, and
switching stations forming a “grid” which extends across
the industrial facility
2.
Load centers
C.
Transform incoming high voltage down to a value usable
by the facility and distribute it to electrical equipment
throughout the facility
3.
Distribution
system
D.
Equipped with a main supply breaker to receive power
and several feeder breakers to supply power to various
motor control centers in the facility and larger electrical
loads
4.
Transmission
system
Correct answers are: 1-C, 2-D, 3-A, and 4-B.
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Single-Phase Connections
ELO 4.3 - Describe the two methods of connecting single-phase loads to
a three-phase power source and the advantages of three-phase systems.
Single-Phase Load Connections
• The source of single-phase power in all facilities is by generation
from a single-phase generator or by utilization of one phase of a
three-phase power source
• Each phase of the three-phase distribution system is a single-phase
generator electrically spaced 120 degrees from the other two
• Therefore, a three-phase power source is convenient and practical to
use as a source of single-phase power
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Single-Phase Connections
• Single-phase loads can connect to three-phase systems utilizing two
methods
• Phase-to-ground scheme (part A) provides connection of the load
from a phase leg to a ground point
• Phase-to-phase scheme (part B) connects the single-phase load
between any two legs of the three-phase source
• The choice of schemes, allows several voltage options depending on
whether the source system is a three-phase delta or wye
configuration
Figure: Three-Phase to Single-Phase Connection
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Single-Phase Connections
Advantages of Three-Phase Systems
• The design is more efficient at producing and utilizing an AC voltage
• Combination of three single-phase systems
– Power comes from a three-phase AC generator that produces
three separate and equal voltages
– Each voltage is 120° out of phase with the other two voltages
Figure: Three-Phase AC
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Single-Phase Connections
Single-Phase Load Connections
• The source of single-phase power is
– Generation from a single-phase generator
– Utilization of one phase of a three-phase power source
• Each phase of the three-phase distribution system is a single-phase
generator electrically spaced 120 degrees from the other two
– Therefore, is convenient and practical to use as a source of
single-phase power
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Single-Phase Connections
Knowledge Check
Which of the following are common three-phase to single-phase
connection methods? (select all that are correct).
A. Phase to line
B. Phase to ground
C. Phase to phase
D. Neutral to ground
Correct answers are B and C.
© Copyright 2016 – Rev 2
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Wye and Delta-Connected
ELO 4.4 - Given a diagram of a wye or delta-connected three-phase
system, describe the voltage/current relationships of the circuit.
Wye-Connected
• The three common ends of each
phase connect at a common
point (neutral)
• The other three ends connect to
a three-phase line
• Three-phase systems can
connect in two different ways
– Wye (Y)
Figure: Wye-Connected
– Delta (Δ)
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Wye and Delta-Connected
Delta-Connected
• The three phases connect in series to form a closed loop
Figure: Delta Connected
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Wye and Delta-Connected
Balanced Three-Phase System
• A three-phase system, that has identical impedance in each
secondary winding, has balanced loads
– The impedance of each winding in a delta load is shown as Z∆
– The impedance in a wye load is shown as ZY (part b)
• For either the delta or the wye connection, the lines A, B, and C
supply a three-phase system of voltages
Figure: Three-Phase Balanced Loads
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Wye and Delta-Connected
Voltage and Current in Delta-Connected Systems
• The table below contains the formulas for calculating line and phase
voltage and current for delta-connected systems
Delta-Connected Systems
Formula
Line Current
𝐼𝐿 = 3𝐼𝑝ℎ𝑎𝑠𝑒
Line Voltage
𝑉𝐿 = 𝑉𝑝ℎ𝑎𝑠𝑒
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Wye and Delta-Connected
Voltage and Current in a Wye-Connected System
• The table below contains the formulas for calculating line and phase
voltage and current in a Wye-connected system
Delta-Connected Systems
Formula
Line Current
𝐼𝐿 = 𝐼𝑝ℎ𝑎𝑠𝑒
Line Voltage
𝑉𝐿 = 3𝑉𝑝ℎ𝑎𝑠𝑒
Phase Power
• Because the impedance of each phase of a balanced delta or wye
system has equal current, phase power is one third of the Total
Power
𝑃Φ = 𝑉Φ 𝐼Φ 𝑐𝑜𝑠 𝜃
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Wye and Delta-Connected
Total Power
• Total Power (PT) is equal to three times the single-phase power
PT = 3VΦIΦcosθ
Delta-Connected System 𝑉𝐿 = 𝑉𝑝ℎ𝑎𝑠𝑒 and 𝐼𝑝ℎ𝑎𝑠𝑒 =
• In a delta-connected system, so: 𝑃𝑇 =
3
3𝑉𝐿 𝐿𝐿 cos 𝜃
Wye-Connected System 𝐼𝐿 = 𝐼𝑝ℎ𝑎𝑠𝑒 and 𝑉𝑝ℎ𝑎𝑠𝑒 =
• In a wye-connected load, so: 𝑃𝑇 =
3𝐼𝐿
3𝑉𝐿
3
3𝑉𝐿 𝐿𝐿 cos 𝜃
The above equations demonstrate that Total Power formulas for deltaand wye-connected systems are identical
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Wye and Delta-Connected
Apparent Power and Reactive
Power
• Total Apparent Power (ST) in
volt-amperes and total Reactive
Power (QT) in volt-amperesreactive are related to total True
Power (PT) in watts
• A balanced three-phase system
has True, Apparent, and
Reactive powers given by the
following equations:
𝑃𝑇 =
3𝑉𝑇 𝐼𝐿 cos 𝜃
𝑆𝑇 =
3𝑉𝑇 𝐼𝐿
𝑄𝑇 =
3𝑉𝑇 𝐼𝐿 sin 𝜃
© Copyright 2016 – Rev 2
Figure: Power Triangle
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Wye and Delta-Connected
Calculating Voltage and Current Demonstration
• In a Wye-connected power system, phase voltage is 150 volts, and
phase current is 5 amps
• Calculate line voltage and line current
• Solution:
𝐿𝑖𝑛𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 = 𝑃ℎ𝑎𝑠𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 = 5 𝑎𝑚𝑝𝑠
𝐿𝑖𝑛𝑒 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 = 3 𝑃ℎ𝑎𝑠𝑒 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 = 260 𝑣𝑜𝑙𝑡𝑠
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Wye and Delta-Connected
Knowledge Check
A three-phase power system has the following parameters. Line
current 12 amps. Phase current 12 amps. This system is a
_____________________.
A. DC system
B. Delta-connected system
C. Wye-connected system
D. X-connected system
Correct answer is C.
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Unbalanced Loads 3-Phase
ELO 4.5 - State the indications of an unbalanced load in a three-phase
power system.
Unbalanced Loads
• An important property of a three-phase balanced system is that
– The phasor sum of the three line or phase voltages is zero
– The phasor sum of the three line or phase currents is zero
• When the three load impedances are not equal to one another, the
phasor sums and the neutral current (In) are not zero, and the load is
unbalanced
– An imbalance occurs when an open or short circuit appears at the
load
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Unbalanced Loads 3-Phase
• In a fault condition, the neutral connection in a wye-connected load
will carry more current than the phase under a balanced load
• Abnormally high currents in one or more of the phases indicate an
unbalanced three-phase circuits
• If allowed to continue, this may cause damage to equipment
Figure: Three-Phase Systems
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Wye and Delta-Connected
Knowledge Check
A three-phase Wye-connected system with an unbalanced load will
have a neutral current of zero.
A. False
B. True
Correct answer is A.
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Power Distribution Schemes
ELO 4.6 - Describe the purpose of common power distribution schemes.
Three-Wire, Single-Phase Edison System
• 3-wire, single-phase Edison system – approved method of wiring
single-phase power from a three-phase system
• The illustration depicts a center-tapped transformer providing half
voltage (120 V) connections on either side or full voltage (240 V)
across both sides
Figure: Three-Wire Edison Scheme
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Power Distribution Schemes
• The physical connections to the secondary involve
– Two insulated conductors – current-carrying legs or neutral legs
requiring insulation
– One bare conductor – serves as a safety ground, bonded to the
ground point of the system
• Safety ground connects to each junction box, or device, in the system
• For (120 V) use, the intended path of the current is from the supply
leg through the load and back to the source via the neutral leg
– Ground carries no current unless a fault occurred in the system
Figure: Three-Wire Edison Scheme
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Power Distribution Schemes
• For (240 V), the insulated conductors are connected across the full
winding of the transformer, and the un-insulated conductor is again
bonded to the grounded center tap
• In a balanced system, all currents will flow on the insulated
conductors
• In case of either an unbalanced load or a fault in the system, the bare
conductor will carry current, but the potential will remain at zero volts
because it connects to the ground point
Figure: Three-Wire Edison Scheme
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Power Distribution Schemes
Three-Phase Wiring Schemes
• The three-phase system needs
neither a separate neutral nor a
ground to operate safely
• To prevent an unsafe condition,
all 3 and 4-wire, three-phase
systems can include an effective
ground path
Three-Wire, Three-Phase Delta
System
• The simplest three-phase
system is the 3-wire Delta
configuration
• Normally used for transmission
of power in the intermediate
voltage class from approximately
15,000 volts to 600 volts
© Copyright 2016 – Rev 2
Figure: Three-Wire, Three-Phase Delta Scheme
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Operator Generic Fundamentals
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Electrical Distribution System
• Upper diagram is an ungrounded
Delta
• Lower diagram shows a ground point
affixed to one corner of the Delta
– Lowers one phase’s reference to
ground, but retains a phase-tophase voltage
– Similar to the grounded neutral
of the single-phase Edison
system
• Has economy in wiring costs
• Grounded phase can physically
protect the other two phases from
accidental grounding or lightning
strikes
• Rarely used for low voltage (under
600 V), however, because of the
absence of a safety ground
© Copyright 2016 – Rev 2
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Figure: Three-Wire, Three-Phase Delta Scheme
Operator Generic Fundamentals
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Electrical Distribution System
Four-Wire, Three-Phase Delta System
• Combines the ungrounded Delta discussed above for three-phase
loads with the Edison system for single-phase loads
• One side of the Delta has a grounded-neutral conductor connected to
a center tap winding on one phase
• Provides the same half-or full-voltage arrangement seen in the
normal Edison scheme with a grounded neutral
• At any location in the system, either three-phase power at full voltage
or single-phase power with half or full voltage is equally possible
Figure: Four-Wire Delta System
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Electrical Distribution System
Four-Wire, Three-Phase Delta System
• Several strict procedures are required in system operation
– Carefully balance all loads on both the single-phase and threephase legs
– Measurement between the neutral and the phase must be taken
to identify the high leg, or abnormal voltage
o Voltage between one leg and grounded neutral is considerably
higher than the rest of the system
– Never use the high leg as a single-phase source because no
ground or grounded neutral exists for this circuit
Figure: Four-Wire Delta System
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Power Distribution Schemes
4-Wire, Three-Phase Wye System
• Completely different voltage
characteristics from the Delta
system
• In the Wye system, the ground
voltage or voltage available from
phase to ground is the phase
voltage divided by 1.73
• Depending on the selection of
conductors, one of the following is
available:
‒ Reduced-voltage single phase
between a phase leg and
neutral
‒ Full-voltage single-phase circuit
between any two phase legs
‒ Full-voltage three-phase power
© Copyright 2016 – Rev 2
ELO 4.6
Figure: Four-Wire, Three -Phase Wye System
Operator Generic Fundamentals
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Power Distribution Schemes
4-Wire, Three-Phase Wye System
• Size the full load ampacity of the
neutral to 1.73 times the highest
phase ampacity
• Avoids
– Over-current condition if a fault
is present
– Operation of single-phase
loads at reduced voltage if an
accidental interruption causes
severely unbalanced loads
© Copyright 2016 – Rev 2
Figure: Four-Wire, Three -Phase Wye System
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Operator Generic Fundamentals
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Power Distribution Schemes
Knowledge Check
The _________________________ is normally confined to protected
environments such as fully enclosed ducts or overhead transmission
lines that cannot be reached by personnel without extraordinary means.
A. 4-wire, three-phase delta system
B. Ungrounded delta system
C. Edison system
D. 4-wire, three-phase Wye system
Correct answer is B.
© Copyright 2016 – Rev 2
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Electrical Test Equipment
TLO 5 - Given an electrical measuring device or piece of test equipment,
describe the use of that equipment including the electrical parameter
measured.
5.1 Describe the use of the following electrical test meters: ground
detector, multimeter, megger, and synchroscope.
5.2 Describe the operation and electrical parameter measured of the
following electrical test equipment: ground detector, multimeter,
megger and synchroscope.
© Copyright 2016 – Rev 2
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Operator Generic Fundamentals
136
Use of Common Test Equipment
ELO 5.1 - Describe the use of the following electrical test meters: ground
detector, multimeter, megger, and synchroscope.
Ground Detectors
• Instrument used to detect conductor insulation resistance to ground
• An ohmmeter, or a series of lights, can detect the insulation strength
of an ungrounded distribution system
• The grounded varieties comprise most power distribution systems in
use today; however, some ungrounded systems are still used
Multimeter
• Portable, single instrument capable of measuring various electrical
values including
– Voltage
– Resistance
– Current
© Copyright 2016 – Rev 2
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Use of Common Test Equipment
Meg-Ohm Meter (Megger)
• Portable instrument used to
measure insulation resistance
• Hand-driven (DC generator and a
direct reading ohmmeter) and
newer digital versions
Synchroscope
• Indicates when two AC generators
are in the correct phase relation
for connecting in parallel
• Shows whether the incoming
generator is running faster or
slower than the on-line generator
© Copyright 2016 – Rev 2
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Wye and Delta-Connected
Knowledge Check
Parameters normally measured by installed meters include _________.
(select all that are true)
A. hysteresis
B. power (watts)
C. voltage
D. Current (amps)
Correct answers are B, C, and D.
© Copyright 2016 – Rev 2
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Operation of Common Test Equipment
ELO 5.2 - State the electrical parameters measured by a ground detector,
multimeter, megger and synchroscope.
Ground Detectors
• Measures insulation resistance to ground in ohms
Ohmmeter Method of Ground Detection
• DC voltage is applied to the conductor, if a leakage path exists
between the conductor insulator and ground, a current will flow
through the ground to the ohmmeter proportional to the insulation
resistance of the conductor
Figure: Simple Ohmmeter Ground Detector
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Operation of Common Test Equipment
Lamp-Type Ground Detector
• Set of three lamps connect through transformers to the system to
check for grounds
• The switch is closed and the brilliance of the lamps observed
– Lamps are equally bright: no ground, all the lamps receive the
same voltage
– One of the three lamps is dark, and the other two lamps are
brighter: the darkened lamp has a short to ground
Figure: Lamp-Type Ground Detector Circuit
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Operation of Common Test Equipment
Multimeter
• Most commonly used is the volt-ohm-milliammeter (VOM)
• A single meter movement indicates current, AC and DC voltage, and
resistance
• These instruments can be analog or digital devices
– Range switches provided for scale selection on analog-type
– Many digital multimeters are auto-ranging and will automatically
respond to indicate the measured parameter on the correct scale
Voltmeters
• By placing a resistor (RS), called a multiplier, in series with the
ammeter movement, and marking the meter face to read voltage as
shown in next slide
• Voltmeters are connected in parallel with the load (RL) being
measured
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Operation of Common Test Equipment
• The voltmeter draws current from
circuit causing a voltage drop
across the resistance of the meter
– Meter subtracts this voltage
drop from the voltage
measured
– Loading effect – can have a
serious effect on voltage
measurement accuracy,
especially for low current
circuits
• Countered by constructing with an
extremely high resistance to limit
the current flow through the
voltmeter
• The accuracy of a voltmeter is the
ratio of measured voltage when the
meter is in the circuit to the voltage
measured with the meter out of the
circuit
© Copyright 2016 – Rev 2
Figure: Simple DC Voltmeter
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Operator Generic Fundamentals
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Operation of Common Test Equipment
Ammeter Operation
• Measures electric current
• It may read in units of amperes, milliamperes, or microamperes
• The ammeter must be in series with the circuit to be tested, in order
to measure current
Figure: Ammeter
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Operation of Common Test Equipment
• When ammeter is in series with a circuit, it will increase the
resistance of that circuit by an amount equal to the internal resistance
of the meter Rm
• The equation without the meter installed is: 𝐼𝑜 =
𝑉
𝑅𝑜
• The next equation is the mathematical representation of the current
𝑉
with the meter installed in the circuit: 𝐼𝑤 =
𝑅𝑜 +𝑅𝑚
• The accuracy of the ammeter KA is the ratio of the current when the
meter is in the circuit, Iw, to the current with the meter out of the
circuit, Io
© Copyright 2016 – Rev 2
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Operation of Common Test Equipment
Ammeter Shunts
• An ammeter with a full scale current deflection (Im) can be shunted
with a resistor (RSH) in order to measure currents in excess of full
scale deflection current (Im) as shown below
• The reason for shunting an ammeter is to extend the range of the
ammeter and, thereby, measure currents higher than the original full
scale value
Figure: Ammeter with Shunt Installed
© Copyright 2016 – Rev 2
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Operation of Common Test Equipment
Ohmmeters
• When used as a test device, an
ohmmeter aids the troubleshooter
in determining if a ground or a short
exists in a circuit
• A simple ohmmeter consists of a
battery, a meter movement
calibrated in ohms, and a variable
resistor
• To obtain an accurate
measurement of a component’s
resistance, connect the ohmmeter
to a component removed from the
circuit
• If the component remains in the
circuit, and a parallel path exists in
the circuit, the current will flow in
the path of least resistance and
give an erroneous reading
• Ro is used to zero the ohmmeter
and correct for battery aging
– It also helps to limit current
along with Rm
• Zeroing the ohmmeter is
accomplished by shorting the
ohmmeter terminals a and b and
adjusting Ro to give full-scale
deflection
Figure: Simple Ohmmeter Circuit
© Copyright 2016 – Rev 2
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Operation of Common Test Equipment
Meg-Ohm Meter (Megger)
• Consists of two coils, A and B rigidly mounted to a pivoted central
shaft, free to rotate over a C-shaped core
– Coils connect by means of flexible leads
• Current is provided by the hand-driven generator through coil B
• With the test terminals open (infinite R) no current flows in coil A
• Thereby, coil B governs the motion causing it to move to the extreme
counter-clockwise position, which is marked as infinite resistance.
Figure: Simple Megger Circuit
© Copyright 2016 – Rev 2
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Operation of Common Test Equipment
Meg-Ohm Meter (Megger)
• With the terminals marked line and earth shorted (zero R) the current
flow through the coil A is sufficient to produce enough torque to
overcome the torque of coil B
• The pointer then moves to the extreme clockwise position, which is
marked as zero resistance
– Resistance (R2) will protect coil A from excessive current flow in
this condition
Figure: Simple Megger Circuit
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Operator Generic Fundamentals
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Operation of Common Test Equipment
Meg-Ohm Meter (Megger)
• When the test terminals, line and earth, connect across an unknown
resistance, the opposing torques of coils A and B balance each other
so that the instrument pointer comes to rest at some point on the
scale
• The scale is calibrated such that the point indicates the value of
resistance being measured, usually in MΩ
Figure: Simple Megger Circuit
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ELO 5.2
Operator Generic Fundamentals
150
Operation of Common Test Equipment
Synchroscope
• Consists of a two-phase stator
– Windings are at right angles to one another
– Current in one phase leads the current of the other phase by 90°,
thereby generating a rotating magnetic field
– The stator windings connect to the incoming generator, and a
polarizing coil connects to the running generator
• The rotating element is unrestrained and is free to rotate through
360°
• It consists of two iron vanes mounted in opposite directions on a
shaft, one at the top, and one at the bottom, magnetized by the
polarizing coil
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Operator Generic Fundamentals
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Operation of Common Test Equipment
Synchroscope
• If the frequencies of the incoming and running generators are
different, the synchroscope will rotate at a speed corresponding to
the difference
– If incoming frequency is higher than running frequency, it will
rotate in the clockwise direction
– If incoming frequency is less than running frequency, it will rotate
in the counterclockwise direction
• When the synchroscope indicates 0° phase difference, the pointer is
at the 12 o’clock position and the two AC generators are in phase
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Operator Generic Fundamentals
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Operation of Common Test Equipment
Knowledge Check
A ground detector measures __________________________.
A. insulation resistance to ground
B. difference in phase currents
C. difference in phase voltage
D. current on the neutral connection
Correct answer is A.
© Copyright 2016 – Rev 2
ELO 5.2
Operator Generic Fundamentals