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ELECTRICAL BASICS
(Chapter 8)
Electrical terms
Electricity & magnetism
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Electricity
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Circuits
Magnetism
Electrical units
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Electric potential or eletromotive force
Electric current
Resistance
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resistance of a conductor is affected by (additional information):
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type of material
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length: directly proportional to the length of a conductor
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cross-section area or thickness
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temperature: directly proportional to the temperature of a conductor
Electrical power
Energy (additional information)
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The term energy is used to express work
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Energy = Power*Time
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Unit of energy is kilowatt-hour (kWh)
ELECTRICAL BASICS
Calculations and examples
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Ohm's Law (additional information)
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Current (I or A), electric potential (V or E), and resistance
(R) are related to each other, and a variance in one will
affect the others. This relationship is known as Ohm's
Law.
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Current (I or A) is directly proportional to voltage (V or E)
Current (I or A) is inversely proportional to resistance (R)
Power formulas
GENERATION AND
DISTRIBUTION (Chapter 9)
Power, work, and energy
Demand and consumption
Electric current: DC or AC
Generating efficiency
Cogeneration
GENERATION AND
DISTRIBUTION (Chapter 9)
Direct current (DC) (additional information)
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In DC, electrons always move in the same direction
Polarity of the generator remains always the same.
Alternating current (AC) (additional information)
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polarity of the generator or alternator reverses periodically
current varies periodically in value and directions, first flowing in one
direction in the circuit and then flowing in the opposite direction
Generation of AC (additional information)
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generated using a combination of physical motion and magnetism
simplest form of AC generator is constructed using a single loop of wire placed
between the poles of a permanent magnet and then rotating it by some suitable
mechanical means.
magnetic lines of force are interrupted with the rotation of loop
an electromagnetic force is induced in the loop
EMF thus produced exists between the two ends of the loop
slip rings and brushes attached to each end of the loop apply the generated EMF
to an external circuit.
GENERATION AND
DISTRIBUTION
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How the current alternates (additional information)
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Cycle of AC (additional information)
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Each complete rotation by the loop of wire (or armature coil) within the poles of magnet is called
a cycle.
Frequency of AC (additional information)
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When the loop is parallel to the magnetic lines of force, no magnetic flux is interrupted and no
EMF is induced to the loop. As it begins to rotate, it cuts the magnetic flux at an increasing rate
reaching a maximum when it has rotated a quarter turn (i.e. 90°).
As the rotation continues, the EMF is still in the same direction but is decreasing in value. A half
revolution (i.e. 180° ), the loop is again parallel to the magnetic lines and no magnetic flux is
interrupted; EMF at this point equals 0. As the rotation continues further, the sides of the loop
reverse position and the induced EMF reverses polarity; therefore, direction of the flow of
current also reverses. The EMF increases to a maximum again at 3/4 turn (i.e. 270° ) and
declines to 0 when the rotation is completed.
The number of cycles per second is known as the frequency of the voltage or current
The unit used to measure this frequency is Hertz
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In the United States, the frequency for alternating current is 60 Hertz.
Use of AC results in the reduction of transmission loss.
GENERATION AND
DISTRIBUTION (Chapter 9)
Example of transmission loss reduction
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Watt = Current2 x Resistance
If current (A or I) = 20 and resistance (R) = 4, then
transmission loss (W) = 202x4 = 1600
The loss can be reduced by decreasing R. If R were
decreased by half to 2, then W = 202x2 = 800
The loss can also be reduced by decreasing current. If it
were decreased by half to 10, then W = 102x4 = 400
It is obvious that if current is decreased by half, transmission
loss would be reduced by a factor of 4.
In case AC, the stepping up and stepping down of current
and voltage can be achieved by using a transformer.
GENERATION AND
DISTRIBUTION (Chapter 9)
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Single-phase and three-phase power
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Single-phase (additional information)
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a single armature coil creates a complete cycle of voltage
and current
requires one 'hot' wire and a neutral wire
Three phase (additional information)
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use of three separate coil conductors equally spaced (@
120° ) around generator armature
in order to obtain the value of line voltage, phase voltage
(either neutral-to-phase or phase-to-phase) has to be
multiplied by 3 or 1.73 (e.g. if single-phase voltage is 208,
then the corresponding three-phase voltage would be 208*
3 = 360)
a three-phase system with an Y-connection requires 3 'hot'
wires and a neutral wire.
GENERATION AND
DISTRIBUTION (Chapter 9)
Transformers
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Power distribution
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Transformers (additional information)
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Used for stepping up or stepping down voltages and
current
Consists of iron core surrounded by two circuit loops
(windings)
Capacity rated in kVA
Power factor (PF)
Grounding
ELECTRICAL RATING OF
EQUIPMENT (Additional
information)
Voltage rating
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Maximum voltage that can be safely applied continuously to an equipment.
The rating is primarily determined by:
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Type and quantity of conductor insulation used
Physical spacing between electrically energized parts of the equipment
Current rating
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Determined by the maximum operating temperature at which the
components of an equipment can operate continuously and properly
maximum operating temperature is determined by the type of insulation of
the conductor
the maximum safe operating temperature of conductors having cotton braid
as insulation would be 65° C
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Current which will be producing this temperature would be the maximum
permissible current for the conductor
the maximum safe operating temperature of conductors having silicone or
glass compound would be 150° C
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Consequently, the maximum permissible temperature for the same conductor
would increase