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BASIC OPERATION OF A TRANSFORMER
In its most basic form a transformer consists of:



A primary coil or winding.
A secondary coil or winding.
A core that supports the coils or windings.
Refer to the transformer circuit in figure 5-1 as you read the
following explanation: The primary winding is connected to a 60
hertz ac voltage source. The magnetic field (flux) builds up
(expands) and collapses (contracts) about the primary winding. The
expanding and contracting magnetic field around the primary winding
cuts the secondary winding and induces an alternating voltage into
the winding. This voltage causes alternating current to flow through
the load. The voltage may be stepped up or down depending on the
design of the primary and secondary windings.
Figure 5-1. - Basic transformer action.
Q.2 What are, the three basic parts of a transformer?
THE COMPONENTS OF A TRANSFORMER
Two coils of wire (called windings) are wound on some type of core
material. In some cases the coils of wire are wound on a cylindrical
or rectangular cardboard form. In effect, the core material is air
and the transformer is called an AIR-CORE TRANSFORMER. Transformers
used at low frequencies, such as 60 hertz and 400 hertz, require a
core of low-reluctance magnetic material, usually iron. This type of
transformer is called an IRON-CORE TRANSFORMER. Most power
transformers are of the iron-core type. The principle parts of a
transformer and their functions are:




The CORE, which provides a path for the magnetic lines of
flux.
The PRIMARY WINDING, which receives energy from the ac source.
The SECONDARY WINDING, which receives energy from the primary
winding and delivers it to the load.
The ENCLOSURE, which protects the above components from dirt,
moisture, and mechanical damage.
CORE CHARACTERISTICS
The composition of a transformer core depends on such factors as
voltage, current, and frequency. Size limitations and construction
costs are also factors to be considered. Commonly used core
materials are air, soft iron, and steel. Each of these materials is
suitable for particular applications and unsuitable for others.
Generally, air-core transformers are used when the voltage source
has a high frequency (above 20 kHz). Iron-core transformers are
usually used when the source frequency is low (below 20 kHz). A
soft-iron-core transformer is very useful where the transformer must
be physically small, yet efficient. The iron-core transformer
provides better power transfer than does the air-core transformer. A
transformer whose core is constructed of laminated sheets of steel
dissipates heat readily; thus it provides for the efficient transfer
of power. The majority of transformers you will encounter in Navy
equipment contain laminated-steel cores. These steel laminations
(see figure 5-2) are insulated with a nonconducting material, such
as varnish, and then formed into a core. It takes about 50 such
laminations to make a core an inch thick. The purpose of the
laminations is to reduce certain losses which will be discussed
later in this chapter. An important point to remember is that the
most efficient transformer core is one that offers the best path for
the most lines of flux with the least loss in magnetic and
electrical energy.
Figure 5-2. - Hollow-core construction.
Q.3 What are three materials commonly used as the core of a
transformer?
There are two main shapes of cores used in laminated-steel-core
transformers. One is the HOLLOW-CORE, so named because the core is
shaped with a hollow square through the center. Figure 5-2illustrates
this shape of core. Notice that the core is made up of many laminations
of steel. Figure 5-3 illustrates how the transformer windings are
wrapped around both sides of the core.
Figure 5-3. - Windings wrapped around laminations.
Shell-Core Transformers
The most popular and efficient transformer core is the SHELL CORE, as
illustrated in figure 5-4. As shown, each layer of the core consists of
E- and I-shaped sections of metal. These sections are butted together
to form the laminations. The laminations are insulated from each other
and then pressed together to form the core.
EFFECT OF A LOAD
When a load device is connected across the secondary winding of a
transformer, current flows through the secondary and the load. The
magnetic field produced by the current in the secondary interacts with
the magnetic field produced by the current in the primary. This
interaction results from the mutual inductance between the primary and
secondary windings.
MUTUAL FLUX
The total flux in the core of the transformer is common to both the
primary and secondary windings. It is also the means by which energy is
transferred from the primary winding to the secondary winding. Since
this flux links both windings, it is called MUTUAL FLUX. The inductance
which produces this flux is also common to both windings and is called
mutual inductance.
Figure 5-11 shows the flux produced by the currents in the primary and
secondary windings of a transformer when source current is flowing in
the primary winding.
Figure 5-11. - Simple transformer indicating primary- and secondarywinding flux relationship.
When a load resistance is connected to the secondary winding, the
voltage induced into the secondary winding causes current to flow in
the secondary winding. This current produces a flux field about the
secondary (shown as broken lines) which is in opposition to the flux
field about the primary (Lenz's law). Thus, the flux about the
secondary cancels some of the flux about the primary. With less flux
surrounding the primary, the counter emf is reduced and more current is
drawn from the source. The additional current in the primary generates
more lines of flux, nearly reestablishing the original number of total
flux lines.
TURNS AND CURRENT RATIOS
The number of flux lines developed in a core is proportional to the
magnetizing force (IN AMPERE-TURNS) of the primary and secondary
windings.
The ampere-turn (I X N) is a measure of magnetomotive force; it is
defined as the magnetomotive force developed by one ampere of current
flowing in a coil of one turn. The flux which exists in the core of a
transformer surrounds both the primary and secondary windings. Since
the flux is the same for both windings, the ampere-turns in both the
primary and secondary windings must be the same.
Therefore:
By dividing both sides of the equation by IpN s, you obtain:
Notice the equations show the current ratio to be the inverse of the
turns ratio and the voltage ratio. This means, a transformer having
less turns in the secondary than in the primary would step down the
voltage, but would step up the current. Example: A transformer has a
6:1 voltage ratio.
Find the current in the secondary if the current in the primary is 200
milliamperes.
The above example points out that although the voltage across the
secondary is one-sixth the voltage across the primary, the current in
the secondary is six times the current in the primary.
The above equations can be looked at from another point of view.
The expression
<figureeq4">
is called the transformer TURNS RATIO and may be expressed as a single
factor. Remember, the turns ratio indicates the amount by which the
transformer increases or decreases the voltage applied to the primary.
For example, if the secondary of a transformer has two times as many
turns as the primary, the voltage induced into the secondary will be
two times the voltage across the primary. If the secondary has one-half
as many turns as the primary, the voltage across the secondary will be
one-half the voltage across the primary. However, the turns ratio and
the current ratio of a transformer have an inverse relationship. Thus,
a 1:2 step-up transformer will have one-half the current in the
secondary as in the primary. A 2:1 step-down transformer will have
twice the current in the secondary as in the primary.
Example: A transformer with a turns ratio of 1:12 has 3 amperes of
current in the secondary. What is the value of current in the primary?
Q.20 A transformer with a turns ratio of 1:3 has what current ratio?
Q.21 A transformer has a turns ratio of 5:1 and a current of 5 amperes
flowing in the secondary. What is the current flowing in the primary?
(Assume no losses)
Losses in Transformer
Transformer losses are produced by the electrical current flowing in the coils and the magnetic field
alternating in the core. The losses associated with the coils are called the load losses, while the losses
produced in the core are called no-load losses.
What Are Load Losses?
Load losses vary according to the loading on the transformer. They include heat losses and eddy currents in
the primary and secondary conductors of the transformer.
Heat losses, or I2R losses, in the winding materials contribute the largest part of the load losses. They are
created by resistance of the conductor to the flow of current or electrons. The electron motion causes the
conductor molecules to move and produce friction and heat. The energy generated by this motion can be
calculated using the formula:
Watts = (volts)(amperes) or VI.
According to Ohm’s law, V=RI, or the voltage drop across a resistor equals the amount of resistance in the
resistor, R, multiplied by the current, I, flowing in the resistor. Hence, heat losses equal (I)(RI) or I2R.
Transformer designers cannot change I, or the current portion of the I 2R losses, which are determined by the
load requirements. They can only change the resistance or R part of the I 2R by using a material that has a
low resistance per cross-sectional area without adding significantly to the cost of the transformer. Most
transformer designers have found copper the best conductor considering the weight, size, cost and
resistance of the conductor. Designers can also reduce the resistance of the conductor by increasing the
cross-sectional area of the conductor.
What Are No-load Losses?
No-load losses are caused by the magnetizing current needed to energize the core of the transformer, and
do not vary according to the loading on the transformer. They are constant and occur 24 hours a day, 365
days a year, regardless of the load, hence the term no-load losses. They can be categorized into five
components: hysteresis losses in the core laminations, eddy current losses in the core laminations, I2R
losses due to no-load current, stray eddy current losses in core clamps, bolts and other core components,
and dielectric losses. Hysteresis losses and eddy current losses contribute over 99% of the no-load losses,
while stray eddy current, dielectric losses, and I2R losses due to no-load current are small and consequently
often neglected. Thinner lamination of the core steel reduces eddy current losses.
The biggest contributor to no-load losses is hysteresis losses. Hysteresis losses come from the molecules in
the core laminations resisting being magnetized and demagnetized by the alternating magnetic field. This
resistance by the molecules causes friction that results in heat. The Greek word, hysteresis, means "to lag"
and refers to the fact that the magnetic flux lags behind the magnetic force. Choice of size and type of core
material reduces hysteresis losse
Voltage Regulation
Many people mistake this to mean that a transformer with 10% regulation will keep the output voltage to a
value within 10% of nominal. That's simply not so. Let's take a look at what transformer voltage regulation is,
and why it's useful to you.
In any step down transformer, the secondary current produces voltage drop across the resistive and reactive
components of the transformer's secondary side. On the other side, the primary current produces voltage
drops across the resistive and reactive components of the transformer's primary side. From this, it's easy to
see the primary voltage will be less than the supply voltage, and the secondary (output) will be less than
either of those.
Let's assume you have no load connected to your transformer. In such a case, no secondary current flows.
With no current, you have no voltage drop across those resistive and reactive components of the
transformer's secondary side. But, another thing happens. Without a secondary current, the primary current
drops to the no-load current—which is nearly zero. This means the voltage drop across the resistive and
reactive components of the transformer's primary side becomes very small. What's the net effect? In a noload situation, the voltage on the primary is almost equal to the supply voltage, and the secondary voltage
nearly equals the supply voltage times the ratio of primary windings to secondary windings.
You might assume the transformer's output voltage is highest at no load. It would then make sense that
(under loaded conditions) the transformer's resistive and reactive components cause the output voltage to
drop below its no-load level. This is a logical assumption, but one that's not necessarily so. Depending on
the power factor of the load, the output full-load voltage may actually be larger than the no-load voltage.
The voltage regulation of the transformer is the percentage change in the output voltage from no-load to fullload. And since power factor is a determining factor in the secondary voltage, power factor influences
voltage regulation. This means the voltage regulation of a transformer is a dynamic, load-dependent
number. The numbers you see in the nameplate data are fixed; the number of primary windings won't
change; the number of secondary windings won't change, etc. But, the voltage regulation will vary as power
factor varies.
Ideally, there should be no change in the transformer's output voltage from no-load to full-load. In such a
case, we say the voltage regulation is 0%. To get the best performance out of your transformer, you need
the lowest possible voltage regulation. You should calculate the voltage regulation and save the result as a
troubleshooting and predictive maintenance benchmark. Suppose the percentage change is too high. What
do you do? You now know you need to look at power factor correction for the loads on that transformer. A
power factor meter can be very helpful in this case
INTRODUCTION
In electrical engineering, it is often useful to use an equivalent circuit model to describe
the non-ideal operation of a device such as a transformer. While an ideal model may be
well suited for rough approximations, the non-ideal parameters are needed for careful
transformer circuit designs. Knowing the non-ideal parameters allows the engineer to
optimize a design using equations rather than inefficiently spending time testing physical
implementations in the lab.
If all dimensions and material properties of a transformer are known, the non-ideal
parameters can be directly calculated. However, this is usually not the case, and a simple
technique for obtaining the parameters can be used. A method for determining the
parameters of the equivalent circuit model using two simple tests is described.
Expressions for calculating the parameters are derived in terms of laboratory
measurements. The procedure is performed in the lab for a transformer. As an example of
the usefulness of the non-ideal equivalent circuit, the parameters found in the lab are used
to calculate one important transformer characteristic, maximum efficiency.
1.0 ANALYSIS
1.1 MODEL
The equivalent circuit model for the non-ideal transformer is shown in Figure 1. An ideal
transformer with resistors and inductors in parallel and series replaces the non-ideal
transformer. This model is called the high side equivalent circuit model because all
parameters have been moved to the primary side of the ideal transformer. The series
resistance, Req, is the resistance of the copper winding. The series inductance, Xeq,
accounts for the flux leakage. That is, a small amount of flux travels through the air
outside the magnetic core path. The parallel resistance, Rm, represents the core loss of the
magnetic core material due to hysteresis. The parallel inductance, Xm, called the
magnetizing inductance, accounts for the finite permeability of the magnetic core.
Figure 1. High side transformer equivalent circuit model.
It is easy to see how each parameter of the equivalent circuit model could be adjusted by
changing the transformer design. For example, increasing the diameter of the wire in the
windings decreases the series resistance. Therefore, the equivalent circuit model
parameters can be used as a way to evaluate a transformer, or compare transformers.
The parameters can be found in the same way that Thevenin equivalent circuit parameters
are found: open circuit and short circuit tests. The parallel parameter values are found
with no load connected to the secondary (open circuit) and the series parameter values are
found with the secondary terminals shorted (short circuit). It is possible, for convenience
in the lab, to make the tests on either the primary or the secondary. Figure 2 shows the
equivalents circuits for the two tests. For the open circuit test, the series parameters are
neglected for convenience. This is reasonable since the voltage drops are across Req and
Xeq are normally small.
Figure 2. Equivalent circuits for tests. (a) Open circuit. (b) Short circuit.
Expressions for the non-ideal transformer parameters are derived from the equivalent
circuits shown in Figure 2. The results are Equations (1), (2), (3), and (4). All parameters
are expressed in terms of quantities measured in the open circuit and short circuit tests.
(1)
(2)
(3)
(4)
1.2 SAMPLE CALCULATIONS
For open circuit measurements of Voc=114.81 VAC, ioc=0.24 A, and Poc=6.4 W, the
parallel parameters of the transformer are calculated in Equations (5) and (6).
(5)
(6)
For short circuit measurements of Vsc=11.14 VAC, isc=3.88 A, Psc=6.1 W, the series
parameters of the transformer are calculated in Equations (7) and (8).
(7)
(8)
2.0 EXPERIMENT
2.1 Description of THE EXPERIMENT AND SETUP
A 1:1 transformer was tested in the lab to determine its non-ideal parameter values.
Figure 3 shows the wiring diagram used to make the open circuit test. With the secondary
open, the primary voltage was increased from zero to rated voltage, where the rated
voltage is the name plate stamp. A digital multimeter was used as an ammeter to measure
the open circuit current. A wattmeter was used to measure the open circuit power. The
power measured was the power dissipated in Rm, the core losses.
Figure 3. Wiring diagram for open circuit test.
The short circuit wiring diagram is shown in Figure 4. With the secondary terminals
shorted, the primary voltage was increased from zero until the rated current was reached
in the primary. At this point the primary voltage was measured. It was much less than
rated voltage. Again, the power and current were measured.
Figure 4. Wiring diagram for short circuit test.
2.2 PRESENTATION OF MEASURED DATA
Using the parameters of the non-ideal transformer equivalent circuit model, the peak
efficiency of the transformer can be calculated. For the transformer tested in the lab, the
results are shown in are Equations (5), (6), (7), and (8). The values for Req and Rm can be
used to find the minimum current, IF, and the maximum current, IM.
(9)
(10)
The maximum efficiency is calculated in Equation (11).
(11)
3.0 RESULTS AND COMPARISON
The experimental results obtained from the open circuit and short circuit tests were not
evaluated. It would be possible to test the maximum efficiency of the transformer by
setting the load so that the transformer is operating at maximum efficiency. The actual
efficiency of the transformer could be found by dividing the power out by the power in.
This value should be close to the value found in Equation (11).
4.0 CONCLUSIONS
The procedure used to find the parameter values of the non-ideal transformer equivalent
circuit model allows the engineer to more efficiently design transformer circuits.
Modeling and simulation are more accurate when the non-ideal parameters are used. This
means that designs can be optimized prior to implementation.
Three Phase Transformers Introduction:
Three phase transformers are used throughout industry to change values of
three phase voltage and current. Since three phase power is the most common
way in which power is produced, transmitted, an used, an understanding of how
three phase transformer connections are made is essential. In this section it will
discuss different types of three phase transformers connections, and present
examples of how values of voltage and current for these connections are
computed.
Three Phase Transformer Construction:
A three phase transformer is constructed by winding three single phase
transformers on a single core. These transformers are put into an enclosure
which is then filled with dielectric oil. The dielectric oil performs several functions.
SInce it is a dielectric, a nonconductor of electricity, it provides electrical
insulation between the windings and the case. It is also used to help provide
cooling and to prevent the formation of moisture, which can deteriorate the
winding insulation.
Three-Phase Transformer Connections:
There are only 4 possible transformer combinations:
1.
2.
3.
4.
Delta to Delta - use: industrial applications
Delta to Wye - use : most common; commercial and industrial
Wye to Delta - use : high voltage transmissions
Wye to Wye - use : rare, don't use causes harmonics and balancing
problems.
Three-phase transformers are connected in delta or wye configurations. A wyedelta transformer has its primary winding connected in a wye and its secondary
winding connected in a delta (see figure 1-1). A delta-wye transformer has its
primary winding connected in delta and its secondary winding connected in a
wye (see figure 1-2).
Figure 1-1: Wye-Delta
connection
Figure 1-2: Delta-Wye
connection
Delta Conections:
A delta system is a good short-distance distribution system. It is used for
neighborhood and small commercial loads close to the supplying substation.
Only one voltage is available between any two wires in a delta system. The delta
system can be illlustrated by a simple triangle. A wire from each point of the
triangle would represent a three-phase, three-wire delta system. The voltage
would be the same between any two wires (see figure 1-3).
Figure 1-3:
Wye Connections:
In a wye system the voltage between any two wires will always give the same
amount of voltage on a three phase system. However, the voltage between any
one of the phase conductors (X1, X2, X3) and the neutral (X0) will be less than
the power conductors. For example, if the voltage between the power conductors
of any two phases of a three wire system is 208v, then the voltage from any
phase conductor to ground will be 120v. This is due to the square root of three
phase power. In a wye system, the voltage between any two power conductors
will always be 1.732 (which is the square root of 3) times the voltage between the
neutral and any one of the power phase conductors. The phase-to-ground
voltage can be found by dividing the phase-to-phase voltage by 1.732 (see figure
1-4).
Figure 1-4:
Connecting Single-Phase Transformers into a Three-Phase Bank:
If three phase transformation is need and a three phase transformer of the proper
size and turns ratio is not available, three single phase transformers can be
connected to form a three phase bank. Whenn three single phase transformers
are used to make a three phase transformer bank, their primary and secondary
windings are conected in a wye or delta conection. The three transformer
windings in figure 1-5 are labeled H1 and the other end is labeled H2. One end of
each secondary lead is labeled X1 and the other end is labeled X2.
Figure 1-5:
Figure 1-6 shows three single phase transformers labeled A, B, and C. The
primary leads of each transformer are labeled H1 and H2 and the secondary
leads are labeled X1 and X2. The schematic diagram of figure 1-5 will be used to
connect the three single phase transformers into a three phase wye-delta
connection as shown in figure 1-7.
Figure 1-6:
Figure 1-7:
The primary winding will be tied into a wye coneection first. The schematic in
figure 1-5 shows, that the H2 leads of the three primary windings are connected
together, and the H1 lead of each winding is open for connection to the incoming
power line. Notice in figure 1-7 that the H2 leads of the primary windings are
connected together, and the H1 lead of each winding has been connected to the
incoming primary power line.
Figure 1-5 shows that the X1 lead of the transformer A is connected to the X2
lead of transformer c. Notice that this same conection has been made in figure 17. The X1 lead of transformer B is conected to X1, lead of transformer A, and the
X1 lead of transformer B is connected to X2 lead of transformer A, and the X1
lead of transformer C is connected to X2 lead of transformer B. The load is
conected to the points of the delta connection.
Open Delta Connection:
The open delta transformer connection can be made with only two transformers
instead of three (figure 1-8). This connection is often used when the amount of
three phase power needed is not excessive, such as a small business. It should
be noted that the output power of an open delta connection is only 87% of the
rated power of the two transformers. For example, assume two transformers,
each having a capacity of 25 kVA, are connected in an open delta connection.
The total output power of this connection is 43.5 kVA (50 kVA x 0.87 = 43.5 kVA).
Figure 1-8: Open Delta
Connection
Another figure given for this calculation is 58%. This percentage assumes a
closed delta bank containing 3 transformers. If three 25 kVA transformers were
connected to form a closed delta connection, the total output would be 75 kVA (3
x 25 = 75 kVA). If one of these transformers were removed and the transformer
bank operated as an open delta connection, the output power would be reduced
to 58% of its original capacity of 75 kVA. The output capacity of the open delta
bank is 43.5 kVA (75 kVA x .58% = 43.5 kVA).
The voltage and current values of an open delta connection are computed in the
same manner as a standard delta-delta connection when three transformers are
employed. The voltage and current rules for a delta connection must be used
when determining line and phase values of voltage current.
Closing a Delta:
When closing a delta system, connections should be checked for proper polarity
before making the final connection and applying power. If the phase winding of
one transformer is reversed, an extremely high current will flow when power is
applied. Proper phaseing can be checked with a voltmeter at delta opening. If
power is applied to the transformer bank before the delta connection is closed,
the voltmeter should indicate 0 volts. If one phase winding has been reversed,
however, the voltmeter will indicate double the amount of voltage.
It should be noted that a voltmeter is a high impedance device. It is not unusual
for a voltmeter to indicate some amount of voltage before the delta is closed,
especially if the primary has been conected as a wye and the secondary as a
delta. When this is the case, the voltmeter will generally indicate close to the
normal output voltage if the connection is correct and double the output voltage if
the connection is incorrect.
Motor
AC Generator (Alternator)
Transformer
An electrical Generator is a machine which converts mechanical energy
(or power) into electrical energy (or power).
Principle :
It is based on the principle of production of dynamically (or motionally) induced
e.m.f (Electromotive Force). Whenever a conductor cuts magnetic flux, dynamically
induced e.m.f. is produced in it according to Faraday's Laws of Electromagnetic
Induction. This e.m.f. causes a current to flow if the conductor circuit is closed.
Hence, the basic essential parts of an electric generator are :
A magnetic field and
A conductor or conductors which can so move as to cut the flux.
Construction :
A single-turn rectangular copper coil abcd moving about its own axis in a
magnetic field provided by either permanent magnets or electromagnets. The two
ends of the coil are joined to two split-rings which are insulated from each other and
from the central shaft. Two collecting brushes (of carbon or copper) press against
the slip rings.
Secondary Winding
from which energy is drawn out, is
called secondary winding.
In it's simplest form it consist of, two inductive coils which are electrically
separated but magnetically linked through a path of low reluctance. If one coil
(primary) is connected to source of alternating voltage, an alternating flux is set up
in the
laminated core, most of which is linked with the other coil in which it produces mutuallyinduced e.m.f. (according to Faraday's Laws of Electromagnetic Induction. If the second
coil (secondary) circuit is closed, a current flows in it and so electric energy is
transferred (entirely magnetically) from the first coil to the second coil.
DC Generator
AC Generator (Alternator)
Motor
A transformer is a static piece of apparatus by means of which electric power
in one circuit is transformed into electric power of the same frequency in another
circuit. It can raise or lower the voltage in a circuit but with a corresponding
decrease or increase in current.
Principle :
Transformer Principle
The basic principle of a transformer is mutual induction
between two circuits linked by a common magnetic flux.
In brief, a transformer is a device that
transfers electric power from one circiut to another.
it does so without a change of frequency.
it accomplishes this by electromagnetic induction and
where the two circuit are in mutual inductive influence of
each
other.
Let
N1 = No. of turns in primary
N2 = No. of turns in secondary
m = Maximum flux in core in webers = Bm x A
f = Frequency of a.c. input in Hz.
The flux increases from it's zero value to maximum value m in one quarter of the
cycle i.e. in 1/4 f second.
Therefore, r.m.s value of e.m.f./turn = 4.44 m volts
Now, r.m.s value of induced e.m.f in the whole primary winding
= ( induced e.m.f. / turn ) x No. of primary winding
E1 = 4.44 f N1m
------------------- ( i )
Similarly, r.m.s. value of e.m.f. induced in secondary is,
E2 = 4.44 f N2m
------------------- ( ii )
From the above equations (i) and (ii), we get
(i)
If
= K
K>1, then the transformer is called step-up transformer.
(ii)
If
K<1, then the transformer is called step-down transformer.
THE ELEMENTARY GENERATOR
The simplest elementary generator that can be built is an ac generator.
Basic generating principles are most easily explained through the use
of the elementary ac generator. For this reason, the ac generator will
be discussed first. The dc generator will be discussed later.
An elementary generator (fig. 1-2) consists of a wire loop placed so
that it can be rotated in a stationary magnetic field. This will
produce an induced emf in the loop. Sliding contacts (brushes) connect
the loop to an external circuit load in order to pick up or use the
induced emf.
Figure 1-2. - The elementary generator.
The pole pieces (marked N and S) provide the magnetic field. The pole
pieces are shaped and positioned as shown to concentrate the magnetic
field as close as possible to the wire loop. The loop of wire that
rotates through the field is called the ARMATURE. The ends of the
armature loop are connected to rings called SLIP RINGS. They rotate
with the armature. The brushes, usually made of carbon, with wires
attached to them, ride against the rings. The generated voltage appears
across these brushes.
The elementary generator produces a voltage in the following manner
(fig. 1-3). The armature loop is rotated in a clockwise direction. The
initial or starting point is shown at position A. (This will be
considered the zero-degree position.) At 0° the armature loop is
perpendicular to the magnetic field. The black and white conductors of
the loop are moving parallel to the field. The instant the conductors
are moving parallel to the magnetic field, they do not cut any lines of
flux. Therefore, no emf is induced in the conductors, and the meter at
position A indicates zero. This position is called the NEUTRAL PLANE.
As the armature loop rotates from position A (0°) to position B (90°),
the conductors cut through more and more lines of flux, at a
continually increasing angle. At 90° they are cutting through a maximum
number of lines of flux and at maximum angle. The result is that
between 0° and 90°, the induced emf in the conductors builds up from
zero to a maximum value. Observe that from 0° to 90°, the black
conductor cuts DOWN through the field. At the same time the white
conductor cuts UP through the field. The induced emfs in the conductors
are series-adding. This means the resultant voltage across the brushes
(the terminal voltage) is the sum of the two induced voltages. The
meter at position B reads maximum value. As the armature loop continues
rotating from 90° (position B) to 180° (position C), the conductors
which were cutting through a maximum number of lines of flux at
position B now cut through fewer lines. They are again moving parallel
to the magnetic field at position C. They no longer cut through any
lines of flux. As the armature rotates from 90° to 180°, the induced
voltage will decrease to zero in the same manner that it increased
during the rotation from 0° to 90°. The meter again reads zero. From 0°
to 180° the conductors of the armature loop have been moving in the
same direction through the magnetic field. Therefore, the polarity of
the induced voltage has remained the same. This is shown by points A
through C on the graph. As the loop rotates beyond 180° (position C),
through 270° (position D), and back to the initial or starting point
(position A), the direction of the cutting action of the conductors
through the magnetic field reverses. Now the black conductor cuts UP
through the field while the white conductor cuts DOWN through the
field. As a result, the polarity of the induced voltage reverses.
Following the sequence shown by graph points C, D, and back to A, the
voltage will be in the direction opposite to that shown from points A,
B, and C. The terminal voltage will be the same as it was from A to C
except that the polarity is reversed (as shown by the meter deflection
at position D). The voltage output waveform for the complete revolution
of the loop is shown on the graph in figure 1-3.
Figure 1-3. - Output voltage of an elementary generator during one
revolution.
ELECTROMAGNETIC POLES
Nearly all practical generators use electromagnetic poles instead of
the permanent magnets used in our elementary generator. The
electromagnetic field poles consist of coils of insulated copper wire
wound on soft iron cores, as shown in figure 1-6. The main advantages
of using electromagnetic poles are (1) increased field strength and (2)
a means of controlling the strength of the fields. By varying the input
voltage, the field strength is varied. By varying the field strength,
the output voltage of the generator can be controlled.
Figure 1-6. - Four-pole generator (without armature).
Q.9 How can field strength be varied in a practical dc generator?
COMMUTATION
Commutation is the process by which a dc voltage output is taken from
an armature that has an ac voltage induced in it. You should remember
from our discussion of the elementary dc generator that the commutator
mechanically reverses the armature loop connections to the external
circuit. This occurs at the same instant that the voltage polarity in
the armature loop reverses. A dc voltage is applied to the load because
the output connections are reversed as each commutator segment passes
under a brush. The segments are insulated from each other.
In figure 1-7, commutation occurs simultaneously in the two coils that
are briefly short-circuited by the brushes. Coil B is short-circuited
by the negative brush. Coil Y, the opposite coil, is short-circuited by
the positive brush. The brushes are positioned on the commutator so
that each coil is short-circuited as it moves through its own
electrical neutral plane. As you have seen previously, there is no
voltage generated in the coil at that time. Therefore, no sparking can
occur between the commutator and the brush. Sparking between the
brushes and the commutator is an indication of improper commutation.
Improper brush placement is the main cause of improper commutation.
Figure 1-7. - Commutation of a dc generator.
THE ELEMENTARY DC GENERATOR
A single-loop generator with each terminal connected to a segment of a
two-segment metal ring is shown in figure 1-4. The two segments of the
split metal ring are insulated from each other. This forms a simple
COMMUTATOR. The commutator in a dc generator replaces the slip rings of
the ac generator. This is the main difference in their construction.
The commutator mechanically reverses the armature loop connections to
the external circuit. This occurs at the same instant that the polarity
of the voltage in the armature loop reverses. Through this process the
commutator changes the generated ac voltage to a pulsating dc voltage
as shown in the graph of figure 1-4. This action is known as
commutation. Commutation is described in detail later in this chapter.
Figure 1-4. - Effects of commutation.
For the remainder of this discussion, refer to figure 1-4,parts A
through D. This will help you in following the step-by-step description
of the operation of a dc generator. When the armature loop rotates
clockwise from position A to position B, a voltage is induced in the
armature loop which causes a current in a direction that deflects the
meter to the right. Current flows through loop, out of the negative
brush, through the meter and the load, and back through the positive
brush to the loop. Voltage reaches its maximum value at point B on the
graph for reasons explained earlier. The generated voltage and the
current fall to zero at position C. At this instant each brush makes
contact with both segments of the commutator. As the armature loop
rotates to position D, a voltage is again induced in the loop. In this
case, however, the voltage is of opposite polarity.
The voltages induced in the two sides of the coil at position D are in
the reverse direction to that of the voltages shown at position B. Note
that the current is flowing from the black side to the white side in
position B and from the white side to the black side in position D.
However, because the segments of the commutator have rotated with the
loop and are contacted by opposite brushes, the direction of current
flow through the brushes and the meter remains the same as at position
B. The voltage developed across the brushes is pulsating and
unidirectional (in one direction only). It varies twice during each
revolution between zero and maximum. This variation is called RIPPLE.
A pulsating voltage, such as that produced in the preceding
description, is unsuitable for most applications. Therefore, in
practical generators more armature loops (coils) and more commutator
segments are used to produce an output voltage waveform with less
ripple.
ARMATURE REACTION
From previous study, you know that all current-carrying conductors
produce magnetic fields. The magnetic field produced by current in the
armature of a dc generator affects the flux pattern and distorts the
main field. This distortion causes a shift in the neutral plane, which
affects commutation. This change in the neutral plane and the reaction
of the magnetic field is called ARMATURE REACTION.
You know that for proper commutation, the coil short-circuited by the
brushes must be in the neutral plane. Consider the operation of a
simple two-pole dc generator, shown in figure 1-8. View A of the figure
shows the field poles and the main magnetic field. The armature is
shown in a simplified view in views B and C with the cross section of
its coil represented as little circles. The symbols within the circles
represent arrows. The dot represents the point of the arrow coming
toward you, and the cross represents the tail, or feathered end, going
away from you. When the armature rotates clockwise, the sides of the
coil to the left will have current flowing toward you, as indicated by
the dot. The side of the coil to the right will have current flowing
away from you, as indicated by the cross. The field generated around
each side of the coil is shown in view B of figure 1-8. This field
increases in strength for each wire in the armature coil, and sets up a
magnetic field almost perpendicular to the main field.
Figure 1-8. - Armature reaction.
Now you have two fields - the main field, view A, and the field around
the armature coil, view B. View C of figure 1-8 shows how the armature
field distorts the main field and how the neutral plane is shifted in
the direction of rotation. If the brushes remain in the old neutral
plane, they will be short-circuiting coils that have voltage induced in
them. Consequently, there will be arcing between the brushes and
commutator.
To prevent arcing, the brushes must be shifted to the new neutral
plane.
Q.11 What is armature reaction?
COMPENSATING WINDINGS AND INTERPOLES
Shifting the brushes to the advanced position (the new neutral plane)
does not completely solve the problems of armature reaction. The effect
of armature reaction varies with the load current. Therefore, each time
the load current varies, the neutral plane shifts. This means the brush
position must be changed each time the load current varies.
In small generators, the effects of armature reaction are reduced by
actually mechanically shifting the position of the brushes. The
practice of shifting the brush position for each current variation is
not practiced except in small generators. In larger generators, other
means are taken to eliminate armature reaction. COMPENSATING WINDINGS
or INTERPOLES are used for this purpose (fig. 1-9). The compensating
windings consist of a series of coils embedded in slots in the pole
faces. These coils are connected in series with the armature. The
series-connected compensating windings produce a magnetic field, which
varies directly with armature current. Because the compensating
windings are wound to produce a field that opposes the magnetic field
of the armature, they tend to cancel the effects of the armature
magnetic field. The neutral plane will remain stationary and in its
original position for all values of armature current. Because of this,
once the brushes have been set correctly, they do not have to be moved
again.
Figure 1-9. - Compensating windings and interpoles.
Another way to reduce the effects of armature reaction is to place
small auxiliary poles called "interpoles" between the main field poles.
The interpoles have a few turns of large wire and are connected in
series with the armature. Interpoles are wound and placed so that each
interpole has the same magnetic polarity as the main pole ahead of it,
in the direction of rotation. The field generated by the interpoles
produces the same effect as the compensating winding. This field, in
effect, cancels the armature reaction for all values of load current by
causing a shift in the neutral plane opposite to the shift caused by
armature reaction. The amount of shift caused by the interpoles will
equal the shift caused by armature reaction since both shifts are a
result of armature current.
Generator Losses
In dc generators, as in most electrical devices, certain forces act to decrease the
efficiency. These forces, as they affect the armature, are considered as losses and
may be defined as follows:
1. Copper loss in the winding 2. Magnetic Losses 3. Mechanical Losses
Copper loss
The power lost in the form of heat in the armature winding of a generator is known
as Copper loss. Heat is generated any time current flows in a conductor.
loss is the Copper loss, which increases as current increases. The amount of
heat generated is also proportional to the resistance of the conductor. The resistance
of the conductor varies directly with its length and inversely with its cross- sectional
area. Copper loss is minimized in armature windings by using large diameter
wire.Copper loss is again divided as
(i) Armature copper loss
= Armature copper loss. Where Ra =resistance of armature and interpoles
and series field winding etc. This loss is about 30 to 40% of full -load losses.
(ii) Field copper loss : It is the loss in series or shunt field of generator.
is the field copper loss in case of series generators, where Rse is the resistance of
the series field widing.
is the field copper loss in case of shunt generators.
This loss is about 20 to 30% of F.L losses.
(iii) The loss due to brsh contact resistance.It is usually inluded in the armture
copper loss.
Magnetic Losses (also known as iron or core losses)
(i) Hysteresis loss (Wh)
Hysteresis loss is a heat loss caused by the magnetic properties of the armature.
When an armature core is in a magnetic field, the magnetic particles of the core tend
to line up with the magnetic field. When the armature core is rotating, its magnetic
field keeps changing direction. The continuous movement of the magnetic particles,
as they try to align themselves with the magnetic field, produces molecular friction.
This, in turn, produces heat. This heat is transmitted to the armature windings. The
heat causes armature resistances to increase. To compensate for hysteresis losses,
heat-treated silicon steel laminations are used in most dc generator armatures. After
the steel has been formed to the proper shape, the laminations are heated and
allowed to cool. This annealing process reduces the hysteresis loss to a low value.
(ii) Eddy Current Loss (We)
The core of a generator armature is made from soft iron, which is a conducting
material with desirable magnetic characteristics. Any conductor will have currents
induced in it when it is rotated in a magnetic field. These currents that are induced in
the generator armature core are called EDDY CURRENTS. The power dissipated in
the form of heat, as a result of the eddy currents, is considered a loss.
Eddy currents, just like any other electrical currents, are affected by the resistance of
the material in which the currents flow. The resistance of any material is inversely
proportional to its cross-sectional area. Figure, view A, shows the eddy currents
induced in an armature core that is a solid piece of soft iron. Figure, view B, shows a
soft iron core of the same size, but made up of several small pieces insulated from
each other. This process is called lamination. The currents in each piece of the
laminated core are considerably less than in the solid core because the resistance of
the pieces is much higher. (Resistance is inversely proportional to cross-sectional
area.) The currents in the individual pieces of the laminated core are so small that
the sum of the individual currents is much less than the total of eddy currents in the
solid iron core.
As you can see, eddy current losses
are kept low when the core material is made up of many thin sheets of metal.
Laminations in a small generator armature may be as thin as 1/64 inch. The
laminations are insulated from each other by a thin coat of lacquer or, in some
instances, simply by the oxidation of the surfaces. Oxidation is caused by contact
with the air while the laminations are being annealed. The insulation value need not
be high because the voltages induced are very small.
Most generators use armatures with laminated cores to reduce eddy current losses.
These magnetic losses are practically constant for shunt and compound-wound
generators, because in their case, field current is constant.
Mechanical or Rotational Losses
These consist of
(i) friction loss at bearings and comutator.
(ii) air-friction or windage loss of rotating armature
These are about 10 to 20% of F.L losses.
Careful maintenance can be instrumental in keeping bearing friction to a minimum.
Clean bearings and proper lubrication are essential to the reduction of bearing
friction.Brush friction is reduced by assuring proper brush seating, using proper
brushes, and maintaining proper brush tension. A smooth and clean commutator also
aids in the reduction of brush friction.
Usually, magnetic and mechanical losses are collectively known as Stray Losses.
These are also known as rotational losses for obvious reasons.
As said above, field Cu loss is constant for shunt and compound generators.Hence,
stray losses and shunt Cu loss are constant in their case.These losses are together
known as standing or constant losses Wc.
Hence, for shunt and compound generators,
Total loss = armature copper loss + Wc
Armature Cu loss is known as variable loss because it varies with the load current.
Total loss = Variable loss + constant losses Wc