Download Elect.machine digita..

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.
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
The
The
and
The
and
CORE, which provides a path for the magnetic lines of flux.
PRIMARY WINDING, which receives energy from the ac source.
SECONDARY WINDING, which receives energy from the primary winding
delivers it to the load.
ENCLOSURE, which protects the above components from dirt, moisture,
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 52) 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.
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.
Figure 5-4. - Shell-type core construction.
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 secondary-winding 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 ampereturns 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 onesixth 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 onehalf 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?
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 I2R losses, which are
determined by the load requirements. They can only change the resistance or R part of the I2R
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 no-load 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 full-load. 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 fullload. 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
EQUIVALENT CIRCUIT
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 wye-delta 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 1-7. 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 mutually-induced 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
(ii) If
= K
K>1, then the transformer is called step-up transformer.
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 seriesadding. 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. 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 brush contact resistance. It is usually included in the armature
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
BASIC OPERATION OF A TRANSFORMER