Building up voltage self excited shunt generator

DIN ALI MADI-SAIF AL
Department of Mechanical Engineering/ College of Engineering/ University of Baghdad
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[Electricity Laboratory II]
University of Baghdad
Name: - Saif Al-din Ali -B-

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TABLE OF CONTENTS
Experiment Name................................................................I
Experiment Aim..............................................................II
Theory............................................................................III
Procedure........................................................................V
Data and processes........................................................VI
DISCUSSION……………….................................................VII

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1. Experiment Name: building up voltage of Self-Excited Shunt
generator
2. Experiment Aim: The object of this experiment is to verity the
factors which affect , the process of voltage build - up in a self- excited
shunt generator
3. Theory:
D.C. Machines may be magnetized by current obtained from a separate
source, such as a battery or an auxiliary dynamo called an exciter, or
may generate their own magnetizing current. These tow method of
excitation are then called 'separate excitation' and 'self- excitation'
respectively
When the field is so connected to the armature that the flux
produced by the field current assists the flux of residual magnetism then
the generated voltage build - up and the process continuous until an
equilibrium point is reached between the magnetization curve and the
field resistance line.
When the field is separately excited the generator will build up
for either polarity of field and either direction of rotation, but when the
field is self- excitation, the generator to build up its field;
1. There must be residual magnetism in the field system.
2. The connexion of the field circuit to the armature must be sueh that
the direction of the field current established is such as to tend to
increase the field already existing.
3. The total resistance of the field winding circuit must not exceed i.e.
'critical value' if appreciable, building up is to take place; this f critical
resistance increases in direct proportion to speed.
4. The speed must be higher than the critical speed for the field
resistance used

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4. Procedure:
Take the name plate data of the generator used in the experiment
 Run (1):
1 .Connect the circuit shown in fig.1 with the field circuit open (i.c. the
switch open)
2. Operate the generator at rate speed and no - load and read the voltmeter
reading due to residual magnetism.
Note which armature terminal is positive relative to this initial polarity.
Keep the speed constant..
 Run (2):
Connect the field circuit by closing the switch and change the field
regulator (FI F2) as in the following table Note. The direction of rotation if
clockwise or counter clockwise. Record all reading in the table In the fifth
expression
 Run (3):
1. Reveres the residual magnetism voltage by separate excitation
so, connect the circuit shown in fig. 2.
Note. The direction of rotation remain the same (e,g ew) but the
polarity of armature voltage will be reversed
2. Repeat step 2,4,6,8 of run 2 with fig 1
FIG 1

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5. Data and processes:
STEPS
Direction
of
rotation
Direction
of switch
The field
regulator
Armature
polarity
Direction
of field
Va If
BUILD
UP
1 CW Right IN A- ZZ-Z 55 0.05 YES
2 CW Right OUT A- ZZ-Z 220 0.36 YES
3 CW Left IN A- Z-ZZ 7 0 NO
4 CW Left OUT A- Z-ZZ 5 0 NO
5 CCW Right IN A+ Z-ZZ 4 0 NO
6 CCW Right OUT A+ Z-ZZ 4 0 NO
7 CCW Left IN A+ ZZ-Z 45 0.05 YES
8 CCW Left OUT A+ ZZ-Z 220 0.4 YES

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6. DISCUSSION:
1. Draw circuit diagram for the generator for step 3,5,7 of run 2
NO3
NO5
NO7

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2. Explain how the polarity of the generator may be kept the same with
reversed rotation
2 Factors affecting brush polarity:
(A) Reversing Armature Rotation Reverses Brush Polarity;
as dc current flows through the coil of wire wound around the iron core, a
magnetic field is produced. The amount of field current is controlled by the
resistance of the field winding and the variable resistor, known as the field
rheostat. By adjusting the field current, the strength of-the magnetic field is
controlled. The field flux or magnetic strength of the magnetic poles is
increased as the field current is increased until magnetic saturation occurs.
Saturation of the magnetic field means that no more magnetic flux can be
produced even with an increase in field current. The magnetic polarity of the
field poles is controlled by the direction of the dc field current.
The output voltage of the generator is developed as an induced voltage in the
armature conductors. This induced voltage appears at the brushes and the
generator output terminals designated as A and AA in ill.
The output voltage is directly proportional to the speed of the rotation and the
strength of the magnetic field. As the speed of the rotor is increased, the
output voltage will also increase. There is, however, a limit to the safe
operating speed of the rotor before physical damage occurs. Likewise, the
output voltage can be controlled up to a point by adjusting the field current.
(B) Reversing Field Current Reverses Brush Polarity
BRUSH POLARITY
When the armature is driven in either direction, an electrical polarity is
established at the generator output terminals and at the brushes. If the
machine is stopped and then driven in the opposite direction, the field flux is
cut in the opposite direction and the brush polarity changes, .
If the direction of rotation isn't changed and the field current is reversed, the
same effect is obtained; that's , if the armature conductors maintain a rotation
in one direction and field flux is established in the opposite direction, then the
brush polarity also changes,
As a result, the brush polarity in a separately-excited generator can be
changed by reversing the rotation of the armature or the direction of the field
current. However, if both the armature direction and field current change, the
brush polarity would remain the same (unchanged).

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3. Explain what is meant by the, critical field resistance for rated speed of
a generator.
It is the maximum resistance of the shunt field which allows the resistance line or
the characteristics between voltage across and current flowing through the shunt
field to intersect or coincide with open circuit/magnetic characteristic of a shunt
dc generator. The magnetic characteristic is variation of no load emf/voltage
across armature with rise in field excitation current (If), for a constant speed.
So, while increasing shunt field resistance, the slope between V and I in the
shunt resistance may get so as to not intersect the said open circuit
characteristic line, at which there would be failure to build up voltage inside the
dc shunt generator.Thus, field resistance should be less than that critical value.
Among these curve, three speeds are shown. For N1 speed, the straight line
B can be said to be the critical field resistance line as it can’t intersect the
open ckt characteristic curve, the slope would have been lesser, had the
shunt resistance been lesser, at which voltage build up would have been
possible. Similarly, for N2 and N3 speeds, line C and D are respective critical
field lines.
PHYSICAL CAUSE:
The underlying cause is that residual magnetism can allow only a small
voltage to build up without flow of field current, but for buildup of the rated
voltage, the armature has to cut increased flux, and this flux is provided by
increase in field current.However, if the resistance in shunt field is so high as
to constrict the shunt current, flux developed is inadequate, and emf induced
in armature by cutting that flux is inadequate

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4. Calculate the field copper loss at no -load and rated voltage for with
reversed rotation. speed of a generator. CW rotation of the armature
and with the field regulator out as found from run 2
Copper Losses
These losses occur in armature and field copper windings. Copper
losses consist of Armature copper loss, Field copper loss and loss due
to brush contact resistance.
Armature copper loss = Ia
2
Ra (where, Ia = Armature current and
Ra= Armature resistance)
This loss contributes about 30 to 40% to full load losses. The armature
copper loss is variable and depends upon the amount of loading of the
machine.
Field copper loss = If
2
Rf (where, If = field current and Rf =
field resistance)
In the case of a shunt wounded field, field copper loss is practically
constant. It contributes about 20 to 30% to full load losses.
Brush contact resistance also contributes to the copper losses.
Generally, this loss is included into armature copper loss.

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5. . Explain the methods for creating residual magnetism in generator
whose field poles were demagnetized
Self excitation is obtained by connecting capacitors across the stator terminals of the generator. When
driven by an external prime mover, a small current will be induced in the stator coils as the flux due to
the residual magnetism in the rotor cuts the windings and this current charges the capacitors. As the
rotor turns, the flux cutting the stator windings will change to the opposite direction as the orientation of
the remanent magnetic field turns with the rotor. The induced current in this case will be in the opposite
direction and will tend to discharge the capacitors. At the same time the charge released from the
capacitors will tend to reinforce the current increasing the flux in the machine. As the rotor continues to
turn the induced EMF and current in the stator windings will continue to rise until steady state is
attained, depending on the saturation of the magnetic circuit in the machine. At this operating point the
voltage and current will continue to oscillate at a given peak value and frequency determined by the
characteristics of the machine, the air gap , the slip, the load and the choice of capacitor sizes. The
combination of these factors sets maximum and minimum limits on the speed range over which self
excitation occurs. The operating slip is generally small and the variation of the frequency depends on
the operating speed range.
If the generator is overloaded the voltage will collapse rapidly (see diagram above) providing a measure
of built in self-protection.
 Control
In variable-speed operation, an induction generator needs a frequency converter to adapt the variable
frequency output of the generator to the fixed frequency of the application or the electricity supply grid.
During operation the only controllable factor available in a self excited induction generator to influence
the output is the mechanical input from the prime mover, so the system is not amenable for effective
feedback control. To provide a controllable output voltage and frequency, external AC/DC/AC converters
are required. A three-phase diode bridge is used to rectify the generator output current providing a DC
link to a three-phase thyristor inverter which converts the power from the DC link to the required voltage
and frequency.
Open Circuit Characteristic (O.C.C.) (E0/If)
Open circuit characteristic is also known as magnetic characteristic or no-load
saturation characteristic. This characteristic shows the relation between generated
emf at no load (E0) and the field current (If) at a given fixed speed. The O.C.C. curve
is just the magnetization curve and it is practically similar for all type of generators.
The data for O.C.C. curve is obtained by operating the generator at no load and
keeping a constant speed. Field current is gradually increased and the corresponding
terminal voltage is recorded. The connection arrangement to obtain O.C.C. curve is
as shown in the figure below. For shunt or series excited generators, the field winding
is disconnected from the machine and connected across an
external supply.

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Now, from the emf equation of dc generator,
we know that Eg = kɸ. Hence, the generated
emf should be directly proportional to field
flux (and hence, also directly proportional to
the field current). However, even when the
field current is zero, some amount of emf is
generated (represented by OA in the figure
below). This initially induced emf is due to the
fact that there exists some residual magnetism
in the field poles. Due to the residual
magnetism, a small initial emf is induced in the
armature. This initially induced emf aids the
existing residual flux, and hence, increasing
the overall field flux. This consequently increases the induced emf. Thus, O.C.C.
follows a straight line. However, as the flux density increases, the poles get saturated
and the ɸ becomes practically constant. Thus, even we increase the If further, ɸ
remains constant and hence, Eg also remains constant. Hence, the O.C.C. curve looks
like the B-H characteristic The above figure shows a typical no-load saturation curve
or open circuit characteristics for all types of
DC generators.

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