final year main pdf ee- 4207
TRANSCRIPT
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Department of Electrical Engineering, SGSITS, Indore
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SGSITS-Machines Lab
Electrical Machines-III (EE-4201)
SHRI G.S. INSTITUTE OF TECHNOLOGY AND SCIENCE, INDORE
DEPARTMENT OF ELECTRICAL ENGINEERING
ELECTRICAL MACHINE LABORATORY
Expt. No. .. Date: .
Remarks(If any) ... Signature of the staff member
OBJECT:To determine the transfer function of D.C. machine.
SPECIFICATIONS OF THE D.C. MACHINE:
REQURIMENT/EQUIPMENTS:
THEORY:
Most of the D.C. motors in servo applications are of separately excited
type. The output of D.C. amplifier can be connected to either field terminals or to the
armature terminals of the motor. When the field is energized by the amplifier signal,the motor is said to be field controlled and if the armature is supplied by the
amplifier, it is said to be armature controlled.
A
AA
Z
ZZ
MD.C.
Amplifier
Error
Signal
Ia(constant)
LaLf af
Figure 2.1- Field controlled DC motor
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A
AA
Z
ZZ
MD.C.
Amplifier
LaLf af
If (constant)
Error
Signal
Figure 2.2- Armature controlled DC motor
Transfer function of any system in general is a ration of Laplace
transform of the output to the input with all initial conditions zero. It can be a pure
numeric having no units or may be real or complex and may have unit depending upon
the choice of input and output quantities. In the most simplified form, an open loop
system can be described as shown in figure below:
G(s)R(s) C(s)
1 2
Figure 2.3- Block Diagram of control system
Where the number 1 and 2 represent the input and output nodes, R(s) - input and C(s)
-output. (The arrow represents the direction of signal flow.)
The transfer function of such a system is G(s), defined by following relation:
G(s) =C(s)
R(s)
In more complicated closed loop systems, there will be one or more than on
nodes in addition to the input and output nodes as well as feedback loops. The
solution of overall transfer function can be found out by applying basic signal flow
algebra or Massions Gain formula. The signal flow technique forms an essential part
of a feedback system and therefore necessitates the determinations of the transfer
function.
In this experiment we shall determine the transfer function of an armature
controlled D.C. motor. (See figure 2.2)
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We wish to determine the transfer function
G(s) =(s)
Va (s)
We have, Ia(s) = (Va sEb s)Ra (1+sRa ) ..(1)
Where Ta= La/Ra
Eb= K.If where K = ZP/2A
Taking Laplace transform, Eb(s) = sK.If(s).m(s) ..(2)
Also the developed torque, Tm(s) is given by
Tm(s) = K.Ia(s).If(s) ..(3)
But
Tm(s) = TL(s) + Jm.s2.m(s) + sFm.m(s)
Or Tm(s) = TL(s) + sFm(1+sT)m(s) ..(4)
Where T = Jm/Fm= motor time constant
If the machine is running at no load, TL(s) = 0
Hence from equation (2) and (3)
m(s) = K.IfsFm .s(1+sT)Ia(s)
From equation (1), (2), (3) and (4), a block diagram can be draw as shown below:
+- )a1(
1
sTRa K If(s)
Ia(s)
)'1(
1
sTsPm
Tm(s)
sK If(s)
Va(s) m(s)
Eb(s)
Figure 2.4- Closed loop block diagram of DC motor
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From the block diagram, the overall transfer function, G(s) of the motor can be
determined as:
G(s) =m (s)
Va (s)=
K If(s)
s.K2If2+ s Fm Ra (1+sTa )(1+sT
)
Thus the experiment consists of the determination of K, Fm, Jm, Raand La.
PRE EXPERIMENTAL QUESTIONS:
1. What is the moment of inertia? On what physical parameters does it depend?
2. What do you understand by polar moment of inertia?
3. Can the retardation test be conducted under load condition, comment?
4. In retardation test will the time required for the speed to drop increase or
decrease when fly wheel is removed from the motor shaft?
1. Determination of K:
From equation (1), K is defined as
K =induced emf /field current
speed of the machine
Make the connection as shown in figure 2.8, run the machine as a generator at
constant speed and note down the voltage induced voltage for the different values of
the field current. Plot these values of voltage against the field current which will give
the magnetization characteristics of the machine (figure 2.5). From this, determine the
value of K.
=constant
E
If
k=(E/If)/
Figure 2.5-Magnetization characteristics of DC machine
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2. Determination of Fmand Jm:
Perform the retardation test on the D.C. machine. Run the machine at a little
over the rated speed and cut the supply, not down the fall of speed with time. Repeat
the same test with armature circuit connected to a known resistance. Two speed time
curves will have two time constants T1and T2respectively (see figure 2.9). These time
constants bear the following relations with Fmand Jm.
T1=Jm
Fm
And
T2=Jm
Fm +K 2I
f2
Rext .
Where Rext is known resistance connected across the armature in the latter
part. Determine Fm and Jm from these relations. For retardation test, make the
connections as shown below:
A
L1
L2
A
AA
Z
ZZ
Rext
AVM
K
Figure 2.6
3. Determination of Raand La:
Laof armature circuit is usually very small and hence can be neglected. Measurethe resistance of the armature by ammeter voltmeter method (Figure 2.8).
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A
L1
L2
A
AA
Z
ZZ
AVM V
L1
L2
Figure 2.7
Figure 2.8- Measurement of armature resistance
T1 T2
Speed
N
Time
t Figure 2.9- Speed time curve of DC machine
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OBSERVATION:
1. Magnetization Characteristics:
Speed of machine = . RPM, K =
If(A)
E (V)
2. Retardation test:
Field current = .. A
(i)
Rext.= .
Time
(sec)
Speed
(RPM)
(ii)Rext. = ..
Time(sec)
Speed
(RPM)
(iii)
Determination of Ra by ammeter voltmeter method
Va(V)
Ia(A)
Ra()
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REPORT:
1. Find out the transfer function(s)
Va (s)?
2. Find out the time response of the system, if the input is a unit step voltage?
3.
Discuss the effect of various time constants on the response of the system and
comment on its stability?
4. Derive the transfer function of a field controlled motor and give the block
diagram representation?
5. What are the assumptions made in this experiment?
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Department of Electrical Engineering, SGSITS, Indore
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SGSITS-Machines Lab
Electrical Machines-III (EE-4201)
SHRI G.S. INSTITUTE OF TECHNOLOGY AND SCIENCE, INDORE
DEPARTMENT OF ELECTRICAL ENGINEERING
ELECTRICAL MACHINE LABORATORY
Expt. No. .. Date: .
Remarks(If any) ... Signature of the staff member
AIM:To draw the torque-slip characteristics of a 3-phase induction motor
APPARATUS: Watt meter-2, ammeter-1, voltmeter-1, tachometer-1, resistance bank-
1, Variable rotor resistance,
SPECIFCATIONS OF THE MOTOR:
CIRCUIT DIAGRAM:
DC
GENERATOR
TWO SPEED
MOTOR
SLIP RING
INDUCTION
MOTOR
A
V
L
O
A
D
A BC
D E F
Ra RbRc
EXTERNAL ROTOR
RESISTANCE
LM
M L
CV
C V
LINE AMPS
LINE
VOLTS
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THEORY:
Induction motors, as they are widely used on account of their low cost, reduced
maintenance and robust construction or a very important class of electrical machines.
Mechanical characteristics of the motors fully define their use to various types of
loads. The load torque variation and the developed torque variation define the point
of operation.
The torque in an induction machine can be formulated in different ways as shown
below
Torque
=222/22+222 =
22/2
2+22 =
Rotor input or air gap power
synchronous speed in rad /sec =
32
The above equation defines the torque slip characteristics.
The torque as it is proportional to the rotor input is calculated from the rotor input
less than stator losses, speed being measured at each load. The load on the induction
motor is put indirectly by putting n electrical load on the d-c generator coupled to it.
The difference between the synchronous speed and rotor speed can be expressed as a
percentage of synchronous speed, known as the slip.
=
s = slip, Ns = synchronous speed (rpm), Nr = rotor speed (rpm)
At no-load, the slip is nearly zero (
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Power flow in induction machine:
Complete Torque slip characteristics of an induction motor
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MODEL GRAPHS:
Torque slips characteristics of IM at different rotor resistances
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PROCEDURE:
Connect the electrical circuit as shown in the circuit diagram.
Start the motor after inserting the full rotor resistance in the circuit.
Note down the power input, current and speed at various loads measure the
stator resistance per phase. (0.81 per phase).
Open circuit the rotor and switch the supply on to the stator. The power gives
approximately the core losses in rotor and stator both (like the open circuit Test
of transformer). Assuming it to be equally divided, find the stator iron loss.
Calculate the torque from power input to the motor that is the copper and
stator loss in the stator, and also slip from the observer values of the
corresponding speed.
OBSERVATIONS:
A)Full load rotor resistance in the circuit
I W1 W2 Win 32R STATORIRON
LOSS
ROTOR
INPUT
WATT
TORQUE SPEED SLIP
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B) Intermediate rotor resistance in the circuit
I W1 W2 Win 32R STATORIRON
LOSS
ROTOR
INPUT
WATT
TORQUE SPEED SLIP
C) All rotor resistance out
I W1 W2 Win 32R STATORIRON
LOSS
ROTOR
INPUT
WATT
TORQUE SPEED SLIP
d) D-C resistance (stator) per phase _____________ ohm.
e)Total iron loss with rotor open circuited ____________watt.
REPORTS:
Plot the torque slip characteristics and the current speed characteristics of the given
induction machine.
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SGSITS-Machines Lab
Electrical Machines-III (EE-4201)
SHRI G.S. INSTITUTE OF TECHNOLOGY AND SCIENCE, INDORE
DEPARTMENT OF ELECTRICAL ENGINEERING
ELECTRICAL MACHINE LABORATORY
Expt. No. .. Date: .
Remarks(If any) ... Signature of the staff member
OBJECT: To study the operation of an induction motor on unbalanced three phase
supply.
SPECIFICATIONS OF THE MACHINE:
REQUIREMENTS/EQUIPENTS:
THORY:
Three phase induction motors are most widely used in the industries .Very frequentlythese motor are called upon to operate in the unbalanced voltage supply under
certain fault conditions. the unbalanced voltages can be resolved into their
symmetrical components i.e. , two set of balanced voltage , one in forward direction
and one in backward direction .if rotor slip relative to the forward field be s , it will be
(2-s) with respect to backward field .This will give rise to two torques opposing each
other i.e.
And TF = I2
R2/S
Tb =I2b R2/ (2-S)
The worst condition of unbalance is when the supply is out of from one of the phase if
this happen when the motor is in operation it will continue to run but with reduced
efficiency and power factor.
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PRE-EXPERIMENTAL QUIZ:
1. What do you mean by unbalancing & single phasing?
2. With the reduction in supply voltage stator current increased why?
3. What is the effect of unbalancing over the net electromagnetic torque?
CIRCUIT DIAGRAM:
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PROCEDURE:
Make the connection as per the circuit diagram keep the variable terminal of the auto
transformer (single phase) to the maximum position. Adjust the auto transformer to
give a balanced three phase voltage and start the induction motor on no load with thehelp of star delta starter. Take the precaution of short circuiting the ammeter and
current coil of the wattmeter when starting the motor.
Give the full 400 volt balanced supply across the lines to the motor and perform the
load test. Record the three line currents, voltage, power speed etc. in the observation
table.
Reduced the load to zero and then reduced the voltage one phase by single phase
autotransformer near to about 150 volt. Load the motor gradually and take the
observations.
Remove the load again.
Now disconnect one phase to give single phase operation. Take the observations.
Again load the machine slightly under this condition & take the observations.
OBSERVATION
a)
Balanced supply voltages:
V1n= V2n= V3n= VL-L=
Parameters At No Load At Load
Current
W1
W2
Power
Speed
Torque
Efficiency
Power Factor
Slip
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b)
Unbalanced Operation:
V1n= V2n= V3n= VL-L=
Parameters At No Load At Load
V1nV2n
V3n
I1
I2
I3
W1
W2
Input
Output
EfficiencyTorque
Speed
Slip
c)
Single Phasing:
V1n= V2n= V3n= VL-L=
Parameters At No Load At Load
V1n
V2n
V3n
I1
I2
I3
W1
W2
Input
OutputEfficiency
Torque
Speed
Slip
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REPORT:
1) For balanced operation plot the curve between-
a) Efficiency and output
b) Power factor and output
c) Slip and output
2) Explain the effect of single phasing over the machine performance.
Derive the equivalent circuit of induction motor under unbalance and balanced supply
voltage.
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SGSITS-Machines Lab
Electrical Machines-III (EE-4201)
SHRI G.S. INSTITUTE OF TECHNOLOGY AND SCIENCE, INDORE
DEPARTMENT OF ELECTRICAL ENGINEERING
ELECTRICAL MACHINE LABORATORY
Expt. No. .. Date: .
Remarks(If any) ... Signature of the staff member
OBJECT:No load speed characteristics of Schrage motor
SPECIFICATIONS OF THE MACHINE:
REQUIREMENTS/EQUIPENTS
THEROY AND PROCEDURE:
Schrage motor is a 3-phase induction machine where speed control and
improvement of power factor are possible by voltage injection; this emf is obtained
from the brushes on the commutator which collect the emf at slip frequency from
commutator winding
Open out the slip ring side and commutator side covers of the machine and study
the construction details of the machine and various arrangements provided
The speed of a Schrage motor depends upon the magnitude of the injected emf,
which can be increased by increasing the brush separation.
Measure the no load speed of the Schrage motor the brush separation .Also measure
the value of E, The injected emf .perform the experiment with the two possible
arrangement of secondary connection Namely three poles parallel when D1 ,DS etc.
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connected together &Six poles in parallel (When D1 D4&d2 d5etc. are connected
together ) . Measure the max value of injected voltage Emax& also the secondary stand
still emf per phase (E20). The approximate speed of motor can be given by:
Nr =Ns(1-(EMAX/ E20) sin/2)
PRE-EXPERIMEMENTAL QUIZE
1 What are the other methods of speed control of induction motor?
2 can Schrage motor run at synchronous speed?
3 How is the desired frequency of the injected voltage achieved at all speed?
4 what are the other methods of generating Ej for controlling the speed of induction
motor?
OBSERVATION FOR THREE POLES IN PARALLEL
S .NO Injected voltage Brush separation () Observed speed Calculated
speed
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FOR SIX POLES IN PARALLES
S.NO Injected voltage Brush separation () Observed speed Calculated
speed
REPORT:
1
Give the report about the connection of primary, secondary and tertiarywinding of the machine and also arrangement of the brushes on the
commutator.
2 Draw the speed VS brushes separation characteristics of motor in the two
arrangements of secondary connection
(1) From the observation.
(2) By calculation
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CIRCUIT DIAGRAM:
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Equivalent circuit is the representation of the actual motor by fictitious elements forming a
simple electrical network. Equivalent circuit so obtained is capable of giving the performance
characteristics of the motor under various operating conditions obtained.
The equivalent circuit can be determined by performing (a) Light running test (b) Blockedrotor test (c) DC measurement of stator resistance.
PRE-EXPERIMENTAL QUIZ:
1. Why single phase motors are not self-starting?
2. What are the different methods of starting single phase Induction motor?
3. Why is the resultant forward flux stronger than the backward flux in a single phase
induction motor at all speeds?
4. What is centrifugal switch?
5. What are the theories used for the analysis of single phase motors? Indicate basic
differences?
CIRCUIT DIAGRAM:
PROCEDURE:
Connect up the circuit as shown. Perform light running test at rated voltage. Note down the
current and the power input.
Perform the blocked rotor test on the motor applying a reduced voltage to the main
winding so that rated current flow through it, for this test insulate the starting winding at the
centrifugal switch s by a piece of paper (Note the ammeter and wattmeter readings).
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OBSERVATIONS:
Test Voltage Current Power
Light Running Test
Blocked Rotor Test
DC resistance of the stator winding ..
REPORT:
1. Draw the equivalent circuit referred to stator for
(a)Light running test
(b)
Blocked rotor test
2. Compute the values of various parameters from the test results and indicate them on the
equivalent circuit for slip s.
3. Determine for slip, s=0.05
(a)Input current
(b)Forward and backward components of rotor current referred to stator.
(c)Forward and backward torques and gross motor torque in synchronous watts.
(d)Combines Iron, friction and windage loss, and net output in watts.
(e)
Efficiency of the motor.4. Draw the torque-slip curve for a single phase induction motor with its forward and
backward components.
5. What are the approximations made while evaluating the equivalent circuit parameters?
How are these approximations justified?
6. How does the Efficiency of test motor compare with that of the three phase motor of
same rating?
While evaluating the performance from the equivalent circuit for different values of
slip, the backward impedance need only be found for one value of slip & the same can
be assumed constant for other normal operating values of slip. Why?
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Electrical Machines-III (EE-4201)
SHRI G.S. INSTITUTE OF TECHNOLOGY AND SCIENCE, INDORE
DEPARTMENT OF ELECTRICAL ENGINEERING
ELECTRICAL MACHINE LABORATORY
Expt. No. .. Date: .
Remarks(If any) ... Signature of the staff member
OBJECTIVES:-
To determine the performance characteristics of a 3-phase induction generator as
a) Grid connected induction generator
b)
Isolated(self-excited) induction generatorConduct power generation test on grid connected induction generator and for self-
excited induction generator and draw
1) Efficiency vs. power
2) Power factor vs. power
3) Stator current vs. power
4) The speed vs. power curves at rated voltage and frequency
MOTIVATION:-Over the past few decades, there has been an increasing use of squirrel cage type
induction generators, particularly in wind energy conversion systems and micro-hydro
power systems. The grid provides frequency and voltage regulation, as well as the
reactive power required by the generator. Their advantages in these applications are
that they are rugged, require less maintenance, high power/weight ratio and self short
circuit protection and cheap. It is not essential to operate precisely at synchronous
speed.
THEORY:-
The set up mainly consists of an I.M coupled with D.C.M. The three phase supply is
given to the starter of the I.M it will start working as a motor as it is coupled with the
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D.C.M it starts working as generator. Now with the help of a voltmeter, measure the
voltage and check the polarity of the generated emf. Its value is made equal to the dc
supply available.
Now the D.C.M is connected with the supply and it starts running as a motor. At an
instant the induction machine and the D.C.M will run as motors. They drive each other
now the I.M crosses its synchronous speed and starts generating power to the grid and
it draws reactive power from the grid.
Induction machine in both of its motoring mode and generating mode draws reactive
power for the shunt branch. Now if we remove the grid from the machine it will stop
generating the power. As the machine is not having the reactive power supply. So by
removing the grid and giving the external reactive power to I.M will make machine to
generate power and this mode of operation is termed as self excited mode of
operation and machine is called self excited machine.
EQUIPMENTS AND COMPONENTS:-
(a) A three phase squirrel cage induction motor coupled with separately excited dc
motor
(b) One AC Ammeter (0-5/10A)
(c) One AC Voltmeter (0-500V)
(d)
Two Low pf wattmeter (600V,10A)
(e) Two unity pf wattmeter (600V,10A)
(f) Suitable dc loads (440V, 10A)
(g) Tachometer
DATA SHEET
Note down the name plate details of the both machines and identify the terminals.
Observe the constructional features. Note the type of rotor used and the winding
connections.
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Name Plate Details of the Machine
Name of the manufacturer:
Rated output:
Rated voltage:
Supply Frequency:
Rated speed:
No. of poles:
Rated current:
Type of rotor:
Type of starting method:
Ic Ic Ic
A
V
STAR
DELTA
STARTER
AFrequency
meter
R
Y
B
3 PHASE
AC
SUPPLY
MAIN
SWITCH
N
L1
L2
L3
STATOR ROTOR
INDUCTION MACHINE
A
AA
Z
ZZ
DC MACHINE as prime
mover
GRID CONNECTED
INDUCTION
GENERATOR
LOAD II
MCB
LOAD I
MCBML
C
V
VC
M
MCB
L1
L2
L3
SELF EXCITED
INDUCTION
GENERATOR
L M
V C
ML
CV
A
Capacitor bank delta
connected
L
A
B
C
Grid connection and self excitation of an induction machine
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PROCEDURE
Power Generation Test
Make the connections as shown in Fig above
Switch on the field winding DC supply and set field current to its rated value.
Start the dc motor using starter and set the field current up to its rated current.
Record the speed of motor/generator set.
Now adjust the field current and increase the speed up to synchronous speed.
Now switch on the induction machine supply using autotransformer gradually.
Monitor carefully the ammeter and Wattmeters readings.
Use field voltage control for further speed increment of DC motor and tabulate
the observations in Table. Take care about the ratings of both machines.
OBSERVATIONS
Winding connections for stator/rotor:
(b) Average stator winding resistance/phase=_____ohm
(c) Average rotor winding resistance/phase=______ohm
Data Processing and Analysis
a) Grid connection
S. No V IL W1 W2 F N Vdc Idc If
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b) Self excited below rated speed
S. No V IC IL W1 W2 f N Vdc Idc If
c) At rated speed
S. No V IC IL W1 W2 f N Vdc Idc If
d) At above rated speed
S. No V IC IL W1 W2 f N Vdc Idc If
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REPORT:-
a) Plot using measured data efficiency vs. output Power
b) Plot using measured data power factor vs. output Power
c) Plot using measured data stator current vs. output Power
d) Plot using measured data Efficiency vs. output Power
e) Plot using measured data speed vs. output Power
f) What are the advantages and disadvantage of induction machine as a
generator?
g)
Draw and explain torque speed characteristics of induction generator.
h)
How the voltage and frequency control takes place in grid connected
induction generator? Explain.
i)
Critically comment on the characteristics you obtained? j) Induction generator draws leading VAR? Justify.
PRECAUTIONS:
Check the direction of rotation of both machines before conducting the power
generation test. It must be same.
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SHRI G.S. INSTITUTE OF TECHNOLOGY AND SCIENCE, INDORE
DEPARTMENT OF ELECTRICAL ENGINEERING
ELECTRICAL MACHINE LABORATORY
Expt. No. .. Date: .
Remarks(If any) ... Signature of the staff member
OBJECT:To determine the equivalent circuit parameters and also perform the routine
test and type test for three phase induction motors as per ISS i.e. IS 325:1996
APPRATUS REQUIRED:
SPECIFICATIONS:
THEORY:-
To know the performance of an induction motor the motor is translated in terms of an
electrical/circuit where the power output etc are represented by the dissipation in
some fictitious element in the circuit which is called the equivalent circuit.
The equivalent circuit can be determined by performing:-
(a) Light running (No load) test
(b) Blocked rotor test and
(c) d-c measurement of the stator resistance.
LIGHT RUN TEST
The no load test on an induction motor gives information with respect to
exciting current and no-load losses. The test is performed at rated frequency andbalanced poly-phase voltages applied to the stator terminals. Readings are taken at
the rated voltage, after the motor runs long enough for bearings to be properly
lubricated.
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At no load, the rotor current is only the very small value needed to produce sufficient
torque to overcome the friction and windage losses associated with rotation. The no-
load rotor I2R loss is, therefore, negligibly small. Unlike the continuous magnetic core
in a transformer, the magnetizing path in an induction motor includes an air gap which
significantly increases the required exciting current. Thus, in contrast to the case of a
transformer, whose no-load primary I2R loss is negligible, the no-load stator I
2R loss
of an induction motor may be appreciable because of this larger exciting current.
Neglecting rotor I2R losses, the rotational loss Prot for normal running conditions can be
found by subtracting the statorI2R losses from the no-load input power.
P = constant losses + cu losses
Constant losses=mech. Losses + core loss
Cu losses =..
Mechanical losses can be separated by graphical method as given below
Core loss
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BLOCKED ROTOR TEST:-
Like the short-circuit test on a transformer, the Blocked-Rotor test on an induction
motor gives information with respect to the leakage impedances. The rotor is blocked
so that it cannot rotate (hence the slip is equal to unity), and balanced polyphasevoltages are applied to the stator terminals.
In some cases, the blocked-rotor torque also is measured. The equivalent circuit
for blocked-rotor conditions is identical to that of a short circuited transformer. An
induction motor is more complicated than a transformer, however, because its
leakage impedance may be affected by magnetic saturation of the leakage-flux paths
and by rotor frequency.
The blocked-rotor impedance may also be affected by rotor position, although this
effect generally is small with squirrel-cage rotors. The guiding principle is that the
blocked-rotor test should be performed under conditions for which the current and
rotor frequency are approximately the same as those in the machine at the operating
condition for which the performance is later to be calculated. For example, if one is
interested in the characteristics at slips near unity, as in starting, the blocked-rotor
test should be taken at normal frequency and with currents near the values
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encountered in starting. If, however, one is interested in normal running
characteristics, the blocked-rotor test should be taken at a reduced voltage which
results in approximately rated current; the frequency also should be reduced, since
the values of rotor effective resistance and leakage inductance at the low rotor
frequencies corresponding to small slips may differ appreciably from their values at
normal frequency, particularly with double-cage or deep-bar rotors. The total leakage
reactance at normal frequency can be obtained from this test value by considering the
reactance to be proportional to frequency. The effects of frequency often are
negligible for normal motors of less than 25-hp rating, and the blocked impedance can
then be measured directly at normal frequency. The importance of maintaining test
currents near their rated value stems from the fact that these leakage reactances are
significantly affected by saturation. Based upon blocked-rotor measurements, the
blocked-rotor reactance can be found from the blocked-rotor reactive power
PRECAUTION:-
In Blocked rotor test we must apply reduced voltage (10-12% of v rated) so that the
rated current could flow.
PROCEDURE:-
Connect up the circuit as shown. Perform light running test at the rated voltage. Note
down the power input and the current.
Apply a reduced voltage across the motor so that approximately the rated current
flows through the circuit.
Measure the dc value of the resistance.
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CIRCUIT DIAGRAM
THREE PHASE
AUTO
TRANSFORMER
Star/delta
startor
Three
phase
induction
motorv
A
c v
LM
M L
C V
R
Y
B
L1
L2
L3
OBSERVATIONS:
S. NO TEST VOLTAGE CURRENT POWER INPUT
D-C resistance of the winding per phase =ohms
Equivalent A-C resistance per phase =..ohms
REPORT:
1. Draw the equivalent circuit of the induction motor and put the values for
various elements in the circuits.
2. Calculate x and plot for the rated voltage line current power factor,
efficiency, torque speed vs power output on the same graph.
3. Draw the equivalent circuit for all load operation.
4. Draw torque slip characteristics.
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ROUTINE TEST AND TYPE TEST
22.2 Test certificates
22.2.1 Unless otherwise specified when
inviting tenders, the purchaser, if so
desired by manufacturer, shall accept
manufacturers certificate as evidence
of the compliance of the motor with
the requirements(see 10,11,12,13 and
17) of this standard together with a
type test(see 22.3.1) certificate on a
motor identical in essential details withthe one purchased, together with
routine test certificate on each
individual motor is supplied on one
order, type tests, as specified, shall be
made on one of these motors, in
addition to the other certificates if the
purchaser so requires.
22.2.2 Certificates of routine tests (see
22.3.2) shall show that the motor
purchased has been run and has been
found to be electrically and
mechanically sound and in working
order in all particulars.
22.3 Classification of test
22.3.1 Type tests
The following shall constitute type
tests:
a) Dimensions (for motors covered
by IS 1231:1974 and IS2223:1983 only) (see 10).
b) Measurement of resistance of
windings of stator and wound
rotor.
c)
No load test at rated voltage to
determine input current, power
and speed (see 23.1).
d) Open circuit voltage ratio of
wound rotor motors (slip ring
motors)(see 23.3).
e) Reduced voltage running up test
at no load( for squirrel cage
motors up to 37KW only)(see
23.2)
f) Locked rotor readings of voltage,
current and power input at a
suitable reducedvoltage(see23.2)
g) Full load test to determine
efficiency, power factor and
slip.(see 23.5)
h) Temperature rise test (see14).
i) Momentary overload test
(see13.1).
j)
Insulation resistance test(see25)k) High voltage test(see 24)
l) Test for vibration severity of
motor(see15)
m)Test for noise levels of
motor(see16)
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n) Test for degree of protection by
enclosure(see5)
o) Temperature rise test at limiting
values of voltage and frequency
variation.
p) Over speed test(see26)
q) Test on insulation system( see 27
These are optional tests subject to
mutual agreement between purchaser
and the manufacturer.
22.3.1.1 It is recommended that the
reports of type tests be made in theform recommended in Annex C.
22.3.2 Routine tests
The following shall constitute the
routine tests:
a) Insulation resistance test(see25)
b) Measurement of resistance of
winding of stator and woundrotor.
c) No load test(see23.1)
d) Locked rotor readings of voltage,
current and power input at a
suitable reduced voltage(see
23.4)
e) Reduced voltage running up test
(see23.2) (for squirrel cagemotors).
f)
High voltage test(see23.4)
23 PERFORMANCE TESTS
23.1 No load test
The motor shall run at rated voltage
and frequency given on the rating
plate. The motor shall run to its normal
speed and shall not show abnormal
electrical or mechanical noise. The
input power, current and speed shall be
measured and used in the
determination of no load losses and
efficiency at full load.
NOTE- in case proper facilities for
conducting this test at rated voltage are
not available, the method of testing
shall be mutually agreed between the
manufacturer and the purchaser.
23.2 Reduced voltage running up test
The test is applied to squirrel cage
motors. The test is made to check the
ability of motor to run up to its rated
speed at no load. The motor up to
37KW shall be supplied with reduced
voltage 1/of rated value for each
direction rotation. For motors above
37KW, the voltage shall be 1/ of rated
value or less but motor shall run only in
the specified direction of rotation.
23.3 Open circuit voltage ratio test for
wound rotor (slip ring) motors
The stator of the motor is supplied with
rated voltage and open circuit voltage
at the slip rings shall be determined (by
lifting the slip ring brushes). The voltage
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shall comply with the declared values of
the manufacturer.
23.4 Locked rotor test
The tests shall be carried out in
accordance with provision of IS
4029:1967. The test may be carried out
at reduced voltage. The readings of the
input current, power and breakaway
torque shall be determined. The values
of breakaway torque shall be not less
than the value given in IS 8789:1996.
23.5 Full load test
The motor shall be supplied with rated
voltage and load on the shaft shall be
adjusted such that it delivers the rated
output.
The value of voltage, input power,
current and speed shall be measured.
The efficiency determined for full load
shall not be less than the values
specified in IS 8789:1996. The detailed
procedure of testing three phase
induction motors shall be in accordance
with IS 4029:1967.
NOTE- In case proper facilities for
conducting this test at rated voltage are
not available; the method of testing
shall be mutually agreed between the
manufacturer and the purchaser.
24 HIGH VOLTAGETEST
24.1 The requirements specified in IS
4029:1967 shall apply.
25 INSULATION RESISTANCE TEST
25.1 The requirements specified in 30.2
of IS 4722:1992 shall apply.
26 OVER SPEED TEST
26.1 All motors shall be designed to
withstand 1.2 times the maximum
rated speed.
26.2 An over speed test is not normally
considered necessary, but may be
performed when this is specified and
has been agreed between the
manufacturer and the purchaser at the
time of the order. An over speed test
shall be considered as satisfactory, if no
permanent abnormal deformation is
apparent subsequently and no other
weakness is detected which may
prevent the motor from operating
normally, and provided the rotor winding
after the test comply with the required
high voltage test. The duration of any over
speed test shall be two minutes.
27 TESTS ON INSULATION SYSTEM
NOTE- Unless otherwise specified when
inviting tenders, the purchaser, if so
desired by the manufacturer, shall accept
manufacturers test certificate as evidence
of the compliance of the motor/ insulation
system with the requirements of the
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following tests together with the tests
certificate as stated in 22.2.
27.1 Tangent delta and delta tangent delta
test
The requirement and method of test shall
be as per IS 13508:1992.
27.2 Impulse voltage withstand test
The requirements and method of test
should be as per IS 14222:1995
ANNEX A
(Clause 2.1)
LIST OF REFFERED INDIAN STANDARDS
900:1992- code of practice for installation and
maintenance of induction motors
(revised)
996:1979- Single phase small AC and universal
electric motors (second revision)
1076(in parts) - Preferred numbers
1231:1974- Dimensions of three phase foot
mounted induction motors (third
revision)
1271:1985- Thermal evaluation and
classification of electrical
insulation(first revision)
1885(part 35): 1973/IEC 50(411):1993-
Electro technical vocabulary:
rotating machinery
2223:1983- Dimensions of flange mounted AC
induction motors (second revision)
2254:1985- Dimensions of vertical shift motors
for pumps(second revision)
3043:1987- Code of practice for earthing
(second revision)
3855(in parts) - Rectangular and square
enameled copper conductors
4029:1967- Guide for testing three phase
induction motors
4691:1985- Degrees of protection provided by
enclosures for rotating electrical
machinery (first revision)
4722:1992- Rotating electrical machines (first
revision)
4728:1975- Terminal marketing and direction of
rotation for rotating electrical
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machinery(first revision)
4800(part5):1968- Enameled round winding
wires: Part 5 wires for
elevated temperatures.
4889:1968- Method of determination of
efficiency of rotating electrical
machines.
6362:1995 IEC Pubs: 34-8:1991- Designation of
methods of cooling for rotating
electrical machines
6455:1972- Single row radial ball bearings
6457:1972- Dimensions and output ratings for
foot mounted rotating electrical
machines with frame numbers
355 to 1000
8544(in parts) -Motor starters for voltages not
exceeding 1000V
8789:1996- Values of performance
characteristics for three phase
induction motors
12065:1987- Permissible limits of noise level for
rotating electrical machines
12075:1986- Mechanical vibration of rotating
electrical machines with shaft
heights 56mm and higher
measurement, evaluation and
limits of vibration severity
12360:1988- Voltage bands for electrical
installations including preferred
voltages and frequency
12661(Part1):1988-High voltage motor starter:
Part 1 direct on line (full
voltage) AC starters
12802:1989- Temperature rise
measurement of rotating electrical machines
12824:1989- Type of duty and classes of
Rating assigned (second revision)
13947(Part4/Sec1):
1993/IEC947-4-1(1990) LV Switchgear and
control gear: Part4 contactors and motor
starters, sec1 Electromechanical
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ANNEX C
FORM FORTYPE TEST REPORT OF THREE PHASE INDUCTION MOTOR
(Clause 22.3.1.1)
MANUFACTURER CERTIFICATE NO.
PURCHASER
PURCHASE ORDER NO. ORDER ACCEPTANCE NO.
NAME PLATE DATA
OUTPUT PHASE VOLTAGE CONNECTION FULL LOAD
CURRENT
FREQUENCY FULL
LOAD
SPEED
FRAME DUTY INSULATION EFFICIENCY
NOMINAL
PERCENT
MANUFACTURERS
NUMBER/REFERENCE
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TESTS
LOAD CONNECTION LINE
VOLTAGE
LINE CURRENT POWER SLIP LOAD POWER FACTOR EFFICIENCY
GUARANTEED TEST GUARANTEED TEST
TEMPERATURE RISE TEST RUN
HOURS RUN VOLTAGE CURRENT INPUT
POWER
CALCULATED
OUTPUT POWER
COOLING AIR(C) TEMPERATURE RISE(C)
STATOR ROTOR
Core Winding Core Winding
BREAKAWAY TORQUE AND STARTING CURRENT
VOLTAGE BREAKAWAY STARTING CURRENT TORQUE POWER
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INSULATION RESISTANCE STATOR ROTOR
HIGH VOLTAGE TEST
OPEN CIRCUIT ROTOR VOLTS:
RESISTANCE OF WINDING PER PHASE
1) STATOR- Ohms at C
2) ROTOR- Ohms at C
REDUCED VOLTAGE RUNNING UP TEST:
OVERLOAD
A) MOMENTARY EXCESS TORQUE TEST
B) PULL UP TORQUE
VIBRATION SEVERITY
NOISE LEVEL
DEGREE OF PROTECTION BY ENCLOSURE
TESTED BY APPROVED BY
DATE DATE
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1. INTRODUCTION
The classical way of teaching electrical machinery theory to undergraduates to start
off with the DC machines and finish with the elementary principles of operation of
single phase motors, covering the transformer, the synchronous machine, theinduction machine and the AC commutator motors. Mostly the steady state
operation of these machines is dealt with and the transient operation is either
completely omitted or confined to the synchronous machine. With the result the
student is left with the impression that each type of electrical machine is a class by
itself having nothing in common with other types of electrical machines.
Kron pointed out 32years ago that all electrical machines can be mathematically
reduced to a basic two axis machine. Further work by Gibbs, Adkins, White and
Woodson and others created interest among the academic circles in the generalized
theory of electrical machines as a possible method of teaching electrical machinery
theory. The important requirement in adopting the new method is to provide thephysical meaning for the various mathematical transformations employed and to
physically show that the different types of electrical machines are simply different
terminal conditions of one and the same physical phenomenon pertaining to the
primitive machine of Kron. It is primary function of the generalized
electromechanical energy converter (GEMC) to meet this requirement. After the
advert of the GEMEC more interest is now being shown in India and abroad, in the
teaching of generalized theory of electrical machines to undergraduates.
The GEMEC is as yet a newcomer to them market and these are only a handful of
manufactures in the whole world who make such machines. The PSG industrial
institute, India is one of those few. It is not sufficient for GEMEC to operate only as a
primitive two axis machine. It must also be able to physically simulate the various
mathematical transformations used in the generalized theory and consequently be
able to operate as a primitive two axis machine and as various type of machines as
many types as possible consistent worth the manufacturing possibilities, size, cost,
utility etc. The GEMEC made by PSG industrial institute provides facilities for such
transformations. This manual gives a description of GEMEC manufactured by the PSG
Industrial Institute. Some of the possible modes of operation, erection and
maintenance instructions and connection diagrams and performance characteristics
for certain modes of operation. The modes of operation listed in this manual aresome of the possibilities and many other modes can be devised by the user, taking
care that the current ratings are within limits as specified under technical data. The
machine is so designed as to give maximum accessibility for demonstration and
maintenance purposes.
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2. DESCRIPTION OF THE MACHINE
The stator of the machine has 48slots accommodating 4coils of 12 slot pitch. All the
coil ends are brought out to terminals arranged in four concentric circles on a
terminal board for easy connection. The terminals are numbered and coils indicated.This enables the stator to be connected for different number of poles and phases.
Single search conductors are provided in six suitably chosen stator slots so that
search coils are provided in six suitable chosen stator slots so that search coils of
various pitches can be arranged.
The rotor has 36 slots accommodating a double layer closed lap winding with a coil
pitch of 8 slots. The winding is provided with a 144 segment commutator on one side
and slip ring tapings on the other side. There are two sets of slip rings tappings:
One set of six symmetrical tappings and another of four.
Two of the tapping being common to the two set. All the eight tappings are brought
to the slip rings. A rotor search coil whose terminals are brought to two more slip
rings is also provided.
The commutator is equipped with a set of rocking brushed and a set of rotating
brushes. The rocking brush set is arranged on two rings, each ring carrying 6brushes,
equally spaced along the periphery.
The rings can be rocked opposite to one another by bevel gear and the whole set can
be rocked relative to the stator winding. Locking arrangement is provided for each of
the movements. Graduated scales indicate the position of the set as a whole and the
angular displacement between the two rings. The rotating brush set consists of four
brushes arranged in quadrature and can be driven round the commutator in eitherdirection by an external drive. The shaft extension for coupling this external drive is
arranged to be co-axial with the main shaft of the PSG GEMEC but independent of
the same. The rotating brush set can also be locked in any desired position.
Diagrammatic representation of the rocking brush set and the rotating brush set
with terminals is provided on the panel board. Connections from the rotating brush
set are brought through slip rings terminals on the panel board. Leads from the
rocking brushes are brought to terminals on the brush carriage and these could be
connected, when required, to the corresponding terminals. Diagrammatic
representation by flexible leads provided.
A short circuiting ring, which can be easily fitted on to the commutator, is provided.When so short circuited, the rotor behaves like a semi cage winding, responding to
the various pole numbers that can be produced by the stator winding.
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The nominal rating of the PSG GEMEC as a #-phase, 50 Hz, 4 pole, slip ring induction
motor is 5.5KW at 230V per phase. The machine will however withstand appreciable
overloads.
A DC work machine coupled to the main shaft and a variable speed drive motor for
rotating brushes would be required for normal use of PSG GEMEC. It is preferable tomount the DC work machine on trunnion bearings so that it could be used also as a
swinging frame dynamometer. If desired, a shaft torque meter, a tachometer. And
an accelerometer may also be added which would increase the versatility of the
machine as a research unit.
3. TECHNICAL DATA
Nominal rotoring a 3-Phase, 4 pole, slip ring induction motor:
Line voltage : 400V
Frequency : 50Hz
Output : 5.5KW
Speed : 1440 RPM
Line current : 12.5A
Stator:
Outer diameter : 344mm
Inner diameter : 230mm
Core length : 102mm
Number of slots : 48
Number of coils : 48
Conductor size : 16SWG
Coil pitch : 12Slots
Conductor current : 7.5A
Rotor:
Number of slots : 36
Number of commutator : 144Segments
Winding : double layer
Lap coil pitch : 8 slots
Conductor current : 11A
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Rotating and rocking brushes:
Carbon brushes : 6mm*20mm*35mm
Current per brush : 10A
Maximum operating speeds
Rotor : 3000RPM
Rotating brushes : 2000RPM
4. INSULATION AND MAINTENANCE
Normally PSG GEMEC is supplied separately as shown in the cover photograph and in
a sectional view. Because of improvements made from time to time the actual
machine supplied may vary to some extent from these illustrations and descriptions.For the use of the machine it must be coupled to a drive motor of about 6.5KW
rating and a brush drive motor of 1KW rating with speed control as desired. Care
must be taken in coupling the three machines as any misalignment can damage the
bearings of PSG GEMEC. All the commutator brushes are locked in position before
dispatching. The rotating brushes can be released by unscrewing two radial screws
on the outer cover. The rocking brushes can be released by unscrewing the knurled
head screw follow the operating instructions and in any case. Do not exceed any of
the ratings of the machine. The commutator and slip rings should be cleaned
periodically. The bearings must be cleaned and lubricated with lithium soap based
grease.
Trade names: Shell Alvania grease, Shell Multipurpose Grease or Mobilux Grease)
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OPERATION
GENERAL
Some of the modes in which the machine can be operated and the corresponding
ratings are listed in appendix 1 and the connection diagrams and some of the
operating characteristics in the form of graphs are given in appendix2. Other modes
of operation like the induction generator, synchronous converter etc are possible
and many more modes can be devised by the user. In all cases the following points
may be noted:
1. Any of the ratings specified should not be exceeded.
2. Commuatator brushes when not in use must be lifted and the carriage locked in
position
INDUCTION MACHINES
Since the entire stator coil terminals are brought out, the stator can be connected
for various numbers of poles and 2 or 3 phase operation, the rotor may either be
short circuited through the slip rings or the commutator may be short circuited.
When the stator is connected for other pole numbers, the commutator has to be
short circuited. The machine can also be operated as 2 or 4 pole, or 2 or 3-phase
rotor fed induction machine.
The stator can also be connected for various types of single phase operation.
SYNCHRONOUS MACHINES
It can be operated either as rotating armature or rotating field synchronous machine
of different pole and phase combinations. However, it is not possible to introducethe effects of saliency, but by introducing symmetry in the field circuit, it is possible
to introduce a difference in the reactance of the tow axes. It can be operated as a
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synchronous converter by supplying the machine through a suitable transformer
bank and by making use of the commutator.
AC COMMUTATOR MACHINES
The machine can be very easily connected as a polyphase 2pole or 4pole shunt
commutator motor. When connecting the 2-phase operation, it will be necessary to
use the rotating brush set, locking it a definite position. An autotransformer may be
used to provide controlled voltage to the brushes. With this arrangement, the
machine can operate also as a frequency converter.
It is possible to operate the machine as a rotor fed synchronous motor as a Schrage
motor at reduced voltage. Taking care not exceed the current rating of the statorconductor the machine can also be operated as a series commutator motor.
Various modes of single phase operation, such as repulsion motor, repulsion start
induction motor can also be arranged.
DC MACHINES
PSG GEMEC can be operated as a shunt, series, compound or separately excited DCmachine of different pole numbers. When operating for poles other than 2 or 4, care
must be taken to carefully set and lock the rocking and rotating brush sets and to
choose the proper brushes. It is also possible to use the machine as a DC
transformer, a Rosenberg Generator etc. however, it is not possible to introduce
saliency.
UNCONVENTIONAL MODES
Operation of the so called DC induction motor can be demonstrated by supplying
direct current to the quadrature brushes and then rotating these by means of the
brush carriage rotor.
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Many other unconventional modes of operation may be devised by the user. One
such mode of operation developed in the PSG laboratory by Charlu results in a
variable speed adjustable PF motors. (Trans A.I.E.E., P.A.S., Vol. 78, 1959, PP 407-
413)
LIMITATIONS
The coil pitch of the stator coils as 12 slots corresponding to the full pitch for 4 pole
operation. This has been chosen so that at least for 1 pole number the winding is full
pitched. This choice makes it impossible for the machine to operate as an 8pole
machine.
Another is the uniform air gap. It is not possible to introduce physical saliencynormally met with the DC machines and in some synchronous machines. As has
already been suggested this effect can be simulated to some extent by the use of
asymmetrical connections in the excitation circuit.
TRANSFORMATION
One important feature of PSG GEMEC as already pointed out, is that it can be used
as a physical model to demonstrate the mathematical transformation of a particular
machine to the generalized machine or of one machine to another. Particular mode
of operation are simulated in PSG GEMEC by connecting same active conductors in
different ways and joining them to terminals via solid leads, slip rings or commutator
and fixed or rotating brushes. Each mode of operation is distinguished by its two
components field patternstator and rotor field. A field pattern can be setup by the
different components arrangements, current and voltage. Therefore currents,
voltages and impedance of 1 type of winding (and hence machine) can be
transformed to those of another taking into account the invariance of power.
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SGSITS-Machines Lab
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SHRI G.S. INSTITUTE OF TECHNOLOGY AND SCIENCE, INDORE
DEPARTMENT OF ELECTRICAL ENGINEERING
ELECTRICAL MACHINE LABORATORY
Expt. No. .. Date: .
Remarks(If any) ... Signature of the staff member
OBJECT: To separate the no load losses of a D.C. shunt machine.
SPECIFICATIONS OF THE D.C. MACHINE:
REQURIMENT/EQUIPMENTS:
THEORY AND PROCEDURE:
The mechanical losses (no load) of a D.C. shunt machine can be separated into
hysteresis, eddy current, friction and windage losses by the method given below:
Wnl = Wcore + Wfrictionand windage
= (Whysterisis +Weddy current) + Wbrushfriction +Wbearingfriction +Wwindage
Now, Wh B1.6
N, and We Be N2
Therefore at constant excitation, the total core losses are
Wc = (KhN + KeN2)
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Also,
Wbrushfriction N , Wbearing friction N, and Wwindage N2
Bearing friction loss is small as compared to the other two losses (i.e. Wbrush friction and
Wwindage) and can be neglected.
The total friction and windage losses are
= (KfN + Kw N2)
Thus the total mechanical loss can expressed as
Wnl = (KhN + KeN2) + (KfN + Kw N
2)
= (Kh+ Kf).N + (Ke+ Kw) ..(1)
Separation of loss therefore requires the determination of four constants K h, Ke, Kfand Kw.
Let C = Kh+ Kf and D = Ke+ Kw ..(2)
So Wnl = C.N + D.N2
Or Wnl/N = C + D.N ..(3)
This is a straight line equation. Hence C and D can be determined by observing Wnl
for different values of N and then plotting (Wnl/N) against N on a graph (with Ifconstant).
Determination of C and D will not be sufficient to calculate all four constant.
Therefore, repeat the above test and perform at some other value of field current. The core
loss constants will change to some other values. Hence
C = Kh+ Kf and D = Ke+ Kw ..(4)
Since hysteresis loss and eddy current loss is proportional to B1.6
and B2respectively, so
(Kh/Kh) = (B/B)1.6
= (E/E)1.6
& (Ke/Ke) = (B/B)2= (E/E)
2 ..(5)
So, C = (E/E)1.6
Kh+ Kf and D = (E/E)2Ke+ Kf ..(6)
Now all four constants can be calculated from the equation excitation constant (2) and (6).
Make the connection as shown in the circuit diagram. Keeping excitation constant,
for all observation, note down the voltage and armature current at different speed of
motor. Speed variation should be obtained by varying the voltage across the armature.
Repeat the same procedure with some other value of excitation. Measure the armature
resistance.
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PRE EXPERIMENTAL QUESTIONS:
1. What are friction, windage and iron loss? Why and where do they occur?
2. What are the effects of excitation, speed and load on the friction & iron losses?
3. How iron losses are related to the input voltage and speed?
4.
Is it necessary to have armature of a D.C. machine laminated?
5. Is it necessary to have field also laminated?
CIRCUIT DIAGRAM:
DC shunt machine
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OBSERVATION:
Armature resistance, Ra = . Field current, If= .
Speed
Va
Ia
I2
a.Ra
Wnl
Wnl/N
E at speed .. RPM = .. Volts
Armature resistance, Ra = . Field current, If= .
Speed
Va
Ia
I2
a.Ra
Wnl
Wnl/N
E at speed .. RPM = .. Volts
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REPORT:
1. Calculate all the four constants. Give all calculations.
2. Calculate the constituents of no load losses at the rated speed.
3. What are the pole face losses? Are these caused by hysteresis and eddy currents or
both?
4. What are alternating and rotational hysteresis losses? Differentiate between them?
Is it possible to separate friction and iron losses by making a single plot of input power with
exciting current at a constant speed? Suggest procedure?