V CORVE OF 3 PHASE INDUCTION MOTOR
AIM: 1) To measure direct axis synchronous reactance of synchronous machine.
2) To measure quadrature axis synchronous reactance by slip test.
THEORY :Direct axis synchronous reactance and quadrature axis synchronous reactance are
the steady state reactance of the synchronous machine. These reactances can be measured by
performing open circuit, short circuit tests and the slip test on synchronous machine.
(A ) Direct-axis synchronous reactances (Xd ):The direct axis synchronous reactance of synchronous
machine in per unit is equal to the ratio of field current. Ifsc at rated armature current from the
short circuit test, to the field current, Ifo at rated voltage on the air gap line.
Direct axis synchronous reactance (Xd) = Ifsc /Ifoc per unit
This direct-axis reactance can be found out by performing open circuit and short test on
alternator.
(B) Quadrature-axis synchronous reactance, Xq by slip test:For the slip test, the alternator should be driven at a speed, slightly less than the synchronous
speed, with its field current open. 3 phase balanced reduced voltage of rated frequency is
applied to armature (stator) terminals of the synchronous machine. Applied voltage is to be
adjusted, so that the current drawn by the stator winding is full load rated current. Under these
conditions of operation, the variation of the current drawn by the stator winding, voltage across
the stator winding and the voltage across the field winding will be as shown in fig. The wave
shape of stator current and stator voltage clearly indicate that these are changing between
minimum and maximum values. When the crest of the stator mmf wave coincides with the direct
axis of the rotating field, the induced emf in the open field is zero, the voltage across the stator
terminals will be the maximum and the current drawn by the stator winding is the minimum.
Thus approximate value of direct axis synchronous reactance Xds is given by
Xds = Emax/Imin
------ eq.(A)
When the crest of stator mmf wave coincides with the quadrature axis of the rotating field, the
induced emf in the open circuit field is maximum, the voltage across the stator terminals will be
minimum and the current drawn by the stator winding is maximum. Hence, approximate value of
the quadrature-axis synchronous reactance, Xqs is given by,
Xqs = Emin / Imax
----- eq.(B)
For best result, these values are not taken as the final values. The most accurate method of
determining the direct axis synchronous reactances, Xd is the one, that has already been
described in (A) above, The most accurate value of quadrature axis synchronous reactance, Xq
can now be found out using the above information i.e Xds and Xd
Quadrature axis synchronous reactance, Xq = (Xqs/Xds)×Xd
= (Emin/Imax)(Imin/Emax) Xd per unit
Hence the accurate value of Xq can be found out by recording minimum and maximum values of
the above quantities. Accurate results can be obtained, if the osillograms are taken during
experimentation for stator current, stator voltage and injected voltage across the field.
It may be noted clearly, that for synchronous machine Xd is greater than Xq i.e. Xd > Xq
Important caution for conducting slip test:1) Slip should be extremely low during experimentation. In case of high slip (more than
about 5%) following effects may be observed:(a) Current induced in the damper winding of alternator will produce an appreciable
error.
(b) Induced voltage in the open circuit field may reach dangerous values.
2) It should be assured that the induced voltage in the open circuit is less than the rating of
the voltmeter connected in the circuit.
CIRCUIT DIAGRAM:-
PROCEDURE:-
(a) Open circuit test:-
AMP.
DC
AMP.
0-1A DC
0-5 AMP.
V
0-600 V AC.
3POINTSTARTER
+
DC 220 V
-
V
F
45-55 Hz
0-300 V DC.
Rehostate
Rehostate
Rehostate
AMP.
DC
AMP.
0-1A DC
0-5 AMP.
V
0-600 V AC.
3POINTSTARTER
+
DC 220 V
-
V
F
0-300 V DC.
Rehostate
Rehostate
Rehostate
45-55 Hz
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Connect the circuit as per the diagram.
Ensure that the external resistance in the field circuit of dc motor acting as a prime mover
for alternator is minimum and the external resistance in the field circuit of alternator is
maximum.
Switch in DC supply to DC motor and the field of alternator.
Start the DC motor with the help of starter. The starter arm should be moved slowly, till the
speed of the motor builds and finally all the resistance step are cut out and the starter arm
is held on position by the magnet of no volt release.
Adjust the speed of the dc motor to rated speed of the alternator by varying the external
resistance in the field circuit of the motor.
Record the field current of the alternator and its open circuit voltage per phase.
Increase the field current of alternator in steps by decreasing the resistance and record the
field current and open circuit voltage of alternator for various values of field current.
Field current of alternator is increased, till the open circuit voltage of the alternator is 25 –
30 percent higher than the rated voltage of the alternator.
Decrease the field current of alternator to minimum by inserting the rheostat fully in the
field circuit.
(b) Short circuit test:With the dc motor running at rated speed and with minimum field current of alternator,
close the switches, thus short- circuiting the stator winding of alternator.
Record the field current of alternator and the short circuit current.
Increase the field current of alternator in steps, till the rated full load short circuit current.
Record the readings of ammeters in both the circuits at every step 4 – 5 observations are
sufficient, as short circuit characteristic is a straight line.
Decrease the field current of alternator to minimum and also decrease the speed of dc
motor by field rheostat of the motor
Switch off the dc supply to dc motor as well as to alternator field.
(c)
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Slip test:-
Connect the circuit of alternator as shown in fig. keeping the connection of the dc motor
same
Ensure that the resistance in the field circuit of dc motor is minimum.
Switch on the dc supply to the motor
Repeat steps 4 described in (a)
Adjust the speed of the dc motor slightly less than the synchronous speed of the alternator
by varying the resistance in the field circuit of the motor. Slip should be extremely low,
preferably less tha motor
Repeat steps 4 described in (a)
Adjust the speed of the dc motor slightly less than the synchronous speed of the alternator
by varying the resistance in the field circuit of the motor. Slip should be extremely low,
preferably less than 4 percent.
Ensure that the setting of 3 phase variac is at zero position
Switch on 3 phase ac supply to the stator winding of alternator.
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Ensure that the direction of rotation of alteration 4 percent.
Ensure that the setting of 3 phase variac is at zero position
Switch on 3 phase ac supply to the stator winding of alternator.
Ensure that the direction of rotation of alteration, when run by the dc motor and when run
as a 3 phase induction motor at reduced voltage (alternator provided with damper winding
can be run as a 3 phase induction motor) is the same.
Adjust the voltage applied to the stator winding, till the current in the stator winding is
approximately full load rated voltage.
Under these conditions, the current in the stator winding. The applied voltage to the stator
winding and the induced voltage in the open field circuit will fluctuate from minimum values
to maximum values which may be recorded by the meters included in the circuit. For better
results, oscillogram may be taken of stator current, applied voltage and induced voltage in
the field circuit.
Reduce the applied voltage to the stator winding of alternator and switch off the 3 phase
supply.
Decrease the speed of dc motor and switch off dc supply.
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OBSERVATION TABLE:S. No. Armature voltage (V)
Max.
Min.
Armature current (A)
Max.
Xd(Ω)
Xq(Ω)
Min.
1.
2.
CALCULATIONS:RESULT:The average value of direct-axis reactance X d = …..
The average value of quadrature-axis reactance Xq= ……..
PRECAUTIONS:1. All connections should be right and tight.
2. The zero setting of all the meters should be checked before connecting them in the
circuit.
3. The slip of the synchronous machine should be kept below 5%.
4. At all the times during this experiment, the voltage induced in the rotor circuit should
remain below the rating of the rating of the voltmeter connected there.
5. 3-phase A.C. voltage applied to the stator of the synchronous machine should be
appreciably below the rated voltage.
PROCEDURE:1) Connection to be made as per the given circuit.
2) Ensure that the motor is unloaded and the variac is set at zero output voltage.
3) Switch on 3 phase as mains and start the motor at reduced applied voltage. Increase the
applied voltage, till its rated value.
4) Observe the direction of rotation of the motor. In case, it is reverse. Change the phase
sequence of the applied voltage.
5) Take down the reading of all the meters and the speed under no load running
6) Increase the load on the motor gradually by turning of the hand wheels, thus, tightening the
belt. Record the readings of all the meters and the speed at every setting of the load.
Observations may be continued up to the full load current rating of the motor.
7) Reduce the load on the motor and finally unload it completely.
8) Switch off the supply to stop the motor.
9) Note down the efficiency diameter of the brake drum.
OBSERVATION TABLE:S. N.
V1 (V)
I1(A)
W (Watt)
W1 (kg)
W2 (kg)
N (rpm)
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CALCULATION:-
RESULT: We show the efficiency of induction motor on various load is …….
PRECAUTIONS:
All connections should be proper & tight.
The range of instruments should be carefully selected.
The zero setting of the instruments should be checked carefully in the beginning.
The range of instruments, particularly wattmeter should be chosen carefully.
Meters
used should be of proper range.