This set of Electrical Machines test focuses on “Starting of Polyphase Induction Motors-II”.
1. A 3-phase induction motor is fed at the stator at a frequency f. If the rotor speed is Nr and synchronous speed is Ns,
then the absolute speed of the resultant flux in space is
b) (Ns+Nr) if the flux rotates in the same same direction as the rotor
c) (Ns-Nr) if the flux rotates in the same same direction as the rotor
Explanation: The flux speed in space will be Ns irrespective of any rotor speed.
2. I. Leakage reactance is dependent on supply current.
II. Magnetizing reactance is dependent on the air gap flux.
Which of the following is correct?
d) None of the mentioned
Explanation:Leakage flux depends on the load current and the magnetizing reactance depends on the air gap flux.
3. I. Rotor resistance is dependent on the speed.
II. Core loss does not depend on the supply frequency and voltage.
d) None of the mentioned
Explanation: Core loss does depend on the supply ‘V’ and ‘f’.
4. The stator of a 3-phase, six pole induction motor’s stator is connected to 50 Hz supply while its rotor is fed from a 20 Hz supply.
The possible speeds at which the machine will run will be
(i) 1600rpm (ii) 600rpm (iii) 1400rpm (iv) 400 rpm
Explanation: Stator speed = 120*50/6 = 1000 rpm
Rotor speed = 120*20/6 = 400 rpm
Hence the possible speeds could be 1400rpm or 600rpm.
5. For a 3-phase 4-pole induction motor has open circuited slip rings. The frequency across the slip rings is 75 Hz. Then we can conclude that the motor is run at a speed of
(i) 750rpm (ii) 1500rpm (iii) 2250rpm (iv) 3750 rpm
Explanation: Stator speed = 120*50/4 = 1500 rpm
Rotor speed = 120*50/4 = 2250 rpm
Hence the possible speeds could be 750rpm or 3750rpm.
6. A 3-phase slip ring IM is has its copper bars replaced by the aluminium bars in the machine. With the above change,
a) starting torque increases, maximum torque remains same, smT increase
b) starting torque increases, maximum torque remains same, smT remains same
c) starting torque increases, maximum torque remains increases, smT increase
d) starting torque decreases, maximum torque remains same, smT decrease
Explanation: As the resistance has been changed as the aluminium has replaced copper, it will not affect the maximum torque.
7. A 3-phase slip ring IM is has its copper bars changed by the aluminium bars in the machine. With the above change,
a) starting torque remains same
b) starting torque increase
c) starting torque decrease
d) maximum torque increase
Explanation: Resistance increase, so the starting torque increase.
8. A center ammeter connected to the rotor end circuit of a 6-pole,50 Hz, induction motor makes 45 complete oscillations in a minute.
Then the rotor speed and the speed of stator field w.r.t. rotor is
a) 985, 15
b) 970, 1000
c) 985, 985
d) 985, 970
Explanation: Rotor frequency = s*f1 = 45/60 = 0.75 Hz.
s = 0.75/50 = 0.015
rotor speed = 1000(1-0.015) = 985 rpm.
Speed of stator field w.r.t. rotor = Ns-Nr = 1000-985=15 rpm.
9. Choose from the below which can be obtained by the equivalent circuit of an electrical machine?
a) Complete performance characteristics of the machine
b) Temperature coefficients of the machine components
c) Type of protection to be used in the machine
d) Design parameters of the windings
Explanation: Equivalent circuit can help to give the complete performance characteristics of an electrical machine.
10. If one of the supply fuses blows off and gets open circuited, then for the connected induction machine continues to run with increased slip.
Explanation: It is true because as one of the phase is missing, it will lead to decrease in speed assuming machine was providing the rated load.
11. Due to the line to ground fault, phase ‘a’ in the induction motor phase supply gets cut, then machine
a) continues to run with excessive supply current
b) continues to run with reduced slip
d) all of the mentioned
Explanation: It continues to run at lowered speed but with increased current.
12. For 3-phase induction motor, as load increases from no load towards the full load,
a) power factor improves
b) power factor remains same
c) power factor varies linearly
d) power factor increases till 40% load and then decreases
Explanation: Overall during the operation power factor improves as the reactance reduces and the power factor angle reduces.
13.For 3-phase induction motor, as load increases from no load towards the full load, torque _______ slip.
a) increases in proportion to
b) decreases in proportion to
c) remains constant to
d) increases hyperbolically to
Explanation: For the lower slip, torque will increase proportionally to the slip as verified by the torque slip characteristic.
14. As a 3-phase induction motor, as load increases from no load towards the full load,
A. power factor improves
B. torque increases
C. air gap flux falls steeply
a) A, B
b) B, C
c) A, B, C
Explanation: Air gap flux increases as the load increase.
15. Torque-slip characteristic of an induction motor is linear in the smaller slip values, because
a) effective rotor resistance is very large compared to reactance
b) rotor resistance is equal to stator resistance
c) rotor resistance is equal to rotor reactance
d) rotor reactance almost equal to stator reactance
Explanation: It is because effective resistance value(r2/s) is far more than reactance and so the torque is linear in this region.
16. If a 3-phase induction motor is fed from the rotor short circuiting the stator terminals, frequency of the current flowing in the short-circuited stator is
a) slip frequency
b) supply frequency
d) frequency corresponding to the rotor speed
Explanation: In the stator the flux developed will revolve with a speed of slip frequency.
17. A constant torque variable power drive of induction machine has
a) constant air gap flux.
b) constant slip
c) variable air gap flux, constant slip
d) constant slip, constant air gap flux
Explanation: A constant torque drive should have constant flux and hence constant air gap flux. Also it should have constant slip.
18. If an emf at slip frequency is injected at an angle α with existing rotor induced emf, then
a) power factor only improves
b) speed changes
c) power factor improves and speed changes
d) power factor worsens and speed changes
Explanation: Power factor increases as the net angle decreases.
Sanfoundry Global Education & Learning Series – Electrical Machines.
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