Electrical Machines Questions and Answers – Starting of Polyphase Induction Motors-I

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Starting of Polyphase Induction Motors”.

1. Voltage induce in the induction motor is highest at
a) starting
b) standstill
c) rated speed
d) any of the mentioned
View Answer

Answer: a
Explanation: At starting when slip is 1, the voltage induced is maximum.
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2. Starters are required in the induction motor because
a) of high starting current
b) they are not self starting
c) torque produced is very low at starting to overcome inertia
d) all of the mentioned
View Answer

Answer: a
Explanation: Starters are needed in induction motors to limit the starting current.

3. Full voltage starting of induction motor may cause
a) dip in the voltages
b) high inrush current
c) high losses
d) all of the mentioned
View Answer

Answer: a
Explanation: Starting induction motor on full load will cause high inrush current which furthe causes dip in the bus voltage.

4. For a 4-pole, 50 Hz induction motor has starting current as 5 times to that of the full load current working at a speed of 1450rpm.
The ratio of starting to full load torque will be
a) 0.83
b) 0.17
c) 0.67
d) 1
View Answer

Answer: a
Explanation: Tst/Tfl = (Ist/Ifl)^2*slip
Tst/Tfl= 25*0.033
=0.83

5. Starting methods applicable to both squirrel-cage and slip ring induction motors is/are
A. DOL starting
B. Auto transformer starting
C. Rotor resistance starting
a) A, B
b) A, B, C
c) B, C
d) A, C
View Answer

Answer: a
Explanation: Rotor resistance method can only be used with slip ring induction motor.

6. The method which employs reduction of the voltage while induction motor starting is/are
A. DOL starting
B. Auto transformer starting
a) Only A
b) A, B
c) Only B
d) Neither A nor B
View Answer

Answer: a
Explanation: Direct online starting uses reduction in voltage.
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7. The method which employs raising of the voltage while induction motor starting is/are
A. DOL starting
B. Auto transformer starting
a) Only A
b) A, B
c) Only B
d) Neither A nor B
View Answer

Answer: d
Explanation: None of the starting methods use increase in the starting voltage.

8. External methods of induction motor starting is employed to
a) Reduce the voltage
b) Reduce the current
c) improve efficiency
d) all of the mentioned
View Answer

Answer: a
Explanation: External methods like auto transformer is used to mainly reduce voltage.

9. A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced by reactance method of starting with a fraction of ‘x’, then the
starting current reduces by
a) x
b) x^2
c) 1/x
d) no change in the starting current
View Answer

Answer: a
Explanation: The current will be reduced by a fraction of x.

10. A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced by reactance method of starting with a fraction of ‘x’, then the
starting torque reduces by
a) x
b) x^2
c) 1/x
d) no change in the starting current
View Answer

Answer: b
Explanation: Tst/Tfl = (x^2)(Ist/Ifl)^2*slip.

11. Series reactor method of starting has main disadvantage of
a) lowering the starting power
b) increasing the starting current
c) increasing starting torque
d) all of the mentioned
View Answer

Answer: a
Explanation: Main disadvantage of using reactors is that ot reduces the starting power of the machine and so the efficiency.

12. Series reactor method of starting has main disadvantage of lowering the starting power. This can be overcome by
a) using series resistance
b) using parallel reactor
c) using parallel resistance
d) using series reactor
View Answer

Answer: a
Explanation: Adding series resistance instead od reactors will help improving the starting power.
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13. A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced by auto transformer method of starting with a tapping ratio of ‘x’, then the
starting current reduces by
a) x
b) x^2
c) 1/x
d) no change in the starting current
View Answer

Answer: b
Explanation: The starting current reduces by a fraction of x^2.

14. A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced by auto transformer method of starting with a tapping ratio of ‘x’, then the
starting torque will be
a) x *Tfl
b) x^2 *Tfl
c) 1/x *Tfl
d) no change in the starting current
View Answer

Answer: a
Explanation: Tst/Tfl = (x^2)(Ist/Ifl)^2*slip.

15. For the application requiring high starting torque and minimum starting current, the most suitable starting method will be
a) Auto transformer starting method
b) Reactance starting method
c) DOL method
d) Star-Delta method
View Answer

Answer: a
Explanation: Auto transformer provides the maximum starting torque.

16. Auto transformer method of starting has most acceptable method for the starting of the induction motor due to
a) maximum torque
b) maximum reduction of inrush current
c) variable tapping
d) all of he mentioned
View Answer

Answer: a
Explanation: Maximum torque, reduction of inrush current and variable turns ratio makes it popular method of starting of IM.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn