Electrical Machines Questions and Answers – Circle Diagrams of Induction Machines

«
»

This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Circle Diagrams of Induction Machines”.

1. Circle diagram of an induction motor is graphical representation of ____________
a) its equivalent circuit
b) its rotor equivalent
c) its stator equivalent
d) its stationary equivalent circuit
View Answer

Answer: a
Explanation: Circle diagram gives information about the equivalent circuit of the induction motor.
advertisement

2. Circle diagram depicts the relation between ____________
a) stator current and the slip variation
b) rotor current and stator current
c) slip variation and the power factor
d) slip variation and the losses of the machine
View Answer

Answer: a
Explanation: Circle diagram shows extermeties of stator current and the slip variation.

3. A 10kW, 50 Hz, 3-phase induction motor develops the rated torque at 1440rpm. If the load torque is reduced to half, then the motor speed is?
a) 1470rpm
b) 1410rpm
c) 1400rpm
d) 1444rpm
View Answer

Answer: a
Explanation: Slip = 0.04
T2/T1 = s2/s1
s2 = (1/2)*0.04 = 0.02
Nr = (1-0.02)*1500 = 1470 rpm.
Sanfoundry Certification Contest of the Month is Live. 100+ Subjects. Participate Now!
advertisement
advertisement

4. A 10kW, 50 Hz, 3-phase induction motor develops the rated torque at 1440rpm. If the load torque is reduced to half, then the power output that can now be obtained is?
a) 5 kW
b) 5.3 KW
c) 4.6 kW
d) 8 kW
View Answer

Answer: a
Explanation: s2 = (1/2)*0.04 = 0.02
P = (10000*60*2*3.14*1470)/(2*2*3.14*1460*60)
P = 5.03 kW.

5. In a 3-phase induction motor, the flux per pole will vary with the slip is?
a) The flux per pole will vary with the slip in a 3-phase induction motor - option a
b) The flux per pole will vary with the slip in a 3-phase induction motor - option b
c) The flux per pole will vary with the slip in a 3-phase induction motor - option c
d) The flux per pole will vary with the slip in a 3-phase induction motor - option d
View Answer

Answer: a
Explanation: Flux remains constant throughout the operation of the induction motor.
advertisement

6. The rotor mmf per pole in the 3-phase, 4-pole induction motor varied with slip is?
a) The rotor mmf per pole in the 3-phase, 4-pole induction motor varied with slip - option a
b) The rotor mmf per pole in the 3-phase, 4-pole induction motor varied with slip - option b
c) The rotor mmf per pole in the 3-phase, 4-pole induction motor varied with slip - option c
d) The rotor mmf per pole in the 3-phase, 4-pole induction motor varied with slip - option d
View Answer

Answer: a
Explanation: As slip decreases, the rotor current decreases so the rotor mmf.

7. Blocked rotor test is conducted to find the ____________
a) leakage impedance
b) leakage reactance
c) stator impedance
d) rotor impedance
View Answer

Answer: a
Explanation: Blocked rotor test determines the leakage impedance and so the core losses.
advertisement

8. In the below circle diagram, OG represents?
OG is the rated torque in the circle diagram
a) Rated torque
b) Maximum torque
c) Rated power output
d) Losses
View Answer

Answer: a
Explanation: OG is the rated torque.

9. In the following circle diagram of the induction motor, QN is the ____________
QN is rotor current referred to stator in the circle diagram of the induction motor
a) rotor current referred to stator
b) stator current
c) stator current referred to rotor
d) no load input to rotor
View Answer

Answer: a
Explanation: It is rotor current referred to stator.
advertisement

10. Stator resistance can be measured using Kelvin’s double bridge.
a) True
b) False
View Answer

Answer: a
Explanation: Stator has very low resistance, so it is most accurately observed by using a kelvin’s double bridge.

11. If the no load test is conducted at its rated frequency but less than the rated voltage, then the wattmeter reading will be ____________
a) reduced
b) more than previous reading
c) remain same
d) none of the mentioned
View Answer

Answer: a
Explanation: No load losses will reduce, so the power consumption.

12. If the no load test is conducted at its rated frequency but less than the rated voltage, then the mechanical losses will _____ and the constant losses will _________
a) constant, reduce
b) constant, rise
c) reduce, rise
d) constant, constant
View Answer

Answer: a
Explanation: Mechanical losses remain always constant, but the stator losses will reduce as the voltage has been decreased.

13. No load power factor of the induction motor will __________ if the no load test is conducted at its rated frequency but less than the rated voltage.
a) improve
b) degrade
c) remain same
d) none of the mentioned
View Answer

Answer: a
Explanation: It will improve.

14. If the no load test is conducted at its rated voltage but less than the rated frequency.

A. The wattmeter reading will be more than previous.
B. The power factor will also improve.

a) Only A is true
b) Only B is true
c) Both A as well as B are true
d) None of the mentioned
View Answer

Answer: a
Explanation: Flux increases as flux = V/f. So the magnetizing current also increases. Power factor decreases.

15. Circle diagram for the fixed rotor resistance and variable reactance is most appropriately shown by?
a) Circle diagram for the fixed rotor resistance & variable reactance - option a
b) Circle diagram for the fixed rotor resistance & variable reactance - option b
c) Circle diagram for the fixed rotor resistance & variable reactance - option c
d) Circle diagram for the fixed rotor resistance & variable reactance - option d
View Answer

Answer: a
Explanation: |I| = |V/Z|
θ = atan(x/r), Here reactance is variable, which maps a semicircle in its locus diagram.

16. Circle diagram for the variable rotor resistance and fixed reactance is most appropriately shown by
a) Circle diagram for the variable rotor resistance & fixed reactance - option a
b) Circle diagram for the variable rotor resistance & fixed reactance - option b
c) Circle diagram for the variable rotor resistance & fixed reactance - option c
d) Circle diagram for the variable rotor resistance & fixed reactance - option d
View Answer

Answer: a
Explanation: |I| = |V/Z|
θ = atan(x/r), Resistance is varying, so the locus will be a semi circle.

17. Use the below circle diagram to choose the correct option.
OG is the rated torque in the circle diagram
a) CF: Rotor copper loss, FG: Stator copper loss, GE: Fixed losses.
b) FG:Rotor copper loss, CF: Stator copper loss, GE: Fixed losses.
c) GE:Rotor copper loss, FG: Stator copper loss, CF: Fixed losses.
d) FG:Rotor copper loss, CF: Stator copper loss, CF: Core losses.
View Answer

Answer: a
Explanation: GE is fixed losses. From the F to top the rated rotor copper losses occur.

18. In the following circle diagram, what is CC’?
OG is the rated torque in the circle diagram
a) full load condition
b) no load condition
c) fixed losses
d) rated full load losses
View Answer

Answer: a
Explanation: It is full load condition.

Sanfoundry Global Education & Learning Series – Electrical Machines.

To practice all areas of Electrical Machines, here is complete set of 1000+ Multiple Choice Questions and Answers.

advertisement
advertisement
Subscribe to our Newsletters (Subject-wise). Participate in the Sanfoundry Certification contest to get free Certificate of Merit. Join our social networks below and stay updated with latest contests, videos, internships and jobs!

Youtube | Telegram | LinkedIn | Instagram | Facebook | Twitter | Pinterest
Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

Subscribe to his free Masterclasses at Youtube & technical discussions at Telegram SanfoundryClasses.