# Electrical Machines Questions and Answers – Circle Diagrams of Induction Machines

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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

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

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

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

Explanation: Slip = 0.04
T2/T1 = s2/s1
s2 = (1/2)*0.04 = 0.02
Nr = (1-0.02)*1500 = 1470 rpm.
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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

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

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

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

Explanation: OG is the rated torque.

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

Explanation: It is rotor current referred to stator.

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

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
c) remain same
d) none of the mentioned

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

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
c) remain same
d) none of the mentioned

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

Explanation: Flux increases as flux = V/f. So the magnetizing current also increases. Power factor decreases.
Explanation: |I| = |V/Z|
θ = atan(x/r), Here reactance is variable, which maps a semicircle in its locus diagram.

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. 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.

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’? c) fixed losses

Explanation: It is full load condition.

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