# Electric Drives Questions and Answers – Induction Motors – Voltage/Frequency Control

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Induction Motors – Voltage/Frequency Control”.

1. Calculate the minimum value of the active power in the cylindrical rotor synchronous machine? (Eb represents armature emf, Vt represents terminal voltage, δ represents rotor angle, X represents reactance)
a) Eb×Vt×sinδ÷X
b) 0
c) Eb2×Vt×sinδ÷X
d) Eb×Vt÷X

Explanation: The real power in the cylindrical rotor machine is Eb×Vt×sinδ÷X. It is inversely proportional to the reactance. The stability of the machine is decided by the maximum power transfer capability. Its minimum value occurs for delta=0°.

2. Cylindrical pole machines are less stable than salient rotor machines.
a) True
b) False

Explanation: Cylindrical pole machines are less stable than salient rotor machines because of the less short circuit ratio and less real power transfer capability. The air gap length in cylindrical pole machines is less as compare to salient rotor machines.

3. The unit of area is m2.
a) True
b) False

Explanation: Area is defined as the product of length and breadth. The length and breadth are expressed in the meter. The unit of area is m2.
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4. Calculate the power factor if the power angle is 45°.
a) .707
b) .407
c) .608
d) 1

Explanation: Power factor is the ratio of the real power to the apparent power. It measures the useful power contained in the total power cosΦ=cos(45°)=.707.

5. Calculate the reactive power in a 451.26 Ω resistor.
a) 1 VAR
b) .6 VAR
c) 0 VAR
d) .9 VAR

Explanation: The resistor is a linear element. It only absorbs real power and dissipates it in the form of heat. The voltage and current are in the same phase in case of the resistor so the angle between V & I is 0°. Q=VIsin0° = 0 VAR.
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6. What is the unit of displacement?
a) m/s
b) m
c) atm/m
d) Volt

Explanation: Displacement is the difference between the final and initial point. It is a vector quantity. It is expressed in the meter. It is not a tensor quantity. It has direction.

7. Calculate the reactive power in a 725.45 Ω resistor.
a) 122.1 VAR
b) 261.1 VAR
c) 0 VAR
d) 199.7 VAR

Explanation: The resistor is a linear element. It only absorbs real power and dissipates it in the form of heat. The voltage and current are in the same phase in case of the resistor so the angle between V & I is 0°. Q = VIsin0° = 0 VAR.

8. The slope of the I-V curve is 180°. Calculate the value of resistance.
a) inf Ω
b) .81 Ω
c) 45 Ω
d) 41.2 Ω

Explanation: The slope of the I-V curve is the reciprocal of the resistance. The slope given is 180° so R=1÷tan(180°)=inf Ω. The slope of the V-I curve is resistance. It behaves as an open circuit.

9. Calculate the value of the duty cycle if the system is on for 1 sec and off for inf sec.
a) 0
b) .89
c) .148
d) .46

Explanation: Duty cycle is Ton÷Ttotal. It is the ratio of time for which the system is active and the time taken by the signal to complete one cycle. D = Ton÷Ttotal=1÷inf = 0.

10. The phase difference between voltage and current in the purely inductive coil.
a) 20°
b) 90°
c) 40°
d) 88°

Explanation: In the case of a purely inductive coil, the voltage leads the current by 90° or the current lags the voltage by 90°. The phase difference between voltage and current is 90°.

11. Armature reaction is purely demagnetizing in nature due to a capacitive load in the synchronous motor.
a) True
b) False

Explanation: Due to a capacitive load, armature current is in opposite and 90° phase with the field magneto-motive force. Armature magneto-motive force produced due to this current will be in 180° phase and 90° phase with the field flux. The nature of the armature reaction is partially demagnetizing and cross magnetizing.

12. Armature reaction is cross-magnetizing in nature due to a resistive load in the synchronous generator.
a) True
b) False

Explanation: Due to a purely resistive load, armature current is in quadrature with the field magneto-motive force. Armature magneto-motive force produced due to this current will be in quadrature with the field flux. It will try to increase the net magnetic field. The nature of the armature reaction is cross magnetizing.

13. Calculate the value of the short circuit ratio if Voc=78 V, Isc=15 A with field current = 5 A.
a) 5.2
b) 4.8
c) 3.2
d) 1.8

Explanation: The value of the short circuit ratio is the ratio of open circuit voltage to the short circuit current with same field current. SCR=Voc÷Isc=78÷15=5.2.

14. SCR determines the stability of the synchronous machine.
a) True
b) False

Explanation: The value of the short circuit ratio determines the stability of the synchronous machine. It is inversely proportional to the per unit value of reactance. Stability of the synchronous machine depends upon the maximum power transfer capability.

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