# Electric Drives Questions and Answers – Solid State Controlled Drives – Switch-Reluctance Motor Drives

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Solid State Controlled Drives – Switch-Reluctance Motor Drives”.

1. Reluctance motors are singly-excited.
a) True
b) False

Explanation: Reluctance motors are singly-excited. Stator side of the motor is excited by an AC source. Due to excitation, a revolving magnetic field is a created and rotor rotates with the synchronous speed.

2. Reluctance motor can operate on DC supply.
a) True
b) False

Explanation: Reluctance motor can operate on AC supply. It cannot operate on DC supply. Reluctance motors are singly-excited.

3. An infinite bus-bar has _____________
a) Constant voltage
b) Constant frequency
c) Constant voltage and frequency
d) constant speed

Explanation: An infinite bus-bar has constant voltage and constant speed. It has many types like single bus-bar, double bus-bar. It is connected to the grid.

4. Calculate the value of the Input power factor for 3-Φ Fully controlled rectifier if the firing angle value is 90°.
a) 0
b) .2
c) .3
d) .4

Explanation: The value of the input power factor for 3-Φ Fully controlled rectifier is .95cos(90°)=0. The input power factor is a product of distortion factor and displacement factor.

5. Which of the following motor is used in recording instruments?
a) Reluctance motor
b) Induction motor
c) Synchronous motor
d) DC motor

Explanation: Reluctance motor is used in recording instruments. It is used for many constant speed applications. The starting torque of the motor depends upon the rotor position.

6. Increasing the stator poles ___________ the speed of Reluctance motor.
a) Decreases
b) Increases
c) Remains the same
d) Negative

Explanation: Increasing the stator poles decreases the speed of Reluctance motor. The rotating magnetic field is developed which rotates at the speed of Ns . Ns∝(1÷P).

7. Calculate the circuit turn-off time for 3-Φ Fully controlled rectifier if the firing angle is 27° and supply frequency is 50 Hz.
a) 11.8 msec
b) 14.4 msec
c) 16.1 msec
d) 16.5 msec

Explanation: The circuit turn-off time for 3-Φ Fully controlled rectifier is (240°-α)÷ω. The value of circuit turn-off time for ∝ < 60° is (240°-27°)÷6.28×50=11.8 msec.

8. Calculate the circuit turn-off time for 3-Φ Fully controlled rectifier if the firing angle is 120° and supply frequency is 60 Hz.
a) 1.8 msec
b) 3.2 msec
c) 6.3 msec
d) 2.7 msec

Explanation: The circuit turn-off time for 3-Φ Fully controlled rectifier is (180°-α)÷ω. The value of circuit turn-off time for ∝ ≥ 60° is (180°-120°)÷6.28×60=2.7 msec.

9. Calculate peak-peak voltage if Vmax=180 V and Vmin=60 V.
a) 120 V
b) 150 V
c) 170 V
d) 110 V

Explanation: Peak-Peak voltage is equal to the difference between the maximum and minimum voltage. It is mathematically represented as Vp-p=Vmax-Vmin=180-60=120 V.

10. Calculate the value of Crest factor if Vpeak=798 V and Vr.m.s=489 V.
a) 1.63
b) 1.54
c) 1.59
d) 1.26

Explanation: The value of the crest factor is Vpeak÷Vr.m.s=798÷489=1.63 V. It signifies the peak value is 1.63 times than the r.m.s value.

11. Reluctance motor operates at power factor of __________
a) .8
b) .2
c) .3
d) .9

Explanation: Reluctance motor operates at a power factor of .8. The Reluctance motor works on the principle of an Induction motor.

12. Calculate the reactive power in a 24.5 Ω resistor.
a) 0 VAR
b) 5 VAR
c) 8 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.

13. Calculate the value of the duty cycle if the system is on for 1 sec and off for 17 sec.
a) .055
b) .781
c) .484
d) .947

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÷18=.055.

14. Calculate mark to space ratio if the system is on for 4 sec and off time is 9 sec.
a) .54
b) .44
c) .45
d) .46

Explanation: Mark to space is Ton÷Toff. It is the ratio of time for which the system is active and the time for which is inactive. M = Ton÷Toff = 4÷(9) = .44.

15. Calculate the value of the frequency if the inductive reactance is 60 Ω and the value of the inductor is 4 H.
a) 2.38 Hz
b) 5.54 Hz
c) 4.65 Hz
d) 9.42 Hz 