# Electric Drives Questions and Answers – Methods of Starting Electric Motors

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Methods of Starting Electric Motors”.

1. Calculate the active power in a .45 H inductor.
a) 0.11 W
b) 0.14 W
c) 0.15 W
d) 0 W

Explanation: The inductor is a linear element. It only absorbs reactive power and stores it in the form of oscillating energy. The voltage and current are 90° in phase in case of the inductor so the angle between V & I is 90°. P=VIcos90° = 0 W.

2. A 10-pole, 3-phase, 60 Hz induction motor is operating at a speed of 100 rpm. The frequency of the rotor current of the motor in Hz is __________
a) 52.4
b) 54.8
c) 51.66
d) 51.77

Explanation: Given a number of poles = 10. Supply frequency is 60 Hz. Rotor speed is 100 rpm. Ns = 120×f÷P=120×60÷10 = 720 rpm. S=Ns-Nr÷Ns = 720-100÷720=.86. F2=sf=.86×60=51.66 Hz.

3. Calculate the phase angle of the sinusoidal waveform z(t)=.99sin(4578πt+78π÷78).
a) π÷3
b) 2π
c) π÷7
d) π

Explanation: Sinusoidal waveform is generally expressed in the form of V=Vmsin(ωt+α) where Vm represents peak value, Ω represents angular frequency, α represents a phase difference.
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4. Calculate the moment of inertia of the disc having a mass of 4 kg and diameter of 1458 cm.
a) 106.288 kgm2
b) 104.589 kgm2
c) 105.487 kgm2
d) 107.018 kgm2

Explanation: The moment of inertia of the disc can be calculated using the formula I=mr2×.5. The mass of the disc and diameter is given. I=(4)×.5×(7.29)2=106.288 kgm2. It depends upon the orientation of the rotational axis.

5. Calculate the moment of inertia of the thin spherical shell having a mass of 703 kg and diameter of 376 cm.
a) 1639.89 kgm2
b) 1628.47 kgm2
c) 1678.12 kgm2
d) 1978.19 kgm2

Explanation: The moment of inertia of the thin spherical shell can be calculated using the formula I=mr2×.66. The mass of the thin spherical shell and diameter is given. I=(703)×.66×(1.88)2=1639.89 kgm2. It depends upon the orientation of the rotational axis.

6. Calculate the value of the torque when 89 N force is applied perpendicular to a 78 m length of stick fixed at the center.
a) 6942 N-m
b) 3000 N-m
c) 1000 N-m
d) 4470 N-m

Explanation: Torque can be calculated using the relation T = (length of stick) × (Force applied) = r×F×sin90. F is given as 89 N and r is 78 m then torque is 89×78 = 6942 N-m. (the angle between F and r is 90 degrees).

7. 100 V, 2 A, 90 rpm separately excited dc motor with armature resistance (Ra) equal to 8 ohms. Calculate back emf developed in the motor when it operates on 3th/4 of the full load. (Assume rotational losses are neglected)
a) 100 V
b) 87 V
c) 88 V
d) 90 V

Explanation: Back emf developed in the motor can be calculated using the relation Eb = Vt-I×Ra. In question, it is asking for 3th/4 load, but the data is given for full load so current becomes 3th/4 of the full load current = 2÷1.33 = 1.5 A. 100 V is terminal voltage it is fixed so Eb = 100-1.5×8 = 88 V.

8. The slope of the V-I curve is 16.8°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
a) .324 Ω
b) .301 Ω
c) .343 Ω
d) .398 Ω

Explanation: The slope of the V-I curve is resistance. The slope given is 16.8° so R=tan(16.8°)=.301 Ω. The slope of the I-V curve is reciprocal of resistance.

9. Calculate the value of the torque when 1 N force is applied perpendicular to a 1 m length of chain fixed at the center.
a) 1 N-m
b) 3 N-m
c) 2 N-m
d) 4 N-m

Explanation: Torque can be calculated using the relation T = (length of chain) × (Force applied) = r×F×sin90. F is given as 1 N and r is 1 m then torque is 1×1 = 6942 N-m. (the angle between F and r is 90 degrees).

10. A 3-phase induction motor runs at almost 140 rpm at no load and 50 rpm at full load when supplied with power from a 50 Hz, 3-phase supply. What is the corresponding speed of the rotor field with respect to the rotor?
a) 20 revolution per minute
b) 80 revolution per minute
c) 90 revolution per minute
d) 70 revolution per minute

Explanation: Supply frequency=50 Hz. No-load speed of motor= 140 rpm. The full load speed of motor=50 rpm. Since the no-load speed of the motor is almost 140 rpm, hence synchronous speed near to 140 rpm. Speed of rotor field=140 rpm. Speed of rotor field with respect to rotor = 140-50 = 90 rpm.

11. Calculate the active power in a .7889 H inductor.
a) .123 W
b) .155 W
c) 0 W
d) .487 W

Explanation: The inductor is a linear element. It only absorbs reactive power and stores it in the form of oscillating energy. The voltage and current are 90° in phase in case of the inductor so the angle between V & I is 90°. P = VIcos90° = 0 W.

12. Calculate the active power in a .8 Ω resistor with 2 A current flowing through it.
a) 2.4 W
b) 3.4 W
c) 2.2 W
d) 3.2 W

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 90°. P=I2R=2×2×.8=3.2 MW.

13. The unit of angular frequency is Hz.
a) True
b) False

Explanation: Angular frequency is defined as the rate of change of angular displacement with respect to time. Angular displacement is generally expressed in terms of a radian. The unit of angular frequency is Hz. ω=2×3.14×f.

14. Calculate the active power in a .56 F capacitor.
a) 37.8 W
b) 0 W
c) 15.4 W
d) 124.5 W

Explanation: The capacitor is a linear element. It only absorbs reactive power and stores it in the form of oscillating energy. The voltage and current are 90° in phase in case of the capacitor so the angle between V & I is 90°. P = VIcos90° = 0 W. Current leads the voltage in case of the capacitor.

Sanfoundry Global Education & Learning Series – Electric Drives.

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