This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Energy Relations During Starting”.

1. 440 V, 77 A, 700 rpm DC separately excited motor having a resistance of 0.11 ohms excited by an external dc voltage source of 24 V. Calculate the torque developed by the motor on full load.

a) 453.51 N-m

b) 451.24 N-m

c) 440.45 N-m

d) 452.64 N-m

View Answer

Explanation: Back emf developed in the motor during the full load can be calculated using equation E

_{b}= V

_{t}-I×R

_{a}= 431.53 V and machine constant K

_{m}= E

_{b}÷W

_{m}which is equal to 5.88. Torque can be calculated by using the relation T = K

_{m}× I = 5.88×77 = 453.51 N-m.

2. Calculate the power developed by a motor using the given data: E_{b} = 55 V and I = 6 A.

a) 440 W

b) 220 W

c) 330 W

d) 550 W

View Answer

Explanation: Power developed by the motor can be calculated using the formula P = E

_{b}×I = 55×6 = 330 W. If rotational losses are neglected, the power developed becomes equal to the shaft power of the motor.

3. Calculate the value of the angular acceleration of the motor using the given data: J = 36 kg-m^{2}, load torque = 66 N-m, motor torque = 26 N-m.

a) 1.11 rad/s^{2}

b) 2.22 rad/s^{2}

c) 3.33 rad/s^{2}

d) 4.44 rad/s^{2}

View Answer

Explanation: Using the dynamic equation of motor J×(angular acceleration) = Motor torque – Load torque: 36×(angular acceleration) = 66-26 = 40, angular acceleration = 1.11 rad/s

^{2}.

4. Calculate the moment of inertia of the apple having a mass of .4 kg and diameter of 12 cm.

a) .0008 kgm^{2}

b) .0007 kgm^{2}

c) .0009 kgm^{2}

d) .0001 kgm^{2}

View Answer

Explanation: The moment of inertia of the apple can be calculated using the formula I=mr

^{2}×.5. The mass of the apple and diameter is given. I=(.4)×.5×(.06)

^{2}= .0007 kgm

^{2}. It depends upon the orientation of the rotational axis.

5. Calculate the moment of inertia of the thin spherical shell having a mass of 3.3 kg and diameter of .6 cm.

a) .00125 kgm^{2}

b) .00196 kgm^{2}

c) .00145 kgm^{2}

d) .00178 kgm^{2}

View Answer

Explanation: The moment of inertia of the thin spherical shell can be calculated using the formula I=mr

^{2}×.66. The mass of the thin spherical shell and diameter is given. I=(3.3)×.66×(.03)

^{2}=.00196 kgm

^{2}. It depends upon the orientation of the rotational axis.

6. Calculate the time period of the waveform y(t)=7cos(54πt+2π÷4).

a) .055 sec

b) .037 sec

c) .023 sec

d) .017 sec

View Answer

Explanation: The fundamental time period of the cosine wave is 2π. The time period of y(t) is 2π÷54π=.037 sec. The time period is independent of phase shifting and time shifting.

7. Calculate the useful power developed by a motor using the given data: P_{in}= 1500 W, I_{a}= 6 A, R_{a}=.2 Ω. Assume frictional losses are 50 W and windage losses are 25 W.

a) 1400 W

b) 1660.5 W

c) 1417.8 W

d) 1416.7 W

View Answer

Explanation: Useful power is basically the shaft power developed by the motor that can be calculated using the formula P

_{sh}= P

_{dev}-(rotational losses). P

_{dev}= P

_{in}-I

_{a}

^{2}R

_{a}= 1500-6

^{2}(.2)=1492.8 W. The useful power developed by the motor is P

_{sh}= P

_{dev}-(rotational losses) = 1492.8 –(50+25) = 1417.8 W.

8. The slope of the V-I curve is 15.5°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.

a) .277 Ω

b) .488 Ω

c) .443 Ω

d) .457 Ω

View Answer

Explanation: The slope of the V-I curve is resistance. The slope given is 15.5° so R=tan(15.5°)=.277 Ω. The slope of the I-V curve is reciprocal of resistance.

9. The generated e.m.f from 2-pole armature having 2 conductors driven at 3000 rpm having flux per pole as 4000 mWb, with 91 parallel paths is ___________

a) 8.64 V

b) 8.56 V

c) 8.12 V

d) 8.79 V

View Answer

Explanation: The generated e.m.f can be calculated using the formula E

_{b}= Φ×Z×N×P÷60×A, Φ represent flux per pole, Z represents the total number of conductors, P represents the number of poles, A represents the number of parallel paths, N represents speed in rpm. E

_{b}= 4×2×3000×2÷60×91 = 8.79 V.

10. A 3-phase induction motor runs at almost 888 rpm at no load and 500 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) 388 revolution per minute

b) 400 revolution per minute

c) 644 revolution per minute

d) 534 revolution per minute

View Answer

Explanation: Supply frequency=50 Hz. No-load speed of motor = 888 rpm. The full load speed of motor=500 rpm. Since the no-load speed of the motor is almost 888 rpm, hence synchronous speed near to 888 rpm. Speed of rotor field=888 rpm. Speed of rotor field with respect to rotor = 888-500 = 388 rpm.

11. Calculate the active power in a 157.1545 H inductor.

a) 4577 W

b) 4567 W

c) 4897 W

d) 0 W

View Answer

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 1.2 Ω resistor with 1.8 A current flowing through it.

a) 3.88 W

b) 3.44 W

c) 3.12 W

d) 2.18 W

View Answer

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

^{o}. P=I

^{2}R=1.8×1.8×1.2=3.88 W.

13. Calculate the total heat dissipated in a resistor of 12 Ω when 9.2 A current flows through it.

a) 2.01 KW

b) 3.44 KW

c) 1.01 KW

d) 2.48 KW

View Answer

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=I

^{2}R=9.2×9.2×12=1.01 kW.

14. Calculate mark to space ratio if the system is on for 9 sec and time period is 11 sec.

a) 4.6

b) 4.8

c) 4.5

d) 4.9

View Answer

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=9÷(11-9) = 4.5.

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