This set of Electric Drives Objective Questions & Answers focuses on “Effect of Starting on Power Supply, Motor and Load “.

1. The ferrite cores are used for ____________ transformers.

a) Small transformers

b) Medium transformers

c) Large transformers

d) Medium and small transformers

View Answer

Explanation: Ferrite cores are used for cores of small transformers used in communication circuits at high frequencies and low energy levels. Because ferrites have high resistivity they will have lower eddy current losses.

2. A 2-pole, 3-phase, 50 Hz induction motor is operating at a speed of 400 rpm. The frequency of the rotor current of the motor in Hz is __________

a) 43.33

b) 42.54

c) 43.11

d) 41.47

View Answer

Explanation: Given a number of poles = 2. Supply frequency is 50 Hz. Rotor speed is 400 rpm. N

_{s}= 120×f÷P=120×50÷2 = 3000 rpm. S=N

_{s}-N

_{r}÷N

_{s}= 3000-400÷3000 = .8666. F

_{2}=sf=.8666×50=43.33 Hz.

3. Calculate the phase angle of the sinusoidal waveform w(t)=.45sin(87πt+8π÷787).

a) 2π÷39

b) 8π÷787

c) 5π÷74

d) 42π÷4

View Answer

Explanation: Sinusoidal waveform is generally expressed in the form of V=V

_{m}sin(ωt+α) where V

_{m}represents peak value, ω represents angular frequency, α represents a phase difference.

4. Calculate the moment of inertia of the disc having a mass of 1 kg and radius of 1 m.

a) 1 kgm^{2}

b) .5 kgm^{2}

c) 2 kgm^{2}

d) 3 kgm^{2}

View Answer

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

^{2}×.5. The mass of the disc and diameter is given. I=(1)×.5×(1)

^{2}=.5 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 kg and diameter of 6 cm.

a) .0178 kgm^{2}

b) .0147 kgm^{2}

c) .0398 kgm^{2}

d) .0144 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)×.66×(.03)

^{2}=.0178 kgm

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

6. The power factor of a synchronous motor __________

a) Improves with an increase in excitation and may even become leading at high excitations

b) Decreases with increase in excitation

c) Is independent of its excitation

d) Increase with loading for a given excitation

View Answer

Explanation: From inverted V-curve we can see when the power factor is leading power factor decreases when the excitation increases and it is over-excited conditions and when power factor is lagging if the motor power factor is increasing if excitation increases.

7. The frame of a synchronous motor is made of _________

a) Aluminum

b) Silicon steel

c) Cast iron

d) Stainless steel

View Answer

Explanation: The frame of a synchronous motor is made of cast iron. The power factor of a synchronous motor depends upon maximum power transfer capability.

8. The slope of the V-I curve is 45°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.

a) 2 Ω

b) 3 Ω

c) 4 Ω

d) 1 Ω

View Answer

Explanation: The slope of the V-I curve is resistance. The slope given is 45° so R=tan(45°)=1 Ω. The slope of the I-V curve is reciprocal of resistance.

9. Which one of the following methods would give a higher than the actual value of regulation of the alternator.

a) ZPF method

b) MMF method

c) EMF method

d) ASA method

View Answer

Explanation: EMF method is a pessimistic method of voltage regulation as it gives higher than the actual value of voltage regulation. EMF method will the values that are greater than the actual value.

10. A 3-phase induction motor runs at almost 1100 rpm at no load and 640 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) 430 revolution per minute

b) 440 revolution per minute

c) 460 revolution per minute

d) 450 revolution per minute

View Answer

Explanation: Supply frequency=50 Hz. No-load speed of motor = 1100 rpm. The full load speed of motor=640 rpm. Since the no-load speed of the motor is almost 1100 rpm, hence synchronous speed near to 1100 rpm. Speed of rotor field=1100 rpm. Speed of rotor field with respect to rotor = 1100-640 = 460 rpm.

11. Which of the following core has linear characteristics?

a) Air core

b) Steel core

c) CRGO core

d) Iron core

View Answer

Explanation: Air core coils have linear magnetization characteristics that are they do not saturate. Open circuit characteristics graph is linear in case of synchronous machine.

12. Calculate the active power in a 1 Ω resistor with 2 A current flowing through it.

a) 2 W

b) 4 W

c) 7 W

d) 1 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°. P = I

^{2}R = 2×2×1=4 W.

13. The slope of the V-I curve is 13°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.

a) .2544 Ω

b) .7771 Ω

c) .2308 Ω

d) .5788 Ω

View Answer

Explanation: The slope of the V-I curve is resistance. The slope given is 13° so R=tan(13°)=.2308 Ω. The slope of the I-V curve is reciprocal of resistance.

14. A 2-pole lap wound generator with 44 armature conductors and a flux per pole of .07 Wb has an armature current of 80 A. The developed torque is _________

a) 39.7 N-m

b) 39.2 N-m

c) 38.4 N-m

d) 37.2 N-m

View Answer

Explanation: The developed torque in the motor is K

_{m}×I. The value of machine constant(K

_{m}) is E

_{b}÷ωm = Φ×Z×P÷2×3.14×A = .07×44×2÷2×3.14×2 = .49 Vs/rad. The developed torque is .49×80 = 39.2 N-m.

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