# Electric Drives Questions and Answers – Dynamics – Load Torques that Vary with Time

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Dynamics – Load Torques that Vary with Time”.

1. Servo motors are an example of which type of load?

Explanation: Servo motors are motors with control feedback. The motor can be AC or DC. This is an example of short time loads. They have a high torque to inertia ratio and high-speed.

2. Load torque of the crane is independent of _________
a) Speed
b) Seebeck effect
c) Hall effect
d) Thomson effect

Explanation: The Load torque of the crane is independent of speed. They are short time intermittent types of loads. They require constant power for a short period of time.

3. The unit of angular velocity is rad/s3.
a) True
b) False

Explanation: Angular velocity 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 velocity is rad/s.

4. R.M.S value of the sinusoidal waveform V=Vmsin(ωt+α).
a) Vm÷2½
b) Vm÷2¼
c) Vm÷2¾
d) Vm÷3½

Explanation: R.M.S value of the sinusoidal waveform is Vm÷2½ and r.m.s value of the trapezoidal waveform is Vm÷3½. The peak value of the sinusoidal waveform is Vm.

5. Calculate the time period of the waveform x(t)=24sin(24πt+π÷4).
a) .064 sec
b) .047 sec
c) .083 sec
d) .015 sec

Explanation: The fundamental time period of the sine wave is 2π. The time period of x(t) is 2π÷24π=.083 sec. The time period is independent of phase shifting and time shifting.

6. The turn-off times of the devices in the increasing order is ___________
I. MOSFET
II. BJT
III. IGBT
IV. Thyristor
a) I, III, II, IV
b) I, II, III, IV
c) III, I, II, IV
d) III, II, IV, I

Explanation: Increasing turn-off time implies decreasing speed and majority carrier devices do not have any minority charge carrier storage so they have less turn-off time and hence MOSFET has the least turn off time. So, the increasing order of turn-off time is, MOSFET < IGBT < BJT < Thyristor.

7. Which of the following devices should be used as a switch for high power and high voltage application?
a) GTO
b) MOSFET
c) TRIAC
d) Thyristor

Explanation: Thyristor is used for high power applications but it has a limited frequency range and cannot be used at high frequencies. A thyristor is a unidirectional, bipolar and semi-controlled device.

8. Calculate the useful power developed by a motor using the given data: Pin = 3000 W, Ia = 60 A, Ra = .4 Ω. Assume frictional losses are 200 W and windage losses are 400 W.
a) 970 W
b) 960 W
c) 980 W
d) 990 W

Explanation: Useful power is basically the shaft power developed by the motor that can be calculated using the formula Psh = Pdev-(rotational losses). Pdev = Pin-Ia2Ra = 3000-602(.4)=1560 W. The useful power developed by the motor is Psh = Pdev-(rotational losses)=1560 –(200+400)=960 W.

9. Calculate the phase angle of the sinusoidal waveform y(t)=55sin(4πt+π÷8).
a) π÷8
b) π÷5
c) π÷7
d) π÷4

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.

Sanfoundry Global Education & Learning Series – Electric Drives. 