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?
a) Pulsating loads
b) Short time loads
c) Impact loads
d) Short time intermittent loads
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 _________
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.
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).
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 ___________
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?
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).
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.
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