# Electric Drives Questions and Answers – Modified Speed Torque Characteristics of DC Shunt Motors

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Modified Speed Torque Characteristics of DC Shunt Motors”.

1. In DC chopper, the waveform for input and output voltages is respectively __________
a) Discontinuous and Continuous
b) Continuous and Discontinuous
c) Both continuous
d) Both discontinuous

Explanation: Chopper has a perfect DC at the input which is chopped into pulses which means the output voltage is discontinuous and by varying the duty cycle we can vary the average output voltage.

2. A chopper behaves as a __________
a) DC equivalent of AC switching device
b) DC equivalent of AC transformer
c) DC equivalent of AC relay
d) AC equivalent of circuit breaker

Explanation: A chopper is used to step up or step down the DC voltage whereas the transformer is used to step up or step down AC voltage so chopper is DC equivalent of AC transformer.

3. A DC chopper feeds an RLE load. If the value of E is increased by 20%, the current ripple ________
a) increases by 20%
b) decreases by 20%
c) increases only 20%
d) remains the same

Explanation: For a buck converter current ripple doesn’t depend on the value of E. ΔIL = Vdc×D×(1-D)×T÷L. The expression is independent of the value of E.

4. The conduction loss versus device current characteristic of a power MOSFET is best approximated by a ________
a) Straight line
b) Rectangular hyperbola
c) Parabola
d) Exponential decaying functions

Explanation: A MOSFET in ON state behaves as resistance so the conduction power loss is given by, P=I2R. Hence, the power vs current curve will be a parabola.

5. Which of the following device is NOT suitable for parallel operation?
a) MOSFET
b) BJT
c) IGBT
d) TRIAC

Explanation: BJT has a negative temperature coefficient of resistance. If it is operated in parallel operation thermal run-away will take place and the device will damage.

6. SCR is uni-directional in nature.
a) True
b) False

Explanation: SCR is uni-directional in nature. It only allows current to flow from anode to cathode. If the current is greater than the latching current then only it will work in forwarding conduction mode.

7. Which of the following device should be used as a switch in a low power switched mode power supply (SMPS)?
a) GTO
b) BJT
c) MOSFET
d) TRIAC

Explanation: MOSFET has a low power rating and high-frequency rating and so it can be used in low power switch mode power supply. MOSFET stands for metal oxide silicon field effect transistor.

8. The slope of the V-I curve is 78°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
a) 4.732 Ω
b) 4.608 Ω
c) 4.543 Ω
d) 4.648 Ω

Explanation: The slope of the V-I curve is resistance. The slope given is 78° so R=tan(78°)=4.732 Ω. The slope of the I-V curve is reciprocal of resistance.

9. Calculate mark to space ratio if the system is on for 17 sec and time period is 30 sec.
a) 1.307
b) 1.457
c) 1.478
d) 1.146

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=17÷(30-17) = 1.307.

Sanfoundry Global Education & Learning Series – Electric Drives.

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