DC Motor Speed-Torque Characteristics MCQ [Free PDF] – Objective Question Answer for DC Motor Speed-Torque Characteristics Quiz

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

Answer: B

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

Answer: B

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 the chopper is the DC equivalent of the 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

Answer: D

For a buck converter, the 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

Answer: C

A MOSFET in the 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

Answer: B

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

Answer: A

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

Answer: C

MOSFET has a low power rating and high-frequency rating and so it can be used in a 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

Answer: A

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 to resistance. 

 

9. Calculate mark to space ratio if the system is on for 17 sec and the time period is 30 sec.

A. 1.307
B. 1.457
C. 1.478
D. 1.146

Answer: A

Mark to space is Ton÷Toff. It is the ratio of the time for which the system is active and the time for which is inactive.

M = Ton÷Toff=17÷(30-17) = 1.307. 

 

10. Turn-on and turn-off times of transistor depend on _________

A. Static Characteristic
B. Junction Capacitance
C. Current Gain
D. Voltage Gain

Answer: B

The depletion layer capacitance and diffusion capacitance affect the turn-on and turn-off behavior of transistors. Due to these internal capacitances, transistors do not turn on instantly. 

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11. The generated e.m.f from 45-pole armature having 400 turns driven at 70 rev/sec having flux per pole as 90 mWb, with 17 parallel paths is ___________
A. 13341.17 V
B. 12370.14 V
C. 14700.89 V
D. 15690.54 V

Answer: A

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

Eb = Φ×Z×N×P÷60×A

Where

Φ represents 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.

One turn is equal to two conductors.

Eb = .09×45×400×2×4200÷60×17 = 13341.17 V. 

 

12. The unit of Magnetic flux density is Tesla.

A. True
B. False

Answer: A

Magnetic Flux density is defined as the number of magnetic lines passing through a certain point or a surface. It is generally expressed in terms of Tesla. Its C.G.S unit is Gauss. 

 

13. Calculate the moment of inertia of the hollow cylinder having a mass of 78 kg and a radius of 49 cm.

A. 9.363 kgm2
B. 9.265 kgm2
C. 9.787 kgm2
D. 9.568 kgm2

Answer: A

The moment of inertia of the hollow cylinder can be calculated using the formula

I=miri2÷2.

The mass of the hollow cylinder and radius is given.

I=(78)×.5×(.49)2=9.363 kgm2.

It depends upon the orientation of the rotational axis. 

 

14. Calculate the value of the angular acceleration of the motor using the given data: J = 81 kg-m2, load torque = 74 N-m, motor torque = 89 N-m.

A. .195 rad/s2
B. .182 rad/s2
C. .183 rad/s2
D. .185 rad/s2

Answer: D

Using the dynamic equation of motor

J×(angular acceleration) = Motor torque – Load torque:

81×(angular acceleration) = 89-74=15

angular acceleration=.185 rad/s2

 

15. 340 V, 45 A, 1400 rpm DC separately excited motor having a resistance of .7 ohm excited by an external dc voltage source of 90 V. Calculate the torque developed by the motor on full load.

A. 94.73 N-m
B. 94.52 N-m
C. 93.37 N-m
D. 94.42 N-m

Answer: A

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

Eb = Vt-I×Ra = 308.5 V

machine constant Km = Eb÷Wm which is equal to 2.1053.

Torque can be calculated by using the relation

T = Km × I = 2.1053×45 = 94.73 N-m. 

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