DC Machines Questions and Answers – DC Motor Control Through Converters

This set of DC Machines Multiple Choice Questions & Answers (MCQs) focuses on “DC Motor Control Through Converters”.

1. Half wave converters are used for controlling DC motor of ________________
a) Below 400 W
b) 400 W – 4000W
c) More than 4000W
d) Anywhere

Explanation: Half wave converter requires a single thyristor and a free-wheeling diode. In half wave converter circuit the motor current will always be discontinuous. Ultimately it results in poor motor performance. This type of converter is employed only for motors below 400W.

2. How many thyristors does we need in half wave converter?
a) 1
b) 2
c) 3
d) Many

Explanation: Only one thyristor is sufficient for the half wave converter purpose, this is employed for small motors only. This thyristor is connected to the single-phase AC supply and with parallel combination of free-wheeling diode with motor armature.

3.Semi-converters can’t be used when regeneration is required.
a) True
b) False

Explanation: According to the one-quadrant converter, it gives voltage and current of one polarity at DC terminals. Therefore, it does not provide support for regenerative braking, i.e. power flow from DC motor to the AC supply.

4. How many thyristors does we need in semi-converter?
a) 1
b) 2
c) 3
d) Many

Explanation: Semi-converter requires two thyristors. Thyristor Th1 is fired at angle of α and Th2 at angle π+α with respect to the supply voltage v and the process is repeated continuously. Circuit also requires two diodes and one free-wheeling diode.

5. When armature current becomes discontinuous?
a) Small firing angles
b) Large firing angles
c) Infinite firing angle
d) Does not depend on firing angle

Explanation: There are various reasons when armature current becomes discontinuous armature current, prominent reasons are large values of firing angle, high speed and sometimes low torque can also affect.
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6. Which of the following is the effect of non-uniform armature current?
a) Ratio of peak to average and rms to average armature current decreases
b) Ratio of peak to average and rms to average armature current increases
c) Ratio of peak to average increases and rms to average armature current decreases
d) Ratio of peak to average decreases and rms to average armature current increases

Explanation: The armature current sometimes due to some reasons may become non-uniform. In this conditions motor performance deteriorates with discontinuous armature current. So, the ratio of peak current to average current increases. Also, rms to average armature current increases.

7. How many quadrants does full-converter work?
a) One
b) Half
c) Two
d) Can be any

Explanation: A full-converter is a two-quadrant converter in which the voltage polarity of the output can reverse, but the current remains unidirectional because of the unidirectional thyristors. It requires no free-wheeling diode.

8. How many thyristors are required for full converter?
a) 1
b) 2
c) 3
d) 4

Explanation: Full converter requires 4 thyristors. The only different thing about the full converter is, it does not require free-wheeling diode. A full converter can be used to feed a separately excited DC motor. Each thyristor is fired accordingly.

9. Full-converter can be used in DC motor for regenerative braking in ______________
a) Constant operation
b) Variable operation
c) Inversion operation
d) Opposite operation

Explanation: When α is greater than 900, the average motor voltage becomes negative. If the motor terminals are reversed, then the circuit will act as a generator in order to feed power back to the AC supply. This is the inversion operation of the converter and used in regenerative braking.

10. Dual converter operates in _________________
a) 1
b) 2
c) 3
d) 4

Explanation: The dual-converter is the modified one and can operate in all the four quadrants. Its circuit contains two full-converters converting to the DC in either direction. The dual converter provides virtually instantaneous reversal of voltage at DC terminals.

11. Which converter/s can be used for DC series motor control?
a) Semi-converters
b) Half-wave converter
c) Full-converter
d) Semi converters and full converter

Explanation: Semi-converters and full-converters are also employed for control of a DC series motor. The total circuit inductance is high because of the series field, as a consequence of which the armature current is mostly continuous.

12. Three phase converters are employed for _______________
a) Large kW motors
b) Small kW motors
c) In all motors
d) Never used

Explanation: Three phase converters are employed when DC motor of very high ratings is used. In three phase converters, the advantage is the ripple frequency of the motor terminal voltage is generally higher than that in single-phase converters.

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