DC Machines Multiple Choice Questions 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
Answer: a
Clarification: 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
Answer: a
Clarification: 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
Answer: a
Clarification: 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
Answer: b
Clarification: 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
Answer: b
Clarification: 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.
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
Answer: b
Clarification: 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
Answer: c
Clarification: 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
Answer: d
Clarification: 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