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DC Machines Mcqs

Q:

Which of the following method is employed when regenerative braking is necessary?

A) DC chopper B) Variable resistor
C) Inverter rectifier D) Motor-generator
 
Answer & Explanation Answer: A) DC chopper

Explanation: A chopper is essentially an electronic switch that turns on the fixed-voltage DC source for short time intervals and applies source potential to motor terminals in series of pulses. This process controls the average voltage applied to the motor.

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59
Q:

Which of the following method is adopted for controlling a DC motor?

A) Resistance control B) Motor-generator set
C) Inverter Rectifier D) DC chopper
 
Answer & Explanation Answer: D) DC chopper

Explanation: All methods listed above are the methods which can be used to control DC motor, but resistive method makes lots of wastage of energy, while motor-generator set and inverter rectifier set are bulky and costly.

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56
Q:

Three phase converters are employed for _______________

A) Large kW motors B) Small kW motors
C) In all motors D) Never used
 
Answer & Explanation Answer: A) Large kW motors

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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61
Q:

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
 
Answer & Explanation Answer: 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.

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50
Q:

Dual converter operates in _________________

A) 1 B) 2
C) 3 D) 4
 
Answer & Explanation Answer: 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.

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35
Q:

Full-converter can be used in DC motor for regenerative braking in ______________

A) Constant operation B) Variable operation
C) Inversion operation D) Opposite operation
 
Answer & Explanation Answer: C) Inversion operation

Explanation: When ? 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.

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79
Q:

How many thyristors are required for full converter?

A) 1 B) 2
C) 3 D) 4
 
Answer & Explanation Answer: 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.

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38
Q:

How many quadrants does full-converter work?

A) One B) Half
C) Two D) Can be any
 
Answer & Explanation Answer: C) Two

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.

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