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DC Machines Questions and Answers for Campus interviews on “DC Motor Control Through Choppers”.
1. 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: d
Clarification: 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.
2. 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: a
Clarification: 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.
3. Which of the following statement is not true regarding to DC chopper?
a) Cheap
b) Fast response
c) Regeneration
d) AC to DC control
Answer: d
Clarification: The chopper provides stepless and efficient control of DC voltage and it is less expensive. It is having another advantage of faster response and regeneration. The chopper is relatively new technology and equivalent to DC-DC transformer.
4. Which of the following correctly suits with chopper?
a) Thyristor in series with load
b) Thyristor in parallel with load
c) Thyristor switch in series with load
d) Thyristor switch in parallel with load
Answer: c
Clarification: A chopper is essentially a thyristor switch in series with the load. A shunting diode is provided across the load because free-wheeling the load current when thyristor is off. The thyristor can be shown enclosed by a dotted square can be turned-off over a time period and the cycle is repeated.
5. The average value of chopper output waveform is given by _____________
a) αV
b) V/α
c) α2V
d) V/α2
Answer: a
Clarification: Here the average value of the chopper output voltage waveform is given by Va=V tON/(tON+tOFF) = V (tON/T) =αV. So, by varying duty cycle α one can vary the average voltage at output.
6. How output voltage of chopper can be varied?
a) By using constant frequency system
b) By using variable frequency system
c) By using constant and variable frequency systems
d) By using constant or variable frequency systems
Answer: c
Clarification: The average voltage of the chopper output depends on α, thus, α can be varied by either changing the complete time period or by changing an on time or off time. These two methods are named as constant and variable frequency system.
7. What is the formula for step up chopper?
a) V(1-α)
b) V/(1-α)
c) αV
d) v/α
Answer: b
Clarification: With the help of conventional chopper one can reduce the average voltage, so by doing slight modification in the circuit one can use chopper method to increase the average voltage applied, by the formula V/(1-α).
8. Which of the following is not the method of auxiliary commutation?
a) Voltage commutation
b) Resistance commutation
c) Current commutation
d) Load commutation
Answer: b
Clarification: Thyristor in a chopper has to be turned off by auxiliary commutation. This can be done by two methods-Forced commutation and Load commutation. Forced commutation can also be done by two ways, current and voltage.
9. In voltage commutation, in chopper circuit we use _____________
a) 2 auxiliary thyristors
b) 2 diodes
c) 1 auxiliary thyristor
d) Many diodes
Answer: a
Clarification: Two auxiliary thyristors are connected in the chopper circuit along with one diode, one inductor, one capacitor in series with the voltage supply which is then connected in parallel with diode and DC motor armature.
10. In current commutation, in chopper circuit we use _____________
a) 2 auxiliary thyristors
b) 2 diodes
c) 1 auxiliary thyristor
d) 2 auxiliary thyristors and 2 diodes
Answer: d
Clarification: Two auxiliary thyristors are connected in the chopper circuit along with two diodes, one inductor, one capacitor in series with the voltage supply which is then connected in parallel with diode and DC motor armature.