DC Machines Questions and Answers – BLDC Motors

This set of DC Machines Multiple Choice Questions & Answers (MCQs) focuses on “BLDC Motors”.

1. In BLDC motor field winding is kept on _______________
a) Stator
b) Rotor
c) Can be placed anywhere
d) Absent
View Answer

Answer: b
Explanation: Unlike conventional DC motor, BLDC motor has its field winding rotating. Unlike PMDC motor field winding is present. Thus, it is placed on rotor. In BLDC motor we have fixed commutators, with brush gear rotating with speed of rotor field.

2. By electronic circuitry BLDC can be controlled for both constant and variable torque operation.
a) True
b) False
View Answer

Answer: a
Explanation: In BLDC because of rotor position feedback triggering of thyristors/transistors, the stator and rotor field always remain in synchronism as the frequency of triggering automatically adjusts to motor speed. The length of on-time of the transistors determines motor torque magnitude.

3. Which of the following is not an advantage of BLDC motor over conventional DC motor.
a) Less maintenance
b) Long life
c) No risk of explosion or possibility of RF radiation
d) Low cost
View Answer

Answer: d
Explanation: The brushless DC motor being more expensive compare to DC motor of same kW ratings, serves many other advantages like long life, less maintenance, rapid response, linear characteristic, no sparking and many more.

4. In BLDC motor driver module, we do not require ______________
a) SCRs
b) Power transistors
c) FETs
d) Transistors
View Answer

Answer: d
Explanation: The stator is connected to a variable voltage current source through an inductor and an inverter comprising six SCRs (S1 to S6). In place of SCRs, power transistor or FETs could be used according to power rating of the motor.

5. In medical fields which DC motor is widely used?
c) Brushed DC motor
d) Cannot be determined
View Answer

Answer: b
Explanation: BLDC motors are widely used in various applications of medical industry. Sensor less BLDC motor and with sensor BLDC motors are used because of easy operation and high reliability compare to conventional motors.

6. Construction of BLDC is exactly similar to the ______________
a) Conventional DC motor
b) Induction motor
c) Permanent magnet synchronous motor
d) Totally different construction
View Answer

Answer: c
Explanation: Construction of BLDC motor is exactly similar to the PMSM i.e. permanent magnet synchronous motor. BLDC is considered as machine working like DC with construction similar to the AC synchronous machine.

7. Typical brushless motor doesn’t have ______________
a) Commutator
b) Permanent magnet
c) Electronic controller
d) Fixed armature
View Answer

Answer: a
Explanation: A typical brushless motor has permanent magnets which rotate around a fixed armature. An electronic controller replaces the brush-commutator assembly of the brushed conventional DC motor, which continually switches the phase to the windings to keep the motor turning.

8. BLDC can be used instead ____________
a) Synchronous motor
b) Normal brushed DC motor
c) Induction motor
d) Air motor
View Answer

Answer: b
Explanation: Brushless motors fulfil almost all functions originally performed by brushed DC motors, but cost and control complexity prevent brushless motors from replacing brushed motors completely in the lowest-cost areas. Brushless motors are now dominating many applications, particularly devices such as computer hard drives and CD/DVD players.

Sanfoundry Global Education & Learning Series – DC Machines.


To practice all areas of DC Machines, here is complete set of 1000+ Multiple Choice Questions and Answers.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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