Electric Drives Questions and Answers – Solid State Controlled Drives – Brushless DC Motors

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Solid State Controlled Drives – Brushless DC Motors”.

1. The Hall effect sensor is used as the rotor position sensor for the BLDC motor.
a) True
b) False
View Answer

Answer: a
Explanation: The Hall effect sensor is used as the rotor position sensor for the BLDC motor. It is used to measure the strength of the magnetic field.
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2. In BLDC motor armature windings are placed on the stator side.
a) True
b) False
View Answer

Answer: a
Explanation: In BLDC motor armature windings are placed on the stator side. It is more economical and less maintenance is required. Permanent magnets are placed on the rotor side.

3. BLDC motor is analogous to ______________
a) Permanent magnet synchronous motor
b) DC motor
c) Rotating Transformer
d) Single-phase Induction motor
View Answer

Answer: a
Explanation: BLDC motor is analogous to Permanent magnet synchronous motor. In BLDC motor armature windings are placed on the stator side. It is more economical and less maintenance is required. Permanent magnets are placed on the rotor side.

4. The speed of a BLDC motor can be controlled by __________
a) Changing input DC voltage
b) Changing temperature
c) Changing wind direction
d) Cannot be controlled
View Answer

Answer: a
Explanation: The speed of a BLDC motor can be controlled by changing the input DC voltage or the current using PWM. It can be achievable by using a transistor and chopper.

5. Which are the advantages of BLDC motor?
I. Low cost
II. Simplicity
III. Reliability
IV. Good performance
a) I, II, III, IV
b) I, II
c) I, II, IV
d) II, III, IV
View Answer

Answer: a
Explanation: The BLDC motor is used in low power application. They have many advantages like low cost, simplicity, reliability, good performance, long life.
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6. Due to low inertia, BLDC motors have __________
a) Faster acceleration
b) Slower acceleration
c) High-cost
d) Low cost
View Answer

Answer: a
Explanation: Due to low inertia, BLDC motors have faster acceleration. BLDC motors have less weight. They can run at high speed than a conventional DC motor.

7. Calculate the electrical angle for 6 pole machine. (θm represents mechanical angle)
a) 4×θm
b) 5×θm
c) 2×θm
d) 3×θm
View Answer

Answer: d
Explanation: The electrical angle for 6 pole machine is 3×θm. One electrical angle is equal to 180° mechanical angle. θe=(P÷2)×θm.

8. Which of the following are the types of BLDC motor?
a) Unipolar, Bipolar
b) Unipolar, PWM
c) Bipolar, PWM
d) Synchronous, Induction
View Answer

Answer: a
Explanation: Unipolar and Bipolar are the types of BLDC motor. They use the Hall effect rotor position sensor and optical rotor position sensor.

9. Calculate peak-peak voltage if Vmax=45 V and Vmin=45 V.
a) 60 V
b) 30 V
c) 50 V
d) 0 V
View Answer

Answer: d
Explanation: Peak-Peak voltage is equal to the difference between the maximum and minimum voltage. It is mathematically represented as Vp-p=Vmax-Vmin=45-45=0 V. It signifies the signal is DC signal.
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10. Calculate the value of Crest factor if Vpeak=141 V and Vr.m.s=100 V for sinusoidal voltage.
a) 1.41
b) 2.38
c) 4.42
d) 5.58
View Answer

Answer: a
Explanation: The value of the crest factor is Vpeak÷Vr.m.s=141÷100=1.41. It signifies the peak value is 1.41 times than the r.m.s value.

11. In the biomedical instruments like artificial heart pumps, the motor used is ____________
a) DC shunt motor
b) DC series motor
c) Induction motor
d) BLDC motor
View Answer

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

12. Calculate the output power of the Buck converter if the supply voltage is 4 V and duty cycle value is .1 for 4 Ω load.
a) 40 mW
b) 20 mW
c) 50 mW
d) 60 mW
View Answer

Answer: a
Explanation: The output voltage of the buck converter is Vo = Vin×(D)=.4. The value of the duty cycle is less than one which makes the Vo < Vin. The buck converter is used to step down the voltage. The output power is Vo2÷R=40 mW.

13. Calculate the compensator rating required for cos(Φ)=.1.
a) 0.91 P.U
b) 0.99 P.U
c) 0.97 P.U
d) 0.92 P.U
View Answer

Answer: b
Explanation: The compensator rating can be calculated using the relation QP.U=√1-cos2(∅)=√1-.01=.99. This per unit value VAR compensator is required to improve the power factor of the system.
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14. Calculate the average inductor current of the Boost converter if the load current is 7 A and duty cycle value is .8.
a) 36 A
b) 35 A
c) 34 A
d) 31 A
View Answer

Answer: b
Explanation: The average inductor current of the Boost converter is Io÷(1-D)=7÷.2=35 A. The average value of the inductor current can be calculated using ampere-sec balance method.

15. Calculate the output voltage of the Buck-Boost converter if the supply voltage is 14 V and duty cycle value is .85.
a) 79.3 V
b) 45.5 V
c) 86.5 V
d) 54.7 V
View Answer

Answer: a
Explanation: The output voltage of the buck-boost converter is Vo = D×Vin ÷ (1-D)=.85(14)÷.15=79.3 V. It can step up and step down the voltage depending upon the value of the duty cycle. If the value of the duty cycle is less than .5 it will work as a buck converter and for duty cycle greater than .5 it will work as a boost converter.

Sanfoundry Global Education & Learning Series – Electric Drives.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn