Unit 3 - Part B Answers
Q5. Compare the characteristics of BLDC Motor with PMSM Motor
Introduction:
Both Brushless DC Motor (BLDC) and Permanent Magnet Synchronous Motor (PMSM) are
widely used in modern electrical drives. They look similar in construction because both use
permanent magnets on the rotor and require electronic control for operation. However,
they have important differences in working principle, torque production, and applications.
Comparison in Detail:
1. Operating Principle:
- BLDC Motor works on the trapezoidal back EMF principle. It means the voltage induced in
the stator winding is in trapezoidal shape.
- PMSM Motor works on the sinusoidal back EMF principle. It means the induced voltage
waveform is sinusoidal.
2. Current Waveform:
- BLDC requires a trapezoidal current for smooth torque.
- PMSM requires a sinusoidal current for smooth torque.
3. Control Method:
- BLDC control is simpler and uses six-step commutation with electronic switches.
- PMSM control is more advanced and generally uses vector control or field-oriented control
(FOC).
4. Torque Production:
- BLDC produces ripple torque because of trapezoidal back EMF.
- PMSM produces smooth torque with almost no ripple.
5. Efficiency:
- BLDC has high efficiency at lower and medium speeds.
- PMSM has better efficiency at high speeds due to smooth operation.
6. Applications:
- BLDC is used in fans, pumps, home appliances, drones, and low-cost EVs.
- PMSM is used in high-performance drives, robotics, electric vehicles, and industrial
automation.
BLDC Motor PMSM Motor
Trapezoidal back EMF Sinusoidal back EMF
Trapezoidal current waveform Sinusoidal current waveform
Simple six-step commutation Advanced field-oriented control
Torque ripple present Smooth torque
High efficiency (medium speed) High efficiency (all speeds)
Lower cost Higher cost
Used in appliances, drones, budget EVs Used in robotics, industrial automation,
premium EVs
Conclusion:
BLDC and PMSM look similar but differ in EMF waveform, control, torque smoothness, and
applications. BLDC is cost-effective and simpler, while PMSM provides smoother and high-
performance operation.
Q6. Compare the characteristics of PMSM Motor with SRM Motor
Introduction:
Permanent Magnet Synchronous Motor (PMSM) and Switched Reluctance Motor (SRM) are
both used in modern industries, especially in electric vehicles. PMSM uses permanent
magnets on the rotor, while SRM works based on variable reluctance principle without
magnets or windings in the rotor.
Comparison in Detail:
1. Rotor Construction:
- PMSM rotor has permanent magnets fixed on it.
- SRM rotor has only laminated steel with salient poles, no magnets or windings.
2. Stator Construction:
- PMSM stator has three-phase windings similar to induction motor.
- SRM stator also has windings, but they are concentrated on the stator poles.
3. Working Principle:
- PMSM works on synchronous principle where rotor runs at synchronous speed with the
stator field.
- SRM works on reluctance principle, rotor aligns itself to the position of minimum
reluctance.
4. Torque Production:
- PMSM torque is produced by interaction of stator field and rotor magnets, giving smooth
torque.
- SRM torque is produced by reluctance alignment, but it has more torque ripple.
5. Efficiency:
- PMSM has high efficiency because of permanent magnets.
- SRM also has good efficiency, but at lower speed the torque ripple reduces efficiency.
6. Cost:
- PMSM is expensive due to permanent magnets (rare-earth materials).
- SRM is cheaper, since it has no magnets or rotor windings.
7. Applications:
- PMSM is used in high-performance EVs, robotics, CNC machines.
- SRM is used in low-cost EVs, pumps, compressors, and aerospace actuators.
PMSM Motor SRM Motor
Rotor with permanent magnets Rotor with laminated steel poles
Requires magnets No magnets required
Works on synchronous principle Works on reluctance principle
Very low torque ripple High torque ripple
Complex control (vector control) Complex control (needs position sensors)
High cost Low cost
Used in premium EVs, robotics, automation Used in budget EVs, pumps, aerospace
Conclusion:
PMSM and SRM are both useful motors. PMSM is chosen where smooth torque and high
performance are needed, though it is costly. SRM is rugged, cheaper, and magnet-free,
making it suitable for cost-sensitive applications.
