The Permanent Magnet Synchronous Motor (PMSM) is an AC synchronous motor whose field excitation is provided by Permanent Magnets. The PMSM is a close relative of the brushless DC (BLDC) motor. Both motors have a permanent magnet rotor and windings on the stator. The principle difference between these two motors is the kind of drive signals that are supplied to the motor from the inverter.
The BLDC motor is commonly driven with a technique known as six-step commutation. As the motor turns, the electrical phases are energized two at a time in different combinations to produce motor torque. For a 3-phase BLDC motor, the six-step commutation method produces square-wave currents that approximate a 3-phase sinusoidal input. In contrast, the PMSM is driven with sinusoidal currents.
The sinusoidal currents can provide the following advantages and characteristics:
- No sparks -> safer in explosive environments
- Cleaner, faster, more efficient
- Less noisy, more reliable
- High Efficiency & Reliability
- Designed for high-performance Servo Applications
- Runs with/without Position Encoders
- More compact, efficient and lighter than ACIM
- Coupled with FOC control produces optimal torque
- Smooth low and high speed performance
- Low audible noise and EMI
PMSM and BLDC motors can usually be driven with either six-step commutation or sinusoidal commutation. A motor must have the proper mechanical and electrical design to get the benefits of sinusoidal operation listed above. It is best to ask the manufacturer which drive method is best for a given motor.
Sinusoidal operation requires more precise rotor position feedback from the motor and a more complex inverter switching algorithm