The terminology for brushless, permanent magnet motors is confusing.
If you look in academic/technical literature like IEEE papers, then generally BLDC refers to brushless PM motors that have a trapezoidal back-emf and is driven by a six-step, trapezoidal drive, while PMSM refers to brushless PM motors that have a sinusoidal back-emf and are driven by sinusoidal waveforms. Be aware that brushless motors with a trapezoidal back-emf can be driven by sinusoidal waveforms and vice versa. And also be aware that trapezoidal and sinusoidal back-emf's are ideals and you can never really get either one. Of course, I've also seen IEEE papers that refer to BLAC motors and use other terminology, so this isn't strict across the board.
Industry hasn't really adopted this terminology completely. You often will see companies refer to BLDC motors, as you've already pointed out. And generally by BLDC they mean exactly what the academics mean - a brushless motor with a trapezoidal back-emf. However, I've also seen these referred to DC brushless (DCB) motors, brushless PM (BPM) motors, or even PMSM's.
With what academic literature refers to as PMSM's, I've seen them called PMSM's, brushless AC (BLAC) motors, AC servo motors, brushless servomotor (BLSM) and others.
Some manufacturers may not make a distinction between the 2 because in reality it isn't an either/or thing. You can't make a brushless motor with a perfect trapezoidal back-emf and you can't make one with a perfect sinusoidal back-emf. Your best bet is to talk directly to manufacturers and tell them what you want to do and they will guide you in the right direction.
In reality: Most so called BLDC motors on the market have sinusoidal
back EMF, and can be controlled by the same FOC method as PMSM motor.
But I think they are still BLDC motor, not PMSM.
This may or may not be true. In my experience, BLDC motors do not have sinusoidal back-emf; they are much closer to trapezoidal. Keep in mind that we are talking about the phase back-emf, not the line-to-line back-emf. Sometimes the line-to-line back-emf looks close to sinusoidal while the phase back-emf doesn't.
In both PMSM and BLDC motors, the speed and frequency are indeed locked to each other. However:
- The PMSM has a fixed input frequency in the sense that it's not affected by the motor itself and therefore a fixed speed in relation to that frequency. Speed is controlled directly by frequency; the voltage and current limits are simply high enough to provide some headroom so that it doesn't stall under load. (but not so high as to fry the motor) It's also designed to run directly from AC power, which is sinusoidal, so it needs a sinewave inverter to control the speed well.
- The BLDC has a controller/inverter that varies the frequency to match whatever the motor is doing at the time. Think of a brushed motor that uses transistors instead of brushes, and you're really close. Therefore, the BLDC has a Kv.
It's designed to run exclusively from an inverter, so the flat-topped trapezoid is more transistor-friendly and allows the inverter to be cheaper yet still just as effective.
These two kinds of motors are actually interchangeable in a pinch; they just won't work as well. Higher noise, less efficient, etc., unless the inverter can drive both and you tell it about the change.
There are two other things to know about motors in general:
- All motors have inductance that increases their impedance with higher speed. AC impedance is somewhat analogous to DC resistance, except that there's now a phase angle involved that makes things more complicated. (There's also a phase angle between a PMSM's rotor and the driving voltage that varies with load and contributes to the impedance.) The increased impedance lowers the current for a given voltage, which reduces the torque. This is why a stalled motor draws maximum current and produces maximum torque.
- All motors generate a voltage while they're running that is proportional to their speed and locked in frequency. This back EMF works against the voltage being applied to further reduce the current. Kv is theoretically the rate of back EMF vs. speed. (it would require a mythical free-energy source to go beyond that)
Best Answer
In a BLDC motor, the windings are concentrated on salient poles, so producing a voltage waveform that's more trapeziodal than sinusoidal.
In a PMSM, the windings are distributed over several poles, so producing a voltage waveform that's more more sinusoidal.
These differences mean that BLDC motors are slightly cheaper to produce, but may be slightly less efficient, noisier and have more cogging, than PMSMs.