What are BLDC motors? How do they differ from AC motors?
Brushless DC motors are widely known over the Internet. They power drones and other RC craft, after all. Furthermore, they also see use in PC fans and the like.
On the other hand, electrical machines people know permanent magnet synchronous machines. They are, well, exactly what the name suggests: AC synchronous machines, with permanent magnets taking care of the magnetization.
But what exactly is the difference between a BLDC and a PMSM?
Not DC at all
Let’s start by clearing a small misnomer: BLDC motors are not DC motors in the truest sense. They are often supplied from batteries, yes, and batteries are very much a DC source. However, between the battery and the motor lies a small block of power electronics, making the motor currents and voltages alternate back and forth.
In other words, the motor is technically an AC motor. Meaning, a rotating magnetic field is created by means of alternating currents.
Sidenote: alright, most of the total field is due to the magnets alone. Meaning, removing the currents wouldn’t change the field much. But there would be a small difference – otherwise the motor couldn’t produce torque.
By contrast, a true DC motor features a static field and rotating conductors. Furthermore, they also utilize brushes – at least I’m not aware of anybody creating a brushless version of a true DC motor, ever.
Sidenote #2: As pointed out by Robert on LinkedIn, even typical (commutated) DC machines actually have alternating currents flowing in their armature windings. Considering this, you could say that only Faraday disks (and railguns) are true DC machines!
What typically separates a brushless DC motor from a typical PM motor are its waveforms.
First of all, it’s back-EMF – the induced voltage – is often closer to trapezoidal rather than sinusoidal. On the other hand, many PMSMs also have rather blocky voltages, so that’s not a fully decisive feature.
More importantly, the phase currents of a BLDC motor are rectangular. Specifically, each phase is assumed to carry a constant positive current for one third of the time, constant negative current for another third, and no current at all for the final third.
This is already quite different from other PM machines, whose currents are typically much more sinusoidal. They also cross zero very briefly – they don’t stay there for third of a period.
Another benefit of the rectangular current waveforms is that the total current is constant. Meaning, the current coming from the battery, in a typical application. So, from the battery’s point of view, a BLDC is indeed a DC motor!
Everybody loves exceptions
There’s one caveat, though. The waveforms I just described occur with the so-called trapezoidal modulation. (Modulation basically means how the voltage fed into the machine by its power electronics is varied with respect to time.)
However, also something called sinusoidal modulation exists. In this case, the current waveforms are closer to sinusoidal than square (and the back-emf waveform is whatever happens to specific to that particular motor; they cannot be changed much by modulation).
In other words, waveforms are not a 100% surefire way of distinguishing between BLDC motors and PMSMs.
Magnet placement, usually
Most commonly, BLDC motors have surface-mounted magnets on the rotor.
But again, not all surface-mounted PM motors are used as BLDC motors. Another stalemate.
Perhaps the most decisive difference comes from the side of power electronics. BLDC motors, especially the smaller ones, are driven by compact inverters (often called simply ‘speed controllers’ in the RC circles). So no cupboard-sized boxes here, like you see with larger motors. Furthermore, the inverters/controllers are almost exclusively intended to be run from a battery or some other DC source.
Lightweight electronics are, in turn, made possible by the structure of the motor itself. Since BLDC motors often have surface-mounted magnets, their inductance will be low (the small size of the typical motor, compared with the low-ish voltage helps, too). Hence, heavy-duty protection against voltage spikes due to switching transients is not needed.
Furthermore, the large DC-link capacitor found in larger drives can often be omitted, too. This is for several reasons. The aforementioned low inductance certainly helps. Furthermore, with trapezoidal modulation the DC-side current will be quite smooth already at the motor terminals – no need to filter it with a capacitance to protect the rest of the power system. And the final reason is that the entire system is supplied by a constant DC voltage (battery, most commonly) rather than a rectifier. So, filtering is not needed to get a smooth DC-link voltage, as is the case with AC-supplied drives.
The exact definition of a BLDC motor is tricky, at least in terms of what separates it from other synchronous machines. Nevertheless, a huge majority of them are
– permanent magnet machines with surface-mounted magnets
– supplied with rectangular current waveforms
– fed from a battery or DC-grid via a compact inverter.
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