Brushless DC Outrunner Motor Control

Question

My question arises from interest mainly in Outrunner motors which are used on RC planes/drones.

With that being said, I understand that these motors are controlled by an Electronic Speed Controller (ESC), which is used to switch DC power into two of the three different phases of the motor at a specific time by monitoring the back EMF (for sensorless operation) on the third phase. I also understand that to regulate speed, the ESC will use PWM to "modulate" the average voltage to the motor.

If my understanding is correct, then I am utterly confused why I have read, in multiple places, that Brushless DC (BLDC) motors are speed controlled by frequency.

For example: "BLAC, BLDC (AC stator, DC rotor) These are basically just Synchronous machines but they have permanent magnets on the rotor. Higher the stator frequency the higher the rotor speed. AC & DC just comes from the type of current control that is used."

Also, I know that Brushless Outrunner motors experience slippage, which means they are definitely not synchronous. Correct?

I am left with the thought that there must be different types of BLDC motors, or different forms of control. May I please have a clarification on this subject or even a correction if I am completely wrong?

EDIT: Changed "Engine Speed Controller" to the correct term: "Electronic Speed Controller"

Answer 1

Brushless DC means exactly that - a DC motor with brushless commutation. The controller's job is to switch the phases in and out at the correct rotor angles (not frequency) just like the commutator in a brushed motor. The motor spins at whatever speed it wants to, depending on supply voltage and load.

If the motor has sensors then the controller can be very simple, since it just has to read the sensors and turn on the appropriate phases depending on rotor angle. It has no direct control over commutation frequency, but it can 'control' motor speed indirectly by varying the effective supply voltage (using either a regulator or PWM).

Sensorless controllers have a harder job because they must monitor the back-emf waveform for zero crossings. At startup there is no back-emf so the ESC cannot detect the rotor's position. To get the rotor spinning it pulses the phases at low power like a stepper motor, gradually increasing speed until it gets a strong enough back-emf to switch into synchronous operation.

During this startup period only, the sensorless ESC controls motor speed by varying commutation frequency. However since it is basically dragging the rotor up to speed, any sudden change in load can cause it to loose sync. Also the motor may start in reverse, then it has to stop and try again. This may result in the rotor jumping back and forth a few times until the ESC sees a good back-emf.

Answer 2

First, as somebody else pointed out, ESC stands for Electronic Speed Control, not Engine Speed Control. And BLDC motors do not experience any slip. They are synchronous motors.

Second, the quote you gave is confused. BLDC motors don't have a "DC" rotor. They have a permanent magnet rotor. "DC rotor" would only make sense if the rotor were wound. The quote isn't wrong that the higher the stator frequency, the higher the speed. But it is misleading because speed isn't controlled by frequency.

BLDC motors are identical to brushed permanent magnet DC motors in how they are controlled. Increase the voltage, increase the speed. Increase the current, increase the torque. The difference comes in how they switch the current in the coils as the rotor rotates. The act of switching current from one coil to another in a motor is called "commutation." In a PMDC motor, the switching is done mechanically with a commutator and brushes. As the rotor turns, the brushes come in contact with different bars on the commutator and that causes the current to switch coils. Note that the faster the rotor turns, the faster the current switches from one coil to the next.

BLDC motors do the same thing but instead of mechanical commutation, they use electronic commutation. The commutation brought into the control so that transistors do the current switching instead of the commutator and brushes. With a PMDC, the timing of the switching was mechanically determined by the width of the brushes and the shape and number of bars on the commutator. With a BLDC, the timing has to be determined by the control. This is done by determining the angular position of the rotor with either an encoder, Hall effect devices or some other means (ESC's are often "sensorless" and estimate the rotor position based on voltage or current waveforms). Note that just like in PMDC motors, the faster the rotor turns, the faster the current switches from one coil to the next.

I hope you see that the frequency is a result of the speed, not determined by it. The switching of the current from one coil to the next occurs when the control "sees" that the rotor has changed angular position. So, yes, the frequency of switching does change as speed changes, but the frequency does not control the speed.

Third, you are right that there are many different forms on control for BLDC motors. The control method I describe above is often called a "six-step) or "trapezoidal" control. But you could have sinusoidal or field-oriented control (FOC) as well. Most hobby grade RC brushless ESC's are six-step controls.