Electrical – How does HFI (high frequency injection) work with BLDC motors

Driving a motor blind is a bad idea for several reasons:

1. It is inefficient. The most efficient way to run the motor is for the magentic field to be 90° ahead of the rotor. Put another way, the torque on the rotor is the cross product of the driving magnetic field and the rotor's magnetic orientation.

   With position feedback, the magnetic field can be kept near the optimum angle, which means the current goes to actually pushing the motor instead of holding it in place. Put another way, the amplitude is just what it needs to be to keep the motor spinning at the desired speed in the maximum torque configuration. When you don't know where the rotor is, you end up over-driving the motor.

   Another way to look at this is that the driving field has a sine and cosine component. Let's say the cosine is the part 90° ahead of the rotor and the sine part is where the rotor currently is. Any phase angle can be thought of as just a different mix of the sine and cosine components. However, only the cosine component moves the motor. The sine component only causes heating and represents wasted power.

2. Once you loose lock, the game is over. With a fixed drive, the drive angle will only be a little ahead of the rotor at low torque. As the speed increases (and the effective drive voltage automatically decreases due to back EMF) or the load increases, the fixed drive will get up to 90° ahead of the rotor.

   However, at this point it's right on the edge and any change will cause less torque. If the load on the motor increases, the rotor will get more than 90° behind, which causes less torque, which causes it to get even more behind. Over the next 1/4 turn of slip, the forward torque will decrease to zero. Then for the next 1/2 turn after that, the driving torque actually pushes the rotor backwards.

   At this point you're totally screwed. Remember you got into this predicament in the first place because the driving torque couldn't keep up with load, and you've just experienced a net negative drive over the last 3/4 turn. If the load is suddenly removed and if you're very lucky, the rotor might be able to speed up to sync up with the drive in the next 1/4 cycle, but certainly not if whatever condition caused the problem in the first place is still present.

   Once the rotor gets out of sync, the net torque over any one rotation is 0. The product of two sine waves of different frequency always averages to 0 regardless of the phase angle between them.

Field Oriented Control (also known as Vector control) of BLDC motors can improve low speed torque and reduce torque ripple at speed. Normally it uses sine wave drive. This works best when matched to motor with sine wave back-emf. However most motors used in multirotor drones are designed for 6 step trapezoid drive.

The following traces show the different back-emfs of a 2 pole coreless ironless BLDC motor and a multipole iron cored motor. The second example might actually perform worse with sine wave drive.

So to get the best out of FOC drive it needs to 'tuned' to the motor. This might explain why the only two FOC ESCs I could find for sale (DJI 1240S/X and EMAX WindTalker) are both intended to be used only with a specific motor.

The vast majority of ESCs used in multirotors don't even use synchronous rectification, so their part-throttle efficiency is already ~5% worse than it could be. Also many of them are not using the latest most powerful processors and high speed drivers, so their switching losses are higher at high frequency (this is more important when using FOC drive, because to produce an accurate waveform requires high frequency PWM).

The only one way to find out what real improvement FOC drive could make is to try it. For a fair comparison you would have to use the same ESC and only change the commutation technique.