Safely Driving a Brushless Motor

In order for it to turn smoothly, a brushless motor must have its winding current switched when the rotor is in a certain position relative to the windings. If the motor passes this point without the windings being switched, the windings will start trying to impede the motor's rotation. If the windings are switched slightly too early, they will impede the motor's rotation until it manages to coast to the proper point, whereupon they will start assisting it. If they are switched too much too early, they will impede the motor's rotation so much that it doesn't reach a point where they would help, and the motor will stall.

If the windings spend any significant amount of time impeding the motor's rotation rather than assisting it, rotation will be jerky, and the fraction of the motor's power consumption which is spent driving the mechanical load rather than heating the motor will be reduced. In order to make a brushless motor rotate smoothly and efficiently, it is generally necessary to sense its rotational position and ensure that the windings are switched at the proper times.

Note that one could make a brushless motor turn smoothly at any speed, even without using positional feedback, by driving its inputs with three-phase simusoidal current- or voltage waveforms, since the position toward which the motor would be trying to move the rotor would move smoothly, but such approaches should generally only be used in cases where one is driving the motor with a tiny fraction of its rated load. The problem with such approaches is that the amount of voltage and current required for the motor to maintain a particular speed will vary depending upon the amount of torque it has to drive, and any power which is fed into the motor and not converted into mechanical energy will be converted into heat. Unlike stepper motors which are generally designed so they can safely dissipate 100% of the energy put into them as heat, brushless motors are not. Putting significant excess power into a brushless motor will destroy it in short order.

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.