Electronic – Brushless DC Motor – Sensoreless Startup Algorithm

In theory, one could use a BLDC motor as a 3-winding (six-phase) stepper motor, controlling rotation speed by controlling the frequency with which the windings are driven. Indeed, for certain situations where would be using a motor at only a fraction of its rated power (e.g. because the cheapest motor one could get was much bigger than necessary) this approach will work fine. A major problem with this approach, however, is that when a motor is driven at a fixed speed, a reduction in mechanical output torque will not cause a corresponding reduction in current draw. Instead, it will cause a massive increase in the amount of heat dissipated by the motor. Although stepper motors are generally able to safely dissipate 100% of rated power as heat, BLDC motors generally aren't.

One might regard the situation as being vaguely analogous to regulating the speed of an automobile purely by using the clutch and brake pedals while the engine was always running at wide open throttle. True, one could for a little while make the car go any desired speed using just two of the three pedals (and if one was going uphill, one could do it with the clutch alone), but fuel economy would be dreadful, and unless one added a truly massive amount of extra cooling to the clutch it would be destroyed in minutes.

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.