Electrical – the effective speed-control range of a BLDC motor

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.

one of the most important points is power handling

using individual MOSFETS will give you a very wide range for how much power it can give you

for example if your BLDC require 30 or even 50 amps to reach its max power(torque and speed) I think it will be hard to find off-the-shelf integrated circuit that can support this amount of power, usually at this case there is no escape from using individual transistors to handle this power

also heat dissipation is a big problem you must handle in your design

in case of using ind. Trans. it would be much easier to distribute the heat over multiple packages instead of one package which would be in the other case the IC

one disadvantage is the size of the final design

sometimes you need the minimal amount of components in your design, but if you will use multiple MOSFET approach you will definitely need a controller also (microcontroller as an example)