Motor – Controlling BLDC Motor Speed with PWM

Pulsing the motor full-on for a few cycles occasionally should work fine if you're OK with the resulting strong torque pulses on the flywheel and backwards on whatever is holding the motor. This will also result in a higher speed ripple. You have to decide whether you can tolerate the speed ripple or not.

One drawback will be lower efficiency. Shorter pulses of higher current versus longer pulses of lower current are the same to the motor, but don't cause the same dissipation in the inevitable effective resistance in series with the windings. The dissipation of the resistance is proportional to the square of the current. Usually you try to pulse motors fast enough so that the inherent inductance smooths out the actual current as a sawtooth about the average. A constant average current minimizes the lossed in the series resistance.

First of all, Vref on the chip controls maximum power = maximum torque. When S1 on eval board is in TRQ mode, then VREF pin on CN4 is connected directly to Vref of the drived. In this mode, it will have very little effect if the motor is unloaded.

In order to control the speed of the motor while maintaining the torque you will need to wrap the whole thing into a control loop -- something measures motor speed, and changes power to motor correspondingly.

For the best performance, you will want to do it per-phase basis, delaying control inputs if speed is exceeded. However, this is quite involved, so I would instead use manufacturer-recommended analog way (see section 9, 'Tachometer'). This is already assembled for you on the eval board -- set S1 to SPEED mode, and VREF should vary motor voltage smoothly between 0 and max speed.