13.21.1.7 PWM Speed Control

The inner commutation loop is a phase-locked loop, which locks to the rotor’s position. This inner loop does not attempt to modify the position of the rotor but modifies the commutation times to match whatever position the rotor has. The outer speed loop changes the rotor velocity, and the inner commutation loop locks to the rotor’s position to commutate the phase at the correct times.

The outer speed loop pulse width modulates the motor drive inverter to produce the desired wave shape and voltage at the motor. The inductance of the motor then integrates this PWM pattern to produce the desired average current, thus controlling the desired torque and speed of the motor. For a trapezoidal BLDC motor drive with six-step commutation, the PWM is used to generate the average voltage to produce the desired motor current and motor speed.

There are two basic methods to PWM the inverter switches. The first method returns the reactive energy in the motor inductance to the source by reversing the voltage on the motor winding during the current decay period. This method is referred to as fast decay or chop-chop. The second method circulates the reactive current in the motor with minimal voltage applied to the inductance. This method is referred to as slow decay or chop-coast.

The preferred control method employs a chop-chop PWM for any situations where the motor is being accelerated, either positively or negatively. For improved efficiency, chop-coast PWM is employed during steady-state conditions. The chop-chop speed loop is implemented by hysteretic control, fixed off time control or Average Current mode control of the motor current. This makes for a very robust controller, since the motor current is always in instantaneous control. The motor speed presented to the chop-chop loop is reduced by approximately 9%. A fixed frequency PWM that only modulates the high-side switches implements the chop-coast loop. The chop-coast loop is presented with the full motor speed, so if it is able to control the speed, the chop-chop loop will never be satisfied and will remain saturated. The chop-chop remains able to assume full control if the motor torque is exceeded, either through a load change or a change in speed that produces acceleration torque. The chop-coast loop will remain saturated, with the chop-chop loop in full control, during start-up and acceleration to full speed. The bandwidth of the chop-coast loop is set to be slower than the chop-chop loop so that any transients will be handled by the chop-chop loop and the chop-coast loop will only be active in a steady-state operation.