Unintended Synchronisation Between Rotational Speed and PWM Frequency in a PM BLDC Drive Unit

In an electric drive with a three-phase PM BLDC motor, tasks of PWM modulation and six-step or twelve-step commutation of phases are usually combined and carried out by an MCU or CPU-controlled MOSFET or IGBT transistor bridge. That approach cuts down electronic element count, decreases size and price of the inverter and reduces power losses. However, at the same time it can also give rise to an undesired phenomenon of a spontaneous synchronisation (locking) between the motor rotational speed and the PWM frequency. An inverter-motor drive system may behave like an unintended phase-locked loop (PLL), endeavouring to keep a constant ratio between the six-step commutation frequency and the PWM modulation frequency. Resulting nonlinearity, discontinuity, hysteresis and nonuniqueness of the drive unit mechanical characteristic makes it difficult to develop a controller algorithm. The paper shows this unexpected and unwanted effect and investigates its causes.