Abstract
In a drive system comprising a high-speed PM BLDC motor and an inverter implementing motor phases commutation and phase voltages PWM modulation, a spontaneous synchronisation between the PWM modulation frequency and the motor rotational speed may occur. The drive behaves like an unintended phase-locked loop. The effect distorts mechanical characteristic of the drive by introducing flat segments where the rotational speed remains constant despite changing load torque. It makes the speed-torque relationship discontinuous, nonlinear, and non-unique. The phenomenon is closely related to the problem of phase current and motor torque ripples. The paper presents experimental results recorded with a laboratory test bench for drives for electric power tools where the phenomenon of the parasitic speed locking has been observed. A root cause of this effect is discussed, and the mechanism of unwanted synchronisation is investigated. Several mitigation techniques are proposed and verified with computer simulations.
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