Beatless control in over-modulation range for permanent magnet synchronous motor drives

In an AC-DC-AC motor drive system, a beat phenomenon of the motor current occurs, especially in the over-modulation range with DC-link voltage ripple and low frequency carrier. We analyze this beat phenomenon and propose two beatless control methods. One modifies the carrier frequency in accordance with the motor speed within a narrow range, and as a result, the beat frequency can be shifted to a high range, and the beat currents can be reduced. The other shifts the modulation waves by using a feedback loop to cancel the influence of DC-link voltage ripple. Both methods do not need the output frequency of an inverter to be modified, so a motor can be driven at any speed. With these proposed methods, the beat currents can be suppressed greatly, and the torque ripple of a motor becomes smaller. Simulation and experimental results verified the effectiveness of the proposed control methods.