Influence of PWM switching frequency on the losses in PM machines

Using PWM converters to control electrical machines brings many benefits, such as improved dynamic behavior and energy savings, but also introduces a current ripple, which induces additional losses in the machine. This paper investigates the extent of these losses in the rotor, windings and laminations of a PM machine as function of switching frequency and dc-link voltage by means of a case study. It is found that particularly the rotor losses are very sensitive to the switching frequency; a high dc-link voltage and low switching frequency increase the rotor losses by an order of magnitude. The additional winding losses are divided in eddy-current losses, which remain a fraction of the dc winding losses at all times; and losses due to unequal current sharing over parallel strands, which can reach the same order of magnitude as the dc copper losses. The iron losses show the smallest increase due to the current ripple.