PWM Loss Analysis in Electrically Excited Synchronous Machines: Implications for Loss-Optimal Operation

This paper investigates the problem of additional PWM losses in electrically excited synchronous machines caused by current harmonics during operation with a voltage source inverter. The behavior of the current harmonics over the entire operating map and the resulting additional losses are investigated. The PWM-induced losses are determined using a combination of transient FEA simulations, a Simulink model to generate the harmonic current waveforms and an optimization routine to establish the operating strategy in Matlab. By comparing the losses of the distorted current waveforms and the ideal, sinusoidal current waveforms, the additional iron losses caused by the PWM can be calculated. These PWM-induced iron losses are particularly significant at high rotational speeds and under operating conditions where the excitation axis flux linkage is reduced, such as in partial load operation. A slight improvement in overall efficiency can be achieved by adapting the control strategy, in particular by specifically increasing the excitation current in critical areas which leads to a better overall efficiency of the machine.