On Medium Frequency Differential Mode Resonance Effects in Doubly Fed Induction Machines

During synchronization phase of a doubly fed induction machine prototype abnormal high voltages were recorded at stator terminals. By evaluating the measured curves, and the frequency response of a similar machine, these overvoltages could be traced back to differential mode resonance effects within the machine itself. The effect of differential mode resonance is not restricted to doubly fed induction machines but also occurs in other machine types such as squirrel cage induction machines. If not considered in the insulation design, such abnormal voltages will lead to a premature ageing of the insulation. Depending on the operating conditions, premature failures may occur. The measured data presented in the article give an illustrative example what could happen if windings of electric machines are excited near series resonance frequency. As the occurence of this effect becomes more probable the higher the switching frequency of the converter, it is thoroughly important to be aware of it.