Examination of additional rotor-losses in inverter-fed induction machines

In the rotor of an inverter-fed induction machine, a considerable power-loss may occur, which is caused by the pulsed output voltages of the inverter. This loss can reach unexpectedly high values and in the worst case it may lead to the destruction of the machine. The major part of that loss is caused by the harmonics of the stator currents, which are compensated by corresponding currents in the rotor´s squirrel cage. Unfortunately, field diffusion effects lead to a very inhomogeneous distribution of current density across the rotor bars. Hence, there will appear high peak values of current density in the rotor bars and, in consequence, a considerable and unexpectedly high power loss. The following paper shows a method for analyzing the distribution of current density across the rotor bars and determining the resulting additional rotor loss, that has to be expected. By this way, the machine itself as well as the inverter and its pulse pattern can be optimized in order to reduce this power-loss