Do Fast-Switching-Inverters endanger the Motor-Insulation System? Laboratory Tests

Abstract:
As an alternative to silicon IGBTs, SiC FET transistors have been available since some years. SiC semiconductors are used because the switching losses with this material are much lower than with Si IGBT. The low switching losses make it possible to increase the switching frequency, for example, to drive motors with high-pole-number at high speed or to shift the noise into a frequency range that is no longer audible to humans. In the experiments described here, switching frequencies up to over 100 kHz were investigated. The lower switching losses are achieved by short switching times with correspondingly steep voltage edges. Tests showed voltage rising rates of up to 100V/ns. It is already known from the widely used IGBT inverters that voltage reflections can occur depending on the cable length, which may lead to a doubling of the pulse amplitude. With the steeper edges of the SiC transistors, these doublings will already occur at cable lengths from approx. 1 m. These pulses lead to a decaying oscillation with a frequency in the range of 0.5-5 MHz, which is at least 5 times the switching frequency. The maximum voltage can be as high as twice the DC-link voltage. When the inverter output voltage is much lower than the DC-Link voltage pulses occur at the motor terminals with twice the switching frequency. These short and numerous pulses put a significant stress on the insulation of electrical machines. This stress can lead to partial discharges (PD) in the motor winding. In tests, both the voltage pulses and the damaging partial discharges were measured. Various sensors, filters and amplifiers have been developed and used to detect the partial discharge. Both inductive and capacitive sensors were investigated. High pass filters are necessary to distinguish the partial discharge signal from the pulse width modulated voltage. If partial discharges occurred, they led to failure of the insulation system of some test motors within a few hours. When a partial discharge is detected it is interesting to know which part of the winding is affected. The location of the PD can be found using a microphone array. The stator has to be removed from the motor to get optical and acoustic access to the winding. The microphones must be suitable for the relevant frequency range. For this purpose, an acoustic camera is used. This device superimposes the acoustic signals on an optical image. Finding damage in concealed areas such as the slots of the stator laminations is very difficult. If the partial discharges occur in the winding head area, the camera can be used to find the fault location.