A method to monitor the thermal stress of power devices for direct stator flux controlled AC drives

Frequency inverter induction motor drives with direct stator flux and torque control have very short torque response times. A typical feature of these control methods is the irregular switching sequence of the inverter legs within a cycle of the stator frequency. Since the average switching frequency does not consider this, it is not a measure for the thermal stress of the power devices. To protect them against thermal destruction, a method of online monitoring is proposed based on a so called `temperature index´ and a `window frequency´. By observing the values of temperature index and window frequency for each switching element, an overload is prevented, including possible cases of faults. When the temperature index or window frequency are reaching their limits, the inverter can be turned off in time. The ensures a safe operation of the drive. In addition, digital simulation results show an improved operational performance, which can be achieved by this method