Safe Torque-Off Technique for Fault Response in Dynamo Load Inverter

This paper proposes a Safe Torque Off (STO) technique for fault response in high-speed dynamometer systems. In such systems, the test motor and dynamo motor are mechanically coupled, and STO operation during a fault critically affects safety. When PWM is turned off, the inverter acts as a diode rectifier, and the motor’s back-EMF induces regenerative braking. This can cause overvoltage in the DC-link capacitor, risking damage to power devices. Assuming the absence of a Discharge Unit (DCU), which is a circuit that actively dissipates regenerative energy during emergency stops, we derive an energy-based equation to predict the capacitor voltage considering motor inertia, speed, and load torque. Using this result, a criterion is proposed for determining safe STO regions under emergency stop conditions. Simulation and experimental results verify that the predicted voltage aligns well with measured values, confirming the algorithm’s effectiveness. The proposed method prevents capacitor failure and fault propagation, enhancing system reliability. Although the algorithm depends on motor and system parameters, it provides a mathematically grounded, safe fault response strategy for high-performance drives.