DSP-based thermal protection for brushless servo motor

The uses of PM brushless servo motors in robotic and other industrial applications has greatly increased, and these can be quite expensive, particularly if they are oversized against thermal overload. A thermal protection controller is presented which allows for a reduced safety factor. The controller can predict the thermal behaviour and protect against overheating, but allows continuous operation for varying load and speed conditions, taking advantage of the long thermal time constants. A critical utilization of the motor size is the result. The motor transient temperature is predicted using a capacitance-resistance thermal model and a real time calculation of the power losses. The main thermal losses in the motor are copper loss created by the armature current and core loss created by the flux revolution. Hence, by real time sampling of the current and speed, the heat generated can be computed.