High-Frequency Inductance-Based Magnet Temperature Estimation in Motor Drives

Permanent magnet synchronous motors are widely used in various industries. This type of motor is vulnerable to magnet temperature variation, monitoring the magnet temperature is important. Accordingly, methods have been developed for estimating the magnet temperature. This study developed a method for magnet temperature estimation at low speeds. At a low speed, high-frequency inductance was obtained by injecting a sinusoidal voltage with a frequency of several 100 Hz, which is utilized in the estimation. This article analyzes the extracted high-frequency inductance, and the high-frequency inductance was found to be affected by the motor speed, and it interfered with the spatial harmonics of the motor. Based on the result, the injection frequency of 2.5 kHz was chosen. Furthermore, a square wave was used as the injection voltage instead of a sinusoidal wave for the implementation. The proposed method is not only much simpler to use than the conventional method but the lookup tables' size is also reduced to 20% of that of the conventional method. In particular, unlike the conventional method, interference caused by spatial harmonics can be avoided in the proposed method. The proposed method was verified by online magnet temperature estimation for 19 000 s (i.e., 5.28 h), and the error was less than 4 °C.