CURRENT REFERENCE ESTIMATOR FOR INVERTER-FED AC MACHINES

Abstract A method of controlling the electrical torque developed in inverter-fed a.c. machines is described. A current reference estimator transforms the torque reference signal to three phase current reference signals using a torque to a current machine model. The estimator is based on a micro computer system and the machine model, for a specific a.c. machine, consists of filed relationships between torque and current amplitude, phase angle and slip (induction machine) stored in EPROM. Furthermore, a filed cosine function is used for producing sinusiodal current references. Since the transformation from the torque reference to the sinusiodal three phase current references consists only of two multiplications and the machine model is filed data, the current reference estimator is easily built by using an ordinary micro computer system. The sampling period becomes shorter than 1 ms. The experimental results, both for the asynchronous and the synchronous machines, indicate a very fast torque response and a good system performance, provided a fast current response in the inverter. This method is therefore, above all, suitable for small power a.c. machines.