Estimation of field current in vector-controlled synchronous machine variable-speed drives employing brushless asynchronous exciters

In wound-rotor synchronous machine variable-speed drives, accurate information about the field winding current is paramount to obtain high dynamic and steady-state performance, such as unity power factor. When brushless excitation is employed and no direct measurement of field winding current is available, it must be estimated. This is a topic which has not received much attention in the literature, despite its importance in high-power synchronous motor drives. This paper presents two different methods for field current estimation, applied in a vector-controlled voltage-source converter-fed drive. Experimental results illustrate the steady-state accuracy and dynamic performance.

I. Introduction

Synchronous machines (SMs) with wound rotors are often used in high-power fixed-speed as well as variable-speed applications. In multimegawatt variable-speed drives (VSDs) the load-commutated inverter (LCI) has been the preferred choice. Cycloconverters have been and are still used, while in recent years voltage-source converters (VSCs) have entered in certain applications, employing vector control or direct torque control (DTC).