Voltage-Hertz Control of the Synchronous Machine with Variable Excitation

In spite of the development of the vector control methods for the AC machines, the scalar control also finds his place in various industrial applications. Generally, the scalar control is used in reduced speed-range applications where is no need for exceptional dynamic behaviour. In order to achieve the highest torque per ampere ratio, the flux amplitude has to be maintained at his rated value. This can be achieved by adjusting in a proper way the U_S amplitude and the f_S frequency of the stator-voltage. The working frequency is determined by the parameters of the application, so the constant flux operation can be maintained by adjusting the supply voltage amplitude. The first control method, which assures loss-less operation for the motor is the so-called constant voltage-Hertz operation. The only control variable is the stator-voltage frequency, while the voltage is computed based on the simplified steady-state equivalent circuit of the stator. Nevertheless, the main drawback of the constant volt/Hertz procedure consists on the effects of the voltage which can cause problems at low speed operation. These voltage drops at low frequencies has the same order of magnitude with the computed voltage, which makes the method inadequate at low speed. This can be eliminated by adopting different techniques, for the voltage drop compensation. The paper deals with current-feedback-based voltage-drop computation procedures for a salient-pole synchronous motor, with variable excitation. In addition to the known current-feedback compensation method, in this case also the excitation voltage is controlled, is order to improve the dynamic behaviour of the system