PWM AC chopper-ready for higher power applications

The brushless excitation of synchronous machines is an attractive solution for wound rotor machines, since no slip-rings are necessary. The paper investigates the rotating transformer method. For controlling the input voltage of the exciter AC chopper is proposed. Two kinds of the AC choppers are investigated, discussed and compared mainly considering the additional losses, which is important at the higher power range where they are applied. These are the choppers constructed using thyristors or IGBTs. The high frequency PWM control of the IGBT AC chopper provides almost ideal sinusoidal supply of the rotating transformer, resulting in significantly less additional copper loss. As a result of the complexity of the system, an approximating calculation should be introduced for the thyristor chopper supply. The limits of the approximation are examined.