A Four-Quadrant Three-Pulse Transistor Rectifier

This paper presents a new power converter which uses 3 bi-directional switches with turnoff capability to connect directly the three-phase source to the load. Each bi-directional switch is composed by power transistors (MOSFET's) working in the switching mode. The converter works with a high switching frequency and for this reason large reactive elements are not needed. The strategy for the selection of the source voltages is based on the concept of Ficticious Bipolar Source and a two-level controller is used to control the load current, which produces a drastical simplification of the control circuit. In addition, in this work it is theoretically demonstrated that with this type of non-linear controller no 150-Hz harmonics are present in the load current. Simulation and experimental results were obtained with passive and active loads. The converter acts as a controlled current source and the low-frequency component of the load current can be varied continuously. The control technique developed in this work allows to change the load frequency between 0 and more than 150 Hz; this output frequency is not limited by the frequency of the three-phase source. The converter presents no restrictions to the direction of power flow between the source and the load and for this reason it is well suited to be used in the four-quadrant speed control of D. C. machines. In addition, in this converter there is no need to identify the different quadrants of operation and to detect the zero crossing point of the load current. Experimental results were obtained in the speed control of a D.C. servomotor.