Conduction Limits of a Three-Phase Controlled Converter in Inversion

The basic six-SCR converter circuit generally used in dc motor drives can provide regenerative braking when operated in the inversion mode. When the motor is operated near rated voltage, however, troublesome commutation failures will occur unless the SCR firing delay angles are severely limited or limited as a function of motor current magnitude. The required delay angles for both light and rated motor currents are discussed, from which it is apparent that a limit on the delay angle should be dependent on the magnitude of the motor current. Since there is a range of values that the delay angle may have, many ways exist in which the delay-angle limit might be implemented. One delay-angle-limit concept is presented to demonstrate the principle.