Universal DC Mode Motor Drive
DC drive for a Universal motor is shown in figure below. In order to supply DC current to the motor, a diode bridge has been added around the motor. The motor current freewheels through the diode bridge after the TRIAC current crosses zero.
To limit the rate of fall of the current as defined in the TRIAC specifications (12A/msec for BTA 12-600BW requiring 3mH in series), it is require an inductance in series with the TRIAC. However, due tot he standards limiting harmonic content of mains current, a larger value may be mandatory. As part of the EMI filter, this serial inductance should be advantageously used. No snubber is needed in normal operation. However, if large mains disturbances occur, the small filter RfCf should be implemented to avoid spurious triggering. [Source: STMicroelectronics Application Note]