Protection circuit for series wound DC traction motor - has diode in series with attenuator protecting against external mains supply short circuits

摘要

The protection circuit protects a series-wound DC traction motor from the effects of an external short circuit in the mains supply. The series combination of an inductor and variable attenuator is coupled in parallel with the motors field winding (2). A diode (5) is connected in series with the variable attenuator (4) and the inductor (3). The diode is forward biased when the motor is running. The variable attenuator may be a tapped resistor whose tappings can be shorted by relay contacts or it may be a resistor shunted by e.g. a thyristor. The diode prevents short circuit current flowing over the resistor, forcing current to pass through the field winding.