I am currently learning about large electric motor protection systems.
In a "stator faults" section there is an opening statement that has confused me.
"DC overcurrent tripping devices on the breaker take care of ground faults or line faults. These devices are thermal or dashpot types giving an inverse time current characteristic, and usually provide instantaneous trip at high current."
What does the above statement refer to? AC or DC motors?
Thank you.
"DC overcurrent tripping devices on the breaker" refers to a DC operated coil in the circuit breaker that allows an external device to trip the circuit breaker. The external device is the "thermal or dashpot" device that monitors the stator current and supplies the DC voltage to trip the breaker. The motor could be any type of large motor, but since "stator faults" are mentioned an AC motor must be the type in this case, because the stator of a DC motor carries the field current rather than the motor load current.
For fail-safe operation, the coil in the breaker should be energized for normal operation and de-energized for trip.
