# Rotor Earth Fault Relay

S

Hello Every one,

I want to know about the working principle for the rotor earth fault relay? and is the working principle same for the earth fault relay in battery chargers? I learned there are different principles for rotor earth fault relay working.

#### PhilCorso

the three prevalent types of protection used are:

1) DC-Voltage Measurement
A voltage-relay is connected between the center-tap of resistor in parallel with the rotor, and earth rotor winding and earth. Virtually any EF will produce a voltage which is detected by the relay, except a fault near the center of the winding. This covered by a second tap on the resistor some distance from center.

2) AC-Current Injection
An auxiliary ac source is connected between the rotor-winding end and an ac voltage-relay, thru a capacitor. This eliminates the situation above when the EF occurs near the center of the winding.

3) DC-Injection
A DC source is provided that sets up a voltage greater than that of the rotor. It will detect an EF anywhere along the rotor-winding. But, its magnitude also varies, i.e., unlimited at the winding end where it's connected, to a minimum value at the opposite end of the winding. Thus, a current limiting resistor is added.

Regards,
Phil Corso