Ernest L Allen

US Patent:

40608417, Nov 29, 1977

Filed:

Apr 19, 1976

Appl. No.:

5/678398

Inventors:

Ernest Duane Allen - Hudson CO

International Classification:

H02H 709

US Classification:

361 33

Abstract:

A motor protector for three-phase electric motors includes two independent sensing transformers for simultaneously sensing the voltage across two phases of a three-phase power supply to detect failures in all of the power lines including either the primary or secondary side of a line transformer. A first relay is actuated by the electric power sensed across the one phase and a second relay is actuated by the electric power sensed across said other phase through a set of contacts of the first relay for disconnecting the second sensing transformer from the power supply when there is a failure that results in a reduction in voltage in the one phase to prevent production of feedback voltages through the phase windings of a line transformer. For a wye-delta line transformer configuration the one phase is a high leg and the other phase is the low leg. At least the second, and optionally both, of said relays have a set of contacts arranged for controlling power to a holding coil of the motor control circuit so that a line failure in said power supply causes a de-energization of said one relay coil and simultaneously disconnects said second sensing coil from the other phase and removes electric power to said holding coil to remove the power from said power system to said electric motor.