Electricity: electrical systems and devices – Safety and protection of systems and devices – Motor protective condition responsive circuits
Patent
1976-04-19
1977-11-29
Miller, J. D.
Electricity: electrical systems and devices
Safety and protection of systems and devices
Motor protective condition responsive circuits
361 92, H02H 709
Patent
active
040608417
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. A failure in the power supply that results in a drop in voltage in the other phase has its contacts de-actuated to directly control the motor circuit to remove the electric power from the electric motor. A rheostat is provided in one or both of the circuits including the respective coils of said relays to adjust the voltage at which the associated relay contacts are de-actuated for greater sensitivity to reductions in line voltage and for adjusting the relays for motor currents of a particular motor installation.
REFERENCES:
patent: T948003 (1976-07-01), Johnson et al.
patent: 1834578 (1931-12-01), Conrad
patent: 2553949 (1951-05-01), Smith
patent: 2609425 (1952-09-01), Turnupseed et al.
patent: 3056067 (1962-09-01), Luber
patent: 3157826 (1964-11-01), Norton
patent: 3260897 (1966-07-01), Parker
Lewis, Jr. Ancel W.
Miller J. D.
Salce Patrick R.
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