Electricity: electrical systems and devices – Safety and protection of systems and devices – Motor protective condition responsive circuits
Patent
1990-01-22
1991-10-15
DeBoer, Todd E.
Electricity: electrical systems and devices
Safety and protection of systems and devices
Motor protective condition responsive circuits
361103, 318783, H02H 708
Patent
active
050579622
ABSTRACT:
A microprocessor-based relay system is used in combination with relay contactors and a control circuit therefor to protect an electric motor from overload currents in addition to phase faults, ground faults, load losses, and load jams. The microprocessor is provided with a data base comprising thermal characteristics of the motor during heating and cooling. The heating data comprise current versus time curves stored as a look-up table for which each data point represents a thermal limit, i.e., the maximum time the motor is permitted to operate at a particular current level. The cooling data comprise motor cooling rates. Phase currents are sampled at preselected fixed time intervals during a predetermined period thereof, and an average motor current value is calculated therefrom. A thermal sum, which is initialized at zero upon first starting the motor, is adjusted depending on the average motor current value relative to a predetermined overload current value. The average motor current value is used as an index to the heating curve look-up table. If the thermal sum exceeds the corresponding thermal limit in the look-up table for a predetermined time interval, then an overload condition is indicated, and the relay is tripped.
REFERENCES:
patent: 3683237 (1972-08-01), Walstad et al.
patent: 4356525 (1982-10-01), Kornrumpf et al.
patent: 4547826 (1985-10-01), Premerlani
patent: 4764744 (1988-08-01), Alley et al.
Boothman, D. R. et al., "Thermal Tracking-A Rational Approach to Motor Protection", IEEE PES-Winter Meeting, New York, Jan. 27-Feb. 1, 1974, pp. 7-15.
Alley Robert P.
Bicknell William H.
Breedlove Jill M.
Davis Jr. James C.
Deboer Todd E.
General Electric Company
Snyder Marvin
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