H - Electricity – 02 – H
Patent
H - Electricity
02
H
342/21, 318/50
H02H 7/085 (2006.01) G05B 15/02 (2006.01) H01H 37/12 (2006.01) H02H 6/00 (2006.01) H01H 9/54 (2006.01)
Patent
CA 2031101
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.
Alley Robert Philbrick
Bicknell William Hull
Company General Electric
Craig Wilson And Company
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