Method and apparatus for providing thermal protection for large motors based on accurate calculations of slip dependent rotor resistance
First Claim
1. A system for determining rotor resistance values of a an induction motor from an input voltage and an input current applied to said induction motor, said system comprising:
- first means for repetitively sampling the input voltage and input current of said induction motor to obtain a plurality of voltage and current samples including at least one pair of voltage and current samples taken at start up of said induction motor;
second means, responsive to said first means, for calculating from said voltage and current samples instantaneous total apparent resistance values including an initial apparent total resistance value obtained at said start up;
third means, responsive to said second means, for deriving from said initial apparent total resistance value a calculated stator resistance;
fourth means, responsive to said first second and third means, for repetitively calculating instantaneous slip values from said instantaneous apparent total resistance values and from said calculated stator resistance;
fifth means, responsive to said second third and fourth means for calculating instantaneous rotor resistance values from said instantaneous slip valuessixth means for calculating, from said instantaneous rotor resistance values and said current input, the quantity of heat generated and the temperature prevailing in said rotor;
seventh means for tripping said motor when said temperature reaches a predetermined value, said seventh means including means for repetitively calculating an instantaneous temperature value;
a microprocessor and an associated control program, said microprocessor and control program comprising said second, third, fourth, fifth and sixth means; and
an interface circuit for interfacing said microprocessor to said motor.
1 Assignment
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Accused Products
Abstract
A microprocessorized thermal protection system and accompanying method measures input voltage and current of an induction motor and derives therefrom continuously updated estimates of the slip dependent rotor resistance during acceleration of the motor. The measurements and calculations of the varying rotor resistance enable determination of the heat generated within the motor based on a thermal model thereof. Phase shift distortions induced by the measurement system are accounted for by determining the value of the stator resistance at start up. In addition, during a brief period following start up, sampling of the voltage/current inputs is suspended to avoid the effects of the initially occurring DC-offset resulting from the application of the voltage input to the inductive motor load.
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Citations
17 Claims
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1. A system for determining rotor resistance values of a an induction motor from an input voltage and an input current applied to said induction motor, said system comprising:
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first means for repetitively sampling the input voltage and input current of said induction motor to obtain a plurality of voltage and current samples including at least one pair of voltage and current samples taken at start up of said induction motor; second means, responsive to said first means, for calculating from said voltage and current samples instantaneous total apparent resistance values including an initial apparent total resistance value obtained at said start up; third means, responsive to said second means, for deriving from said initial apparent total resistance value a calculated stator resistance; fourth means, responsive to said first second and third means, for repetitively calculating instantaneous slip values from said instantaneous apparent total resistance values and from said calculated stator resistance; fifth means, responsive to said second third and fourth means for calculating instantaneous rotor resistance values from said instantaneous slip values sixth means for calculating, from said instantaneous rotor resistance values and said current input, the quantity of heat generated and the temperature prevailing in said rotor; seventh means for tripping said motor when said temperature reaches a predetermined value, said seventh means including means for repetitively calculating an instantaneous temperature value; a microprocessor and an associated control program, said microprocessor and control program comprising said second, third, fourth, fifth and sixth means; and an interface circuit for interfacing said microprocessor to said motor. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. A method for determining rotor resistance values of a three phase induction motor from an input voltage and an input current applied to said three phase induction motor, said method comprising the steps of:
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repetitively sampling the input voltage and input current of said three phase induction motor to obtain a plurality of voltage and current samples including at least one pair of voltage and current samples taken at start up of said three phase induction motor; calculating from said voltage and current samples instantaneous total apparent resistance values including an initial apparent total resistance value obtained at said start up; deriving from said initial apparent total resistance value a calculated stator resistance; repetitively calculating instantaneous slip values for said motor from said instantaneous apparent total resistance values and from said calculated stator resistance; calculating instantaneous rotor resistance values from said instantaneous slip values; and repetitively obtaining instantaneous estimates of the temperature prevailing in said motor and tripping said motor when said temperature reaches a predetermined value. - View Dependent Claims (13, 14, 15, 16, 17)
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Specification