Method and apparatus for sensing a ground fault in a motor control system
First Claim
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1. A method for sensing a ground fault in a motor control system including a motor coupled to at least two conductors, the method comprising the steps of:
- generating a first unrectified signal indicative of the current level in a first conductor;
generating a second unrectified signal indicative of the current level in a second conductor;
summing the first and second unrectified signals to generate an unrectified sum signal indicative of the sum of the current levels;
rectifying the sum signal to produce a rectified sum signal and data representative thereof;
comparing the data to a predetermined limit; and
generating indicia of a ground fault in response to the data exceeding the predetermined limit for more than a predetermined time period.
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Abstract
A motor control unit for protecting a motor against ground fault currents. The unit monitors the currents in each phase of the motor to provide values representative of each phase current. The unit also includes a contactor for opening and closing each phase, wherein the unit controls the contactor based upon the values representative of the phase currents. The contactor is opened when a ground fault situation exists at the motor.
112 Citations
23 Claims
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1. A method for sensing a ground fault in a motor control system including a motor coupled to at least two conductors, the method comprising the steps of:
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generating a first unrectified signal indicative of the current level in a first conductor; generating a second unrectified signal indicative of the current level in a second conductor; summing the first and second unrectified signals to generate an unrectified sum signal indicative of the sum of the current levels; rectifying the sum signal to produce a rectified sum signal and data representative thereof; comparing the data to a predetermined limit; and generating indicia of a ground fault in response to the data exceeding the predetermined limit for more than a predetermined time period. - View Dependent Claims (2, 3, 4, 22)
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5. An apparatus for sensing a ground fault in a motor control system, the apparatus comprising:
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means for generating a first unrectified signal indicative of the current level in a first conductor coupled to the motor; means for generating a second unrectified signal indicative of the current level in a second conductor coupled to the motor; means for summing the first and second unrectified signals to generate an unrectified sum signal indicative of the sum of the current levels; means for rectifying the sum signal to produce a rectified sum signal; means for generating data representative of the rectified sum signal; means for comparing the data representative of the rectified sum signal to a predetermined limit; and means for generating indicia of a ground fault upon the data exceeding the predetermined limit for more than a predetermined time period. - View Dependent Claims (6, 7, 8, 9, 10, 11, 12, 13, 23)
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14. A motor control for sensing a ground fault in an electric motor coupled to a three phase system, comprising:
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a first current transformer disposed to generate an unrectified first voltage substantially proportional to the current level in a first phase; a second current transformer disposed to generate an unrectified second voltage substantially proportional to the current level in a second phase; a third current transformer disposed to generate an unrectified third voltage substantially proportional to the current level in a third phase; means for generating, from the first, second and third unrectified signals, an unrectified sum signal indicative of the sum of the current levels; means for rectifying the sum signal so as to produce a rectified sum signal; means for generating data representative of the rectified sum signal; means for storing data representative of a sum limit and data representative of a predetermined time period; means for comparing the data representative of the rectified sum signal to the data representative of the sum; and means for generating indicia of a ground fault upon the data representative of the rectified sum signal exceeding the data representative of the sum for more than the predetermined time period as defined by the data representative of the predetermined time period; means responsive to a control signal applied thereto, for selectively engaging and disengaging the motor from the first, second and third phases; and means, responsive to the generation of indicia of a ground fault, for applying a control signal to the means for selectively engaging. - View Dependent Claims (15, 16, 17, 18, 19)
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20. An apparatus for sensing a ground fault in a motor control system, the apparatus comprising:
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a plurality of current transformers, each of which is disposed to generate a signal indicative of the current level in one of a plurality of conductors; means for generating, from the signals generated by the current transformers, a sum signal indicative of the sum of the current levels in the conductors; a microprocessor coupled to the means for generating, the microprocessor being programmed to determine whether the sum signal falls within a range defined by at least one designated limit, the microprocessor further comprising means for storing data representative of a predetermined time period based upon a digital model of a motor being controlled by the motor control system; and means, coupled to the microprocessor, for generating indicia of a ground fault upon the sum signal falling outside the designated limit for more than the predetermined time period as defined by the data representative thereof. - View Dependent Claims (21)
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Specification