Overload protection apparatus for emulating the response of a thermal overload
First Claim
1. A thermal overload detector and current interrupter for a device supplying load current to a load which may store heat and can become subject to thermal overload comprising:
- load current measuring means for providing a current value signal proportionate to the load current;
processor means operatively connected to said load current measuring means, said processor means for computing the square of the current value signal to obtain a square current value, for computing a multiplied square current value by multiplying said square current value by a predetermined constant related to the heat transfer characteristics of the load, for computing a present running sum by adding the multiplied square current value to a past test value, for computing a proportionate present running sum by multiplying the present running sum by the predetermined constant, for computing a present test value related to stored heat in the load by subtracting the proportionate present running sum from the present running sum, for determining whether the present test value exceeds a trip value which is related to the thermal overload of the load, and for issuing a turn off signal if the present test value exceeds the trip value; and
interrupting means for responding to the turn off signal to interrupt the load current when the turn off signal is present.
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Accused Products
Abstract
A thermal overload detector and interrupter has a current transformer for monitoring load current and providing a current value signal, a microprocessor for calculating values related to load current and the thermal characteristics of a load, and an interrupter for selectively conducting and interrupting load current. The microprocessor computes a square current value from the current value signal and multiplies the square current value by a predetermined constant to obtain a multiplied square current value. The predetermined constant is related to the heat transfer characteristics of the load. A present running sum is computed in the microprocessor by adding the multiplied square current value to a past test value. The microprocessor then computes a proportionate present running sum by multiplying the present running sum by the predetermined constant. A present test value is computed by subtracting the proportionate present running sum from the present running sum. If the present test value exceeds a trip value, the microprocessor causes the interrupter to trip and interrupt the load current.
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Citations
5 Claims
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1. A thermal overload detector and current interrupter for a device supplying load current to a load which may store heat and can become subject to thermal overload comprising:
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load current measuring means for providing a current value signal proportionate to the load current; processor means operatively connected to said load current measuring means, said processor means for computing the square of the current value signal to obtain a square current value, for computing a multiplied square current value by multiplying said square current value by a predetermined constant related to the heat transfer characteristics of the load, for computing a present running sum by adding the multiplied square current value to a past test value, for computing a proportionate present running sum by multiplying the present running sum by the predetermined constant, for computing a present test value related to stored heat in the load by subtracting the proportionate present running sum from the present running sum, for determining whether the present test value exceeds a trip value which is related to the thermal overload of the load, and for issuing a turn off signal if the present test value exceeds the trip value; and interrupting means for responding to the turn off signal to interrupt the load current when the turn off signal is present. - View Dependent Claims (2, 3, 4)
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5. A method for detecting and interrupting an electric current overload with an apparatus including a load current measuring means for providing a current value signal proportionate to load current, a microprocessor receiving the current value signal, and an interrupter for selectively conducting and interrupting the load current operatively connected to said microprocessor, said method comprising the steps of:
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providing the current value signal with said load current measuring means; receiving the current value signal with said microprocessor; computing a value of the square of the current value to obtain a square current value with said microprocessor; computing a multiplied square current value by multiplying the square current value by a predetermined constant related to the heat transfer characteristics of the load with said microprocessor; computing a present running sum by adding the multiplied square current value to a past test value with said microprocessor; computing a proportionate present running sum by multiplying the present running sum by the predetermined constant with said microprocessor; computing a present test value related to stored heat in the load by subtracting the proportionate present running sum from the present sum with said microprocessor; determining whether the present test value exceeds a trip value which is related to the thermal overload of the load with said microprocessor; issuing a trip signal when the present test value exceeds a trip value with said microprocessor; interrupting load current with said interrupter when the trip signal is present.
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Specification