Modular PTC thermistor overload protection system
First Claim
1. A modular thermistor overload protection system comprising:
- a power circuit including an electrical power supply source and a load device;
switching means operable to connect said load device to said source and to disconnect said load device therefrom thereby to selectively energize or deenergize said load device;
control means for operating said switching means;
and protective means operable to sense an overload trip current in said power circuit and trip said switching means so as to cause said switching means to disconnect said load device from said source comprising;
trip means for directly tripping said switching means without delay;
a thermistor sensor in circuit with said trip means and having a switching temperature level at which its resistance abruptly changes;
a control circuit for applying an electrical bias to said thermistor sensor;
heater means responsive to an overload trip current in said power circuit for heating said thermistor sensor to its switching temperature level whereby its abrupt change in resistance causes direct operation of said trip means;
and the speed of operation of said protective means being fast enough due to said direct operation of said trip means so that said trip means will always trip before said switching means has had time to operate long enough to apply enough power to cause motor energization if said control means is operated to apply power under overload conditions.
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Accused Products
Abstract
A motor overload protection system (OL) that has a speed of operation fast enough so that it will always trip before the power switch (MC) has had time to operate if the turn-on control means (ON) is operated under overload conditions. In one version, modules (OL1,OL2) are provided having PTC thermistor sensors (PTC1,PTC2) bonded to resistance heaters (H1,H2) connected in the motor power circuit (L1,L2,L3), these sensors being supplied from a current source (CT) and being effective to shunt current to a protective trip device (TR) upon occurrence of an overload condition, there being an ambient temperature compensating PTC themistor (PTC3) in series in the sensor circuit (OLC). In another version, a thermal mass (TM) is preheated above ambient temperature by an ambient temperature compensating PTC thermistor (ATC) operated above its switching temperature level and a sensor PTC thermistor (ST) and a heater (HT) are also thermally coupled to the thermal mass (TM) with a bias resistor (R1) connected to the sensor thermistor (ST) to provide a step voltage output (OUT) on overload. The heater may be a PTC thermistor (HT) having a higher switching temperature for self-shutoff on higher than usual overload or may be a resistance heater (H3) with a PTC thermistor (TP) in series therewith for self-shutoff.
47 Citations
14 Claims
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1. A modular thermistor overload protection system comprising:
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a power circuit including an electrical power supply source and a load device; switching means operable to connect said load device to said source and to disconnect said load device therefrom thereby to selectively energize or deenergize said load device; control means for operating said switching means; and protective means operable to sense an overload trip current in said power circuit and trip said switching means so as to cause said switching means to disconnect said load device from said source comprising; trip means for directly tripping said switching means without delay; a thermistor sensor in circuit with said trip means and having a switching temperature level at which its resistance abruptly changes; a control circuit for applying an electrical bias to said thermistor sensor; heater means responsive to an overload trip current in said power circuit for heating said thermistor sensor to its switching temperature level whereby its abrupt change in resistance causes direct operation of said trip means; and the speed of operation of said protective means being fast enough due to said direct operation of said trip means so that said trip means will always trip before said switching means has had time to operate long enough to apply enough power to cause motor energization if said control means is operated to apply power under overload conditions. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
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11. A modular electric current level responsive system comprising:
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a module comprising an insulated thermal mass member representing the thermal mass analog of an electrically powered device to be protected from overcurrent; an electrical power supply source; an ambient temperature compensating PTC thermistor having a first switching temperature level and being in contiguous heat transfer relation to said thermal mass; means for connecting said souroe to said thermistor to heat it to said first switching temperature level so as to limit its self-heating and apply to said thermal mass thermal energy in proportion to the difference between ambient temperature and said first switching temperature thereby to provide a relatively constant temperature environment within said module regardless of the exterior ambient temperature and a stable ambient temperature compensation for the output of the system; a heater means in contiguous heat transfer relation to said thermal mass; means for controlling the heating rate of said heater means in proportion to the current of said electrically powered device to be protected from overcurrent and applying the heat to said thermal mass; a PTC thermistor sensor having a higher switching temperature level in contiguous heat transfer relation to said thermal mass; an output connected to said PTC thermistor sensor; and circuit means for applying an electrical bias from said source to said PTC thermistor sensor whereby a protective signal appears at said output when said PTC thermistor sensor is heated by said thermal mass to its said higher temperature level due to overcurrent in said electrically powered device. - View Dependent Claims (12, 13, 14)
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Specification