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Over-load and light-load protection for electric machinery

  • US 4,541,029 A
  • Filed: 10/03/1983
  • Issued: 09/10/1985
  • Est. Priority Date: 10/06/1982
  • Status: Expired due to Fees
First Claim
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1. An apparatus for protecting machinery driven by electric motors from operating during over-load and light-load conditions, comprising:

  • inductive means, operatively associated with an electrical supply line for the induction motor, for generating a control signal having a magnitude proportional to current flowing in the supply line;

    an upper limitation magnitude comparator for generating an over-load signal output whenever the magnitude of the control signal exceeds a predetermined upper safety value;

    a lower limitation magnitude comparator for generating a light-load signal output whenever the magnitude of the control signal is less than a predetermined lower safety level;

    an inhibitable protection timer for producing a fault persistence output signal whenever one of the over-load and light-load signal outputs is generated for a period of time longer than a predetermined protection time;

    a steady state timer for inhibiting an enable output signal for a predetermined period of time after start-up and during shut-down of the electric motor;

    a start-up/shut-down comparator for producing an output signal for enabling operation of the protection timer and the steady state timer whenever the magnitude of the control signal exceeds a predetermined reference value corresponding to an electric current lower than the unloaded current of the electric motor; and

    an output relay for interrupting operation of the electric motor when both the protection timer and the steady state timer produce time coincident output signals.

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