Over-load and light-load protection for electric machinery

Abstract

An apparatus for protecting machinery driven by electric motors from operating during over-load and light-load conditions, comprising: a current transformer operatively associated with an electrical supply line for the induction motor; a current-voltage transforming circuit 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 steady state timer produce time coincident output signals.

Dependencies are inferred from the claim text.