Load monitoring circuit for electric motors

Claim 1

A load monitoring circuit for monitoring different operational conditions of an electric motor having a stator winding, comprising a measuring resistor connected in series with the stator winding to produce an actual load value signal; a variable reference voltage source; a comparison circuit having one input coupled to said measuring resistor, a second input coupled to said variable reference voltage source and an output; means for successively varying the adjustment of the reference voltage source for automatically applying to the second input of the comparison circuit at least three consecutive nominal reference values corresponding to different operational conditions of the electric motor, namely to a maximum nominal reference value corresponding to the starting phase of the motor, a standard nominal reference value corresponding to normal operation of the motor and zero nominal reference voltage value corresponding to an overload, said comparison circuit includes an operational amplifier having a non-inverting input coupled to said measuring resistor, an inverting input coupled to said variable reference voltage source, and an output, said variable reference voltage source including a voltage divider connected to a power supply for said electric motor and being series connected with a first switching transistor controlled by a timing circuit in response to the starting current of the motor so as to apply during the starting phase a maximum nominal reference value to the comparison circuit and upon completion of the starting process to apply a standard nominal reference value corresponding to normal operational conditions of the motor; and a second transistor connected to the inverting input of the operational amplifier and having its base coupled to the output of the opertional amplifier so as to adjust the nominal reference voltage to a zero value when an overload is detected by the comparison circuit.