Control system and method for motor starting

Abstract

The present invention utilizes the fact that the rate of change of the amplitude of a sinusoidal waveform is substantially constant within an interval of such waveform near its zero crossing. A circuit for measuring peak amplitudes of sinusoidal waveforms includes a voltage differentiator for differentiating between a first and second reference selected within the interval of a sinusoidal waveform wherein the rate of change of the amplitude is substantially constant; a time interval calculator for measuring a time interval between the references; a frequency determinator for determining the frequency of oscillation of the waveform; and a processing unit responsive to the voltage differentiator, the time interval calculator, and the frequency determinator for determining a peak amplitude of the waveform. In accordance with the present invention, a method of controlling the operation of an induction motor of the type having a main winding and at least one start winding utilizes the circuit for measuring peak amplitudes of sinusoidal waveforms. The motor start control system includes a current sensing device for producing a waveform proportional to the main winding current of the motor and a control circuit responsive to the waveform for deenergizing the start winding of the motor when the amplitude of the waveform is less than a threshold level wherein the threshold level is variable in accordance with the start main winding current. By continuously determining peak amplitudes of the waveform which is proportional to main winding current of the motor and storing an initial peak amplitude of the waveform, the start winding is deenergized when a peak amplitude of the waveform becomes less than a defined percentage of the stored initial peak amplitude.

Dependencies are inferred from the claim text.