4,052,648 1 POWER FACTOR CONTROL SYSTEM FOR AC ORIGIN OF THE INVENTION The invention described herein was made by an employee of the United States Government, and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to power input controls for motors, and particularly to a control which varies input power to an AC induction motor proportional to loading on the motor. 2. General Description of the Prior Art 5 10 2 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an electrical schematic diagram of an embodiment of the invention. FIGS. 24-2/ are waveforms illustrating aspects of operation of the invention. FIG. 3 is a plot illustrating power drawn by a motor for different states of loading and with and without the control system of this invention. DETAILED DESCRIPTION OF THE An AC induction motor 10 is powered by an alternating current voltage 12 (FIG. 2a) through switch 14 and 15 connectible at terminals 16. The switched AC power is also applied to transformer 18 and circuit bias power supply 20. Triac 22 is connected in series with motor 10 and is triggered for controlled portions of each half cycle of power input. A small value resistor 24 of 0.010 20 to 0.020 ohms is connected in series with motor 10 and serves to develop a signal 26 (FIG. 26) which is proportional to the current flow through the motor. FIG. 26 illustrates an instantaneous state of operation after initial start-up and with an initial optimum input voltage-load relationship, whereby triac 22 is fully on and where, thereafter, loading is substantially decreased. The initial current-voltage phase lag 28 for such optimum state of operation may vary from motor to motor and would be determined for each motor with which this invention is to be employed. In the present example, initially, optimum phase lag 28 is approximately 30°, and potentiometer 78 is adjusted to provide the zero error output signal for the control of the turn on time of triac 22 to maintain the phase angle of this or another selected value. The occurrence of increased current lag 28a at time T, depicts a sudden decrease in loading of motor 10. The detection of this is used, as will be further explained, to reduce the average amplitude of input voltage and thereby to effect a commanded, optimum, phase lag. The induction motor is perhaps the most rugged, and is certainly one of the most commonly used motors. It runs at an essentially constant speed which, within certain limits, is independent of both load and applied voltage. For efficient operation, the applied voltage should 25 be a function of the load. Heretofore, this has not been practically accomplished. Line voltages are a matter of availability from a local utility. In the case of nominal 115-volt service, line voltage may be typically in the range of 105 to 125 volts and may not be constant with 30 the service from a particular source and often varying significantly over a 24-hour period. In recognition of this, typically a 115-volt motor would be designed to deliver its rated load plus a safety margin at an under voltage condition of 105 to 110 volts. However, in taking care of the ability of the motor to perform its rated job at under voltage conditions, it becomes wasteful when line voltage is in the 120- to 125-volt range. Further, since this type of motor draws essentially the same 40 current whether loaded or unloaded, motor efficiency goes down when less than a rated load is applied to the motor. Thus, where a user employs a motor over-rated for a job or a variable load is applied to the motor, efficiency suffers and waste of electrical power occurs. 3. Object of the Invention It is the object of this invention to provide an electrical device which, when placed in circuit with the power input of an AC induction motor, will effect a reduction in power normally provided the motor when operated in either a condition where line voltage is greater than normal and/or motor loading is less than a rated load. SUMMARY OF THE INVENTION 35 45 50 55 In accordance with the invention, the voltage applied to an AC induction motor and current through that motor are sampled, the phases of the samples are compared, and a control signal representative of the differ- 60 ence is obtained. This signal is then employed to vary the duty cycle portion of each cycle (portion of each cycle of alternating current) applied to the motor, decreasing the duty cycle proportional to phase difference to thereby regulate phase difference and thus improve 65 the power factor to a more optimum state when there is otherwise present less than an optimum relationship between line voltage and motor load. To further examine the circuitry, transformer 18, having center tap secondary 32, provides oppositely phased inputs to square wave shapers 34 and 36, and the resulting oppositely phased outputs, square wave 38 (from shaper 36) shown in FIG. 2c and square wave 40 (from shaper 34) shown in FIG. 2d, which are fed to saw tooth or ramp wave shapers 42 and 44, respectively. The outputs of the wave shapers are combined to provide a ramp wave each half cycle of the alternating current input as shown in waveform 46 of FIG. 2 Waveform 38 is also used as a reference signal for the phase of input voltage and is fed to one input of multiplier 48, functioning as a phase detector, to which is also fed a current reference signal 50 shown in FIG. 2g. The current reference signal is generated as follows. Current signal 26 (FIG. 26) from resistor 24 is fed to isolation transformer 52 and from it to square wave pulse shaper 54, which provides square wave 56 (FIG. 2). This square wave is differentiated in differentiator 58 to provide spike pulses 60 shown in FIG. 2, and the negative pulses (derived from the trailing edge of square wave 56) are used to trigger one-shot 62, which provides as an output the square waveform 50 shown in FIG. 2g. This square waveform commences at a time corresponding to the trailing or zero crossing point of current signal 26 (FIG. 26) and has a duration (determined by the time constant of one-shot 26) corresponding to the length of a half cycle of AC input to the motor. Thus, there is generated a square wave current signal which is of the |