Abstract:
A power control system for an A.C. induction motor is disclosed, comprising a voltage/current phase difference generator for determining a difference in phase between a voltage applied to the motor and a current drawn by the motor, and for generating a phase difference signal as a function of the determined difference in phase, the voltage/current phase difference generator including an integrator, the integrator receiving the phase difference signal and generating an error signal for controlling an amount of power supplied to the motor as a function of the phase difference signal, the integrator being electrically coupled to a potentiometer, the potentiometer providing a bias signal for at least partially controlling the error signal; and a delay circuit for controlling the bias signal provided by the potentiometer so as to cause full available power to be supplied to the motor for a predetermined amount of time. The potentiometer further comprises first and second outer terminals and a center tap terminal, the center tap terminal providing the bias signal. The delay circuit controls the resistance appearing across the first outer terminal and second outer terminal of the potentiometer for the predetermined amount of time.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to AC induction motors, and more particularly to a power factor control system for AC induction motors which includes a timer circuit for delaying the operation of the power factor control system. 
     BACKGROUND OF THE INVENTION 
     The difference in phase between the voltage supplied to an induction motor and the resulting current through the motor, known as the power factor, is indicative of the load on the motor. It is known for a power control system to be connected to a motor in order to detect and compare the supplied-voltage and resulting-current signals. Based upon this comparison, the power control system may control the voltage applied to the motor, which in turn controls the flow of current to the motor, in order to reduce the power consumed by a less than fully loaded motor. 
     U.S. Pat. No. 4,266,177 to Nola, for example, describes a power control circuit for an induction motor (hereinafter “the Nola &#39;177 circuit”), wherein a servo loop is used to control the voltage applied to the motor, which in turn controls the flow of current to the motor, in order to reduce the power consumed by the motor. In particular, a pulse signal is used to control the “on” time of a triac which is in circuit with the motor in order to maintain motor operation at a selected power factor. The pulse signal is based upon the measured current-voltage phase angle. 
     Power factor controllers of the prior art, such as the one just described, use an integrator as part of the processing required to produce the pulse signal. Typically, the integrator includes an operational amplifier and a filter which includes a capacitor and provides a single path of feedback from the output of the operational amplifier to one of the inputs of the operational amplifier. A command signal circuit is also connected to one of the inputs of the operational amplifier, which is typically the same input to which the filter is connected. Conventionally, the command signal circuit contains a potentiometer. The potentiometer must be adjusted for the particular motor being controlled in order to provide a proper bias voltage to the operational amplifier. In effect, it sets a selected power factor (or phase angle between current and voltage) as determined by the greatest power factor (smallest motor current-voltage phase difference) at which the motor will operate over a range of loadings to be encountered. The resulting control signal is a negative signal which shifts positively responsive to the presence of a higher than commanded power factor, and shifts negatively when there is detected a lower than commanded power factor. It is employed in a servo loop to vary the applied voltage and control the input power to the motor. In this way, the motor is forced to operate at the selected power factor. In such circumstances, this enables motors which are less than fully loaded to draw significantly reduced power. 
     Power factor controllers require a power supply in order to provide an operating bias voltage of, for example, 15 volts, to the controller&#39;s active components, such as the operational amplifier of the integrator, so that the pulse signal is provided to the triac Exemplary transformer-less power supplies are employed. However, in such cases, relatively large capacitors are typically necessary, and this slows full voltage output and start-up time of the circuitry. This in turn may prevent a motor from having a sufficient starting voltage (average voltage through triac) initially applied to it for effective starting. To compensate for this, a delay circuit is employed which delays any power from being applied to motor until operating biases are essentially at full operating levels. 
     Unfortunately, the delays provided by such delay circuits may be too short during times of high temperature stresses, as would occur during summer months for air conditioning system and refrigeration systems. In such circumstances, the voltage supplied by the power company is lowered in response to the heavy loads produced by the very same air conditioning systems. The voltage supplied by the power companies is lowered to just above the level of adequate operation of such systems. Further, these air conditioning and refrigeration systems need even more time for the pressures in the compressor portion of such systems to stabilize and for back pressures to have been eliminated. It is therefore necessary to apply as much of the full supply voltage as possible for longer periods of time than can be provided by the time delay circuits employed in existing power factor controllers based on the Nola &#39;177 circuit. Any energy saving circuit/power factor controller operating in a similar fashion to the Nola &#39;177 circuit would lower the voltage supplied to AC induction motors even further, often leading to catastrophic failures of the AC induction motors. 
     Accordingly, what would be desirable, but has not yet been provided, is a system for delaying the operation of energy savings/power factor controller, such as the Nola &#39;177 circuit, or conversely causing such circuits to apply maximum available supply voltage for a longer period of time that has been previously provided until the system employing the AC induction motor has stabilized. 
     SUMMARY OF THE INVENTION 
     The above-described problems are addressed and a technical solution is achieved in the art by providing a power control system for an A.C. induction motor, comprising: a voltage/current phase difference generator for determining a difference in phase between a voltage applied to the motor and a current drawn by the motor, and for generating a phase difference signal as a function of the determined difference in phase, the voltage/current phase difference generator including an integrator, the integrator receiving the phase difference signal and generating an error signal for controlling an amount of power supplied to the motor as a function of the phase difference signal, the integrator being electrically coupled to a potentiometer, the potentiometer providing a bias signal for at least partially controlling the error signal; and a delay circuit for controlling the bias signal provided by the potentiometer so as to cause full available power to be supplied to the motor for a predetermined amount of time. The potentiometer further comprises first and second outer terminals and a center tap terminal, the center tap terminal providing the bias signal. The potentiometer further comprises first and second outer terminals and a center tap terminal, the center tap terminal providing the bias signal. The delay circuit controls the resistance appearing across the first outer terminal and second outer terminal of the potentiometer for the predetermined amount of time. 
     The delay circuit further comprises: a DC power supply having a DC supply voltage derived from an alternating current source; a delay timer having an input for receiving the DC supply voltage from the DC power supply and an output for outputting at least a portion of the DC supply voltage for the predetermined amount of time; a comparator having an input, the input of the comparator being electrically connected to the output of the delay timer, the comparator being configured to operate in a low state for the predetermined amount of time after receiving the at least a portion of the DC supply voltage from the delay timer and to operate in a high state thereafter; and a relay having a drive coil and a pair of output contacts, the drive coil being in electrical communication with and driven by the comparator, the output contacts being in a closed state across the first and second outer terminals of the potentiometer when the comparator is in the low state and being in an open state otherwise. 
     The value of the predetermined amount of time is based on the value of the parallel combination of the current limiting resistor, the timing resistor, and the timing capacitor within the delay timer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more readily understood from the detailed description of exemplary embodiments presented below considered in conjunction with the attached drawings, of which: 
         FIG. 1  is a partial schematic/block diagram of a power factor control system for AC induction motors that employs the Nola &#39;177 circuit and a timer circuit for delaying the operation of the Nola &#39;177 circuit, constructed in accordance with an embodiment of the present invention; 
         FIG. 2  is an electrical schematic diagram of a representative preferred embodiment of the Nola &#39;177 circuit and the energy saver delay circuit of  FIG. 1 ; and 
         FIG. 3  shows a series of waveforms illustrating aspects of the operation of the Nola &#39;177 circuits of  FIGS. 1 and 2 . 
     
    
    
     It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , a power factor control system for AC induction motors that employs the Nola &#39;177 circuit and a timer circuit for delaying the operation of the Nola &#39;177 circuit is depicted, constructed in accordance with an embodiment of the present invention, and generally indicated at  2 . The system  2  includes the Nola &#39;177 circuit  4 , and an energy saver delay circuit  6 . The energy saver delay circuit  6  includes a DC power supply  80  derived from an alternating current source (115 volts AC and shown as waveform (a) of  FIG. 3 ), a delay circuit  82 , a comparator  84 , a relay driver  86 , and a normally closed, single pole-single throw relay  88 , connected as shown. 
     The operation of the Nola &#39;177 circuit  4  is herein described as follows. The alternating current source is connected across terminals  10  and  12 . This supplies the voltage/current associated with the alternating current source to circuit bias supply  13  and across a series circuit including a winding or windings of motor  14 , triac  16 , and current resistor  18 . The input voltage signal is also applied to voltage squaring wave shapers  20  and  22 , shaper  22  providing a first phased, full wave, rectangular wave output as shown in waveform (b) of  FIG. 3 ; and voltage squaring wave shaper  20  providing an oppositely phased, full wave, rectangular wave output as shown in waveform (c) of  FIG. 3 . A signal voltage across resistor  18 , shown as waveform (g) in  FIG. 3 , and representative of motor current, is applied to the inputs of full wave current squiring wave shapers  24  and  26 , shaper  24 , being responsive only to the positive half cycle of current waveform (d), providing a first phased, full wave, rectangular wave output as shown in waveform (h) of  FIG. 3 . Wave shaper  26 , being responsive only to the negative half cycle of current waveform (d), provides an oppositely phased, full wave, rectangular wave output as shown in waveform (i) of  FIG. 3 . The bias outputs of supply  13  power the indicated bias requirements as well as generally supply bias power to all active elements in the Nola &#39;177 circuit  4  by connections not shown. 
     An output of each of voltage square wave shapers  20  and  22  are fed to negative going pulse detector  28  which provides a negative spike output (waveform (d) of  FIG. 3 ). The spike pulses are then fed to ramp generator  30  which provides a ramp waveform as shown in waveform (e) of  FIG. 3 . This ramp waveform is applied to the (−) (inverting) input of differential or operational amplifier  32 , which functions as a zero crossing detector responsive to the combination of the ramp waveform and a control signal applied to the positive (non-inverting) input of amplifier  32 , as will be further explained. 
     The control signal is a function of phase difference between the current and voltage and a command signal. This phase difference, that is, the phase difference between the current and voltage applied to motor  14 , is detected each half cycle. Phase difference is detected by a selected combination of the outputs of square wave shapers  20 ,  22 ,  24 , and  26 . Thus, the outputs of shapers  20  and  24  are summed in summing device  34  to provide a signal output as shown in waveform (j) of  FIG. 3 , and the outputs of wave shapers  22  and  26  are summed in summing device  36  to provide an output as shown in waveform (k) of  FIG. 3 . As a result, each of the outputs of summing devices  34  and  36  provides superimposed pulses, pulses p 1  (waveform j) and p 2  (waveform k), which are of a duration representative of the phase difference between applied voltage to and current through the motor  14 . This occurs because, in effect, the turn-off of a summed voltage square wave established the rise point of those superimposed pulses, and the trailing edge of the current derived square wave produces the trailing edges of the superimposed pulses. 
     As will be noted in waveforms (j) and (k), a superimposed pulse occurs each half cycle of the AC input. In order to use them both, the outputs of summing devices  34  and  36  are fed to OR circuit  38  which presents both at its output. The output of the OR circuit  38  is fed through rectifier  40  preparatory to integration, and there is applied at junction  42  pulses p 1  and p 2  appearing each half cycle of signal voltage. They are of a constant height, but of variable width, width varying as a direct function of phase angle between current and voltage. Although pulses p 1  and p 2  are fed to summing junction  42 , they alone do not appear at this point as there are two other influential circuits connected to it. The first of these is integrator  44 . It functions to provide an averaged or integrated value for pulses p 1  and p 2 , which thus converts the pulse width modulated pulses to an amplitude modulated signal. 
     Integrator  44  comprises an operational amplifier  46  and capacitor  48  (e.g., 1 uF), the inverting input of operational amplifier  46  being connected to summing junction  42 , and capacitor  48  being connected between this point and the output of operational amplifier  46 . 
     A third circuit connected to summing junction  42  is command signal circuit  50  comprising potentiometer  52 , across which is connected a negative 15 volts source. This circuit is connected to summing junction  42  via summing resistor  54  and provides a bucking (opposite polarity) voltage to that provided by the amplitude of the detected phase difference signal. The (+) or non-inverting input of amplifier  46  is connected to a ground reference. 
     A command signal from circuit  50  is set with motor  14  unloaded and by adjustment of potentiometer  52 . In effect, the command signal sets a selected power factor (or phase angle between current and voltage) as determined by the greatest power factor (smallest motor current-voltage phase difference) at which the motor will operate over a range of loadings to be encountered. The command signal, which appears at the output of operational amplifier  46 , is a negative signal which shifts positively responsive to the presence of a higher than commanded power factor, and shifts negatively when there is detected a lower than commanded power factor. It is employed in a servo loop to vary the applied voltage and control the input power to motor  14 . In this way, motor  14  is forced to operate at the selected power factor. In a typical case, it might be found that without the present control, the motor would operate with a power factor of 0.75 when fully loaded and a power factor of 0.15 when unloaded, but that with the control system of this invention, it may be operated at a relatively constant power factor of 0.85 regardless of loading conditions. In such circumstances, this enables motors which are less than fully loaded to draw significantly reduced power. 
     The control signal from the output of operational amplifier  46  is coupled through resistor  55  and is applied to the positive (non-inverting) input of operational amplifier  32 , operating as a zero crossing detector. This control signal has the effect of varying the response of operational amplifier  32  to the ramp signal shown in waveform (e) and which is applied to the negative or inverting input of amplifier  32 . Thus, with a basically zero level of control signal, represented by reference r 1 , being in the top position, operational amplifier  32  would be essentially fully held positive by the ramp signal, resulting in a triggering output from amplifier  32  which stays on. As a typical value of negative control signal, represented by reference r 2 , the relative position of reference r 2  with respect to the ramp signal is such that operational amplifier  32  will be triggered on during the latter portion of each ramp signal, commencing with the intersecting of the ramp signal with reference line r 2 . This produces a positive pulse output from operational amplifier  32  as indicated in waveform (f) shown with approximately 50% “on” time. The output of operational amplifier  32  is applied through diode  56  and resistor  58  to the gate input of triggering or buffer triac  60 , and its output is connected to the trigger input of triac  16  which is in circuit with motor  14 . Accordingly, as shown, triac  16  is turned on each cycle for the pulse width of the positive pulse shown in waveform (f) for about 50% of the time of each half cycle of the AC input to motor  14 . This state would typically occur for a medium loading of motor  14 . If motor load should increase, this would be detected by a decreased phase angle; and in correcting this, the feedback system of the circuit would raise the control voltage and increase the turn-on time of triac  16 . If the motor load is shifted downward, the opposite would occur, and turn-on time of triac  16  would be reduced. The net result is that when motor  14  is less than fully loaded, it is driven by a substantially lower average voltage, and thereby draws substantially less power than were the Nola &#39;177 circuit  4  not employed. 
     As discussed above, the Nola &#39;177 circuit  4  may be powered by relatively inexpensive transformerless power supplies for bias power supply  13 . However, in such circumstances, relatively large capacitors are typically necessary, and this slows full voltage output and start-up time of the circuitry. This in turn may prevent a motor from having a sufficient starting voltage (average voltage through triac  16 ) initially applied to it for effective starting. To compensate for this, delay circuit  62  is shown which delays any power from being applied to motor  14  until operating biases are essentially at full operating levels. The circuit employs operational amplifier  64  and a +15 volts bias terminal  65  (from power supply  13 ) which is connected through 1 uF capacitor  66  to the inverting input of amplifier  64 . The output of amplifier  64  is connected through diode  68 , poled as shown, to the (+) input of operational amplifier  32 . 
     With the application of power across terminals  10  and  12  and to an input of power supply  13  providing bias potential to the circuit elements, the bias potential on terminal  65  will commence rising, and this change will appear through capacitor  66  and on the inverting terminal of operational amplifier  64 . This occurs as capacitor  66  is charged through resistor  69  (500K ohms); and during this occurrence, the rising positive potential on this input of amplifier  64  will produce an increasing negative signal at the output of amplifier  64 . Diode  68  will then couple this potential to the (+) (non-inverting) input of operational amplifier  32  which will swamp any control output from operational amplifier  46  and initially hold a significant negative (turn-off) potential on the input of amplifier  32 . This thus will prevent triac  60  from triggering triac  16 . In this manner, motor  14  is initially prevented from being turned on. This state of prevention will continue until operating biases of the circuit, including the +15 volts on terminal  65 , are essentially up to full potential. When this occurs, as evidenced by an essentially full +15 volts on terminal  65 , capacitor  66  will become fully charged, and the input applied to the negative input of operational amplifier  64  will drop. When this occurs, the +15 volts potential on terminal  65  will be effectively applied through 51K ohm resistor  70  and across 3.9K ohm resistor  72  to the (+) input of operational amplifier  64 , with the effect that amplifier  64  will be transitioned to provide a +15 volts output. This process is accelerated by positive feedback resistor  74  connected between the output and (+) input of amplifier  64 . With the output positive, diode  68  is blocked, and the normal operating output from amplifier  46  is applied to the (+) input of amplifier  32 . Since initially there is no current feedback from motor  14  (via resistor  18 ), the full command voltage would be effected to cause a full positive potential to be applied to amplifier  46  to the (+) terminal of amplifier  32  which then would cause a full turn-on potential to be applied to triac  60 , and thereby to triac  16 . Thus at this point, motor  14  would be enabled to quickly turn full on. 
     As discussed above, the delays provided by delay circuit  62  may be too short during times of high temperature stresses. In such circumstances, more time is needed for the pressures in the compressor portion of air conditioning/refrigeration systems to stabilize and for back pressures to have been eliminated. The maximum available alternating voltage should be supplied to the motor  14  for longer periods of time. One way of supplying the maximum available alternating voltage is to further prevent triac  60  from triggering triac  16  so as to prevent the motor  14  from being turned on for a longer period of time than is provided by the delay circuit  62 . This can be achieved by employing the energy saver delay circuit  6 . 
     The operation of the energy saver delay circuit  6  is herein described as follows. A DC power supply  80  derives a DC supply voltage and ground potential from the alternating current source. A separate DC power supply  80  is needed as opposed to deriving DC power from the existing bias supply  13  of the Nola &#39;177 circuit  4  because the bias supply  13  cannot supply an adequate amount of DC power to both the Nola &#39;177 circuit  4  and the energy saver delay circuit  6  simultaneously. The DC power supply  80  supplies power to the delay timer  82 , the comparator  84 , the relay driver  86 , and the relay  88 . The delay timer  82  applies at least a portion of the DC supply voltage to the comparator  84  for a predetermined amount of time that is greater than the maximum time needed for the pressures in the system operating under the control of the motor  14  to stabilize. This causes the comparator  84  to operate in a “low” state, which causes the relay driver  86  to output about 0 Volts, which supplies about 0 Volts to the normally-closed relay  88 . The relay  88  is then configured to be in a closed position, which shorts the leads, A and B, of the potentiometer  52  of the command signal circuit  50 . This a second potential to be applied to the summing junction  42  such that the command signal goes to about 0 Volts, which in turn prevents triac  60  from triggering triac  16 . In this manner, motor  14  is initially prevented from being turned on, thereby causing the motor  14  to receive the maximum available voltage from the AC current source. 
     When the predetermined amount of time has elapsed, the output of the delay timer  82  goes to low potential, which causes the comparator  84  to switch to a “high” state, which causes the relay driver  86  to supply about 15 Volts to energize the relay  88 . The relay  88  switches to the open position, thereby allowing the potentiometer  52  to return to its normal bias position, which causes the Nola &#39;177 circuit  4  to return to normal operation indefinitely. 
     In most air conditioning and refrigeration systems, AC power is cycled on and off periodically. Therefore the energy saver delay circuit  6  can operate whenever power is resupplied to the AC induction motor. The time delay of the delay timer  82  should be greater than both the delay time introduced by the delay circuit  62  and the worst case stabilization time of refrigeration and air conditioning compressors. In a preferred embodiment, the delay time of the delay timer  82  is set to about 20 seconds, but in other embodiments can range between about 0 seconds to about 60 seconds. 
     A more detailed schematic of a preferred embodiment of the Nola &#39;177 circuit  4  and the energy saver delay circuit  6  is depicted in  FIG. 2 . Focusing on the implementation of the energy saver delay circuit  6 , the DC power supply  80  includes  100  ohm current limiting resistor  90  and 0.68 uF AC coupling capacitor  92 , which feeds AC current/voltage from the alternating current source. This AC voltage is rectified by bridge diodes  94 ,  96 , and filtered by a 20V 100 uF capacitor  98 . The voltage across capacitor  98  is further limited by 15V zener diode  100  to provide a +15 volt DC voltage to the remainder of the energy saver delay circuit  6 . 
     The +15 V supply voltage is applied to the delay timer  82 . The delay timer  82  includes a voltage divider comprising a 175K current limiting resistors  102  and a  100 K resistor  104 . A 100 uF polarized timing capacitor  106  is coupled in parallel with the resistor  104 . One node of each of resistors  102  and  104  and capacitor  106  is electrically coupled to the gate of an N-channel depletion mode MOSFET  108 , which is configured as a switch. A diode  110  is coupled across the drain and source leads of the MOSFET  108 , with the source of the MOSFET  108  connected to ground potential. The drain of the MOSFET  108  is coupled to the base of an NPN transistor  112 , which is configured as an emitter follower. A resistor  114  is coupled between the base of the NPN transistor  112  and the gate of the MOSFET  108 , while a resistor  116  is coupled between the collector of the NPN transistor  112  and the gate of the MOSFET  108 . The emitter of the NPN transistor  112  is coupled to one terminal of a 1M resistor  118 . 
     The emitter of the NPN transistor  112  is connected to the non-inverting (+) input of an operational amplifier  120 , while the inverting input (−) of the operational amplifier  120  is coupled to the +15 V DC supply via a 56K resistor  122 . The operational amplifier  120  is configured to operate as both the comparator  84  and the relay driver  86  of  FIG. 1 . The output of the operational amplifier  120  is coupled to the anode of a diode  124 . The cathode of the diode  124  is coupled to a 0.68 uF capacitor  126  and to the input coil of the relay  88 . The diode  124  blocks back EMF current from the input coil of the relay  88  from flowing into the output of the operational amplifier  120 . The other side of the input coil of the relay  88  is connected to ground potential. The output contacts of the relay  88  are connected to points A and B across the potentiometer  52  of the circuit  50  of the Nola &#39;177 circuit  4 . 
     In operation, when power is supplied by DC power supply  80  to the delay timer  82  as a result of the alternating current source being activated by the air conditioning/refrigeration system, the voltage across the timing capacitor  106  is initially about 0 volts. As a result, the N-channel MOSFET  108  is initially turned off. The NPN transistor  112  is turned on so that the voltage across the 1 Megaohm resistor is set to about +15 Volts. The operational amplifier is configured to be in a a “low” state so as to output about 0 Volts to the relay  88 , which, in turn, operates in the closed state, thereby shorting points A and B connected to the potentiometer  52 . 
     The timing capacitor  106  begins charging with a time constant equal to the value of the capacitance times the value of the parallel combination of resistors  102 ,  104 . When the capacitor  106  is nearly charged, the MOSFET  108  switches “on” so as to “turn off” the NPN transistor  112  so that the non-inverting input of the operational amplifier  120  is set to about 0 Volts. As a result, the operational amplifier switches to a “high” state such that its output supplies about +15 Volts to the relay  88 , thereby energizing it. The output contacts of the relay  88  open so as to allow the potentiometer and therefore the Nola &#39;177 circuit  4  to return to normal operation. This process repeats as power to the motor  14  cycles off and on during operation of the refrigerator/air conditioner. 
     It is to be understood that the exemplary embodiments are merely illustrative of the invention and that many variations of the above-described embodiments may be devised by one skilled in the art without departing from the scope of the invention. It is therefore intended that all such variations be included within the scope of the following claims and their equivalents.