TECHNICAL FIELD 
     This application relates generally to permanent split capacitor (PSC) electrical motors, and, in particular, to circuits for inserting a start capacitor into the power circuit for such motors at the initial powering of the motor. 
     BACKGROUND 
     Single-phase permanent split capacitor (PSC) motors are typically used in domestic appliances and air conditioner compressors. These motors have two windings usually denoted as a phase shift winding and a run winding. The phase shift winding has two capacitors, which are arranged in parallel so their capacitances add together, connected to it. One of these capacitors remains connected to the phase shift winding throughout operation of the motor, while the other capacitor is removed from the circuit once the motor reaches its operational speed. These capacitors are known as run and phase shift capacitors, respectively. The phase shift capacitor adds phase shift of the current through the run winding at low speed to increase the starting torque of the rotating field produced by the run winding to enable the rotor to commence rotation. The increased capacitance that helps generate the starting torque, however, does not optimize the performance of the motor once operational speed is reached. Therefore, the phase shift capacitor is removed from the circuit when the operational speed of the motor is reached. 
     The selective coupling of the phase shift capacitor into and out of the circuit supplying power to the motor windings is performed with different types of components. In some PSC motors, a mechanical switch is mounted to the output shaft of the motor with springs that bias the switch to a closed position, which couples the phase shift capacitor to the power circuit for the motor. As the output shaft approaches the operational speed of the motor, centripetal force acting on the springs stretches the springs and opens the switch, which decouples the phase shift capacitor from the circuit. As long as the output shaft rotates at a speed near the operational speed, the switch remains open and the capacitor remains out of the power circuit for the motor. Other PSC motors use relays to couple the phase shift capacitor selectively to the power circuit and still other PSC motors use sophisticated controllers that monitor the power circuit and remove the phase shift capacitor in response to predetermined conditions being detected in the circuit. 
     All of the previously known circuits for selectively decoupling a phase shift capacitor from the power circuit are relatively expensive. The mechanical switch is an additional component mounted to the output shaft, which not only is expensive to produce, but it also contributes to motor failure as the springs and the switch age and deteriorate. The relays are also costly items as are the microprocessors and other solid state control devices used to control the coupling and decoupling of the phase shift capacitor to the motor circuit. Retail prices for domestic appliances and compressors are closely related to the production costs for these devices. Reducing production costs in the motors used to run these devices would be beneficial. 
     SUMMARY 
     A method of coupling and decoupling a phase shift capacitor into the power circuit for a PSC motor predominantly uses passive components for that purpose. The method includes connecting a line voltage of a single phase power supply across a first winding of the PSC motor and to a second winding of the PSC motor, providing a first capacitor and a second capacitor in parallel to one another at a common node electrically connected to the second winding of the PSC motor and also electrically connecting the first capacitor to a neutral line of the single phase power supply, providing a triac with a first anode connected to the second capacitor and a second anode connected to electrical ground, generating a signal indicative of an expiration of a predetermined time period commencing at the connection of the line voltage with a timing circuit having a plurality of passive electrical components and a single active comparator, the timing circuit having an input electrically connected to the line voltage, and operating the triac with a switching circuit to electrically connect the second capacitor to the second winding during the predetermined time period and to disconnect electrically the second capacitor from the second winding in response to the signal generated by the timing circuit, the switching circuit having a plurality of passive electrical components and a single active switch, an input of the switching circuit being electrically connected to an output of the timing circuit and an output of the switching circuit being electrically connected to a gate of the triac. 
     A PSC motor couples and decouples a phase shift capacitor into the power circuit with a circuit primarily made with passive components. The PSC motor includes a connector configured to couple electrical a line voltage of a single phase power supply across a first winding of the PSC motor and to a second winding of the PSC motor, a first capacitor and a second capacitor connected to one another in parallel and at a common node to the second winding of the PSC motor, the first capacitor also being electrically connected to a neutral line of the single phase power supply, a triac having a first anode connected to the second capacitor and a second anode connected to electrical ground, a timing circuit having a plurality of passive electrical components and a single active comparator, the timing circuit having an input electrically connected to the line voltage and the timing circuit being configured to generate a signal indicative of an expiration of a predetermined time period from application of the line voltage to the connector, and a switching circuit having a plurality of passive electrical components and a single active switch, an input of the switching circuit being electrically connected to an output of the timing circuit and an output of the switching circuit being electrically connected to a gate of the triac, a signal on the output of the switching circuit operating the triac to electrically connect the second capacitor to the second winding during the predetermined time period and to disconnect electrically the second capacitor from the second winding in response to the signal generated by the timing circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The foregoing aspects and other features of a PSC motor that is configured to control the coupling and decoupling of a phase shift capacitor into a power circuit for the motor are described in connection with the accompanying drawings. 
         FIG. 1  is a block diagram of a circuit for selectively coupling a phase shift capacitor to a phase shift winding in a PSC motor. 
         FIG. 2  is an electrical schematic diagram of a circuit that implements the timer of  FIG. 1 . 
         FIG. 3  is an electrical schematic diagram of a circuit that implements the switching circuit of  FIG. 1 . 
         FIG. 4  is a flow diagram of a process for selectively coupling a phase shift capacitor to a phase shift winding in a PSC motor. 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the environment for the system and method disclosed herein as well as the details for the system and method, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements. 
       FIG. 1  is a block diagram of an electrical circuit that electrically connects a capacitor to the phase shift winding of a PSC motor for a predetermined period of time to increase the starting torque and then electrically disconnects the capacitor until the line voltage to the motor is removed. The circuit  100  is connected by a switch (not shown) to a single phase power supply, such as a 120V, 60 Hz source (not shown), although other power sources can be used. In the figure, L 1  represents the line voltage and N represents the neutral line, although the motor operates in the same manner as described below if these lines are reversed. A thermal overload protector  104  and a metal oxide varistor  108  are provided as shown to protect the motor in the event that the motor heats to a temperature that could possibly damage components and to remove transients on the line voltage that could damage the electronic components of the circuit, respectively. 
     The remainder of the circuit of  FIG. 1  includes a main motor winding MW, a phase shift motor winding SW, a run capacitor  112 , a start capacitor  116 , an inductor  120 , a resistor R 1    124 , a triac  128 , a timer  132 , a switching circuit  136 , and a resistor R 2    140 . The thermal load protector  104 , metal oxide varistor  108 , timer  132 , and the motor windings MW and SW are electrically connected at node N 1 . Run capacitor  112  is serially connected to one end of the phase shift winding SW and to the neutral N. Start capacitor  116  is electrically connected in parallel to the run capacitor  112  so their capacitances add when the start capacitor  116  is electrically connected to electrical ground through inductor  120 , resistor  124 , and triac  128 , when the triac  128  is in its “on” state. The operation of triac  128  is controlled by timer  132  and switching circuit  136 . In brief, timer  132  detects electrical power being applied to the motor at node N 1  at start-up and enables the switching circuit  136  to generate a signal on the gate of triac  128  that electrically connects start capacitor  136  to electrical ground. This operation enables the capacitance of the start capacitor  116  to be added to the capacitance of the run capacitor  112  to add phase shift to the current in the windings and increase the starting torque. The inductor  120  and the resistor  124  limit the maximum current through the triac  128 . When the predetermined time period expires, the timer  132  changes the state of the signal to the switching circuit  136  and the signal to the gate of the triac  128  no longer enables the electrical connection of the start capacitor  116  to electrical ground. The connection from one end of the start capacitor  116  to the switching circuit keeps the voltage on the start capacitor  116  from retriggering the triac  128  once the start capacitor has been removed from the circuit. The resistor R 2    140  provides a path to ground for electrical noise and helps immunize the triac from being triggered by such noise. 
       FIG. 2  is an electrical diagram of a circuit  200  that can be used to implement the timer  132 . Diode  202  and resistor  204  electrically connect capacitor  208  to the line voltage at node N 1  to rectify the voltage and charge the capacitor. The resistor  204  is depicted as a series connection of three resistors, although other configurations of resistors can be used. These resistors are arranged as shown in  FIG. 2  to prevent the voltage limits for the individual resistors from being exceeded. The voltage on the capacitor is provided to the inverting input of the operational amplifier  212 . A voltage V 1  is derived from the line voltage by rectifying the voltage and reducing it to an appropriate level for operating the operational amplifier  212  and the transistors of the switching circuit  136  shown in  FIG. 3 . In one embodiment, this voltage V 1  is 4.7V, although other voltages can be used depending upon the components to be operated by the voltage. The voltage V 1  is dropped across the voltage divider formed by resistors R 6    216  and R 7    220  to provide a reference signal for the non-inverting input of the operational amplifier  212 . The anode of schottky diode  224  is electrically connected to the voltage on capacitor  208  and the cathode of schottky diode  224  is electrically connected to the node of the resistor divider formed by resistor  216  and resistor  220 . This schottky diode operates to keep the voltage on the inverting input of the amplifier  212  from exceeding the voltage limit for the input and to allow capacitor  208  to discharge when electrical power is removed from circuit  100  so the capacitor  208  is reset to a voltage close to zero volts. When the voltage on the capacitor  208  builds to a level at a time corresponding to the time constant of resistor  204  and capacitor  208  that exceeds the reference signal on the non-inverting input of operational amplifier  212 , the output of the operational amplifier  212  goes from a logical “1” to a logical “0.” The resistor R 8    228  adds hysteresis to keep the output of the operational amplifier, which is operating as a comparator, to the logical zero state. The change in the output of the amplifier  212  to a zero state causes the switching circuit  136  to disconnect the start capacitor from the electrical circuit of  FIG. 1  as explained below. As used in this document, the term “active comparator” refers to an operational amplifier or other active electrical component that is configured to compare a signal at one input of the active electrical component to a reference signal and to generate a signal indicative of whether the signal at the one input is equal to or greater than the reference signal. 
       FIG. 3  is an electrical diagram of a circuit  300  that can be used to implement the switching circuit  136 . The output of the operational amplifier  212  from the timer  132  is provided to a first circuit  304  having an output electrically connected to the node between a resistor  312  and a capacitor  316  electrically connected to one end of the start capacitor  116  and a second circuit  308  having an output electrically connected to the node between the resistor  312  and the capacitor  316 . Again, the resistor  312  is shown as a series of resistors for reasons similar to those stated above with regard to resistor  204 . When the operational amplifier  212  is a logical zero, the first circuit  304  is configured to shunt charge on the capacitor  316  to electrical ground during a negative half cycle of the voltage at the junction of the phase shift capacitor  116  and inductor  120 , and the second circuit  308  is configured to shunt charge on the capacitor  316  to electrical ground during a positive half cycle of the voltage at the junction of the capacitor  116  and the inductor  120 . Capacitor  316  is kept below the trigger voltage for the diac  348  so the diac  348  does not activate the triac  128 . 
     In more detail, the output of the amplifier  212  is electrically connected by the voltage divider formed by resistor  320  and resistor  324  to the base of NPN transistor  328 . The collector of transistor  328  is electrically connected to voltage V 1  through resistor  330  and the emitter is electrically connected to electrical ground. The voltage V 1  through resistor  330  and the collector of transistor  328  are electrically connected to the base of NPN transistor  332  and a resistor  336  is electrically connected between the base of the transistor  332  and electrical ground. The emitter of transistor  332  is connected to electrical ground and the collector of transistor  332  is electrically connected to the cathode of diode  340 . The anode of diode  340  is electrically connected to resistor  312  through resistor  344 . This configuration enables the logical one output of the timer  132  while the start capacitor  116  is electrically connected to the circuit  100  to activate transistor  328  and pull the V 1  voltage to electrical ground through the transistor  328 . This operation disables transistor  332  so the positive cycle of the voltage from capacitor  116  enables the voltage across capacitor  316  to trigger diac  348  to the gate of triac  128  and connect capacitor  116  to node N through inductor  120 , resistor  121 , and triac  128 . When the output of the timer  132  goes to a logic low, the transistor  328  is turned off and the voltage V 1  through resistor  330  forward biases the base-emitter leg of transistor  332  so transistor  332  conducts the positive cycle of voltage from the capacitor  116  through resistor  344  and diode  340  to electrical ground so capacitor  316  does not charge to a voltage that exceeds the trigger voltage for the diac  348 . This operation prevents the voltage on capacitor  316  from activating the diac  348  and the gate of the triac  128 . 
     With regard to circuit  308  in  FIG. 3 , the output of the amplifier  212  is electrically connected through resistor  352  to the base of PNP transistor  356 . The emitter of transistor  356  is electrically connected to voltage V 1  and to the base of transistor  356  through resistor  360 . The collector of transistor  356  is electrically connected to the anode of diode  364  and the cathode of diode  364  is electrically connected to the node between the resistor  312  and the capacitor  316  through resistor  368 . This configuration enables the logical one output of the timer  132  to disable transistor  356  so the negative cycle of the voltage from capacitor  116  enables the voltage across capacitor  316  to decrease to the negative trigger voltage for the diac  348  so the diac  348  turns on the gate of triac  128  and connects capacitor  116  to node N through inductor  120 , resistor  121 , and triac  128 . When the output of the timer  132  goes to a logic low, the voltage at the base of the transistor  356  forward biases the emitter-base leg of the transistor  356  and the voltage V 1  passes through the collector of the transistor  356 , diode  364 , and resistor  368  so the transistor  356  conducts during the negative half cycle of the voltage at capacitor  316 . Thus, the capacitor  316  is electrically connected to the voltage V 1  while the transistor  356  conducts so capacitor  316  does not charge to a negative voltage that exceeds the negative trigger voltage for the diac  348 . Thus, the negative cycle of the line voltage does not result in the activation of the triac  128 . 
       FIG. 4  depicts a process  400  for selectively coupling a phase shift capacitor to a phase shift winding in a PSC motor. For purposes of illustration, process  400  is described in conjunction with the embodiment of the PSC motor circuit  100 , but alternative embodiments are also suitable for use with process  400 . In the process, the application of a line voltage of a single phase power supply to the main winding and the phase shift winding of the PSC motor is detected (block  404 ). One end of the start capacitor serially connected to one end of the phase shift winding is electrically connected to electrical ground because the output of timer circuit  132  remains a logical low during a predetermined time period, which commences at the application of electrical power. During this predetermined time period, the switching circuit  136  enables capacitor  316  to charge to a voltage that triggers diac  348  and turns on triac  128  to electrically connect the phase shift capacitor to the phase shift winding (block  408 ). The predetermined time period corresponds to the time constant of the resistor  204  and the capacitor  208 . When the voltage on capacitor  208  exceeds the reference voltage on comparator  212 , the timer circuit output generates a signal indicative of the time period expiring (block  412 ) and, upon expiration of the time period, the signal from the timer circuit  132  causes the switching circuit  136  to deactivate the diac and, consequently the triac  128  to disconnect the phase shift capacitor from electrical ground and therefore from the phase shift winding (block  416 ). Thereafter, the voltage at the anode of the diac  128  connected to the node between the resistor  312  and the capacitor  316  is held at a level that prevents the diac  348  from triggering (block  420 ). In the circuits described above, this holding of the voltage at the diac  128  is achieved during positive cycles of the voltage at the phase capacitor  116  by shunting the voltage across capacitor  116  to ground through transistor  332  in the switching circuit  136  and, during the negative portions of the voltage cycle, electrically connecting that voltage to the voltage V 1  through transistor  356  in the switching circuit  136 . This phase shift capacitor  116  remains disconnected from the phase shift winding and the voltage on the diac stays below the trigger voltage (blocks  416 ,  420 ) until power to the motor is terminated (block  424 ) and the process waits for the next powering of the motor (block  404 ). 
     Those skilled in the art will recognize that numerous modifications can be made to the specific implementations described above. Therefore, the following claims are not to be limited to the specific embodiments illustrated and described above. The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.
A power circuit is configured with mostly passive electrical components to connect a phase shift capacitor to a phase shift winding of a PSC motor selectively. The power circuit includes a timing circuit, a switching circuit, and a triac having a first anode connected to the second capacitor and a second anode connected to electrical ground. The timing circuit has a plurality of passive electrical components and a single active comparator configured to generate a signal indicative of an expiration of a predetermined time period after application of a line voltage to the motor. The switching circuit has a plurality of passive electrical components and a single active switch, which generates a signal to operate the triac to electrically connect the second capacitor to the second winding during the predetermined time period and to disconnect electrically the second capacitor from the second winding in response to a signal generated by the timing circuit indicating the predetermined time period has expired.