Abstract:
A motor start switch includes a temperature responsive resistor element and a bimetal cutout switch electrically connected in series with a start winding of an electric motor. The cutout switch is operable between an open position disconnecting the temperature responsive resistor from the start winding, and a closed position electrically connecting the temperature responsive resistor to the start winding. The cutout switch transitions from the open position to the closed position due to current flowing through, and heating, a bimetal element in the cutout switch.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to electric motors and, more particularly, to a start winding cutout switch for a refrigerator compressor motor. 
     Electric motors typically include a start winding, a run winding and a magnetized rotor. The start winding is used to initiate rotation of the rotor. The run winding has a high inductive reactance relative to the start winding, so that the magnetic fields generated in the respective windings are out of phase with one another. The geometric time phase relationship between the magnetic fields causes the rotor to begin to rotate from a standstill condition when the windings are energized. Once the rotor has sufficient torque to attain its normal operating speed, the start winding is “cut-out” of the motor circuit so that the magnetic field generated by the start winding does not adversely affect motor operation. Alternatively, the start winding may be utilized as an auxiliary run winding after motor start-up by connecting a run capacitor in series with the start winding. Often, utilizing an auxiliary run winding results in better motor efficiency and power factor. 
     Low power current relays have been used to switch a start winding out of a motor circuit. However, the relays contacts are often short lived and susceptible to sticking together when switching the current, which would continuously energize the motor and cause burnout. 
     A positive temperature coefficient resistor (PTCR) may be used in lieu of a relay to regulate the current flowing through the motor start winding. A PTCR is a temperature responsive resistor element that has a low resistance in a cool state, and a very high resistance when heated to an “anomaly temperature” or “Curie Temperature.” When a PTCR is connected in series with a start winding, the low initial resistance in the cool state allows the start winding to draw a relatively large current to accomplish initial motor rotation. As current flows through the PTCR, the current heats the PTCR, ultimately causing the PTCR to reach the Curie Temperature and the corresponding very high resistance state. Consequently, very little current flows into the start winding. Thus, the PTCR restricts or “chokes off” the current to the start winding to negligible levels. By selecting a PTCR so that the Curie Temperature is reached at approximately the same time when the motor running speed is achieved, a PTCR effectively regulates current flow into the start winding more reliably than a current relay. 
     A PTCR, however, consumes 2-3 watts of power to maintain the high resistance state at the Curie Temperature. In light of stringent energy consumption standards, PTCR energy consumption is a factor in the efficiency rating of a compressor motor. Therefore, energy savings could be realized, and efficiency ratings increased, by cutting the PTCR out of a circuit. While relay switches have been used in series with a PTCR to switch the PTCR out of the circuit, relay switches require power to keep the switch open, which affects the efficiency rating of the motor. Also, relay switches suffer from reliability problems with the switching contacts. 
     Accordingly, it would be desirable to provide a reliable cutout switch to remove a PTCR from a motor circuit. Further, it would be desirable to provide a cutout switch which does not consume power to keep the switch open. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the invention, a motor start switch includes a temperature responsive resistor element and a cutout switch in series with the start winding of a motor. The temperature responsive resistor element is a positive temperature coefficient resistor (“PTCR”). The cutout switch is operable between an open position disconnecting the PTCR from the start winding, and a closed position electrically connecting the PTCR to the start winding. 
     When electrical power is supplied to the motor, current flows through the run winding to energize the run winding, and the closed cutout switch allows current to flow through the start winding and the PTCR through the cutout switch. The PTCR is cool and has a low resistance, which allows large startup currents to flow through the PTCR and into the start winding to accomplish initial rotor rotation. 
     As current flows through the PTCR and the cutout switch, both the PTCR and the cutout switch are heated by the current flowing through them. As the PTCR heats, its resistance increases, and less current flows through the start winding. When the PTCR reaches its Curie Temperature, its resistance is high enough that the current running through the start winding is negligible. At approximately the same time the PTCR Curie Temperature is reached, a bimetal element in the cutout switch reaches a temperature which causes the bimetal element to deflect and break electrical contact with the PTCR. Current continues to flow through the bimetal element in the deflected position, generating heat in the bimetal element and keeping the cutout switch open. Because no current flows through the PTCR when the cutout switch is open, the PTCR consumes no power and begins to cool. As the PTCR cools, it returns to the low resistance state. 
     When electric power to the motor is switched off, the bimetal element in the cutout switch cools and resets to a closed position in electrical contact with the PTCR. When electrical power is returned to the motor, the closed cutout switch allows current to flow to the PTCR in the low resistance state, which allows large startup current to flow through the start winding. The current heats the PTCR and the switch until the PTCR reaches the Curie Temperature and the cutout switch opens. 
     Thus, a reliable motor start switch disconnects the PTCR from the start winding and eliminates power consumption by the PTCR. In contrast to a relay switch, the bimetal cutout switch does not require external power to open the switch and disconnect the PTCR from the motor circuit due to its mechanical nature. In addition, current flowing through the cutout switch bimetal element generates heat to maintain the switch in the open position, so external electrical or mechanical elements are not required to keep the switch open. While the cutout switch bimetal element dissipates power as heat generated from current flowing through the bimetal element when the cutout switch is open, the power consumed by the bimetal element is a small fraction of the power consumption of the PTCR. Thus, an increased percentage of electrical power supplied to the motor is dissipated in the run winding and the start winding, and the efficiency rating of the motor is increased. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit schematic of a refrigerator compressor circuit including a start switch in a closed position; and 
     FIG. 2 is a circuit schematic of the compressor circuit shown in FIG. 1 with the start switch in an open position. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a circuit schematic of a refrigerator compressor  10  including a motor control  12 , a motor  14 , and a motor start switch  16 . Motor control  12  is electrically connected in series with motor  14  and includes a thermostat  18  and an overload protector  20 . Thermostat  18  connects compressor  10  to an electrical power source (not shown) and cycles motor  14  from an “ON” state to an “OFF” state upon occurrences of selected conditions. 
     Overload protector  20  includes a heater element  22  in thermal contact with a bimetal element  24 . When motor  14  is “ON,” current flows through resistance heater element  22  and bimetal element  24  to motor  14 . In instances of excessive, or prolonged, current flowing through heater element  22  to motor  14 , or upon other specified fault conditions, heater element  22  reaches a predetermined temperature that causes bimetal element  24  to open and break electrical contact with motor  14 . Overload protector  20  therefore protects motor  14  from overheating and burnout. The predetermined temperature which causes bimetal element  24  to open is selected to keep the motor winding temperature to less than a maximum temperature specified by the compressor motor manufacturer. 
     Motor  14  includes a run winding  26 , a start winding  28 , and a run capacitor  30  electrically connected to one another. Run capacitor  30  is electrically connected in series with start winding  28  so that start winding  28  remains in the motor circuit as an auxiliary run winding. In an alternative embodiment, motor  14  includes one or more auxiliary windings electrically connected in series with start winding  28 . 
     Start switch  16  includes a cutout switch  32  and a temperature responsive resistor  34  electrically connected in series with start winding  28  and in parallel with run capacitor  30 . Cutout switch  32  includes a stationary contact  36 , a movable contact  38 , and a bimetal element  40  connected to moveable contact  38 . Cutout switch  32  has an open position and a closed position. In the closed position, stationary contact  36  and movable contact  38  form an electrical connection through cutout switch  32 . In the open position, stationary contact  36  and movable contact  38  are separated from one another which prevents an electrical connection, i.e., prevents current flow, through cutout switch  32 . 
     Bimetal element  40  includes two metallic strips (not shown) connected to one another. Each metallic strip has a different coefficient of thermal expansion so that each strip expands at a different rate. As current flows through bimetal element  40  heat is generated within bimetal element  40 , causing the metallic strips to expand. However, since the metallic strips expand at different rates, bimetal element  40  bends or curls as the metallic strips are heated. Therefore, at a predetermined temperature, i.e., the transition temperature, bimetal element  40  curls or deflects, causing moveable contact  38  to separate from stationary contact  36  and to open or break the electrical circuit through cutout switch  32 . Bimetal element  40  may be a snap action or creep type bimetal element. Unlike relay and other electrical switches, which require external electrical power to open and close the contacts, bimetal element  40  mechanically opens and closes cutout switch  32 . Therefore, in contrast to a relay switch, additional power in not required to open or close the switch. 
     Temperature responsive resistor  34  is a positive temperature coefficient resistor (PTCR) having a low resistance when in a cool state and a high resistance when in a heated state. An exemplary temperature responsive resistor  34  is a disk type PTCR, such as a PTCR disk available from CeraMite, a company located in Grafton, Wis. PTCR  34  is selected to be the minimum size which satisfies resistance/current/voltage conditions to optimize the cooling rate of PTCR  34 . PTCR is separated from, or external to, start winding  28 . In an alternative embodiment, PTCR  34  is internal to, or part of, start winding  28 . 
     Exemplary ratings of PTCR  34  are set forth below: 
     Cold Resistance, 5 ohms to 15 ohms, 180 vac, 12 amp 
     Maximum Curie Temperature 125 C. 
     5.5 ohms resistance at 25 C. 
     Life: 50,000 starts (minimum). 
     PTCR  34  which satisfies the above ratings effectively disconnects, or takes out, start winding  28  in less than 0.75-1.0 seconds at 8.0 amps during the motor startup for a cold start, and resets within 80 seconds in the event that power is cut-off. 
     Cutout switch bimetal element  40  is selected to reach its predetermined transition temperature so that bimetal element  40  opens and breaks the circuit at approximately the same time as PTCR  34  reaches the Curie Temperature. Suitable bimetal-type switches for this application are commercially available from Otter Controls, Limited, Hardwick Square South, Buxton, Derbyshire, SK17, 6LA, England. The transition temperature of bimetal element  40  is selected based on motor current and application conditions so that bimetal element  40  deflects after PTCR  34  reaches the Curie temperature but before overload protector  20  breaks electrical power to motor  14 . 
     When electrical power is initially delivered to motor  14  through thermostat  18  and overload protector  20 , cutout switch  32  is closed so that an electrical circuit is completed through stationary contact  36  and moveable contact  38 . Also, PTCR  34  is well below the Curie Temperature and in the state of low resistance, so that when power is delivered to motor  14 , relatively large startup currents flow through start winding  28  to generate the start winding magnetic field, and thus the desired torque, which causes the rotor (not shown) to begin rotating from a standstill condition. Upon startup, both run winding  26  and start winding  28  are energized, and the resistance of PTCR is sufficiently low so that run capacitor  30  is substantially electrically disassociated from run winding  26  and start winding  28 . 
     The current flowing through cutout switch  32  causes bimetal element  40  and PTCR  34  to heat up. As PTCR  34  heats up, less current flows through PTCR  34  and start winding  28  and the magnetic field generated in start winding  28  is accordingly reduced. As current continues to flow through cutout switch  32  and PTCR  34 , the current continues to heat bimetal element  40  and PTCR  34 . Eventually, PTCR  34  will reach a steady state of high resistance at the Curie Temperature that prevents any appreciable current from flowing into start winding  28  and substantially electrically disassociates PTCR  34  from start winding  28 . When PTCR  34  is substantially disassociated with start winding  28 , run capacitor is substantially associated with start winding  28 . Under such conditions, start winding  28  functions as an auxiliary main winding. 
     The PTCR  34  steady state is reached at approximately the same time motor  14  is brought up to speed. Subsequently, bimetal element  40  reaches its transition temperature so that bimetal element  40  deflects and moves movable contact  38  away from stationary contact  36  and opens the circuit through cutout switch  32 . 
     FIG. 2 is a schematic of compressor  10  with cutout switch  32  in an open position. Power is supplied to motor  14  through thermostat  18  and overload protector  20 . Thus, run winding  26  is energized and maintains rotor rotation. Run capacitor  30  remains in the motor circuit and is electrically connected to run winding  26  and start winding  28 . Start winding  28  therefore functions as an auxiliary winding. 
     Current continues to flow through run winding  26  and bimetal element  40 , and consequently heat is generated in bimetal element  40  to keep bimetal element  40  in its deflected position where moveable contact  38  and stationary contact  36  are separated and electrical contact through cutout switch  32  is broken. PTCR  34  is thus disconnected from the circuit and power consumption by PTCR  34  is therefore avoided. While bimetal element  40  dissipates electrical power as heat when cutout switch  32  is open, power dissipation of cutout switch  32  is a small fraction of the power consumption of PTCR  34 , and is generally on the order of minor power losses occurring in wires and electrical connectors. Therefore, as run capacitor  30  generally does not dissipate power, an increased percentage of electrical power delivered to motor  14  through thermostat  18  and overload protector  20  is dissipated in run winding  26  and start winding  28 . When cutout switch  32  is in the open position electrical power to motor  14  is dissipated only in run winding  26 , start winding  28 , and bimetal element  40 . Therefore, the energy efficiency rating of compressor  10  is increased. 
     Once cutout switch  32  is opened, PTCR  34  begins to cool and return to its initial state of low resistance. After power is disconnected to motor  14  via thermostat  18  or overload protector  20 , current ceases to flow through bimetal element  40 . Bimetal element  40  therefore begins to cool and reset to its initial closed position where an electrical connection is established through stationary contact  36  and movable contact  38 . Start switch  16  is then ready for motor restart. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.