Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1999-07-01
2001-05-29
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C318S786000, C318S788000, C318S753000, C318S792000
Reexamination Certificate
active
06239523
ABSTRACT:
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.
REFERENCES:
patent: 5952811 (1999-09-01), Hamatani
Fogel Larry C.
Janicek Alan Joseph
Armstrong Teasdale LLP
General Electric Company
Horton Esq. Carl B.
Jones Judson H.
Ramirez Nestor
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