1. Field of the Invention.
The present invention relates to control circuits for refrigeration systems such as air conditioners, heat pumps, and refrigerators. More specifically, the field of the invention relates to such control circuits for such systems having a single phase compressor.
2. Description of the Related Art.
Motor control and protection devices for electric motors generally include electro-mechanical or solid state electronic devices for activating and deactivating the electric motor based on various operating conditions and the loading on the electric motor. Generally, the motor protection devices sense the current in the motor and interrupt current to the motor when the current exceeds a predetermined threshold.
A method of interrupting the current in the event of an over current condition uses a snap disc placed in series with the windings of the compressor motor. The snap disc is composed of bimetallic layers which are in physical contact with the contact points which close the circuit. Typically, a resistive heating element which heats the bimetallic layers is connected in series with the contact point such that when the heat generated by current passing through the resistive element exceeds a predetermined threshold, the different metals of the bimetallic layer expand at different rates, causing the disc to bend and break the connection to the contact points. Another arrangement involves placing the snap disc device in close proximity to the motor so that the snap disc device may open and close in response to the temperature of the compressor motor.
However, the snap disc arrangement includes a number of disadvantages including: long actuation delay from the time the over current condition occurs and the time the snap disc interrupts the current, imprecise set points, non-dynamic response, inability to account for different environments or motor load conditions, and long restoration time until the snap disc returns to the original condition and re-establishes electrical contact.
Other methods of controlling current to an electric motor include using solid state devices which permit or interrupt current from an AC power source to the motor. Control circuits using such devices have the advantage of precision, reliability and self-regulation. Along with such arrangements, it is advantageous to inductively couple the control circuit with the power output stage. The power output stage regulates the application of power to the motor. Such inductive coupling provides the benefit of electrical isolation of the control logic circuit and the power output circuit thereby reducing the effect of noise in the system. One method of inductive coupling the control circuit with the power output stage, which is applicable to a single phase application, is disclosed in U.S. Pat. No. 5,463,874, entitled "Inductively Activated Control and Protection Circuit for Refrigeration Systems," issued Nov. 7, 1995, which is assigned to the assignee of the present invention, the disclosures of which are explicitly incorporated by reference. However, this inductively coupled control circuit cited above does not contain a current sensing circuit and so does not provide protection against the effect of lack of air flow over the condenser except by the sole means of a thermistor mounted on the compressor housing. A limitation of only using thermistor based protection is that the thermistor may become detached from the compressor housing.
Another inductively coupled control circuit, disclosed in copending U.S. Pat. No. 5,784,232, entitled "Multiple Winding Sensing Control and Protection Circuit for Electric Motors," which is assigned to the assignee of the present invention, the disclosures of which are explicitly incorporated by reference. This circuit provides a current detector circuit having a plurality of current detectors, each current detector associated with one of the motor windings and having a capacitor which provides a voltage level corresponding to the current in the respective winding. The output of the current detectors are combined to provide a combined current output to provide a combined current level signal. The combined current level signal is compared with a current threshold level to provide over current protection. The output of the current detectors are also connected to a single phase detector circuit and light load detector circuit which provide protection against single phase operation and light loading conditions, respectively. This circuitry also includes temperature detector circuits respectively associated with the compressor motor winding, selected compressor components and the interior of the chamber to be cooled. A low voltage detector circuit is also connected to the power supply voltage. However, this circuit does not address the thermostat functions, nor does the circuit address the relay function.
Therefore, what is needed is a control circuit for controlling a compressor motor which incorporates the thermostat and relay functions with protection features.
What is also needed is such control circuit for controlling a compressor motor which incorporates a number of protective features associated with abnormal power supply conditions.
What is also needed is such a control circuit which minimizes the required number of components to provide an inexpensive control and protection circuit in a compact, efficient package.