Low Voltage Protection Circuit for Induction Motors

This invention relates generally to low voltage protection circuits for induction motors such as air conditioner compressors and the like.

Generally speaking, such circuits have been known and used in the prior art. FIG. 1 of the attached drawings illustrates one such conventional low voltage protection circuit wherein an air conditioner compressor motor 10 is connected through a manually operable on/off switch 12, an adjustable thermostatically controlled switch 14 and a thermally activated low voltage protector switch unit 16 to a suitable source 18 of a.c. potential. The switch unit 16 includes a heat sensitive switch 20 thermally coupled to a heater element 22, the latter being connectable directly across the source 18. Now, when the air conditioner is connected to the source 18, the element 22 draws power therefrom and is selected so as to heat up sufficiently to activate the thermally sensitive switch 20 to close the same provided the source potential is at least high enough to safely operate the compressor motor 10. However, should the source potential drop below the safe operating level for the motor 10 for any reason, the current drawn by the element 22 will also drop to thus reduce its heat output sufficiently to deactivate and open the switch 20. The motor 10 is thereby protected against possible damage which might otherwise result if a low voltage condition occurs at the source 18 sufficient to stall the motor 10 and produce damaging locked rotor current.

One difficulty which has been encountered with such prior art circuits results from the fact that the heater element is directly connected across the source at all times while the air conditioner is so connected. This is the case whether or not the air conditioner is turned on and whether or not the thermostat is calling for compressor operation. The load thus presented by the heater element upon the source is constant and can produce a significant amount of power consumption over long periods of time just to keep the low voltage protection switch activated.

By means of my invention, this and other difficulties encountered in the prior art can be substantially overcome.

It is one object of the present invention to provide an improved low voltage protection circuit for induction motors.

It is another object of the present invention to provide a low voltage protection circuit for air conditioner compressors and other induction motors which substantially reduces the power drain on the commercial power source compared with that caused by conventional prior art low voltage protection circuits.

Briefly, in accordance with the present invention, there is provided an improved low voltage protection circuit for an a.c. motor of the type having first switching means for applying a source of a.c. potential across the motor when activated, and resistance means operatively associated with the first switching means for activating the latter when current therethrough is at least equal to a minimum value. The improvement includes second switching means for connecting the resistance means across the source during each half-cycle of the source potential when the first switching means is inactive, and for connecting the resistance means across the source during alternate half-cycles of the source potential when the first switching means is activated.

These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description and attached drawings upon which, by way of example, only the preferred embodiment of the subject invention is illustrated.

FIG. 1 shows a schematic diagram of a typical prior art low voltage protection circuit for an a.c. induction motor, air conditioner compressor or the like.

FIG. 2 shows a schematic diagram of an improved low voltage protection circuit for an a.c. induction motor, air conditioner compressor or the like, thus illustrating one preferred embodiment of the present invention.

Referring now to FIG. 2, there is shown, in one preferred embodiment of the subject invention, a low voltage protection circuit for an a.c. motor 24 such as an air conditioner compressor, for example.

Typically, such a compressor motor 24 is controlled by means of a manually operable on/off switch 26 and an adjustable thermostatically controlled switch 28. Protection against potentially motor damaging low voltage is provided by means of a conventional switch assembly 30 having a normally open heat sensitive switch 32 and a resistive heater element 34 thermally coupled thereto. The assembly 30 may be selected such that the switch 32 will be activated from an open condition, as shown, to a closed condition when the heat generated by a current flowing through the element 34 reaches a desired level, such heat being a function of the r.m.s. value of a.c. current flowing therethrough. The series combination of the motor 24 and the switches 26, 28 and 32 is connected across a suitable source 36 of a.c. potential.

Now, in accordance with my invention, a first pair of suitable diodes 38, 40 is connected in series with the heater 34 on opposite sides thereof and the resulting series combination is connected across the source 36 such that, during alternate half-cycles of the source potential, current flows through the diodes 38, 40 and the heater 34, whether the switch 32 is open, as shown, or closed. A second pair of diodes 42, 44 is series connected to and on either side of the element 34, the series combination of which is connected across the switch 32. The polarities of the diodes 42, 44 are reversed with respect to the polarities of the diodes 38, 40 in their relationship to the source 36. Consequently, the diodes 42, 44 conduct only during alternate half-cycles of source potential opposite those which drive the diodes 38, 40 to conduct, and only when the switch 32 is open. When the switch 32 is closed, as when the source potential is at least equal to a safe operating level for the motor 24 and after the element 34 has warmed up sufficiently, the series combination of the diodes 42, 44 and the element 34 is shunted out of the circuit.

Thus, while the element 34 draws full power from the source 36 at all times while the switch 32 is open, the element 34 consumes substantially less power when the switch 32 is closed. Accordingly, the average power consumed by the element 34 during operation of an air conditioner employing such a motor 24 will be substantially less than where the element 34 is directly connected across the source 36 as in prior art low voltage protection circuit of FIG. 1.

Proper and stable operation of the circuit of FIG. 2 requires that the assembly 30 be selected such that the temperature at which the thermal switch 32 opens is below the temperature generated by the element 34 when the latter is operated under half-wave or half-power conditions, i.e., when the switch 32 is closed, assuming the source potential is within normal limits. For example, a suitable thermal switch for operation in the prior art circuit of FIG. 1 is Model No. 20474F1-40 as manufactured by Texas Instruments, Incorporated which is rated for use with a 230 volt power source at 81/2 watts .+-.1 watt and 6180 ohms, .+-.10 percent. This switch is calibrated to close at 185.degree. F., .+-.5 degrees F., and to open at 175.degree. F., .+-.5 degrees F. However, in the circuit of FIG. 2, where the source potential is 230 volts, the thermal switch should be selected so as to open at a substantially lower temperature, for example, at about 120 degrees F., to avoid the tendency of the switch 32 to cycle on and off in an unstable manner when the source potential is within normal limits.

Although the subject invention has been described with respect to specific details of a certain preferred embodiment thereof, it is not intended that such details limit the scope of the subject invention otherwise than as set forth in the following claims.