GB2086615A - Control system for a multi- capacity refrigerant motor-compressor - Google Patents

Abstract

A control system for a multi- capacity refrigerant motor-compressor comprises control means (74, 76,82-82a, 84-84a, 80-80a, 92-92a,b) (Fig. 2) for effecting compressor operation in a low capacity mode in response to served- space sensed temperatures differing from a given served-space demand temperature up to a predetermined degree, and in a high capacity mode in response to served-space sensed temperatures differing from said demand temperature by more than said predetermined degree, and ambient temperature responsive means (96, 96a) preventing a shifting of the compressor operation from the high capacity mode to the low capacity mode when sensed ambient temperatures differ from the given demand temperature by more than a predetermined number of degrees. Preferably, the system includes timing means (80) for delaying restarting of the compressor motor upon each change of capacity modes. The control system fairly matches the compressor capacity to prevailing demand and temperature conditions, and is particularly suitable for use with heat pumps. <IMAGE>

Description

SPECIFICATION Control system for a multi-capacity refrigerant motor-compressor The invention relates to the art of multicapacity refrigerant compressors for air conditioning and heat pump applications and, particularly, to a control system therefor.

Studies of heat pump economics have shown that, if a heat pump operating in a heating mode were capable of running efficiently at lower capacity on mild days and at higher capacity on cold days, definite economic advantages would be obtained. The problem is, however, that the compressor capacity of heat pumps operating in a heating mode decreases with falling temperatures because the latter cause the suction gas temperature and density to drop so that less refrigerant is fed to the compressor. Thus, the compressor capacity actually is decreasing when, ideally, it should increase in order to compensate for the lower ambient by supplying more heat.Several ways of handling this problem have been proposed, among them the use of multispeed compressors, of compressors having multiple cylinders which are partially unloadable, and of compressors deliberately oversized to meet all heating needs anticipated (which results in rather poor economics, of course, having regard to the fairly moderate cooling needs experienced in northern regions and moderate heating demands to be satisfied on mild days).

It is the principal object of the invention to provide a reasonably simple and inexpensive control system for matching the capacity of a refrigerant compressor to prevailing demand and temperature conditions.

The invention accordingly resides in the combination of a multi-capacity refrigerant motor-compressor including means operable to change the capacity of the compressor, and a control system for controlling said means in accordance with prevailing demand and temperature conditions, characterized in that said control system comprises control means for effecting operation of the compressor in a low capacity mode in response to one range of temperature change needed to satisfy a given served-space temperature demand, and in a high capacity mode in response to a greater range of temperature change needed to satisfy said demand, and ambient temperature responsive means operative, in response to ambient temperatures differing from the servedspace temperature demand by at least a predetermined degree, to prevent shifting of operation of the compressor from the high capacity mode to the low capacity mode.

Typically, heat pumps intended for use in northerly climates are sized to provide the required cooling capacity, and this usually creates a need for supplementary resistance heating to be applied at temperatures of, say, below 35"F (2"C). In other words, the heating capacity of such a heat pump does not match well the required cooling capacity.The arrangement according to the invention alleviates this problem insofar as it enables a heat pump operating in its high capacity mode to provide all of the heat required down to a temperature of, say 20"F(-7"C). Thus, the unit will operate at lower capacity in the air conditioning mode under ordinary conditions, as well as in the heating mode during moderate temperatures, such as down to 35"F (2"C). Most of the high capacity operation would occur in the heating mode during ambient temperature conditions below said moderate temperature level of, say, 35"F (2"C). This unit would be sized, in effect, to have a reduced balance point of about 20"F (-7"C) so that supplemental resistance heat would only be needed below that level. However, the arrangement according to the invention is adaptable also for use with multi-capacity units intended primarily to provide cooling in southerly climates which occasionally may call for lower-capacity heating and perhaps no supplemental resistance heat at all.

Regardless of the manner and the climate in which a unit utilizing a control system according to the invention is to operate, the control system preferably includes means for introducing a time delay sufficient to permit at least some pressure equalization in the refrigerant system after each stopping of the compressor occurring upon a capacity change, and before subsequently restarting it in a manner, e.g., in the opposite direction, which without such time delay would unduly burden the compressor.

A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which; Figure 1 is a side elevational and partially sectional view of a hermetic refrigerant compressor; and Figure 2 is a schematic of a control system embodying the invention and suitable for use with the type of compressor shown in Fig. 1.

Although the compressor as shown in Fig.

1 has two cylinders, it should be understood that the invention is applicable to single-cylinder and multiple-cylinder compressors alike, provided the latter include means enabling their capacity to be changed, for instance, by changing the speed or reversing the operating direction of the compressor motors or with the aid of solenoid-controlled unloading or suction control valves, or the like. The refrigerant compressor illustrated in Fig. 1 is of the type disclosed in U.S. patent specification No.

3,259,307, for example. Briefly, it includes a generally cylindrical, hermetically sealed shell 10 having an inlet 1 2 through which suction refrigerant gas is admitted into the shell, and one or more discharge tubes 1 4 through which compressed refrigerant gas exits from the shell. The upper part of the shell houses an electric motor 1 6 which has its rotor 1 8 affixed to the upper end of a crankshaft 20 for driving pistons 24 movable in cylinders 22 and connected to a lower crankpin portion 28 of the crankshaft by means of connecting rods 26. At its lower end, the crankshaft 20 is provided with oil inlet means 30 for admitting oil from a sump 32 into an oil passage 34 extending axially through the crankshaft.For the purposes of the following description, it will be assumed that the capacity of the illustrated compressor is changed by changing either the speed or the operating direction of the electric compressor motor 1 6 which, of course, would be of the multi-speed type in the former case and of the reversible type in the latter. The motor speed changing or motor reversing means employed may be of any suitable type known in the art. If changes of the compressor capacity are to be effected through directional reversals, the compressor would include suitable means for translating the directional reversals into corresponding capacity changes, for instance by automatically adjusting the stroke length, or displacement, of the pistons of the compressor in a manner such as described in U.S.Patent Specification No. 3,010,339 or in Applicant's copending Patent Application No. (W.E. Case 47,635-BI). Since neither the means for translating directional reversals into capacity changes nor the means for reversing or changing the speed of the compressor motor form part of the invention proper, a detailed description thereof is not thought necessary herein, it being sufficient, for a clear understanding of the invention, to know that the motor compressor includes means, such as mentioned above, which can be operated to change the compressor capacity under the control of the control system of the invention which will now be described with particular reference to Fig. 2.

As illustrated in Fig. 2, the control system is powered from a low-voltage (e.g. 24 volts) source shown as a transformer 72 which has connected to one side thereof a first thermostatic switch 74 adapted to be placed so as to sense the temperature in the space served by the heat pump, and having a temperature set point corresponding to the served-space demand temperature. For the purpose of the following description, and in order that examples of specific temperatures may be used therein, it will be assumed that the system is operating in a heating mode.The circuit also includes a second thermostatic switch 76, also for sensing the temperature in the served space, which has a set point of a temperature different from that of the first thermostatic switch, this different temperature corresponding to a served-space temperature which is more easily satisfied in accordance with whatever air tempering (heating or cooling) operation is being carried out.For example, the thermostatic switch 74 may be set to be closed in a heating operation at a temperature below 70"F (21"C) which will be considered the demand temperature, while the second thermostatic switch 76 may have a set point such that it will be closed at temperatures below 66"F (19"C). Thus, the second thermostatic switch is more easily satisfied in a heating operation than the first thermostatic switch The two thermostatic switches 74 and 76 are connected together at a common point 78 which, in turn, is connected to a common terminal of a timer motor 80 which has a twoposition controlled mode changing switch 80a.Line 78 also leads to two separate relay coils 82 and 84 which control reversing switch means 86 (or other suitable capacity control means if other than motor reversal is used) in the power line 88 to the compressor motor 1 6. If the capacity control is to be effected by a change in motor speed, the motor 1 6 will, of course, be a multi-speed motor.For purposes of explaining the operation, the relay coil 82 will be characterized as the high capacity coil which must be energized for the motor 1 6 to rotate in a direction, or at a speed, as the case may be, causing the compressor to operate at high capacity, while the coil 84 will be referred to as the low capacity coil which must be energized in order for the motor to be driven in the opposite direction, or at a lower speed, causing the compressor to run at lower capacity. In addition, the relay coils 82 and 84 also control the normally closed switches 82a and 84a, respectively, in the timer motor circuit which will be explained hereinafter.The side of second thermostatic switch 76 opposite from the one connected to the common point 78 is connected by line 90 to two parallel circuits, one of which includes a relay coil 92 controlling a normally closed switch 92a and a normally open switch 92b, both in the timer circuit; and the other of which includes, in series, an electrical resistance heater relay coil 94 and an ambient temperature responsive switch 96 which controls a switch 96a in the timer circuit.

Regarding the circuit for the timer motor 80, when all three of the switches 84a, 92a and 96a as well as the first thermostatic switch 74 are closed, the timer motor will be energized for a period to drive the mode changing switch 80a to the position shown in Fig. 8, which results in energization of the compressor motor 1 6 causing the compressor to run at lower capacity. This compressor motor energization occurs without a time dle- lay if the compressor had last run at'lower capacity due to the timer motor controlled switch 80a being at its low capacity terminal position L, as shown in Fig. 8; on the other hand, it would occur with a time delay if the compressor had last run at higher capacity because the timer motor controlled switch 80a was in the high capacity terminal position H.With the energization of the low capacity relay coil 84 and consequential opening of the control switch 84a, the timer motor 80 becomes deenergized and parks the switch 80a in its low capacity position L. If the operation of the compressor motor 1 6 in its low capacity mode provides adequate heat to satisfy the first thermostatic switch 74, the latter will open, and thereafter close again as the room temperature drops, and then it will continue to alternately open and close so as to cycle the compressor off and on, respectively, in a low capacity mode.

If the compressor operating at low capacity does not provide enough heat to maintain the room temperature above the set point of the second thermostatic switch 76, then the latter will close and thereby cause the relay 92 to become energized so as to open its controlled switch 92a and to close its controlled switch 92b. With this condition, the timer motor 80 is energized in a direction to move its controlled switch 80a to the high capacity terminal H. This changing of the switch 80a position, during which the compressor motor 1 6 is deenergized, provides a time delay adequate to permit some equalization of refrigerant pressures in the refrigerant circuit to take place.When the switch 80a reaches its high capacity mode position H, the high capacity relay 82 is energized to start the compressor motor 1 6 in the high capacity mode, and the relay controlled switch, 82a is opened to deenergize the timer motor 80 which thus will park the switch 80a in the high capacity terminal position H thereof.

Operation of the compressor at high capacity will raise the room temperature until it satisfies the second thermostatic switch 76 (the more easily satisfied one), whereupon the latter will open and deenergize the relay 92, whereby the switch 92b is opened and the switch 92a is reclosed to complete the circuit containing the switches 84a, 92a and 96a.

Therefore, and since the first thermostatic switch 74 is still unsatisfied and thus closed, the timer motor 80 becomes energized, and the compressor motor 1 6 consequently is stopped as the energized timer motor 80 moves the switch 80a from its high capacity position H and toward the low capacity position L thereof. When the latter is reached by the timer controlled switch 80a, the low capacity relay 84 is energized to again operate the compressor motor 1 6 in the low capacity mode. Thus, it will be appreciated that in a period of moderate to moderately lower temperatures, the compressor motor 1 6 will be cycied back and forth to operate the compressor in the low or high capacity mode, as demanded, with a time delay introduced between each mode change.

Now assume that there is a rapid drop in the ambient temperature so that ambient thermostatic switch 96 closes and its controlled switch 96a opens. If, when this occurs, the second thermostatic switch 76 is open, and the compressor motor 1 6 is operating in the low capacity mode because the switch 80a is in its low capacity position L, it is possible for the first thermostatic switch 74 of cycle the compressor motor 1 6 on and off in the low capacity mode so long as the room space does not drop in temperature to that at which the second thermostatic switch closes. However, since the compressor operating at low capacity is not likely to maintain the desired room temperature, the second thermostatic switch 76 soon will close.In the manner described above, this will result in movement of the timer controlled switch 80a to its high capacity position H and, consequently, in energization of the compressor motor 1 6 in the high capacity mode. At the same time, it will also result in energization of the electrical resistance heater relay coil 94 which in turn will cause supplemental resistance heating means (not shown ) for heating the space to be energized. As long as the ambient thermostatic switch 96 remains closed to keep the switch 96a open, the timer motor 80 cannot be energized to move the timer control switch 80a from its high capacity position H to its low capacity position L.

When the space temperature has come up far enough to satisfy the second thermostatic switch 76, the latter will open and thereby deenergize the relay 94 which, consequently, will turn off the electrical resistance heat. The compressor, however, will continue to operate in the high capacity mode because the first thermostatic switch 74 still is closed and the switch 96a is still open at this point. If the compressor capacity at this setting is not enough to maintain the space temperature between the two set points of the thermostatic switches 74 and 76, the thermostatic switch 76 will close and thus will cycle the electrical resistance heat on and off. However, it is also possible that the compressor operating at high capacity will raise the space temperature sufficiently to satisfy the thermostatic switch 74, in which event the latter will open and stop the compressor motor.Subsequently, when the switch 74 closes again while the ambient temperature control switch 96 is still closed and the switch 96a hence is still open, the compressor will restart in the high capacity mode because the timer motor 80 will remain deenergized and its switch 80a consequently will remain in its high capacity position H.

This enables the thermostatic switch 74 thereafter to cycle the compressor motor 1 6 on and off in the high capacity mode. Thus, it is seen that cycling of the first thermostatic switch 74 results in cycling of the compressor motor 1 6 in its low capacity mode when the ambient temperature responsive switch 96 is open, and in the high capacity mode when the switch 96 is closed. In other words, the closed ambient temperature responsive switch 96 holding the switch 96a open will ensure that, when the compressor runs, it will be running in a high capacity mode regardless of how temperature conditions in the served space are changing to effect cycling of the second themostatic switch 76 which, under these conditions, will cause electric heat to be cycled on and off with the low outside tem perature conditions.

While not considered likely, it is possible that a heat pump could be used in northern regions without having electrical resistance heating means directly tied into the control circuit of the heat pump. In such a case, the heater relay 94 could be omitted.

As explained hereinbefore, a heat pump designed for operation in northern climates will typically have more than adequate capac ity, with the compressor operating in the high capacity mode, to meet the cooling needs encountered in such northern climate.

If it is desired to adapt the control system according to the invention for operation in the cooling mode, the second thermostatic switch 76 is reset to a set point temperature higher than the set point temperature of the first thermostatic switch 74, and the sense of closing is reversed such that the switches 74, 76 will close upon rising instead of falling temperatures. In the system thus adapted for cooling, the second thermostatic switch 76 is again, as before, the more easily satisfied .switch, and the system will control the com pressor for operation in the high and low capacity modes in the same general manner as described above in connection with heat ing, except that the compressor will be made to run in the high capacity mode when the low capacity mode of operation results in inadequate cooling to cause opening of the second thermostatic switch 76. The ambient temperature responsive switching means 96 and 96a would be set at a relatively high temperature to lock the system into high ca pacity operation whenever the sensed ambient temperature is extraordinarily high.

It is also conceivable that, if the heat pump is to be used in climates of high humidity and high temperatures, and with no need for elec trical resistance heating, dual capacity cooling may be the desirable feature with only the lower capacity heating being required.

Claims (7)

1 The combination of a multi-capacity re frigerant motor-compressor including means operable to change the capacity of the com pressor, and a control system for controlling said means in accordance with prevailing demand and temperature conditions, characterized in that said control system comprises control means for effecting operation of the compressor in a low capacity mode in response to one range of temperature change needed to satisfy a given served-space temperature demand, and in a high capacity mode in response to a greater range of temperature change needed to satisfy said demand, and ambient temperature responsive means operative, in response to ambient temperatures differing from the served-space temperature demand by at least a predetermined degree, to prevent shifting of operation of the compressor from the high capacity mode to the low capacity mode.

2. The combination according to claim 1, characterized in that said control means includes means delaying a restart of the compressor motor for a predetermined time upon each change of capacity modes effected by the control means.

3. The combination according to claim 1 or 2, characterized in that said control means include a pair of thermostatic switches adapted to sense the temperature in the served space, a first one of which switches has a temperature set point corresponding to the served-space demand temperature, and a second one of which switches has a different temperature set point corresponding to a served-space temperature more easily satisfied through operation of the compressor than said served-space demand temperature, said first thermostatic switch being operative to cycle the compressor on and off in the low capacity mode in response to the served-space temperature ranging between the more easily satisfied temperature and a temperature beyond the set point temperature of the first thermostatic switch, and said second thermostatic switch being operative, with said first thermostatic switch closed, to effect operation of the compressor in the high capacity mode when a served-space sensed temperature fails to meet said more easily satisfied temperature.

4. The combination according to claim 3, characterized in that said ambient temperature responsive means comprises a switch assuming a first position when the ambient temperature differs from the served-space demand temperature by at least a predetermined amount, and a second position when the difference between the ambient and demand temperatures is less than said predetermined amount, the ambient temperature responsive switch, in said second position thereof, enabling said first thermostatic switch to cycle said compressor on and off in the low capacity mode in response to the served-space temperature ranging between said more easily satisfied temperature and a temperature beyond said set point temperature, and, in said first position thereof, enabling the first thermostatic switch to cycle the compressor on and off in the high capacity mode in response to the served-space temperature ranging between said more easily satisfied temperature and a temperature beyond said set point temperature following a condition of said served-space temperature failing to meet said more easily satisfied temperature.

5. The combination according to claim 3 or 4, characterized in that said means delaying a restart comprises a timer controlling a mode changing switch having a high capacity mode position and a low capacity mode position, said timer being connected to be energized by said first thermostatic switch to place said mode changing switch in said low capacity mode position when only said first thermostatic switch is closed, and in said high capacity mode position when both said first and second thermostatic switches are closed.

6. The combination according to claim 5, characterized in that said timer is connected in a circuit including energization-preventing switching means for causing the timer to be parked in one position and another position corresponding to said high capacity mode position and said low capacity mode position, respectively, of the timer-controlled mode changing switch.

7. The combination according to claim 5 or 6 including electrical resistance heater and heater relay means for effecting energization of said heaters to add heat to the served space, characterized in that said ambient temperature responsive means comprises a normally-open switch connected in series with said heater relay and said second thermostatic switch, and a normally-closed switch connected in series with said timer to prevent the latter from moving said timer-controlled mode changing switch to said low capacity mode position when said normally closed switch is open.