Compressor control with thermal density sensor

[57] ABSTRACT

The disclosure describes a thermal density sensor for preventing the start-up of a compressor motor operatively connected to the piston and cylinder means of a compression mechanism when the temperature of the compression mechanism is below a predetermined value to prevent damage to the running parts of the compression mechanism due to inadequate lubrication. Control circuitry for operating the compressor motor is contained in a single control box on the compressor so that the wiring of the circuitry is simplified. The control circuitry is entirely factory wired and the circuitry is maintained in a tamper-proof environment.

Claims, 4 Drawing Figures

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PATENTED APR 151975

3,877,837

SHEET 1 OF 2

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THERMAL DENSITY SENSOR SWITCH

HIGH PRESSURE SWITCH

102

MOTOR PROTECTOR MODULE

I-----?'76 )

DISCHARGE L / GAS / THERMOSTAT ⁸⁴ SWITCH

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LOW PRESSURE SW TCH

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PATENTED APR 151975

3,877,837

SHEET 2 OF 2

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3,877,837

COMPRESSOR CONTROL WITH THERMAL DENSITY SENSOR

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to an improved refrigerant compressor of the type housed in an outer casing and more particularly relates to improved apparatus for preventing the start up of the compressor if the temperature of the running parts within the outer casing is below a predetermined value.

Many refrigerant compressors utilize an oil pump which lubricates the compression mechanism by centrifugal force. One such lubrication system is illustrated in U.S. Pat. No. 3,584,980 (Cawley - June 15, 1971). As shown in the Cawley Patent, the oil must travel from an oil sump in a vertical direction through relatively small passages in order to lubricate vital parts. If the compression mechanism is too cold, the viscosity of the oil will be sufficiently thick to prevent adequate flow through the passageways. As a result, bearing and journal parts of the compression mechanism may not receive adequate lubrication and could be severely damaged.

Modern compressors also utilize metals having different coefficients of expansion. For example, it is not uncommon for a compressor to utilize an aluminum piston rod that is fitted with a steel wrist pin. If the temperature of these components .is too low, the diverse coefficients of expansion of the metals may reduce bearing clearances below acceptable tolerances. This condition, coupled with inadequate lubrication, can cause excessive wear or damage to the components.

In addition to the foregoing difficulties, the applicants have noted that a compressor which is started-up in a cold condition may experience valve flutter. The resonant condition set up by the valve flutter can cause annoying noise and also can reduce the operating efficiency of the compressor.

The applicants have discovered that the foregoing undesirable conditions can be alleviated by providing a thermal density sensor which is responsive to the temperature within an outer casing that surrounds the compression mechanism of a refrigerant compressors. Preferably, the thermal density sensor is arranged to prevent the start-up of a motor which drives the compression mechanism when the temperature of the running parts in the compression mechanism is below a predetermined value.

According to another feature of the invention, the control circuitry required to protect and operate the compressor motor is contained within a single control box mounted on the outer casing or within the outer casing itself. This arrangement facilitates the wiring of the circuitry during manufacture and thereby reduces the cost of the compressor. The control box package is factory wired and tested, thus reducing chance of field problems due to the faulty hook-up of the separate elements of the control circuit in the field. In addition, the control box prevents theft of and tampering with the sensitive control circuitry.

Accordingly, it is an object of the present invention to provide apparatus for preventing the start-up of a refrigerant compression mechanism when the temperature of the running parts of the compression mechanism is below a predetermined value.

It is another object of the invention to provide a thermal density sensor in heat transfer relationship with the compression mechanism so that the temperature of the mechanism can be more accurately sensed.

Still another object of the invention is to provide a thermal density sensor in physical contact with the block of a compression mechanism which prevents an electrical motor from driving the running parts of the compression mechanism if the temperature sensed by the thermal density sensor is too low.

Yet another object of the present invention is to enclose all electrical control circuitry needed to operate and protect the motor of a compressor within a single control box or an outer casing surrounding the compressor.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects, advantages, and features of the present invention will hereafter appear in connection with the accompanying drawings wherein:

FIG. 1 is an isometric, fragmentary, partially crosssectional view of a refrigerant compressor embodying control apparatus made in accordance with the present invention.

FIG. 2 is a top plan view of the control box shown in FIG. 1 with the top cover removed;

FIG. 3 is a top plan view of a control box showing an alternative arrangement of the components illustrated in FIG. 2; and

FIG. 4 is an electrical schematic diagram of the motor control circuitry contained within the control box illustrated in FIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, there is illustrated a refrigerant compressor 10 embodying a preferred form of the present invention. Compressor 10 comprises a sealed outer casing 11 which comprises an upper shell 12 (having a top surface 13) that is welded to a lower shell 14. A plurality of legs 16 are suitably secured to compressor 10 to support it in an upright position.

Resiliently supported within the outer casing by resilient coil spring means 20 is a compression mechanism 22. The compression mechanism comprises a compressor block 24 which defines a cylinder 26, together with additional cylinders (not shown). A movable piston 28 reciprocates within cylinder 26 in order to compress a refrigerant vapor. Additional pistons like piston 28, reciprocate in the additional cylinders, not shown. Each of the pistons is driven from a vertical disposed drive shaft 30. The lower portion of the outer casing froms an oil sump in which the oil level is visible through an oil sight glass 31.

An electric motor 32 is used to drive compression mechanism 22. The motor comprises a stator 34 which includes windings 36. A rotatable rotor 38 is inductively coupled to stator 34 and is mechanically coupled to drive shaft 30.

Provided at the end of each cylinder, including cylinder 26, and closing the end of each cylinder cavity, is a valve assembly, such as a valve assembly 40. The valve assembly includes a discharge valve and a suction valve. The suction valve opens on the suction stroke of piston 28 to permit refrigerant gas (suction gas) to enter cylinder 26 through a suction line 46. On the compression stroke of piston 28, the suction valve

3,877,837 closes and the discharge valve opens to permit the flow of compressed refrigerant gas to a discharge muffler in the compression mechanism and then to discharge lines 48. The compressed gas is transmitted through the discharge lines to a conventional condensor (not shown).

One important feature is the provision of a thermal density sensor SO inside the outer casing 11. The sensor preferably comprises a thermostat which is screwed into compressor block 24 in the position shown in FIG. 1. The thermostat is preferably the same type of temperature limiting device shown in U.S. Pat. No. 3,278,1 1 1 (Parker - Oct. 11, 1966) in which switch contacts 58 (FIG. 4) and a temperature responsive element are contained within a single housing. Sensor 50 is set to open electrical contacts 58 at about 22°F, plus or minus 3°F, and to close contacts 58 at about 32°F, plus or minus 3°F. Contacts 58 are electrically connected to other components of the compressor by conductors 54 and 55 that are attached to terminals 54T and 55T (FIG. 2). As previously explained, sensor 50 is an important feature because it prevents the start-up of motor 32 when the temperature of the running parts of the compression mechanism is below a predetermined value which inhibits adequate lubrication.

Another feature is the provision of a control box assembly 62 incorporating all of the electrical control circuitry for the compressor therein. As shown in FIG. 1, the control box assembly 62, which comprises a top 64 and sidewalls 66-69, is physically coupled to top surface 13 of outer casing 11.

FIG. 2 illustrates the layout of an electrical control assembly 72 contained within control box 62. Most of the wiring is not illustrated so that the layout of the components can be made easily seen. The control assembly includes a contactor 74, as well as compressor terminals T1-T6. A motor protection module 76 opens switch contacts 77 (FIG. 4) if detector 78 (shown schematically in the motor windings) senses a temperature higher than a desired value. Detector 78 is electrically connected to protection terminal block through conductors 79 and 80. The operation of switch contacts 77 is such that terminals 79T and 80T are closed if the motor temperature is sufficiently low and are opened if the temperature exceeds a preset value.

Control assembly 72 also comprises a high pressure switch assembly 82 located in the discharge gas spud 99 on the exterior of the outer casing. Switch contacts 84 (FIG. 4) are connected across terminals 86T and 87T (FIG. 2).

The control assembly also includes a low pressure switch assembly 90 that incorporates switch contacts 92. The switch contacts are connected across terminals 94T and 95T (FIG. 2).

Control assembly 72 further comprises a discharge gas temperature sensor 100 which is the same type of device as sensor 50 described above. Sensor 100 includes a set of switch contacts 102 which are connected by conductors 104 and 105 to terminals 104T and 105T located in control box 62. The discharge gas temperature sensor senses discharge gas temperature at its source so as to terminate compressor motor operation if the discharge gas temperature exceeds a predetermined high value (on the order of 300°F) to prevent breakdown of the compressor oil and damage to the running parts of the compression mechanism.

As shown in FIG. 2, the various control terminals are interconnected by conductors 108 to form the series pilot circuit schematically illustrated in FIG. 4. The pilot circuit also includes a 2 ampere fuse 110 which is held by a fuse holder 112. A conductor 114 is connected to a source of AC voltage, such as a low voltage transformer. AC power is supplied to the control box through a shielded cable 116.

In operation, each of the switches shown in FIG. 4 must be in a closed circuit condition in order for motor 32 to be operated. For example, if the temperature of the running parts of the compression assembly, for example, the drive shaft, piston rods, and wrist pin, is too low, thermal density sensor 50 opens switch contacts 58, thereby preventing motor 32 from starting up and damaging the compression mechanism. If high pressure switch assembly 82 detects pressure above approximately 410 PSIG in discharge lines 48, switch contacts 84 are open circuited or opened to stop motor 32. Likewise, if a temperature above approximately 300⁹F. in the discharge manifold is detected by discharge gas temperature sensor 100; switch contacts 102 are opened to stop motor 32. In a similar manner, if detector 78 senses that the temperature of motor 32 is abnormally high, or if low pressure switch assembly 90 detects pressure below approximately 25 PSIG inside outer casing 11, switch contacts 77 and 92, respectively, are open in order to stop motor 32.

Referring to FIG. 2, it should be noted that the electrical terminals necessary to control motor 32 are all contained within control box 62, thereby facilitating the wiring of the controls. Contactor 74 is advantageously placed on top of module 76, thereby conserving space inside the control box.

FIG. 3 illustrates an alternative embodiment in which contactor 74 is placed along side module 76. Otherwise, as indicated by the like-numbered elements, the embodiments of the control boxes shown in FIGS. 2 and 3 are identical.

It will be understood that the thermal density sensor will prevent operation of the compressor motor until the ambient heats up. In one aspect, the compressor will remain inoperative until the sun heats the ambient and by heat transfer heats the running parts and lubricant within the outer casing. In another aspect, a control for external heat may be included in the control circuitry. Should there be a demand for compressor operation negated only by the thermal density sensor sensing too low a temperature, then an external heat source or additional crankcase heat will be actuated to elevate the temperature within the outer casing.

Those skilled in the art will recognize that only two preferred embodiments of the invention have been disclosed herein, and that these embodiments may be altered and modified without departing from the spirit and scope of the invention as defined in the accompanying claims.