GB2075774A

GB2075774A - Motor-compressor unit

Info

Publication number: GB2075774A
Application number: GB8113949A
Authority: GB
Original assignee: Necchi SpA
Current assignee: Necchi SpA
Priority date: 1980-05-12
Filing date: 1981-05-07
Publication date: 1981-11-18
Also published as: IN155178B; KR840001586B1; GB2075774B; IT1209785B; DE3118638A1; SE448197B; SE8102938L; KR830004580A; IT8042911A0; DE3118638C2

Abstract

A starter and thermal protector combination in a hermetic motor- compressor unit comprises a PTC starter (10) and an element (12) for protection against temperature and current overload, the starter (10) and thermal protector (12) being disposed externally of and in proximity to the motor- compressor unit, and a sensor of the protector (12) being substantially in contact with the outer surface of the container (20) of the motor-compressor unit. The operational and constructional characteristics and rating parameters of the starter and protector are chosen in a suitable manner for the correct operation of the combination under all conditions, especially irregular conditions resulting from overloading of any kind. <IMAGE>

Description

SPECIFICATION Mdtor-compressor unit The present invention relates to a motor-compressor unit provided with starter control means and a thermal protector, and has particular-reference to a refrigerator motor-compressor unit.

Such a starter-control means may comprise a positive temperature coefficient resistor (PTCR) which has the advantage overconventional electromechanical starters of not possessing mobile contacts which, due to sparking, notoriously limit component life and cause unacceptable radio disturbance. A PTC starter also has the advantages of considerable standardisation which aids stocking and facilitates maintenance of equipment in which it is included and of offering a supplementary thermal protection for the starting winding.

According to the present invention there is provided a motor-compressor unit provided with starter control means comprising a positive temperature coefficient resistor and with a thermal protector for protection of the unit against excess temperature and current overload, the starter control means and the protector being disposed externally of and in close proximity to the unit and the protector comprising a sensor arranged substantially in contact with the outer surface of a container of the unit, the operational characteristics and rating parameters of the starter control means and the protector being so selected that the starter control means and the protector operate in a predetermined correct manner in all working conditions including irregular conditions due to excess temperature or current overload.

An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a diagram showing the characteristic curve of a PTC starter, Figure 2 is a circuit diagram of a motor compressor unit according to said embodiment, Figure 3 is a schematic view of the arrangement of a starter and protector in the unit of Figure 2, Figure 4 is a diagram illustration operating characteristics of the starter and protector, and Figures 5, 6 and 7 are circuit diagrams of prior art motor-compressor units.

By way of general explanation and with reference to Figure 5 of the drawings, motor-compressor units are fitted with thermal protectors, such as protector 112, and starters, such as starter 138. Briefly, the purpose of the thermal protector 112 is to protect the motor-compressor unit from overloads and' excess temperature, and is consequently normally connected into a common electrical supply feeder 116 of the motor-compressor unit and arranged in the proximity of the container 120 of the unit in such a manner as to measure its temperature.

The electromagnetic starter 138 is in series with a starting circuit 114 of the motor-compressor unit and its purpose is to cut off the supply to this circuit once the motor-compressor unit is in operation.

In the known art, the starter 138 is of the amper ometric mobile armature or solenoid type. Accord ingly, it is specific to each motor-compressor unit and has no temperature response. The characteris tics of a PTCR used as a motor-compressor starter are well known. Briefly and with reference to Figure 1, the PTCR is characterised by a "switch closed" state shown by portion 1 of its R-T curve, a "switch open" state.shown by portion 3 thereof, and an intermediate-transition state shown by portion thereof. The passage from state 1 to state 3 is caused by increase in temperature of the PTCR due to current flowing therethrough.This characteristic is utilised for supplying the starting circuit of the motor-compressor unit with which it is connected in series, and thus cut off the supply to the circuit when the motor-compressor unit is in operation. During the entire period-in which the motor-compressor unit is in operation, the PTCR is in the "switch open" state (portion 3), characterised by a high internal temperature and very high resistance. Only by cutting off the supply to the motor-compressor unit is it possible to restore the initial conditions of the PTCR, namely low temperature (portion 1), after which it is possible to again supply the starting circuit.

The time necessary for the PTCR to cool and to pass from portion 3 to portion 1 is thus its resetting time.

The resetting time for the PRCR must not interfere with the resetting times for other controls such as the thermal protector, or the temperature control system which the motor-compressor unit controls.

Similarly, the operating times of the PTCR starter and controls should not interfer with each other.

It is therefore apparent that in the application of the PTCR to motor-compressor units for refriger ators, special measures are required based on the temperature pattern and the time interaction be tween the PTCR and the protector in order to ensure proper operation under all conditions, especially overload conditions of the motor-compressor unit and irregular-operation conditions which most prob ably arises from the interactions. The arrangement of the PTCR relative to the thermal protector and the arrangement of each of these components relative to the elements of the motor-compressor unit and its container are of great importance for the purpose of correct operation of the system. The situation which has to be prevented is that following an overload of any type, the PTCR starter fails to reset itself or it is inadequately cooled to its initial conditions.If this should happen, it may give rise to continuous cyclic opening and closing of the protector which could continue either indefinitely or for a limited time, but for a sufficient period of time to compromise the basic operation of the motor-compressor unit and the system controlled by the unit.

Two methods are known for controlling and normalising such operating and resetting times. The first of these, as shown in Figure-6, use a PTCR 210 connected into a starting winding 214 and disposed externally of and in proximity to a container 220 of the motor-compressor unit, together with a bimetal lic protector 212, the sensor of which is arranged internally of the motor-compressor unit and in intimate contact with the motor wndings. In this method, the protector sensor is disposed in an environment in which the temperature falls slowly, as the sensor is hermetically enclosed in the motorcompressor unit, whereas the PTCR is in an environment of relatively fasttemperature reduction. This ensures that the PTCR becomes reset before the protector.This method has certain characteristic disadvantages, namely a long stoppage time after operation of the protector, the fact that the protector operates almost exclusively on temperature and not on current, and the fact that if a fault develops in the protector, the entire compressor unit must be replaced.

In the second method as shown in Figure 7, a PTCR starter310 and thermal protector312 are mounted in circuit, spatial and thermal relationship either on the same support or inside a single common housing 336 mounted externally of and in proximity to the container 320 of the motorcompressor unit. In this manner, both the PTCR starter and the thermal protector are in an environment of fast temperature reduction, but in the event of overload, when the thermal protector breaks the supply circuit because of the relative thermal interaction, the cycling transient of the protector is cancelled after a certain time period in favour of a normal start, provided that the cause of the overload has disappeared in the interim.

There are undoubted advantages in a method of this kind, not least of which are ease of assembly and construction and the compactness of the PTCR and thermal protector assembly 336.

However, the thermal interaction between the PTCR and thermal protector for obtaining the aforesaid effect cannot always be accurately determined, and often supplementary correction means are required which considerably complicate the method.

Standardisation is also lost, because each PTCR and thermal protector system 336 is special and has to be designed for each specific type of compressor.

There is accordingly a need for an arrangement in which the PTCR starter and thermal protector are applied to the motor-compressor unit using an arrangement of the kind described, while ensuring proper overall operation, especially in the event of current and temperature overloads of any type.

Up to the present time, no satisfactory method has been found in which the PTCR and thermal protector are mounted externally and in proximity of the container of the motor-compressor unit and are separated spatially such that no direct thermal interaction takes place therebetween.

It has now been found possible to achieve this result by choosing the mass of the PTCR pellet, its dimensions, the materials and construction of the PTCR and finally the heat dispersion coefficient which influences the resetting or cooling rates of the PTCR, these rates being different according to the environment in which the PTCR is located. These choices lead to proper operating characteristics, which are ensured if the PTCR resetting rates are 1.5 and 2.4 sec/ C when the ambient temperature is 60"C and 1 00 C, respectively.

A motor-compressor unit embodying the invention may have considerable advantages of simplicity, assembly and standardisation. The features characterising the embodiment have not been used before the present time because of the aforesaid possible operational drawbacks. Such drawbacks, in the form of interference with the inherent resetting times of each of the two components, namely the PTC and thermal protector, always arose if the two components were not suitably coupled and applied.

For example, it is known that the PTCR starter may be used in some cases as a replacement for the electromagnetic starter. However, when this happens, the person making the replacement does not pay attention to the characteristics of the thermal protector and therefore the aforesaid interference problems can arise especially if the supply voltage, ambient temperature and operational conditions are not perfect.

Figure 2 shows a circuit arrangement of a PTC starter 10 and thermal protector 12, the former being connected to a starting winding 14 and the latter to a common supply feeder 16 of the starting winding 14 and a main winding 18. Both components 10 and 12 are disposed externally of a container 20 of the unit.

Figure 3 shows the spatial arrangement of the thermal protector 12 and casing of the starter 10 on the container 20. The sensor 22, for example a bimetallic strip, of the thermal protector 12 is substantially in contact with the surface of the container 20 so as to be influenced directly by the temperature which is transmitted to it by the windings 14 and 18 by way of, for example, refrigerant gas present in the container.

The casing of the starter 10 is connected to the container 20 byway of supply terminals so that the PTCR pellet 24 of the starter is disposed in the position indicated.

With this arrangement, the thermal behaviour of the two components 10 and 12 is as described hereinafter with reference to Figure 4 should they operate due to overloading of the motor-compressor unit.

If, during a period of operation of the motorcompressor unit in which the conditions are as represented by the suction pressure 26 and delivery pressure 28 and by the temperature 30 of the PTCR of the starter 10, the thermal protector 12 in the state represented by ON interrupts the supply of current to the motor-compressor unit due to overload and thus assumes the OFF state, the suction and delivery pressures begin to balance each other from this moment (t1), in accordance with the illustrated law of change. The temperature of the PTCR begins to decrease from its actual value as indicated, as does the temperature of the bimetallic strip of the thermal protector, which is now in the OFF state. When the bimetallic strip has cooled to the point (t2) at which continuity of the supply circuit is restored, the motor cannot start even though it is supplied with current, because the PTCR has not yet been reset by virtue of its temperature and the pressures are not yet balanced. The temperature of the PTCR begins to rise because of the return of the current supply.

The bimetallic strip ofthe thermal protector, through which the strong short circuit current passes, again reaches cut-out temperature and this alternating condition is repeated for some cycles.

The final result of this cycling is an increase in the temperature of the windings and in the temperatures of the container, to which the heat produced in the windings is transmitted via the refrigerant gas present inside the motor-compressor unit, and thus an increase in the actual temperature of the thermal protector.

If this chain of influence is suitably determined, the cycling transient develops towards successive OFF cycles of increasingly greater duration. At each successive cycle, the cooling of the starter 10 reaches an improved resetting value, so that after a certain number of cycles the PTCR falls to the optimum resetting value and finally allows passage of current through the starting winding 14so that the motor can start. If at this point the suction and delivery pressures have reached a degree of balancing such that the resistant torque of the compressor is less than the static torque of the motor, then normal power is ensured.

As will be apparent from the foregoing, the operation of the described embodiment is based on the heat accumulation and dispersion mechanisms of the protector and PTCR starter, i.e. on the constructional parameters of the components which control these phenomena and also on their spatial relationship with the heat sources which establish the actual temperatures of the components themselves, namely the container surface, the windings and their relative thermal resistances.

Claims

1. A motor-compressor unit provided with starter control means comprising a positive temperature coefficient resistor and with a thermal protector for protection of the unit against excess temperature and current overload, the starter control means and the protector being disposed externally of and in close proximity to the unit and the protector comprising a sensor arranged substantially in contact with the outer surface of a container of the unit, the operational and constructional characteristics and rating parameters of the starter control means and the protector being so selected that the starter control means and the protector operate in predetermined correct manner in all working conditions including irregular conditions due to excess temperature or current overload.

2. A unit as claimed in claim 1, wherein the sensor comprises a bimetallic strip.

3. A unit as claimed in either claim 1 or claim 2, wherein the starter control means and the protector are thermally coupled by and only by the container.

4. A motor-compressor unit substantially as hereinbefore described with reference to Figures 1 to 4 of the accompanying drawings.