Abstract: A scroll type machine incorporates a unique system for monitoring the status of a valve which is used to control the capacity of the compressor. The valve functions to open and close a fluid passage between two areas of the compressor for capacity modulation. By monitoring the temperature of the fluid after the valve, it can be determined whether or not the valve is functioning. If the temperature fluctuates, the valve is functioning. If the temperature is constant, the valve is not operating properly. Another embodiment monitors the pressure within the fluid line controlled by the valve.

Abstract: A scroll compressor is provided with a motor protector for stopping operation of its motor should conditions be indicative of a problem. The motor protector is sensitive to elevated temperature, and stops operation of the motor should a sensed temperature exceed a predetermined maximum. The motor protector is positioned in a chamber in a rear face of a base of the non-orbiting scroll. A port extends through the base of the non-orbiting scroll to connect a motor protector chamber to a suction chamber. Electric wires connect the motor protector to the motor and extend through this same port. In some embodiments, a pressure relief valve also communicates with the motor protector chamber. In one embodiment the motor protector chamber is enclosed by a cap and the pressure relief valve is mounted in that cap. In another embodiment the pressure relief valve extends through a wall of the non-orbiting scroll. In other embodiments, a pair of mating plugs connect the protector to the motor.

Abstract: A compressor diagnostic system incorporates a control which receives a plurality of data streams about various operational features of the compressor. As an example, both temperature and pressure of the suction and discharge refrigerant are taken and sent to the control. Moreover, information with regard to the power being supplied to the motor is taken and stored. All of this information is utilized at a control which compares the information to expected values and determines a fault based upon the evaluation. Moreover, in another feature of this invention, much of this data is stored, and maintained at the compressor. In the event of a compressor failure, this stored information will provide a maintenance worker with a good indication of why the compressor failed.

Abstract: A pump assembly flows pressurized engine oil to HEUI fuel injectors in a diesel engine. The assembly includes an inlet throttle valve which controls the volume of oil flowed to the pump dependent upon the difference between the pump outlet pressure and a desired outlet pressure determined by an electronic control module for the diesel engine. When the engine is cold a valve opens to assure flow of cold oil to the pump.

Abstract: A highly-viscous-fluid applying apparatus including a fluid supply device operable to supply a highly viscous fluid, a delivery nozzle from which the highly viscous fluid is delivered, a pump disposed between the fluid supply device and the delivery nozzle and operable to feed the highly viscous fluid received from the fluid supply device, to the delivery nozzle, and a pump control device operable to control the pump, for controlling an amount of delivery of the highly viscous fluid to be delivered from the delivery nozzle.

Abstract: A method and apparatus for controlling a compressor system by sensing a temperature of compressed fluid in an outlet of a compressor and automatically reducing an output of the compressor when the temperature exceeds a predetermined temperature and a pressure in the outlet exceeds a predetermined pressure. High and low pressure settings may be provided so that the controller operates the compressor system at the appropriate setting based on the temperature of the compressed fluid in the compressor outlet. The output of the compressor may be controlled by limiting an amount of air permitted to enter the compressor's inlet and/or controlling the compressor's throttle. A three-way valve may also be provided to allow a user to chose between the high and low pressure settings.

Abstract: An impeller pump with thermostatically adjustable guide vanes is suitable for use as an automotive coolant pump. The pump is driven by a constant speed electric motor, and flow variation is controlled by varying the orientation of the vanes. Orientation of the vanes is effected by a wax-type thermostat, which senses coolant temperature: flow is increased when the coolant is hot, and decreased as the coolant cools. The variable guide vanes are mounted for pivoting about radial axes, and are located just upstream from the pump impeller.

Abstract: A compressor protection and control subsystem for a rotary compressor provides temperature pressure, mis-wiring and vibrational protection for the scroll machine. The vibrational protection comprises a vibration sensor which is integrated on the circuit board of the protection and control subsystem. The vibration sensor, in conjunction with at least one timer, monitors the vibrations of the scroll machine and will shut down the machine when excess vibrations are sensed over a prespecified period of time. The temperature system monitors operating temperature conditions the pressure system monitors operating pressures and the mis-wiring system monitors the power supplied to the compressor. Once an undesirable characteristic is identified, the operation of the scroll machine is stopped. These protection systems are integrated into a single subsystem which identifies the reason of shutting off the scroll machine in order to simplify repairs needed.

Abstract: An impeller pump with thermostatically adjustable guide vanes is suitable for use as an automotive coolant pump. The pump is driven by a constant speed electric motor, and flow variation is controlled by varying the orientation of the vanes. Orientation of the vanes is effected by a wax-type thermostat, which senses coolant temperature: flow is increased when the coolant is hot, and decreased as the coolant cools. The variable guide vanes are mounted for pivoting about radial axes, and are located just upstream from the pump impeller.

Abstract: To protect a turbocompressor (3) with a downstream process from operation in the unstable working range, a machine controller is used, which optionally contains—besides a surge limit controller (15)—an intake pressure controller (11), an end pressure controller (20) and a bypass controller (9). A control matrix is determined from the position of a control unit (fuel gas control valve 6) determining the flow to the process, optionally taking into account additional influencing variables such as the compressor intake pressure and the compressor discharge pressure.

Abstract: Method and apparatus for heating gases cooled by an aftercooler that receives hot gases from a compressor. The method includes the steps of directing at least a portion of said hot gases to means for bypassing the aftercooler, and using the bypass means to direct hot gases to a location that receives cooled gases from the aftercooler. The hot gases are used at the location to heat the cooled gases when ambient temperature is at or below freezing, and are used downstream from the aftercooler when moisture freezes in the aftercooler.

Abstract: The rated system pressure in a pressure control system is adjusted through a pressure relief valve (2). In addition to the pressure relief valve (2), the system includes a check valve (4), a pressure sensor (3), a position switch (10), a first pressure adjustment member (13) for the relief valve, a second pressure adjustment member (12) for the check valve (4), an actuator (16), and a microcontroller (15) with a memory (6) and a display (7). Faults caused by pressure adjustment interactions are prevented by permitting an adjustment of the pressure relief valve only if the check valve is fully open and a pump provides pressure. For this purpose the actuator (16) of the check valve in its normal position prevents an adjustment of the pressure relief valve. Opening the check valve by the actuator permits adjusting the pressure relief valve (2) and the switch (10) operated through the actuator sends a signal to the microcontroller to drive the pump.

Abstract: A scroll compressor includes a first scroll member and a second scroll member with intermediate spiral wraps. A drive member causes the scroll member to orbit relative to one another to create pockets of progressively changing volume between a discharge pressure zone and a suction pressure zone. One of the scroll members defines a chamber which contains fluid, a pressure intermediate the discharge pressure and suction pressure of the compressor. A temperature responsive valve is located within the chamber to release the intermediate pressure fluid to the suction pressure zone of the compressor when an excessive temperature is sensed.

Abstract: The inside of a vacuum pump is heated by means of a first heating unit to a temperature higher than the temperature at which products of reaction discharged from a process chamber are separated, and the inner surface of an exhaust pipe is heated to a temperature higher than the separation temperature by means of a second heating unit. If a vacuum process is carried out in the process chamber in this state, exhaust gas discharged from the process chamber can pass in a gaseous phase through the exhaust pipe and the vacuum pump without separating its unwanted by-products. Since a trap unit is located on the downstream side of the vacuum pump, moreover, the reaction products and the like can be prevented from adhering to the inside of the vacuum pump so that the maintenance operation is easier, although the conductance of the exhaust pipe is lowered so that the trap unit and the vacuum pump can be reduced in size.

Abstract: A compressor of scroll type or the like, in which, in order to change the discharge capacity automatically in accordance with the rpm (rotational speed) of a shaft (4) by simple means, the centrifugal force exerted on a movable scroll (9) orbiting with the rotation of the shaft (4) is used as a vibratory force to forcibly vibrate a spool (23) constituting a valve body supported on an elastic member (25), thereby opening and closing bypass holes (22) for establishing communication between a working chamber (V) and a suction chamber (15).

Abstract: A system and method for air compressor control wherein the air compressor is loaded only when engine activities require it to be loaded or when free engine power is available to operate the compressor. An air compressor system is an engine-driven, piston-type compressor which operates in a loaded and an unloaded mode and provides air to a vehicle's air powered devices, such as service brakes, air suspension, windshield wipers, etc. The operating mode of the compressor is controlled by a pressure activated air governor which applies an air signal when pressure in a reservoir reaches a set level that activates a cap valve on an unloader stopping the air compressor. When the air pressure in the reservoir drops below a lower set pressure or when energy is "free," such as during downhill operations, the air governor exhausts the air signal allowing the air compressor to resume operation.

Abstract: A method of controlling compressor minimum capacity. The method comprises the steps of: measuring a first condition; comparing the first condition to a first setpoint to determine a first conditioned error; modulating compressor capacity relative to the first condition error; measuring a second condition; comparing the second condition to a second setpoint to determine a second condition error; and modulating compressor capacity relative to the second condition error if the magnitude of the second conditioned error is greater than the magnitude of the first conditioned error.

Abstract: The inside of a vacuum pump is heated by means of a first heating unit to a temperature higher than the temperature at which products of reaction discharged from a process chamber are separated, and the inner surface of an exhaust pipe is heated to a temperature higher than the separation temperature by means of a second heating unit. If a vacuum process is carried out in the process chamber in this state, exhaust gas discharged from the process chamber can pass in a gaseous phase through the exhaust pipe and the vacuum pump without separating its unwanted by-products.

Abstract: A rolling-piston expander has a hermetic casing 7 provided with a suction pipe 3 and a discharge pipe 5, a cylinder 13 disposed in the casing, a roller 31 eccentrically rotated in the cylinder, an expansion chamber 39 defined by the roller and communicating with a suction port 47 and a discharge port 55, a shaft 19 for supporting the roller so that the roller may eccentrically rotate, a suction timing controller 51 consisting of the ports 47 and 51, for controlling the timing of the supply of gas into the expansion chamber, and a bypass for supplying high-pressure gas into the expansion chamber when the suction timing is off.

Abstract: A pump apparatus includes a pump having a suction port and a discharge port for discharging the fluid sucked from the suction port, a return passage connecting the discharge port with the suction port for returning a portion of the fluid discharged from the discharge port into the suction port, a return flow control valve disposed in the return passage for controlling an amount of the portion of the fluid returned to the suction port through the return passage and an expanded chamber formed in the return passage and located between the suction port and the return flow control valve. The expanded chamber has a sectional area larger than that of the return passage.

Abstract: A thermally responsive valve assembly for scroll motor-compressor high temperature protection, which causes a high-side to low-side leak when excessive discharge gas temperatures are encountered, thereby causing the motor protector to trip and de-energize the motor. The valve assembly includes means for ducting the excessive temperature discharge gas to the lower portion of the motor/compressor shell to the motor to circulate the high temperature gas throughout the motor cavity. The excessive temperature discharge gas heats the motor stator and windings which will in turn cause the motor protector to trip and de-energize the motor. A secondary flow passage increases the flow rate or the discharge gas to lower the cycle time of the compressor.

Abstract: In a scroll compressor, an internal space of a sealed container is partitioned by a cylinder-shaped partition wall into a delivery chamber and a suction chamber including a driving motor, and a pressure relief valve is provided with the cylinder-shaped partition wall, and a thermal switch is disposed at the position to receive heat of the high-pressure coolant relieved from the pressure relief valve into the suction chamber, thereby, the pressure relief valve reduces pressure of coolant of the delivery chamber, and thermal switch is operated by the coolant from the pressure relief valve so as to stop the driving motor.

Abstract: A thermally actuated vapor vent valve in a fuel pump module allows hot vapor to escape from the fuel pump module when the module is at a temperature significantly above normal operating temperatures. The vent valve is carried by a retainer in the fuel pump module and, at normal operating temperatures, the vent valve is closed on a valve seat of the vapor outlet to prevent vapor flow through the vapor outlet. The valve and the retainer are formed of materials having different coefficients of thermal expansion such that when the fuel pump module becomes heated to a temperature sufficiently above normal operating temperatures the retainer expands and disengages the valve from the valve seat. This allows the hot vapor within the fuel pump module to flow out of the module and thereby prevents the fuel pump from overheating or reaching temperatures which can damage the electric motor that drives the fuel pump.

Abstract: A water saving device for use with domestic hot water systems of the noncirculating storage tank heater type uses a hydraulic motor to drive a water pump to pump water from the hot water pipe to a cold water pipe. Pumping begins when a hot water tap connected to the device is turned on and the water in the hot water pipe is cold. Pumping continues until a temperature sensing element senses that hot water has reached the water saving device and actuates a valve. When hot water is present at the device, the valve permits the hot water to pass directly to the tap instead of driving the hydraulic motor.

Abstract: A thermally responsive valve assembly (134) for scroll motorcompressor high temperature protection, which causes a high-side to low-side leak when excessive discharge gas temperatures are encountered, thereby causing the motor protector (35) to trip and de-energize the motor. The valve assembly (134) includes means motor ducting (200) the excessive temperature discharge gas to the lower portion of the motor/compressor shell (10) to the motor to circulate the high temperature gas throughout the motor cavity. The excessive temperature discharge gas heats the motor stator (20) and windings (32) which will in turn cause the motor protector (35) to trip and de-energize the motor.

Abstract: One or more thermostatically responsive valves are positioned to regulate the diversion of suction gas flow over the motor to cool the motor. The valves are responsive to motor temperature and only open when motor cooling is required.

Abstract: An apparatus for regulating a compressor lubrication system includes an oil-flooded rotary gas compressor, a reservoir flow connected to the compressor, a heat exchanger for cooling a lubricant, and a controller. A thermal mixing valve regulates the temperature of the lubricant flowing to the compressor. A first temperature sensor measures a discharge temperature at a compressor outlet, and provides a signal corresponding to the discharge temperature to the controller. A second temperature sensor measures a lubricant temperature at a lubricant inlet to the compressor, and provides a signal corresponding to the lubricant temperature to the controller. A pressure sensor measures the pressure at the outlet of the compressor, and provides a corresponding pressure signal to the controller. A valve means continuously regulates the supply of lubricant to the compressor, the valve means being controlled by the controller in response to the temperature signal of the first temperature sensor and the pressure signal.

Abstract: A hydraulic flow control apparatus has shape memory alloy springs which develop substantial actuating forces only at relatively high temperature. As a result of these forces, an only relatively slightly throttled connecting path between delivery side and intake side of a pump is opened directly or indirectly.

Abstract: A fuel flow arrangement has a fuel feed line (14) for feeding fuel from a tank (10) to an engine (12) with a fuel injection pump (20) by means of a pump (18), and a fuel return line (16) for returning excess fuel from the engine to the tank. A bypass passage (21) with an orifice (22) provides a communication between the feed and return lines and a flow restriction (24) is provided in the return line. When the fuel pressure created by the pump (18) is low, at low temperatures, fuel entering the return line will be recycled through the orifice (22) to the engine to increase the temperature of the fuel feed to the engine, and at higher temperatures, the flow through the orifice (22) will be reversed.

Abstract: A thermally responsive valve establishes a discharge to suction bleed responsive to the actuation of a thermal actuator. The thermal actuator is responsive to the temperature of the floor of the fixed scroll in a region near the outlet so that the actuator is responsive to local conditions indicative of impending failure.

Abstract: The first of two serially arranged electromagnetically operated spool type drain control valves is conditioned to select the drain passage of smaller the pump before the second valve is moved from its closed or communication cut-off position, to its open position wherein hydraulic fluid can be drained therethrough, whereby the spool of the second valve is exposed to a relatively small flow generated force which acts thereon and which tends to move the valve spool toward to its closed position. In another embodiment the hydraulic fluid temperature is monitored and used to modify the control schedule via which the switching of the drain control valves is determined. A further embodiment monitors a parameter which is indicative of the prime mover rotational speed and inhibits the closure of the second of the two valves when the rotational speed is above a predetermined limit.

Abstract: A low side scroll compressor is protected from both the potentially damaging effects of improper electrical hookup and the development of high discharge temperatures by the disposition of a valve member in a passage which communicates between the suction pressure portion and a discharge pressure portion of the compressor. The valve operates to permit the flow of gas from the suction to the discharge pressure portion of the compressor through a protective passage when the compressor is caused to run backwards due to miswiring so as to avert damage to the scroll members. The valve permits the flow of gas from the discharge to the suction pressure portion of the compressor through the protective passage when the temperature of the discharge gas produced by the compressor is caused to exceed a predetermined temperature. The resulting flow causes the compressor motor to de-energize.

Abstract: Rotary compressors such as screw compressors and scroll compressors are capable of reverse operation at shutdown as the system pressure seeks to equalize through the running gear. The present invention provides a valved bypass which is opened responsive to the initiation of stopping the compressor. The valve bypass may be located internally or externally with respect to the compressor.

Abstract: A thermally responsive valve arrangement for scroll motor-compressor high temperature protection, which causes a high-side to low-side leak when excessive discharge gas temperatures are encountered, thereby causing the motor protector to trip and deenergize to motor. A unique valve per se is also disclosed.

Abstract: A thermal relief valve is constructed and arranged to sense the fluid temperature of a pump and to open when this fluid temperature becomes too high to discharge pump fluid to prevent the occurrence of an excessive temperature condition of the pump fluid.

Abstract: A system for controlling vehicle passenger compartment comfort. An electronic controller receives input from: (i) a thermistor array sensing blower plenum discharge temperature, (ii) a user movable Level Select Control (iii) a potentiometer sensing the position of a servo driven heater core water valve, (iv) an engine rpm tachometer generator and (v) a blower speed signal. The controller employs proportional-integral-derivative (PID) strategy in a microprocessor to generate a control signal for operating the heater core valve servo motor to maintain the plenum discharge air at a constant temperature irrespective of wide swings in engine rpm/water pump speed or user selector changes in blower speed.

Abstract: In the screw compressing apparatus, by rotating paired male and female rotors as meshing with each other, a gas is suctioned from a suction chamber into a working space, and compressed in the working space. The compressed high pressure gas is exhausted from a exhaust chamber, and then cooled by an aftercooler, thereby producing a low temperature, high pressure gas. In the compression process, the temperature of the gas in the working space becomes high at the exhaust chamber side. When this exhaust gas temperature reaches a predetermined value, a part of the gas cooled by an aftercooler is injected into the working space for lowering the temperture in the working space.

Abstract: A thermal relief valve is constructed and arranged to sense the fluid temperature of a pump and to open when this fluid temperature becomes too high to discharge pump fluid to prevent the occurrence of an excessive temperature conditon of the pump fluid.

Abstract: A refrigeration system is disclosed which incorporates apparatus for preventing overheating of the compressor by selectively feeding liquid refrigerant from the outlet of the condenser to the compressor. In one embodiment the refrigerant fluid from the compressor is injected into the suction manifold of the compressor. In another embodiment this fluid is injected directly into the compression chamber or chambers. Control means are provided which include a temperature sensor located within the compressor discharge chamber and valve means responsive thereto to control the flow of liquid refrigerant to the suction manifold or compression chamber.

Abstract: A hydraulic motor for driving a hydraulically driven fan system in a combustion engine includes both an oil pump and water pump in combination. The pump body and a pump cover together define a hollow chamber within which a shaft is rotatably mounted, the shaft supporting an oil pump rotor. A thrust plate in the hollow chamber coaxially aligns the pump body and pump cover and supports a first bearing for the shaft. The water pump is mounted in the pump cover and includes an impeller mounted on the shaft.

Abstract: A sleeve seal heating system for a multi-stage compressor is disclosed. The multi-stage compressor includes a mechanism for dissipating heat disposed between and in fluid communication with a pair of compressor stages in the multi-stage compressor. The present invention contemplates the inclusion of a mechanism for selectively bypassing the heat dissipating mechanism disposed between and in fluid communication with each pair of compressor stages to thereby warm a succeeding stage, causing the stage to become substantially operative. The mechanism for bypassing comprises a bypass valve. The bypass valve is controlled to bypass the mechanism for dissipating heat when a seal in the succeeding stage is not substantially operative. Each compressor stage comprises a piston reciprocating within a piston chamber, the piston chamber being in valved communication with and between a fluid input and a fluid output.

Abstract: In a device for controlling the flow rate of a hydraulic operating fluid from a pump to a hydraulic motor for driving an engine cooling fan in a vehicle by modifying either the cross sectional opening area of a variable flow constriction means in the fluid conducting passage while maintaining a constant pressure difference across the flow constriction means by a pressure control valve responsive to the pressure difference across the flow constriction means or the pressure difference by selectively partly leaking the pressure transmitted from the downstream side of the flow constriction means to a rear side of the pressure control valve, a cold state relief means is provided to temporarily relieve the rear side of the pressure control valve at cold start of the engine to avoid abnormally high fluid pressure being supplied to the motor due to delayed response of the pressure control valve under high viscosity of the operating fluid at low temperature.

Abstract: Inside the hermetic shell of a scroll compressor, a normally closed solenoid valve seats against the back side of a stationary scroll plate to close a discharge opening through the plate. A coil circuit that actuates the valve has an electrical resistance that increases with temperature. The temperature dependent resistance allows the coil circuit to also function as a discharge temperature sensor. Should the discharge gas over-heat, the compressor motor and the valve are de-energized in response to the resistance exceeding a predetermined limit. The closed valve prevents backflow from rapidly reversing the rotational direction of the compressor, which can be extremely noisy and damaging to the compressor. Both the valve and the compressor motor are energized at the same time, regardless of the compressor's direction of rotation.

Abstract: A lubricating oil supply system has a pump for automatically supplying lubricating oil from an oil tank to a crankcase of an engine through a passage. A bimetallic strip is cantilevered on a wall of the passage. The bimetallic strip is provided so as to reduce the cross-sectional area of the passage at the bimetallic strip when the temperature of the strip rises and to increase the cross-sectional area when the temperature lowers.

Abstract: A variable-delivery compressor having plural compression chambers whose volume is changed as a rotor is rotated in a housing to compress a gas sucked through a suction port and deliver the compressed gas through a discharge port, comprising: a by-pass passage for communication between a compressing and a sucking compression chamber of the plural compression chambers; a device for changing the position of the discharge-side extremity of an opening of the by-pass passage on the side of the compressing compression chamber, in the circumferential direction of the rotor, to retard the compression start timing of the compressing compression chamber; and at least one of a variable flow restrictor device associated with a suction passage communicating with the suction port to adjust a flow of the gas which is sucked through the suction passage, and a pressure relief device including a pressure relief passage and a switching device for closing and opening the pressure relief device.

Abstract: A control apparatus and method for restricting liquid flow in a liquid moving pump, usually of the centrifugal type, to prevent pump cavitation and pump prime mover overloading. The control apparatus includes sensors to detect liquid temperature and pressure at the inlet of the pump. It may further include a device such as a current transformer to develop a signal indicative of power consumed by the prime mover of the pump where the prime mover is an electrical motor. The liquid pressure and temperature indications are used to generate a specific indication of the subcooling of the liquid. The temperature indication is used to derive an indication of the instantaneous required subcooling of the pump. The subcooling indication and the required subcooling indication are introduced to a comparator. Should the subcooling of the liquid fail to exceed the required subcooling, a first control signal is generated.

Abstract: A steering assistance pump includes a temperature sensor valve which communicates with an inlet duct of the pump by way of an additional, normally closed outlet passage. When a pressure relief valve responds, upon blocking of the servo steering means, the temperature of the hydraulic fluid in the system increases to such a degree that it exceeds the limit value of the temperature sensor valve. The valve thus opens to cause the pilot control pressure in the control chamber of the pressure relief valve to be reduced, whereby the pump pressure itself is also reduced.

Abstract: A surge detection system for a compressor having a gas inlet and a gas outlet and experiencing rapid temperature changes in said gas inlet during surges, said detector comprising first and second thermocouples having different temperature response times, Tf and Ts respectively, and producing an electrical output proportional to said rapid temperature change occurring during a surge, means for mounting said first and second thermocouples in said compressor gas inlet such that both thermocouples are subject to the same temperature changes, means for electrically coupling said thermocouples to algebraically sum said electrical outputs, Tf-Ts to represent the rapid change in temperature occurring between said thermocouples, and means coupled to said thermocouples for converting said algebraically summed outputs, Tf-Ts, to control signals representing the number, intensity and duration of said surges.

Abstract: A sliding vane type rotary compressor has at least two suction ports through which a refrigerant is sucked into the vane chamber. A combination of a spring made of a shape memory alloy adapted to expand and contract when supplied with an electric current and a spool operative by the spring is disposed in a flow passage through which the source of the refrigerant is connected to one of the suction ports leading to a region of higher magnitude of compression. The flow passage is opened and closed by the spool which is moved by the expansion and contraction of the spring of the shape memory alloy in response to the heat produced by the self heat build-up action when supplied with an electric current and also to the cooling function of the refrigerant of low temperature, thus attaining a variable capacity control of the compressor with a simple and compact construction.

Abstract: A variable capacity compressor has a low-pressure chamber, a high-pressure chamber receiving the discharge of some cylinders and a discharge chamber receiving the discharge of other cylinders. An on-off valve between the discharge chamber and low-pressure chamber is actuated by an electric device, such as a servo motor. An electric circuit controls the servo motor so that it is driven for a certain time period in response to a signal to change the compressor capacity. A device operative with the on-off valve moves the valve forward or backward according to the rotation of the servo motor and makes the motor idle when it reaches the fore or back end of movable space.