SE439181B - Micro processor controller of compression ratio with full load in equivalence to a screw compressors driving motor current as well as a procedure for controlling an electric motor powered compressor - Google Patents

Abstract

The invention refers to a control device in order to change the inner compression conditions in the compressor (10) when it works under full load conditions and to at the same time sense the compressor motor current. The compression conditions are changed through affecting a composite vent (32, 33) which works together with the compressor's rotors (18, 19). The composite vent is affected in one direction, which is determined by an accompanying computer programme (110, 112), as long as the sensed current decreases. When the current begins to increase the direction is shifted and so forth. Should the intake pressure sink under a pre-stated "best value" the vent sections are separated (32, 33) in order to allow the compressor to work at a lower load.<IMAGE>

Description

U.S. Pat. Nos. 4,249,866 and 4,351,160 further illustrate the state of the art.

The present invention is directed to control devices for changing the internal compression ratio of the compressor when operating under full load conditions and for simultaneously sensing the compressor motor current.

The compression ratio is changed by acting on a composite valve which cooperates with the rotors of the compressor. The composite valve is actuated in a direction which is determined by an associated computer program, as far as the sensed current decreases. When the current starts to increase, the direction is reversed, and so on. Should the intake pressure drop below a predetermined "setpoint", the valve sections are separated to allow the compressor to operate at a lower load.

The invention will now be described in connection with two preferred embodiments shown in the accompanying drawing.

Fig. 1 shows a horizontal section through a screw compressor in accordance with the present invention, certain parts being omitted for the sake of clarity, Fig. 2 is a cross-section through a part of the compressor along the line 2-2 in Fig. 1, Fig. 3 is a view similar to Fig. 1 showing the slide valve and slide stop in positions other than those shown in Fig. 1, Fig. 4 shows a schematic wiring diagram including the control circuit, Fig. 5 is a view of the same type as shown in Fig. 1, but of a modified embodiment and Fig. 6 shows a schematic view including the control circuit of the modified embodiment according to Fig. 5.

As can be seen from the drawings, especially now Figs. 1-4, a screw compressor 10 has a central rotor housing 11, an intake housing 12 and an outlet housing 13, which are tightly connected to each other. The rotor housing has interconnecting bores 15 and 16 which form the working space for interlocking male and female screw rotors or shells 18 and 19 which are arranged to rotate about their mutual parallel axes in suitable bearings.

The rotor 18 is rotatably arranged with a drive shaft 20 accommodated in a bearing (not shown) in the socket housing 13 and in a bearing 22 located in the intake housing 12. The shaft 20 extends outside the socket housing 13 to be connected to a motor (not shown) by means of a suitable coupling.

The compressor has an intake passage 25 in the intake housing 12 which communicates with the working space through a port 26.

An outlet passage 28 in the outlet housing 13 communicates with the work space via the gate 29, which is at least partially located in the outlet housing 13.

From the embodiment shown, it can be seen that in the case of a horizontally positioned machine, the inlet port 26 lies substantially above a horizontal plane through the rotor shafts, and that the outlet port 29 lies substantially below this plane.

In the middle below the rotor cylinders 15 and 16 and axially parallel thereto is a longitudinally extending cylindrical recess 30 which communicates with both the inlet and outlet port.

In the cylindrical recess 30 there is displaceably a valve member comprising a slide valve 32 and a cooperating slide stop 33. The inside 35 of the slide valve and the inside 36 of the slide stop lie opposite the outer periphery of the rotors 18 and 19 in the rotor housing 11.

The right end of the slide valve has, as shown in Fig. 1, an open part 38 which on its upper part forms a radial port which communicates with the outlet port 29. The left end 39 may be flat or shaped as desired to fit the right end 40 of the valve stop for that the two opposite ends of the slide valve and the slide stop should seal the recess 30 against the rotor cylinders 15 and 16. The slide valve has an internal bore 42 and a head 43 at one end. A rod 44 is attached by means of fasteners 45 at one end to the head through which it extends, and at its other end it is attached to a piston 46. The piston is arranged to move back and forth in the barrel 47 of the cylinder 48 which is connected to and extends axially from the intake housing 12. A lid or end 50 is located on the outer end of the cylinder 48.

The intake housing 12 is connected to the cylinder 48 by means of an inlet lid 51, which receives the end portion 52 of the cylinder 48 with a turned diameter.

Inside the inlet cover 51 is a sleeve 54 with a collar 55 at one end and extends longitudinally towards the rotor housing 11. The slide stop 33 has a front part 56 which ends with the end surface 40. Furthermore, the front part has an angled slot 57 on its underside, the bottom of which rises from left to right of the drawing. The axial length of the slot is so adapted that it allows maximum desired movement of the slide stop. Extending from the front of the slide stop is a main part 58 which is slidably received in the sleeve 54. At its other end, the slide stop has a piston 60, which is held by means of a suitable fastening device 61.

A stationary partition 62 is secured in the cylinder 48 between its ends and divides its interior into an outer chamber 64 in which the piston 46 moves and an inner chamber 66 in which the piston 60 moves. The cylinder 48 has fluid ports 67 and 68 close to each side of the partition 62, which communicate with the chambers 64 and 66, respectively. At the outer end of the cylinder 48, a fluid port 70 is provided and communicates with the chamber 64 fixedly on the other side of the piston 46. At its inner end, the cylinder 48 has a port 72 which communicates with a recess 73 in the outer end face of the collar 55 for supplying and diverting fluid from the chamber 66 fixed on the other side of the piston 60, seen from the port 68.

The slide stop has an inner bore 74 with the same inner diameter as the bore 42 in the slide valve 32 which communicates with this bore. At its other end, the slide stop has a ledge 75 against which the piston 60 comes into abutment.

A self-relieving coil spring 76 sits in the coaxial bores 74 and 42 around the rod 44 and tends to force the slide valve 32 to the closed position and to force the slide stop to abut against the partition 62. In such a position, the slide valve and the slide stop are separated at maximum mutual distance.

In use, the working fluid, for example a cooling gas, enters the compressor via the inlet 25 and the port 26 to the rotors 18 and 19. During the rotation of the rotors, V-shaped compression chambers are formed which take up the gas and which progressively reduce their volume as the compression the chambers move towards the inside of the outlet housing 13. The fluid is discharged as the tops of the rotor threads, which form the leading edge of a compression chamber, pass the edge of the port 38 communicating with the outlet 28. A positioning of the slide valve 32 further away from the outlet housing 30 reduces the compression ratio by enlarge the last compression chamber. Positioning in the direction of the outlet housing with the wear stop abutting the slide valve has the opposite effect. In this way, the movements of the slide valve change the compression ratio and the pressure of the gas discharged from the compressor.

The compressor and its control means are operated so that they continuously vary and seek the optimal compression ratio on the basis of the lowest required current to drive the compressor motor under full load conditions. As described, the slide valve and slide stop will be controlled as a composite unit to vary the internal compression ratio of the compressor by sensing the motor current and finding the position that results in the lowest possible motor current. If relief is required, the slide valve and the slide stop will move from each other, as indicated in Fig. 3. The space between them is then connected to the interlocking rotors 18 and 19. to allow working fluid in a compression chamber between the inlet pressure rotors to remain in communication with the inlet through the slot 78 and a channel not shown in the housing 11 thereby reducing the volume of the fluid being compressed and causing the compressor to operate at a lower load .

The present invention also includes a control system for moving the slide valve and the valve stop in accordance with a predetermined program for achieving the above objects. To achieve this, four variables are continuously sensed in the compressor and an electrical circuit is supplied. Thus, the outlet housing 13 has a connection opening 80 which is connected via a line 81 to an outlet pressure transducer 82. The outlet housing 12 has a connection opening 84 which is connected via an connecting line 83 to an intake pressure transducer 86.

The movable member 91 of the potentiometer 90 extends through the wall of the rotor housing 11 to engage the sloping slot 57 in the slide stop 33 and acts as P1 to control a voltage divider 92. The potentiometer 94 has its movable member 95 inserted into the cylinder end 50 to engage with the piston rod 44 of the slide valve 32 and serves as P2 for the voltage divider 96 of the control circuit.

The voltage divider 92 has calibration resistances R1 and R2 and transmits a 1-5 V DC signal to the analog input module 98 via lines 100 and 101.

Similarly, the voltage divider 96 has calibration resistors R3 and R4 and outputs a 1-5 V DC signal to the analog input module 98 via lines 102 and 103.

The output pressure converter 82 and the intake pressure converter 86 convert their respective received signals into a 1-5 V direct current signal and lead it via lines 104, 105, 106, 107 to the analog input module.

The module 98 converts the received signals into digital signals and transmits them to the microcomputer 110.

The microcomputer 110 has a program 112 of the pre-given type so that the computer output provides the intended control of the slide valve 32 and the slide stop 33. A suitable screen or display 114 is connected to the computer 110 for to indicate the current positions of the slide valve and the slide stop on the basis of the signals received from potentiometers 90 and 94.

Four control signals are output from the computer 110 through the outputs 116, 117, 118 and 119. The two signals from the voltage parts 92 and 96 corresponding to the position of the slide stop and the slide valve and the two signals from the output pressure transducer 82 and the intake pressure transducer 86 are connected through the analog input to the microcomputer. are processed there so that they emit suitable outputs 116 - 119. The outputs 116 and 117 are connected to solenoids 120 and 121 via lines 122 and 123, respectively. The outputs 118 and 119 are connected to solenoid valves 125 and 126 through lines 127 and 128, respectively.

The electromagnets 120 and 121 control hydraulic circuits via the control valve 130, which position the slide stop 33. The electromagnets 125 and 126 control hydraulic circuits along the control valve 131 which position the slide valve 32.

The control valve 130 is connected via a line 134 to or other suitable liquid under pressurized lubrication system. Line 135 connects valve 130 to fluid port 72 and line 136 connects valve to fluid port 68. A return line 137 is connected to the compressor inlet area.

The control valve 131 is connected via line 134 to the oil pressure source and via line 137 to the return. Line 138 connects valve 131 to fluid port 70 while line 139 connects valve 131 to fluid port 67.

Activation of the magnet 120 of the valve 130 transmits the valve in such a position that the flow takes place in accordance with the schematic representation on the left side of the valve.

The flow starts at "P", goes to "B" and thus applies the piston 60 and a pressure oil source from the compressor oil pressure via line 136 to the left 8402996-6 10 15 20 25 30 35 8 simultaneously diverts oil from the opposite side of the piston via line 135 to the valve port "A", thence to "T" and to the oil return, this forces the piston and its associated slide stop to the right, as shown in Fig. 1 of the drawing.

Activation of the electromagnet 121 of the valve 130 positions the valve so that the flow takes place in accordance with the schematic representation on the right side of the valve, ie the flow goes from "P" to "A" and in this way applies oil pressure via line 135 to the chamber to the right of the piston 60 and transfers it to the left at the same time as oil is diverted from the opposite side of the piston via line 136 to the valve from "B" to "T" and to the oil return.

Similarly, actuation of the solenoid valve 125 causes valve 131 to be positioned so that port "P" goes to "B" to apply pressure through fluid port 70 and divert fluid through port 67 from "A" to "T" to move slide the slide valve to the right as shown in the drawing.

Magnetizing the electromagnet 126 positions the valve 131 to position from "P" to "A" to apply pressure through the fluid port 67 and divert fluid through the fluid port 70 from "B" to "T" to displace the slide valve to the left.

When the compressor is used in a cooling system, a normal displacement of its slide valve is desired so as to maintain a certain suction which is normally referred to as "setpoint" or "set point". Alternatively, other parameters such as the temperature of the product treated in a The system of the present invention aims to supply the microcomputer 110 with a desired setpoint through suitable switches connected to a control panel (not shown), such as coefficients connected to the compressor cooling system connected to the slide valve and thus to the capacity of the compressor. - "operates with the display 114. The control panel may also include devices for controlling the mode of operation, ie automatically or manually, as well as the mode of operation of the slide stop, the slide valve 10 15 20 25 30 35 8402996-6 9 and the compressor. The displayed values of the display 114 from the microcomputer 110 are based on the received signals. The required electrical connections are made between the control panel and the microcomputer 110 to achieve the desired function by means well known to those skilled in the art.

To achieve the object of the present invention, another variable, the compressor motor current, is also sensed and input to the circuit. Thus, the motor current converter 140 is connected to the motor (not shown). The converter 140 is connected by lines 141 and 142 to the analog input module 98 which in turn is connected to the microcomputer 110. The microcomputer is programmed to relieve the compressor if the motor current exceeds a given value. This is achieved by providing suitable disassembly of the slide valve and the slide stop.

When the microcomputer detects operation at full load, its program causes the slide valve and the slide stop to move together as a unit an extra distance in one direction, as determined in advance by the program. If such a movement at work under full load causes the sensed motor currents to decrease, the computer program causes an additional extra movement in the same direction. This continues until the current has reached its lowest level and begins to increase. Then the program changes direction of movement and seeks again the position in which the current reaches its minimum.

Should the initial movement of the composite valve lead to an increase in current, the program calls for reversal of direction and continues in this direction until the position with minimal current has been passed.

The feedback of the potentiometers for both the slide stop and the slide valve is used to determine whether there is an opposite relationship or harmony between the desired mechanical position of the slide stop and the actual mechanical position of the slide valve. If there is an opposite condition, locate the slide valve temporarily so that the positioning of the slide stop takes precedence. 10 15 20 25 30 35 '3402996- * 6 10 The system also has devices by means of which suitable control measures indicated on the control panel can be performed to allow manual positioning of both the slide valve and the slide stop. Although hydraulic means have been described for displacing the slide stop and the slide valve, it is obvious that other means well known to those skilled in the art can also be used for this purpose. For example, electric stepper motors or stepper motor-controlled hydraulic means can be used if desired.

Figures 5 and 6 show a modification of the embodiment described above. Instead of having a spring 76 which tends to disassemble the slide valve and the slide stop as takes place in the embodiment according to Figs. 1-4, the spring 76 is arranged around the rod 44 in the bore 74 'for the slide stop only. Its left end extends through the slide abutment abutment against the partition 62 and its right end abuts the right end of the bore 74 '.

In this way, the spring assists the movement of the slide stop to the right as shown in Fig. 5 while counteracting its movement to the left. The bore 42 in the slide valve, as shown in Fig. 1, has been omitted in the embodiment according to Fig. 5.

Another difference is that outputs 5 and 6 have been added, which have been given the reference numerals 148 and 143 and are connected to the microcomputer 110. The output 5 is connected via line 144 to a magnetic coil 145 which controls the flow through a shunt line 146 between lines 136 and 135. A non-return valve 147 is also arranged in line 146. The outlet 6 is connected by means of line 150 to the solenoid 151 which controls the flow through a shunt branch 152 between the lines 139 and 138, a non-return valve 153 also being arranged here in the shunt line 152.

When the device according to Figs. 5 and 6 operates and it is indicated that the machine is operating at full load, the program shifts the slide valve and the slide stop together a pre-given extra distance in accordance with what the program determines. The slide valve and the slide stop move together to the right as shown in Figs. 5 and 6 as a result of the actuator of the solenoid A, 120, which causes the hydraulic pressure against the piston 60 to shift it to the right. At the same time, the solenoid 151 in the shunt line 152 between 139 and 138 allows an oil shunt from the right side of the piston 46 to its left side.

When the movement of the combination to the right while maintaining full load causes the current to drop, the program moves the combination further to the right.

This continues until the current reaches its lowest value and begins to rise again. Then the program moves the slide valve and slide stop back in the direction of decreasing current and searches for a lowest value.

The joint movement of the slide valve and the slide stop to the left is effected by activating magnetic coil B, 126, which allows oil pressure to be applied to the right side of the piston 46 and activates the shunt magnetic coil 145, whereby oil on the left side of the piston 60 can flow to the right side of the piston 60.

Claims (9)

84-02996-'6 10 15 20 25 30 12 PATENTKRAV Microprocessor control of the compression ratio at full load corresponding to the drive motor current of a screw compressor, wherein the screw compressor has interlocking screw rotors (18, 19) with mutually parallel shafts in a housing (11) with interconnecting cylindrical bores. (15, 16), a high pressure end wall (13) at one end of the housing (11) and a low pressure end wall (12) at its other end, the low pressure end wall (12) having an inlet opening (26) and the high pressure end wall. an outlet opening (28, 29), an axially extending recess (30) in the housing (11) in open communication with said bores (15, 16), a slide valve member (32) which is axially movable in the recess (30 ), the slide valve means being in sealing relationship with the rotors (18, 19) and having an outlet means (38, 43) at its one end which is opposite the high pressure end wall (13), the displacement of the slide member (32) being arranged to provide a change the compression ratio, feel sensing means (140) for the current of the electric drive motor, sensing means (82) for sensing when the compressor is operating at full load, means (46, 131) for extra displacement of the slide valve means (32) in both directions and means (110, 112) which are suitable for full load operation in order to displace the slide valve means (32) in a first direction when the motor current decreases, -to reverse to the opposite direction when increasing the motor current, to continue the movement in the opposite direction while the motor current decreases and to change back to the first direction when the motor current increases. Device according to claim 1, characterized in that the slide valve means (32) comprises a slide valve (32) or a slide stop (33), the slide valve (32) having a rear side (39) at its outlet. The end (38, 43) and the slide stop has a front side (40) which can be brought into abutment against the back of the slide valve to form a continuous, composite member, the slide valve ( 32) and the slide stop (33) are movable apart to form an opening and selectable size and in the axial inlet opening (26) to provide a variable position therebetween in connection with providing selective relief of the compressor, further with sensing means (86 ) for the pressure at said inlet (26) and actuators (60, 130) for disassembling the slide valve (32) and the slide stop (33) when the inlet pressure drops below a predetermined setpoint. Device according to claim 1 or 2, characterized in the outlet opening (29), second sensing means (86) of first sensing means (82) for sensing the pressure in the inlet opening, third sensing means (94) for position of the slide stop (33), fourth sensing means (90) for the position of the slide valve (32), means (98, 110) which are actuated by said first, second, third and fourth means and are arranged to emit proportional electrical output signals thereto, and means (98) which supply said electrical output signals to a microcomputer (110) which is programmed in accordance with predetermined parameters and arranged to control the movements of the slide valve (32) and the slide stop (33). Device according to claim 2, characterized in that means are provided which continuously press the slide stop (33) in the direction of the slide valve (32). Device according to claim 2, characterized in that the slide stop (33) is connected to a first piston (60) by means of a first member (48), that the slide valve (32) is connected to a second member (44) by a second member (44). second piston (46), that the first and second pistons are movable along the same axis, that a partition 62) separates the pistons so that they move in a first and in a second chamber isolated from each other (64, 66), or ä nnet sign - 84Ü2996 = 6 10 15 20 25 30 35 14 gan (67, 68, 71, 72) for supplying and diverting fluid to and from said chambers on either side of the first and second pistons (46, 60) for controlling their movements, the first member (58) having an axial bore (74) and the second member (44) extending through the slide stop (33), said bore (74), the first piston (60) and the partition wall (62), and that a compressible member (76) in the bore (74) surrounds the other member (44) and supports the sheath stop (33) and the partition wall (62) and tries to press sl the abutment (33) against the slide valve (32). Device according to claim 5, characterized in that the means (67, 68, 70, 72) for supplying and diverting fluid to and from the chambers (64, 66) on characteristics - both sides of the first and second piston (60, 46) includes first means (120) for displacing the first piston (60) in one direction, second means (145) arranged to allow fluid in the second chamber (64) to pass from the second piston (46). ) one to its second side, third means (126) arranged to displace the second piston (46) in the second direction and fourth means (151) arranged to allow fluid in the first chamber (66) to pass from the first piston (60). ) one to its other side. 7. Apparatus according to claim 1, characterized in - means for selectively and further raising and lowering the internal compression ratio of the compressor, means for operating the means for raising and lowering the compression ratio in a manner when the motor current decreases, to change to a different mode of operation with increasing motor current, to continue working in the latter way while the motor current decreases and to return to the first mode with a new increase of motor current. Device according to claim 1, 2 or 3, characterized by a fifth means sensing the current of the motor, pressure transducers controlled by the first and second means (82, 86) and arranged to emit proportional electrical output signals thereto, 10 15 20 25 30 8402996-6 15 electrical circuit means for setting the third (94), fourth (90) and fifth (140) means and are arranged to emit proportional electrical output signals thereto, a means (98) for outputting the output signals to a microcomputer (110), the microcomputer (110) being programmed (112) in accordance with predetermined parameters and arranged to control the movement of the slide stop (33) and the slide valve (32) which can one or more times in both directions, the microcomputer (110) being further programmed to, in response to full load operation, cause the slide valve (32) and the slide stop (33) to move as a single composite member in one direction when the motor current drops, that change offset direction when the motor current increases, to continue the movement in said second direction while the motor current decreases and to return to the first-mentioned direction in the event of a new increase of the motor current. Method of controlling an electric motor-driven compressor with interlocking rotors (18, 19), means for selective loading and unloading, a slide (32, 33) for axial displacement, the position of which determines the compression ratio, characterized therefrom , that the current of the compressor motor is sensed, thereby fully detecting, during full load operation, to displace the slide (32, 33) stepwise in one direction while the drive motor current decreases until the drive motor current begins to increase, to displace the slide (32, 33) stepwise t the other way while the drive motor current decreases and to continuously seek a minimum point for the current through displacement of the valve slide (32, 33).