A WORK GENERATING DEVICE DRIVEN BY AN ELECTRIC MOTOR

Abstract: A work generating device (10; 33) comprises a movable working part (12; 35) which is driven by an electric motor (11) and which during a working cycle has a varying need of torque including at least one maximum value. The motor (11) is an electronically controlled reluctance motor having stator poles (21, 22; 23, 24) which are provided with windings (25), and rotor poles (26a, 26b) which cooperate with the stator poles and which are arranged on a rotor (26) made from soft magnetic material. The motor (11) is connected to the working part (12; 35) in such a way that one of the rotor positions where the pulsating torque of the motor has a maximum value coincides with the position of the working part in which the need of torque is at a maximum.

Description: A work generating device driven by an electric motor The present invention relates to a work generating device of the kind indicated in the preamble of appending claim 1. One example of a work generating device having a varying need of torque is a compressor included in a refrigeration apparatus or in an air conditioning arran- gement. Here, the compressor motor has to be dimensioned in accordance with the maximum need of torque of the compressor. This need of torque appears during the compression phase and in terms of time it takes up only a minor part of the working cycle. If, in a piston compressor the working cycle is converted into one turn of a disc operating the piston and driven by the motor, the maximum need of torque appears during the last 90 degrees of the turn of the disc whereas during the remaining parts of the turn the need of torque is only a fraction of the maximum need. This means that the motor is working in an uneconomical way and that during periods of low need of torque a greater power than necessary is generated by the motor. The type of motor traditionally used in driving refrigeration compressors is the induction motor. This motor has the advantage of being simple and robust at the same time as it does not include any spark generating commutator device having brushes. The construction of the motor and the frequency of the mains voltage deter- mine the revolutional velocity of the motor and the usual way of achieving a constant temperature in a refrigerator or a freezer is to have the compressor operate intermittently. The motor is dimensioned to be safely started even in case of a high back-pressure in the refrigerant system operated by the motor. This means that at operating speed the power factor of the motor (cos ) is low and, accordingly, at this speed the motor suffers from unnecessarily large losses. As there is a demand for torque during about a quarter of the turn only, during three quarters of the turn the motor will unnecessarily consume energy. The object of the invention is to remedy the draw-backs indicated above and to provide a work generating device in which the torque emitted by the drive motor has been adapted to the need of torque. The object is achieved in a device having the features indicated in claim 1. Preferred embodiments appear from the appending sub-claims. The invention will now be described more in detail in connection with two embodiments and with reference to the enclosed drawings. One embodiment refers to a piston compressor while the other one is a rotary compressor. In the drawings Fig. 1 shows a torque diagram for a piston compressor while Fig. 2 shows a corre- sponding diagram for a drive motor designed in accordance with the invention. Fig. 3 is a vertical section through, a piston compressor having a built-in motor. Fig. 4 is a top view of the compressor according to Fig. 3 in which the upper part of a casing enclosing the compressor has been removed. Fig. 5 is a schematic hori¬ zontal section along the line V-V of Fig. 3. Fig. 6 schematically shows the mutual relations between the piston and the motor. Finally, Fig. 7 - 9 schematically shows a rotary compressor with the rotor situated in various positions. In the diagram of Fig. 1 the torque M.^ of a piston compressor has been plotted as a function of the turning angle of a disc which drives the piston and is connected to the drive motor. The working cycle of the piston corresponds to turning of the disc through one turn or 360 degrees. As appears from the diagram the need of torque is very low and even negative during the first 180 degrees and increases only slightly between 180 and 210 degrees. The following increase takes place at an accelerating rate and a torque peak appears in the range just after 270 degrees. Even at 300 degrees there is still a high need of torque but the need then rapidly decreases before a new working cycle starts. Like Fig. 1, Fig. 2 shows a torque diagram, however, referring to a two-phase reluctance motor. The motor used comprises two pairs of stator poles which form a cross. The stator poles cooperate with rotor poles which are disposed diametri¬ cally opposite each other on a rotor consisting of soft magnetic material. As is shown in the diagram the motor has a pulsating torque which has four extreme values or peaks during one turn of the rotor. Now according to the inventive idea the torque graphs of Fig. 1 and Fig. 2, respectively, are to be mutually adapted so that the torque graph of the motor has a peak a the same time as the need of torque of the compressor is at a maximum. Now, the task is to physically interconnect the motor and the compressor so as to achieve the desired adaptation. In Fig. 3 there is shown a vertical section of a compressor 10 which is driven by a reluctance motor 11. The compressor comprises a piston 12 moving in a cylinder 13 closed by a cylinder head 14. Valves, not shown, are disposed in the cylinder head for sucking in a refrigerant into the cylinder and outputting same to a cooling system, not shown. By a piston pin 15 the piston is connected to one end of a connecting rod 16 the other end of which is turnably journalled on a pin 17 which is diposed excentrically on a circular disc 18 secured to a shaft 19 which is at the same time the rotor shaft of the reluctance motor 11. In the usual way a counterweight 20 is arranged to balance the movement of the connecting rod. As indicated by way of introduction the compressor motor is a reluctance motor having the special character of a pulsating torque. The motor chosen comp¬ rises two pairs of stator poles 21, 22; 23, 24, see Fig. 6, of which the pair 21, 22, is also shown in Fig. 3. The stator poles support windings 25 which are activated such that the two poles of each pair are simultaneously operating. A rotor 26 is mounted on the shaft 19, said rotor being made from soft magnetic material and having two diametrically opposite poles 26a, 26b. Each pole comprises two parts 28, 29 of the same overall shape. The part 28 has a greater air-gap whereas the part 29 has a smaller air-gap with respect to the stator poles which are all of iden¬ tical shape. The two pole parts are arranged such that in the direction of rotation the part having the greater air-gap is the leading one. By the rotor pole shape de- scribed it will be possible to drive the rotor during a greater angle of rotation. In addition, the peripheral extension of the pole is chosen such that upon the part 29 of the smaller air-gap being positioned in front of a stator pole, the part 28 of the greater air-gap is about to turn in over one of the poles of the other pair of stator poles. Alternatively, the pole extension can be chosen such that in the posi- tion described the rotor pole part 28 of the greater air-gap to some extent overlaps the stator pole. The stator pole windings 25 of the reluctance motor are magnetized alter¬ nately and in that way the rotor is forced to rotate in the direction determined by the shape of the rotor poles, namely in the direction in which the pole part 28 of the greater air-gap is leading. In order for the motor to operate there is a demand for a magnetizing current to be supplied to the stator windings 25 in correct order and this is effected by an electronic control arrangement, generally referred to by 30. Moreover, a sensing device is required giving rotor position information to the control arrangement. In the example of Figs. 1 - 6 a Hall effect sensor 31 is used which cooperates with a wing 32 secured to the rotor shaft and having essen¬ tially the same shape as the rotor, as seen in Fig. 6. The schematic Fig. 6 shows, in terms of turning, the interrelation between the rotor and the circular disc 18 at the moment of maximum need of torque. This would be expected to take place at the upper dead point of the piston but that is not the case. Instead, the maximum need of torque of the compressor occurs somewhat before the piston reaches the upper dead point which corresponds to the angle °C in the figure. It has proved that for optimum results the angle should be in the area of 20 - 55 degrees and preferably equal 47 degrees. In Figs. 7 - 9 there is shown an example of using the invention in a rotary compressor 33. The compressor comprises a cylindrical space 34 in which a rotor 35 is rotated by a reluctance motor, not shown, for example of the kind described with reference to Figs. 1 - 6. The rotor is journalled excentrically and, suitably, it is fixed directly to the rotor shaft 36 of the motor. A slide 37 is spring biased into sealing engagement with the peripheral surface of the rotor. In that way a space 40 is formed between the cylinder wall 39, the peripheral surface 38 of the rotor and the slide 37, the size of which is continuously changing during the turning of the rotor. In Fig. 7 the rotor is shown in a position corresponding to the upper dead point of the piston compressor. In Fig. 8 the piston has moved some distance and in this position the sucking-in phase has started. In Fig. 9 the rotor has reached a position in which the space 40 is at a minimum and in this position the motor shall provide the maximum torque. Even in this case the interrelation between the compressor rotor 35 and the motor rotor should be such that the compressor rotor is positioned 20 - 55 degrees before the position corresponding to the upper dead point.

Claims: Claims 1. A work generating device comprising a movable working part (12; 35) driven by an electric motor (11) and during a working cycle having a varying need of torque including at least one maximum value, c h a r a c t e r i z e d in that the motor (11) is an electronically controlled reluctance motor having stator poles (21, 22; 23, 24), which are provided with windings (25), and rotor poles (26a, 26b) which cooperate with the stator poles and which are arranged on a rotor (26) made from soft magnetic material, the said motor (11) being connected to the working part (12; 35) in such a way that one of the rotor positions where the pulsating torque of the motor has a maximum value coincides with the position of the working part in which the need of torque is at a maximum. 2. A device according to claim 1, c h a r a c t e r i z e d in that it is a compressor. 3. A device according to claim 2, characterized in that the compressor is a refrigerant compressor. 4. A device according to claim 2 or claim 3, c h a r a c t e r i z e d in that the working part is constituted by a piston (12) of a piston compressor (10), the motor (11) being connected to the piston (12) in such a way that one of the peak^s of the pulsating torque of the motor coincides with a position being passed by the piston (12) on its way to the upper dead point and corresponding to an angular dis- tance for the rotor (26) from the rotor position corresponding to the upper dead point of the piston amounting to 20 - 55 degrees. 5. A device according to claim 4, characterized in that the said angular distance amounts to 47 degrees. 6. A device according to claim 4 or claim 5, c h a r a c t e r iz e d in that the piston (12) moves in a cylinder (13) situated such that the direction of movement of the piston is perpendicular to the rotor shaft (19) of the motor, the piston (12) being connected to a pin (17) via a connecting rod (16), said pin being excentrically positioned on a disc (18) secured to the rotor shaft (19). 7. A device according to claim 2 or claim 3, c h a ra c t e r iz e d in that the working part is a cylindrical rotary part (35) which is journalled excentrically and rotates in a cylindrical space (34) in a rotary compressor (33), a movable slide (37), sealingly engaging with the peripheral surface (38) of the space (34), and the motor (11) being connected with the rotary part (35) such that one of the peaks of the pulsating torque of the motor coincides with a position of the rotary part which, as seen in the direction of rotation, is positioned at an angular distance of 20 - 55 degrees before the position in which the volume delimited by the rotary part (35), the wall (39) of the cylinder space and the slide (37) is at a minimum. 8. A device according to claim 7, characterized in that the cylindrical rotary part (35) is disposed directly on the rotor shaft (36) of the motor.

Inventors: MOREN LARS GUNNAR, MOREN, LARS, GUNNAR
Applicants: ELECTROLUX MECATRONIK AB, ELECTROLUX MECATRONIK AKTIEBOLAG
Application year: 1989
Main classification: H02K7
IPC codes: F04B35, H02K7, H02K19, H02K29