WO1992008277A1 - Synchronous motors - Google Patents
Synchronous motors Download PDFInfo
- Publication number
- WO1992008277A1 WO1992008277A1 PCT/GB1991/001886 GB9101886W WO9208277A1 WO 1992008277 A1 WO1992008277 A1 WO 1992008277A1 GB 9101886 W GB9101886 W GB 9101886W WO 9208277 A1 WO9208277 A1 WO 9208277A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- instantaneous magnitude
- waveforms
- motor
- rotor
- sinusoid
- Prior art date
Links
- 230000001360 synchronised effect Effects 0.000 title claims abstract description 17
- 238000004804 winding Methods 0.000 claims abstract description 25
- 230000010355 oscillation Effects 0.000 claims description 7
- 230000001419 dependent effect Effects 0.000 abstract description 4
- 230000000694 effects Effects 0.000 description 8
- 238000012986 modification Methods 0.000 description 7
- 230000004048 modification Effects 0.000 description 7
- 238000013461 design Methods 0.000 description 4
- 238000013459 approach Methods 0.000 description 3
- 238000010276 construction Methods 0.000 description 3
- 230000003247 decreasing effect Effects 0.000 description 3
- 238000010586 diagram Methods 0.000 description 2
- 238000000034 method Methods 0.000 description 2
- 230000004075 alteration Effects 0.000 description 1
- 238000012937 correction Methods 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 230000003534 oscillatory effect Effects 0.000 description 1
- 238000005482 strain hardening Methods 0.000 description 1
- 230000036962 time dependent Effects 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
- H02P8/00—Arrangements for controlling dynamo-electric motors rotating step by step
- H02P8/42—Arrangements for controlling dynamo-electric motors rotating step by step characterised by non-stepper motors being operated step by step
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
- H02P25/00—Arrangements or methods for the control of AC motors characterised by the kind of AC motor or by structural details
- H02P25/02—Arrangements or methods for the control of AC motors characterised by the kind of AC motor or by structural details characterised by the kind of motor
- H02P25/022—Synchronous motors
- H02P25/024—Synchronous motors controlled by supply frequency
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
- H02P8/00—Arrangements for controlling dynamo-electric motors rotating step by step
- H02P8/14—Arrangements for controlling speed or speed and torque
Definitions
- the present invention relates to synchronous electrical motors, and more specifically to synchronous motors of the press construction type.
- synchronous motors can be operated to rotate by appropriate energisation of coils of the motor and thus it is possible to rotate the motor to a predetermined position by causing the wave forms applied to the coils to take up certain values, usually related to sin ⁇ and cos ⁇ .
- stator coils In a typical synchronous motor ac voltages are applied to stator coils which thereby generate a rotating magnetic field. In high power applications there are commonly three or a multiple of three stator coils and the 3-phase mains supply is used accordingly. In low power applications it is also known to use two or a multiple of two stator coils.
- the rotor in such a synchronous motor has a magnetic field which is fixed in position with respect to the rotor. In high power applications this is typically generated using a d.c. supply. In low power applications this may be produced by the use of a permanent magnet.
- motors having a stator magnetic field which rotates with respect to the stator and a rotor magnetic field which is stationary with respect to the rotor
- motors which have a stator magnetic field which is stationary with respect to the stator and a rotor magnetic field which rotates with respect to the rotor.
- the present invention is equally applicable, with necessary and obvious modifications, to such motors.
- the present invention provides a synchronous motor in which the voltage waveforms applied to the coils of the motor depart from the theoretically required waveforms so as to generate a stator magnetic field which takes account of the non-ideal nature of the motor so as to generate a substantially smooth rotation of the rotor.
- the rotor field should lag behind the stator field by a constant angle S .
- the stator field does not have the constant magnitude and varying phase required by the theory and the rotor time position vector "hunts" along the two differing load lines.
- the rotor torque angle varies as a function of any back emf imbalance and the resultant positional error follows a sinusoidal error pattern having a frequency of twice the basic frequency.
- stator coils each have exactly the same number of turns and hence any imbalance of voltage results from a lack of magnetic symmetry, in particular from the axial position of the rotor .
- this invention also provides a synchronous motor as described above in which the axial position of the rotor may be fixed after the rotor has been allowed to locate the axial position nearest to magnetic equality for all of the stator coils.
- motors according to the present invention produce a smoother rotation of the rotor than prior art motors. This mean ' s they may be advantageously used in instrumentation application for instance in car instrumentation.
- the present invention may also be advantageously used in stepper motors which are a particular form of synchronous motor.
- Figure 1 shows a schematic diagram of a typical two-winding synchronous motor
- Figure 2 illustrates waveforms used in driving motors such as illustrated in Figure 1 ;
- Figure 3 illustrates detent torque as experienced in motors such as illustrated in Figure 1 ;
- Figure 4 illustrates preferred apparatus for determining the waveforms to be used according to a preferred embodiment of the invention
- Figure 5 illustrates an alternative waveform which may be used in an embodiment of the invention
- Figure 6 is a schematic illustration of a motor drive circuit according to the present invention.
- FIG. 7 illustrates in more detail a preferred control circuit according to the invention
- Figure 8 illustrates apparatus implementing a further feature of the invention.
- Figure 9 shows, partially cut away, a motor according to the present invention.
- FIG. 1 shows a schematic diagram of a typical two-winding synchronous motor.
- the rotor 1 is represented as a permanent magnet having a north pole N and a south pole S.
- the two stator coils are represented at A and B.
- the stator positions diametrically opposite to A and B are designated A' and B' respectively.
- FIG. 2 illustrates in solid lines the theoretical voltage waveforms, V a and Vj-, which are applied to coils A and B respectively in order to generate a rotating stator magnetic field.
- rotor 1 and its associated magnetic field rotate in synchronism with the rotating stator field.
- Table 1 overleaf sets out the position taken by rotor pole N for given values of wt for a complete revolution.
- the detent torque is attempting to align the rotor with a line B-B 1 , generating an anti-clockwise torque and acting to decelerate the rotor.
- ⁇ is in the range ⁇ r/4 - T ⁇ /2
- the detent torque is attempting to align the rotor with a line A-A 1 generating a clockwise torque and acting to accelerate the rotor.
- the rotor therefore does not rotate at a constant rate and this is known as cogging.
- the actual torque acting on the rotor is altered from that described above according to this invention, such that it is independent of the angular position of the rotor or at least such that the variation in the nett torque with angular position is substantially reduced.
- This is achieved by altering the torque generated by the windings to be dependent on angular position in a manner inverse to the detent torque in order to compensate for the detent torque.
- the nett sum of the torques acting on the motor is calculated the total torque acting on the motor varies less with angular position.
- the torque generated by the stator windings may be considered to be the sum of the torque generated by coil A and the torque generated by coil B.
- both coils are positively energised .
- coil A is acting to turn the rotor clockwise in figure 1
- coil B is acting to turn it anti-clockwise.
- the detent torque acts in an anti ⁇ clockwise direction, and thus to compensate for this, either the torque generated by coil A must be increased, the torque generated by coil B decreased, or a combination of these.
- FIG. 2 shows the waveforms applied to the coils A and B as V a * and V ⁇ ' respectively.
- V a ' and V- * ' depart from the waveforms V a and Vj-,
- V a ' and Vj-,' are shown in dotted lines.
- V a ' and V ⁇ ' follow the solid lines of V a and V ⁇ respectively.
- the modified waveforms are based on the two theoretical sinusoidal waveforms, out of phase from each other by ⁇ r/2 .
- the modification is such that the waveform which, at a particular moment, would have the lower magnitude according to the theoretical model is the one that is modified, and is such that, during the periods a given waveform is modified, its magnitude is increased above that expected according to the theoretical model.
- a conventional drive circuit for such a motor provides the two sinusoidal waveforms V a and Vfc for application to the coils A and B of the motor.
- the waveforms may be generated digitally with, for instance, 2 ⁇ or 2 ' O steps per electrical cycle, the values provided corresponding to sampled sinusoids.
- the modified waveforms of the present invention may similarly be generated digitally, the values now corresponding to samples of V a ' and Vj j '.
- the actual modifications required depend on the design of each individual motor. Thus the modification required may be determined empirically for each motor and the effectively sampled value may be provided in ROM for sequential application to the coils.
- Motor 100 has its two stator windings energised by respective variable d.c supplies 101a and 101b.
- the mechanical output of the motor is connected to a high precision, low friction encoder 102 which determines the angle of the motor shaft and displays the value by way of display 103.
- one winding is energised at full stator voltage and the other winding is not energised.
- the position the motor takes under these conditions is taken to be zero.
- the currents supplied by the d.c supplies 101a, 101b, are changed to the values theoretically required for the first mechanical step of the motor and the position taken up by the motor is displayed by display 103.
- the current supplied by one or both of the d.c supplies is adjusted if necessary until the motor takes the correct mechanical position for the first step, and the values of the currents from the d.c supplied are recorded, or at least the ratio therebetween. These values can then be stored in ROM as required.
- Input interface 10 may be adapted to accept analog or digital drive inputs. Digital inputs may be in serial binary form or in time dependent (wherein the required motor shaft angle is proportional to the input frequency) form. Interface 10 generates corresponding addresses which are applied to ROMs 12, 14 by way of bus 11. This causes ROMs 12,14 to output appropriate values along the modified waveforms V a ' , V]- respectively. These outputs are input to digital-to-analog converters 13,15 respectively which have outputs connected to coils A and B respectively.
- modified waveforms V a ' and V]-, 1 are applied to coils A and B respectively.
- the form of modification depends on the information stored in ROMs 12 and 14 and this information is selected to be appropriate to the motor being driven as described above.
- Figure 7 illustrates in more detail the control and drive circuitry according to a preferred embodiment of this invention, in which the motor is used for car instrumentation.
- a signal f is provided and it is required that the angle taken by the rotor is proportional to the frequency of signal f. This is the case for instance in a car speedometer drive.
- Signal f is input into frequency/voltage converter 20 which gives an output, the magnitude of which is proportional to the frequency of signal f. This is then compared with a feedback signal on line L1 to give an error signal e.
- This signal is input to converter 21 which give as an output a signal having a value equal to the magnitude of signal e and to converter 23 which gives as an output a binary signal according to the polarity of signal e.
- the output from converter 21 is input to voltage controlled oscillator (VCO) 22 which outputs onto line L2 an oscillating signal the frequency of which is proportional to the magnitude of signal e.
- VCO voltage controlled oscillator
- the output of converter 23 is applied to line L3 via a latch 24 which is clocked by the output from VCO 22.
- Lines L2 and L3 are connected to two up/down (U/D) counters '25, 27, L2 as the clock input and L3 as the direction input.
- the output of U/D counter 25 is input to digital to analog (D/A) converter 26, the output which is provided as the feedback signal on line L1.
- D/A converter 26 digital to analog converter 26
- this feedback loop causes U/D counter 25 to count, up or down as appropriate, until the output of D/A converter 26 is equal to the output of frequency/voltage converter 20.
- VCO 22 the larger the discrepancy between these two outputs, the faster the counter will count to reduce this to zero.
- U/D counter 27 receives the same inputs as U/D counter 25.and hence its output is identical, or at least counts up and down correspondingly.
- the output of U/D counter 27 is input to ROM 28 which has stored therein the values for the waveforms required to be applied to the windings of the motor. The appropriate values are addressed by the input taken from U/D counter
- timing controller 29 which receives the signals on lines L2 and L3 as inputs.
- the digital values are then output via D/A converters 31 , 33 to windings A and B respectively of the motor.
- a further problem which is encountered in low torque motors such as those used form driving a needle on the dial of a car instrument is that of magnetic stiction and mechanical hysteresis effects.
- substantial work hardening of the stator poles occurs. This results in a considered hysteresis effect in the motor and the positional error when the direction of rotor rotation changes may be some 3-4°.
- This problem may be overcome by causing the motor to oscillate a very small amount around its correct position. Such an oscillation may be so small as to be imperceptible to a human eye and hence have no effect on the motor's applicability to car instrumentation.
- the effect of the oscillation signal is to AC demagnetise the pole tips and to provide some mechanical de-stiction.
- signal f is input to voltage / frequency converter 20, the output of which is input to block 40.
- Block 40 represents the feedback loop of figure 6 and comprises items 21-27 of that embodiment.
- the output is input to ROM 28 which has outputs fed to latches 30, 32 and D/A converters 31 , 33 as described above.
- the circuitry of figure 8 further comprises ac current sources 34, 35. These superimpose, on the outputs.of D/A converters 31, 35 respectively a oscillating signal (eg. 16-20 Hz) with a low magnitude. This generally low frequency signal has a frequency which is relatively high compared to the movement of the motor in applications such as car instrumentation.
- ac current sources 34, 35 These superimpose, on the outputs.of D/A converters 31, 35 respectively a oscillating signal (eg. 16-20 Hz) with a low magnitude.
- This generally low frequency signal has a frequency which is relatively high compared to the movement of the motor in applications such as car instrumentation.
- the magnitude of the oscillation provided by the sources 34, 35 is equivalent to approximately 3-5 bits in the output of ROM 28 or an oscillation of the rotor of 0.1 -0 .1 5°.
- the oscillation of the motor rotor may alternatively be achieved by the design of the digital control circuit such as illustrated in figure 7. It is usual when designing servo-control systems to design the deadband to be as small as possible, ie the amount of overshoot when the deadband approaches its "null" position, when the error signal reaches zero, is minimised. This reduces the oscillation and provides the best positional accuracy.
- Figure 9 shows the rotor 1 mounted on rotor axle 3 and stator coils 2 enclosed in casing 4. As can be seen the rotor 1 is manufactured shorter in the axial direction then the stator assembly. Initially the rotor axle 3 and hence rotor 1 is allowed to move freely in the axial direction.
Landscapes
- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Control Of Motors That Do Not Use Commutators (AREA)
- Control Of Stepping Motors (AREA)
Abstract
Description
Claims
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB9023693.6 | 1990-10-31 | ||
GB9023693A GB2249440A (en) | 1990-10-31 | 1990-10-31 | Control system for synchronous motor |
Publications (1)
Publication Number | Publication Date |
---|---|
WO1992008277A1 true WO1992008277A1 (en) | 1992-05-14 |
Family
ID=10684658
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/GB1991/001886 WO1992008277A1 (en) | 1990-10-31 | 1991-10-29 | Synchronous motors |
Country Status (3)
Country | Link |
---|---|
EP (1) | EP0555299A1 (en) |
GB (1) | GB2249440A (en) |
WO (1) | WO1992008277A1 (en) |
-
1990
- 1990-10-31 GB GB9023693A patent/GB2249440A/en not_active Withdrawn
-
1991
- 1991-10-29 EP EP91918836A patent/EP0555299A1/en not_active Withdrawn
- 1991-10-29 WO PCT/GB1991/001886 patent/WO1992008277A1/en not_active Application Discontinuation
Non-Patent Citations (2)
Title |
---|
PATENT ABSTRACTS OF JAPAN vol. 13, no. 348 (E-799)(3696) 4 August 1989 & JP,A,1 107 700 ( FUJI PHOTO FILM CO. LTD. ) 25 April 1989 see the whole document * |
PATENT ABSTRACTS OF JAPAN vol. 13, no. 348 (E-799)(3699) 4 August 1989 & JP,A,1 107 699 ( FUJI PHOTO FILM CO LTD ) 25 April 1989 see the whole document * |
Also Published As
Publication number | Publication date |
---|---|
GB2249440A (en) | 1992-05-06 |
GB9023693D0 (en) | 1990-12-12 |
EP0555299A1 (en) | 1993-08-18 |
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