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
  • 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
  • H02P25/08—Reluctance motors
  • H02P25/092—Converters specially adapted for controlling reluctance motors
  • H02P25/0925—Converters specially adapted for controlling reluctance motors wherein the converter comprises only one switch per phase

Definitions

• the present invention relates to a method of driving an electric motor and to apparatus for carrying out that method.
• the method and apparatus have particular but not exclusive application to driving switched reluctance motors.
• Switched reluctance motors offer a fully-controllable electric motor drive of high torque and simple construction. However, in some potential fields of application switched reluctance motors have been rejected at least partially due to their being relatively acoustically noisy in operation.
• a rotor In a typical switched reluctance motor, a rotor is mounted for rotation within a stator.
• the stator has a plurality of inwardly directed salient poles each of which has a corresponding coil winding.
• the rotor also has a plurality of salient poles directed outwardly towards the poles of the stator.
• Combinations of windings are connected together, typically in series, each combination constituting a phase winding. Most commonly, phase windings comprise pairs of windings on diametrically opposite stator poles.
• a drive circuit known as a power converter, is used to control voltage and current in the windings.
• phase winding When a phase winding is energised by applying a voltage across it, this creates a magnetic flux linking the stator pole, and exerts a torque on the rotor which tends to turn the rotor to bring its poles into alignment with the poles of the stator whose windings are energised.
• the rotor By energising successive phase windings in sequence around the stator, the rotor can be caused to rotate continuously, the speed of rotation being determined by the frequency at which the successive phase windings are energised.
• the power converter controls the current in the phase windings to control the torque.
• the power converter provides an increase in the current before the rotor reaches its aligned position, limits the current magnitude (especially at low speeds) and decreases the current rapidly near to, but usually just before, the aligned position.
• Many different power converters are known, but are usually of two kinds. The first kind can apply positive, negative or zero voltage across the windings; the zero voltage state is known as a zero voltage loop or free-wheeling state. The second kind can apply only positive or negative voltage across the windings.
• the vibrations are initiated by a change in the radial force on the stator; the radial force is related to the flux, which is a result of the voltage applied to the phase winding, and so will change when the applied voltage changes.
• any step change in voltage applied to the winding tends to generate noise and/or vibration.
• the power converter is of the type having a free-wheeling state, and has two switches which operate to control the voltage applied to the windings. The switches are closed simultaneously at a given angle of rotation to connect a winding to a positive voltage. One switch is opened at a fixed angle of rotation to connect the winding to zero voltage, and the other switch is opened at an invariable angle after closure, to apply negative voltage.
• the motor operates for a relatively long time in a zero voltage loop, which minimises core losses, by ensuring that the stored magnetic energy is converted to torque rather than returned to the voltage source. This method of operating the motor may also reduce acoustic noise, as the radial force on the stator is reduced. However, this motor and controller do not primarily address the reduction of acoustic noise.
• the present invention provides, in a first aspect, a method of driving an electric motor comprising a switching operation in which current in a winding is switched from a first state to a second state, the switching operation comprising performing at least first and second switching steps, the interval between the first and second steps being between a quarter and three-quarters of the period of a dominant frequency of vibration of the motor or to an integral multiple of the said period plus a portion between a quarter and three-quarters of the said period.
• the effect of this method is that a first vibration is set up by the first switching step and a further vibration is set up, out of phase with the first vibration, by the second switching step, the result being that the two vibrations tend to at least partially cancel one another.
• the method therefore reduces the noise and/or vibration generated by energisation and de-energisation of the windings. It will be understood that the motor is driven continuously by switching successive phase windings using the method of the invention.
• the interval between the switching steps is approximately equal to a half of the said period, or an integral multiple plus a half of the said period.
• the dominant frequency may be measured experimentally for example by velocity or acceleration sensors mounted on the motor or by using a microphone to analyse the acoustic noise produced by the motor during operation, or it may be calculated theoretically, for example by finite element analysis of the motor structure. It has been found that in the case of switched reluctance motors, the dominant frequency typically corresponds to the natural frequency of the stator or, in the case of "inside-out" motors, the rotor or a harmonic thereof. If more than one dominant frequency is found then further noise reduction may be achieved in some circumstances by adding additional stages to the switching operation the duration of which may be determined in accordance with the principle described above.
• the first and second switching steps may be the only steps of the switching operation. However, there may be additional switching steps before, between, or after the first and second switching steps. Typically, the first and second switching steps are the initial and final steps of the switching operation.
• vibration and acoustic noise may be generated by switching operations which take place during the course of chopping.
• the method of the present invention as described above may be applied to a switching operation which occurs during chopping instead of or as well as a switching which occurs operation during commutation.
• the switching operation employed will depend on when the method is used, and the particular power converter circuit used. Where the method is employed on commutation of the windings, the higher current in the first state is driven in the windings in a higher voltage loop, and the lower current in the second state is driven in the windings in a lower voltage loop. The first and second switching steps cause the current to be driven in different voltage loops.
• the method comprises performing the first switching step to cause the current to freewheel in a zero voltage loop, and performing the second switching step to drive the current in the winding in a negative voltage loop to cause it to diminish.
• an intermediate switching step is introduced, so that performing the first switching step causes the current to be driven in a lower voltage loop, performing the intermediate switching step restores the higher voltage loop and performing the second switching step causes the current to be driven in a lower voltage loop.
• the method comprises performing the first switching step to drive the current in a negative voltage loop, performing the intermediate switching step to restore the positive voltage loop and performing the second switching step to restore the negative voltage loop.
• the modified method may also be used where a power converter circuit with a free-wheeling state is employed.
• the method comprises performing the first switching step to cause the current to free-wheel in a zero voltage loop.
• This three-step method may be used where the motor is required to operate for an extended period in the zero voltage loop, as in WO 90/16111. Performing the three switching steps, rather than the single step described in WO 90/16111 reduces the noise generated by that step.
• the first switching step may drive the current in a negative voltage loop.
• the three-step method may then be used for the second part of the operation of WO 90/16111, when the current is to be reduced to zero, so that performing the first switching step drives the current in a negative voltage loop, performing the intermediate switching step restores the zero voltage loop, and performing the second switching step restores the negative voltage loop.
• the intermediate step may drive the current in a positive voltage loop.
• the three-step method may be modified for different circumstances, so that for example the switching steps may cause the sequence positive, zero, positive and negative voltage loops.
• the method includes one or more pairs of further intermediate steps, before the second step.
• This multi-step method operates in a manner similar to chopping and provides a constant current, or a gradual reduction in current over at least part of the commutation. This may be preferable to having an extended period in a zero voltage loop, which can result in loss of torque.
• the multi-step method comprises the three-step method and performing one or more pairs of further intermediate switching steps of which the first of the pair restores the negative voltage loop and the second of the pair restores the positive voltage loop, before performing the second switching step.
• the multi-step method comprises performing the first switching step to drive the current in a zero or negative voltage loop, performing the intermediate switching step to restore the positive voltage loop, performing one or more pairs of further intermediate switching steps of which the first of the pair drives the current in a zero or negative voltage loop and the second of the pair restores the positive voltage loop, and performing the second switching step to drive the current in a negative voltage loop.
• the number and timing of the pairs of intermediate switching steps are chosen to give the required reduction in current from the first state to the second state. Indeed, the current may be reduced gradually over the whole commutation period, rather than providing a rapid fall at the end, although this results in a loss of torque, which may be disadvantageous.
• a suitable method according to the invention may be employed to reduce the noise generated by any step change in voltage applied to the winding.
• the method has been found to be particularly effective in reducing noise generated by switched reluctance motors. However, it may be effective in reducing noise generated by other types of motor in which switched or rapidly varying current is supplied to a winding, such as permanent magnet brushless motors
• the invention provides a method of driving an electric motor in which a vibration is periodically created during normal operation thereof, the method comprising creating in the motor a second vibration having physical parameters such that it interacts with one of said vibrations created during normal operation of the motor so as to cause at least partial cancellation thereof.
• This method may be carried out in the manner of the first aspect of the invention or it may be carried out in other ways, for example, by energising two phase windings simultaneously such that the vibration produced by each phase winding at least partially cancels the vibration produced by the other.
• the invention provides apparatus for driving an electric motor including at least one coil winding, the apparatus comprising a power source, a power converter circuit including switch means for supplying voltage across the windings to drive current in the windings, and a controller for controlling operation of the power converter circuit, the controller including means for operating the switch means to switch the current in a winding from a first state to a second state, operation of the switch means including performing at least first and second switching steps, the interval between which is between a quarter and three-quarters of the period of a dominant frequency of vibration of the motor, or to an integral multiple of the said period plus a portion between a quarter and three-quarters of the said period.
• the apparatus may employ power converter circuits of many known types.
• the power converter circuits used are operable in a freewheeling state in which current flow is maintained in a phase winding in which energy is neither taken from nor returned to the power supply.
• Conventional asymmetric half bridge circuits may be used.
• a converter with no free-wheeling state such as a split D C converter, may be used.
• other converters are known which have advantages over such conventional converters, such as those disclosed in GB-A-2 208 456 and IEE Proceedings, Vol 137, Pt B, No. 6, PP373-384, may advantageously be used.
• the controller may be of the digital or analogue type.
• Figure 1 is a schematic view along the axis of rotation of a switched reluctance motor
• Figure 2 is a schematic diagram of a control circuit for the motor
• Figures 3a and 3b show simulated and experimental results respectively of measurement of current and stator acceleration in a switched reluctance motor driven by a method of the invention
• Figures 4a and 4b are similar to Figures 3a and 3b, but show the motor driven at a different speed;
• Figures 5a and 5b show simulated results of measurement of current and acceleration of a switched reluctance motor, respectively driven conventionally and by a modified method;
• Figures 6a, 6b and 6c show diagrams of current and voltage for a further modified method of the invention.
• Figure 7 is a schematic diagram of a further control circuit for the motor.
• the switched reluctance motor represented in Figure 1 is a typical four-phase switched reluctance motor which eight stator poles SI,SI' to S4,S4' and six rotor poles R.
• a respective coil winding C1,C1' to C4,C4' is wound around each stator pole SI,SI' to S4,S4', each winding being connected in series with the winding on the diametrically opposite stator pole.
• coils Cl and Cl' are connected in series, as are C2 and C2' , and so forth.
• Each pair of series-connected coils Cn,Cn' together form a phase winding.
• the reluctance of the magnetic flux path between two diametrically opposite stator poles Sn,Sn' varies as a pair of rotor poles R rotates into and out of alignment with them. If current flows in a phase winding Cn,Cn' as the reluctance of the magnetic path seen by the flux associated with that phase winding is decreasing, torque is produced by the tendency of the salient pole rotor to turn into an aligned position where it provides a path of minimum reluctance for the stator flux.
• the current supplied to the winding determines the torque of the motor. It is necessary to increase the current before the rotor reaches the aligned position, limit the current magnitude, and decrease the current rapidly when the rotor nears the aligned position.
• the current is controlled by changing the voltage applied to the winding, and in fact the voltage can be seen to be the basic influence on operation of the motor.
• the voltage determines the rate of change of magnetic flux linkage in the stator pole.
• the flux also determines the radial force on the stator, which in turn determines the radial vibration of the stator.
• the stator vibration is related to the acoustic noise produced by the motor, and it appears to be a change in the gradient of the radial force that initiates vibration and therefore noise.
• Figure 2 shows a control circuit including a power converter circuit of known type able to drive the switched reluctance motor of Figure 1 as described above.
• the control circuit comprises a controller 1, in the form of a microprocessor, operating in response to inputs 2, 3, 4 to drive, through gate drive circuits 5, 6, a power converter circuit 7 for a phase winding. Further power converter circuits (not shown) for the other phase windings are connected in parallel with the circuit 7.
• the power converter circuit 7 comprises two legs connected between positive and less positive DC supply lines +V, OV, each leg having a series-connected switch in the form of an insulated gate bipolar transistor and diode.
• the diode DI is connected between the switch SW1 and the other supply line +V while in the second leg, the diode D2 is connected between the switch SW2 and the negative supply line OV.
• the diodes D1,D2 are connected such that they are reverse-biased by the DC supply on closure of the corresponding switch SW1,SW2.
• a phase winding comprising a pair of series-connected coils Cn,Cn' is connected between the legs of the circuit to points between the respective diodes D1,D2 and switches SW1,SW2.
• the phase winding is represented as a single coil C.
• the controller 1 operates the switches SW1, SW2 to apply voltage across the winding C to energise and de-energise it.
• the switches SW1, SW2 are each operable separately, being driven through a respective gate drive circuit 5, 6, which amplifies the control signals, and may also provide isolation.
• the controller 1 uses the input 2 from the user (usually speed demand), the input 3 giving current feedback from the windings and the input 4 giving feedback of the rotorposition.
• the controller 1 then sends the appropriate signals to the power converter circuit 7.
• the controller 1 also includes, as part of its programming, means 8 for controlling a switching operation of at least first and second switching steps on commutation of the winding. As shown in Figure 2 the means 8 operates to open one switch SW2 as a first switching step, and after a fixed time to open the other switch, SW1.
• the power converter circuit has three main modes of operation. In a first mode, both switches SW1 and SW2 are closed, and a positive voltage loop is formed with the phase winding C connected across the supply lines +V,OV. In this mode, the current in the phase winding C rises rapidly and energy is supplied to the motor.
• a second mode of operation occurs when either one of the switches SW1,SW2 is opened while current is flowing in the phase winding C.
• current continues to flow in a free-wheeling state in the phase winding in a zero voltage loop through the closed switch SW1 or SW2 and one of the diodes D2 or DI respectively, energy being neither taken from nor returned to the DC supply.
• the current in the phase winding will decay slowly with time, in this mode. If one of the switches SW1, SW2 is opened and closed rapidly (an operation known as chopping) the result is to maintain the voltage and the current at an average level for a period of time during which there is a net flow of energy to the motor. If chopping is not used, such as when the motor is driven at high speed or at low torque, the circuit is said to operate in single pulse mode.
• Figure 3a shows simulated traces of current (dotted line 10), the radial force on the stator (chain-dotted line 11) and stator acceleration (solid lines 12, 13) for a motor driven in single pulse mode conventionally by the circuit in Figure 2, and driven according to the method of the invention.
• the conventional method starting at region 15, no current is flowing in the phase winding C and both switches SW1,SW2 are open. .
• both switches SW1,SW2 are closed, and the current rises rapidly to point 17.
• No chopping is required, so at point 17, both switches are opened, the circuit enters its third mode of operation, and the current in the phase winding C falls rapidly to zero, along the line 10'.
• the radial force is shown by line 11', and the acceleration, representative of the acoustic noise generated, by the upper trace 12.
• the power converter circuit is switched to operate in its first mode at point 16 causing the current in the phase winding C to rise rapidly to point 17, the commutation point, whereupon a switching operation is started by the means 8 to reduce the current in the phase winding C to zero.
• the first switching step is to open one of the switches SW1,SW2 causing the current to free-wheel in a zero voltage loop, and therefore, as shown in line 10", to fall at a relatively slow rate.
• the elapsed time T between the first of the switches SW1,SW2 being opened at point 17 and the second of the switches SW1,SW2 being opened at point 18 is selected as being close to half the period of the dominant vibration of the stator. Alternatively, the period may be selected to be an integral multiple of plus half of the said period.
• the step change in the gradient of the radial force (line 11") is reduced by the use of the method, and the acceleration, shown by the lower trace 13, is substantially reduced. It can be seen that the stator acceleration caused by the first switching step and initially directed negatively is substantially cancelled by that caused by the second switching step, also initially directed negatively, but out of phase with the first vibration.
• the graph of Figure 3b shows experimental results for current and acceleration confirming the simulation of Figure 3a.
• Experiments have shown that the acoustic noise, as measured by a microphone, is similarly reduced when the motor is driven in accordance with the invention.
• Figure 4a is similar to Figure 3a, showing a motor again driven in single pulse mode but at a higher speed, conventionally, and by the method of the invention.
• a winding is energised by closing both switches SW1,SW2 of the power converter at point 16 whereupon the current rises. Then, after a predetermined time both switches are opened at point 17, and the current falls to zero. The fall in current between points 19 and 17 is a result of the back-EMF in the phase winding exceeding the voltage of the power converter, which often happens at higher speeds.
• the radial force on commutation is shown by line 11', and acceleration by the upper trace 12.
• the operation of the power converter (and, therefore, the current applied to the phase winding C) is identical to that of the prior art from points 16 to 17, as described above.
• the means 8 starts a switching operation in which a first switching step is to open one of the switches SW1,SW2 of the power converter whereupon the current starts to fall (line 10").
• a second switching step opens the other of the switches SW1,SW2 (at point 18) and the current in the phase winding falls more rapidly to zero.
• the effect of the method on the radial force (line 11") and the acceleration (lower trace 13) is clearly seen, as in Figure 3a.
• Figure 4b shows experimental results for current and acceleration confirming the simulation of Figure 4a.
• the method described above is used to reduce acoustic noise by reducing the vibration of the stator for voltage changes occurring during commutation. Modifications of the method may be used to reduce noise similarly for any step change in voltage applied to a winding, by introducing one or more intermediate switching steps between the first and second switching steps.
• the means 8 of the control circuit will be modified accordingly by programming.
• FIG. 5a A first modified method, the three-step method, involving one intermediate step, is shown in Figure 5.
• Figures 5a and 5b show simulations of motor operation driven respectively conventionally and by the three-step method.
• the reference numerals correspond to those of Figures 3 and 4.
• the voltage step at point 17 causes a change in the gradient of the radial force (line 11) and induces acceleration (line 12) causing the stator vibration (initially directed negatively) of which the frequency is such that several cycles occur during the zero voltage loop.
• the voltage step at point 18 causes a further stator vibration, again initially directed negatively, which is in phase with the first vibration, and so amplifies it. It will be appreciated that the timing between points 17 and 18 will be affected by the speed of the motor, so that at some speeds the second vibration will tend to cancel the first, instead of amplifying it.
• Figure 5b shows the three-step method of driving the motor. Again, both switches are closed at point 16, causing a rise in current.
• the means 8 controls the switching operation, to open both switches as a first switching step, to drive the current in a negative voltage loop, then, before a time £T has elapsed, to close the switches again as an intermediate switching step to restore the positive voltage loop, and slightly before a time T after the first step, to open one switch as a second switching step to allow the current to freewheel in a zero voltage loop.
• the means 8 controls a similar switching operation, as a first step opening the second switch to give a negative voltage loop, closing a switch to restore the zero voltage loop as an intermediate step, and slightly before a time T after the first step, opening that switch to restore the negative voltage loop, causing the current to decay rapidly.
• the first step initiates a vibration (initially directed negatively)
• the intermediate step initiates an intermediate vibration, but initially directed positively since the voltage change is positive, which takes the vibration rapidly back towards zero.
• This step must be performed before the first vibration reaches its negative peak at time £T, otherwise the intermediate vibration will amplify the first one.
• the second step initiates a further vibration directed negatively, which substantially cancels the combined first and intermediate vibrations.
• the acceleration line 12
• the intermediate step is best performed at a time 0.3T after the first step, and the second step at a time 0.7T after the first step, in order to maximise noise reduction. This timing may however be different for different motors.
• the first switching operation may be modified so that the first switching step is to open one switch to allow the current to freewheel in a zero voltage loop, the intermediate step is to close that switch to restore the positive voltage loop, and the second step is to open one switch to restore the zero voltage loop.
• the second switching operation may be modified so that the intermediate step is to close both switches to provide a positive voltage loop, and the second step is to open both switches to restore the negative voltage loop.
• a second modified method may be called the multi-step method, and includes pairs of further intermediate switching steps between the intermediate switching step and the second switching step of the three-step method.
• the pair of further intermediate steps comprises opening one or both switches to provide a zero voltage loop or negative voltage loop , and closing that switch or both switches to restore the positive voltage loop.
• Any number of pairs of further intermediate steps may be included, in a manner similar to chopping, but performed at a duty cycle (ratio of time at a higher voltage to the total period between pairs of intermediate steps) such that the average voltage is less than before point 17.
• the technique of pulse width modulation may be applied in the multi-step method, so that the timing between the intermediate switching steps is chosen to obtain a required current.
• FIGS. 6a to 6c are diagrams showing the use of the multi-step method, and show current I, actual voltage V applied to a winding, and average voltage V, plotted against time t.
• control circuit 1 operates to close both switches at point 16 to cause current to rise.
• the switching operation commences, controlled by means 8, to perform the first switching step to apply zero voltage to the winding, and then the first and further intermediate switching steps alternately restoring the positive voltage loop and the zero loop, and then the second switching step at time T after the first step to apply negative voltage.
• the intermediate steps produce a reduced average voltage and a constant current, that is, conventional chopping for period T.
• Figure 6b shows a similar operation, but with a lower duty cycle to produce a lower average voltage and a current decay.
• Figure 6c is similar to Figure 6b, but shows a period of conventional chopping 20 before the start of the switching operation.
• FIG. 7 shows a modified control circuit with a split-DC type converter 27, which has only one switch SW1 per phase winding and diode DI, and operates either in a positive voltage loop or a negative voltage loop mode. In a motor and controller of this type there are an even number of phases, alternate ones having the position of the switch and diode reversed.
• the control circuit operates the switch through a single gate drive circuit, and has same inputs 2, 3, 4 as Figure 2.
• the means 8 is modified to control the switching operation as a three-step operation on commutation of the winding. As shown in Figure 7 the means 8 operates to open the switch for a predetermined time, to close it for a further predetermined time, and then to open it again.
• the first switching step is to open the switch to go to the negative voltage loop mode.
• An intermediate switching step restores the converter to its positive voltage loop mode.
• the timing of the intermediate switching step is not as critical as the timing of the other pair of first and second switching steps in driving motors in which an energisation of a winding creates less vibration than the de-energisation of a winding.
• the intermediate switching step should be timed to occur up to the time of JT after the first switching step, and preferably at about 0.3T, while the second switching step should be at about 0.7T after the first.
• the multi-step method of Figure 6 may also be applied by the circuit of Figure 7, given an appropriate modification to the means 8.
• the time T is dependent only upon the dominant frequency of vibration and this, for many motors, will remain substantially constant throughout the operating range of the motor.
• the value of T may be measured experimentally by direct analysis of the acoustic noise emitted by the motor.
• the natural frequency of the stator of the motor may be calculated, for example, by finite element analysis, the value of T in both cases being taken to be approximately half the period of vibration at the natural frequency.
• the control circuit for implementing the drive method may be of many forms.
• the power converter circuit of Figure 2 may also be implemented using MOSFET or other transistors, thyristors or any suitable electronic switching device as the switches.
• the controller 1 may be implemented by discrete logic or analogue circuits. A person skilled in the art will be familiar with such controllers and power converter circuits.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Control Of Motors That Do Not Use Commutators (AREA)
• Control Of Electric Motors In General (AREA)

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• 1993
  • 1993-05-29 GB GB939311176A patent/GB9311176D0/en active Pending
• 1994
  • 1994-05-27 EP EP94917721A patent/EP0700594B1/en not_active Expired - Lifetime
  • 1994-05-27 IN IN673DE1994 patent/IN189492B/en unknown
  • 1994-05-27 DE DE69407286T patent/DE69407286T2/de not_active Expired - Fee Related
  • 1994-05-27 US US08/556,927 patent/US5767638A/en not_active Expired - Fee Related
  • 1994-05-27 AU AU69324/94A patent/AU6932494A/en not_active Abandoned
  • 1994-05-27 WO PCT/GB1994/001165 patent/WO1994028618A1/en active IP Right Grant

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