WO2013108014A2

WO2013108014A2 - Reluctance motors

Info

Publication number: WO2013108014A2
Application number: PCT/GB2013/050060
Authority: WO
Inventors: Rajesh Pranay DEODHAR; Adam Alasdair PRIDE; Xu Liu; Zi-Qiang Zhu; Taketoki Maruyama
Original assignee: Imra Europe S.A.S.
Priority date: 2012-01-16
Filing date: 2013-01-14
Publication date: 2013-07-25
Also published as: WO2013108014A3; GB2498394A; GB201200633D0; GB2498394B

Abstract

An apparatus for driving a reluctance motor comprising a stator having a plurality of stator poles. The apparatus comprises AC generating means for generating a substantially sinusoidal alternating current (AC) drive current and DC generating means for generating a direct current (DC) bias current. The apparatus further comprises AC driving means for applying the AC drive current to a stator pole of the stator of the reluctance motor, and DC driving means for applying the DC bias current to the stator pole of the reluctance motor.

Description

RELUCTANCE MOTORS

The present invention relates to reluctance motors and in particular, but not exclusively, to an apparatus for driving a reluctance motor, a reluctance motor, a reluctance motor system and a method for driving a reluctance motor.

Reluctance motors typically tend to be low-cost and have a high power density. As such, they are becoming more popular in applications where these qualities are desirable. However, they can require complex circuitry in order to control them. Additionally, they can suffer from torque ripple at low speed.

A typical switched reluctance motor comprises 6 stator poles and 4 rotor poles (a so- called 6/4 motor). The rotor poles typically do not have any windings. The 6 stator poles are typically driven with a unipolar current supplied by a 3 -phase inverter, each stator pole typically being driven using a square pulse of time period t/3, where t is the time period for three pulses. A switched reluctance motor can be thought of as being driven by DC (direct current) pulses. For switched reluctance motors, control circuitry is usually arranged to generate the unipolar drive current and power the stator poles in an appropriate sequence to cause the rotor to rotate by magnetic reluctance. In this context, "unipolar" is taken to mean flow of current in a coil only in one direction when the motor is in operation. However, switched reluctance motors can suffer from noise and vibration problems due to torque ripple caused by the pulsed driving current.

Another design of reluctance motor is the synchronous AC (alternating current) reluctance motor. Typically, the stator and rotor are arranged in the same way as a switched reluctance motor (e.g. a 6/4 motor) but driven using a 3-phase AC inverter for supplying a 3 phase sinusoidal alternating current to the stator poles. For a three phase synchronous AC reluctance motor, all the phases conduct, and the driving currents are bi-polar. In this context "bipolar" is taken to mean that each coil can have current flowing in either direction in that coil. In synchronous AC reluctance motors, the rotor typically rotates at synchronous speed (i.e. with a rotational frequency which corresponds with the frequency of the AC drive current). Synchronous AC reluctance motors can help reduce noise and vibration compared with switched reluctance motors but tend to have lower torque performance.

Aspects of the invention are defined in the appended claims.

In a first aspect, there is provided an apparatus for driving a reluctance motor comprising a stator having a plurality of stator poles, the apparatus comprising: AC generating means for generating a substantially sinusoidal alternating current (AC) drive current; DC generating means for generating a direct current (DC) bias current; AC driving means for applying the AC drive current to a stator pole of the stator of the reluctance motor; and DC driving means for applying the DC bias current to the stator pole of the reluctance motor.

In a second aspect, there is provided a reluctance motor comprising: a rotor comprising a plurality of rotor poles; and a stator comprising a plurality of stator poles; in which each of the stator poles is arranged to be driven by a substantially sinusoidal alternating current (AC) drive current and a direct current (DC) bias current.

In a third aspect, there is provided a method for driving a reluctance motor comprising a plurality of stator poles, the method comprising: generating a substantially sinusoidal alternating current (AC) drive current; generating a direct current (DC) bias current; applying the AC drive current to a stator pole of the stator of the reluctance motor; and applying the DC bias current to the stator pole of the reluctance motor.

In a fourth aspect, there is provided a reluctance motor comprising: a rotor comprising a plurality of rotor poles; a stator comprising a plurality of stator poles, the stator comprising a plurality of stator teeth; a first winding arranged around a set of stator teeth comprising a subset of the plurality of stator teeth, the first winding being arranged with respect to a first stator pole so as to be driven by the AC drive current, the first winding being; and a second winding different from the first winding, the second winding being arranged around the set of stator teeth and arranged with respect to the first stator pole so as to be driven by the DC bias current, in which: each of the stator poles is arranged to be driven by a substantially sinusoidal alternating current (AC) drive current and a direct current (DC) bias current; and the subset of stator teeth comprises more than one stator tooth.

Examples of the first to fourth aspects are believed to help reduce noise levels as well as providing a greater torque range than switched reluctance motors or AC synchronous motors.

In a fifth aspect, there is provided a reluctance motor comprising: a rotor comprising a plurality of rotor poles; and a stator comprising a plurality of stator poles, each stator pole comprising a stator winding arranged to be driven by a substantially sinusoidal alternating current (AC) drive current so as to cause the rotor to rotate about a rotation axis of the motor; in which each stator winding is arranged to be driven in one of a symmetric connection configuration and an asymmetric connection configuration.

In a sixth aspect, there is provided a method for driving a reluctance motor comprising a rotor comprising a plurality of rotor poles and a stator comprising a plurality of stator poles, each stator pole comprising a stator winding arranged to be driven by a substantially sinusoidal alternating current (AC) drive current so as to cause the rotor to rotate about a rotation axis of the motor, the method comprising: generating the substantially sinusoidal alternating current (AC) drive current; applying the AC drive current to the stator poles; and controlling the AC drive current so as to apply the AC drive current to the respective windings of the stator poles in one of a symmetric connection configuration and an asymmetric connection configuration.

In a seventh aspect, there is provided an apparatus for driving a reluctance motor comprising a rotor comprising a plurality of rotor poles and a stator comprising a plurality of stator poles, each stator pole comprising a stator winding arranged to be driven by a substantially sinusoidal alternating current (AC) drive current so as to cause the rotor to rotate about a rotation axis of the motor, the apparatus comprising: AC generating means for generating the substantially sinusoidal alternating current (AC) drive current; AC driving means for applying the AC drive current to the stator poles; and controlling means for controlling the AC driving means, in which: the controlling means is arranged to apply the AC drive current to the respective windings of the stator poles in one of a symmetric connection configuration and an asymmetric connection configuration.

Examples of the fifth to seventh aspects are believed to help reduce noise levels as well as providing a greater torque range than switched reluctance motors or AC synchronous motors.

Examples of the present disclosure will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of a cross section of a reluctance motor according to examples of the present disclosure;

Figure 2 is a schematic representation of a stator pole and a rotor pole of the reluctance motor;

Figure 3 schematically illustrates an example of drive current for applying to the reluctance motor;

Figure 4 is a schematic diagram of a motor control apparatus for controlling the reluctance motor;

Figures 5A-5C are schematic representations of an example of motor drive circuitry for driving the reluctance motor;

Figure 6 is a graph illustrating torque against current density for different types of reluctance motor; Figure 7 is a graph illustrating noise level against motor speed for different types of reluctance motor;

Figure 8 is a graph illustrating torque against current angle for different AC/DC drive current ratios;

Figure 9 is a graph illustrating line voltage against current angle for different AC/DC drive current ratios;

Figure 10 is a graph illustrating current density against rotor position for a reluctance motor driven with an AC bipolar current waveform;

Figure 11 is a graph illustrating current density against rotor position for a reluctance motor driven with a combined AC + DC current waveform;

Figure 12 is a schematic representation of a reluctance motor with asymmetric winding connections;

Figure 13 is a schematic representation of a reluctance motor with symmetric winding connections;

Figure 14 is a graph illustrating line voltage against current angle of a reluctance motor with asymmetric winding connections and a reluctance motor with symmetric winding connections;

Figure 15 is a schematic representation of a synchronous reluctance motor according to examples of the disclosure;

Figure 16 is a schematic representation of a motor control apparatus for controlling a reluctance motor;

Figure 17 is a flowchart of a method for driving a reluctance motor; and

Figure 18 is a flowchart of a method for driving a reluctance motor.

A reluctance motor according to examples of the disclosure will now be described with reference to Figure 1.

Figure 1 is a schematic representation of a cross section of a reluctance motor 10 according to examples of the present disclosure. The motor 10 comprises a rotor 12 and a stator 14 arranged circumferentially around the rotor 12. The rotor 12 is arranged with respect to the stator 14 so that, in operation, the rotor 12 can rotate with respect to the stator 14 about an axis 15.

In the example shown in Figure 1, the rotor 12 comprises four rotor poles 16a, 16b, 16c, 16d, and the stator comprises six stator poles 18a (pole C), 18b (pole B), 18c (pole A'), 18d (pole C), 18e (pole B'), 18f (pole A). In this example, the reluctance motor is termed a 6/4 reluctance motor in relation to the number of rotor poles and stator poles. The skilled person will appreciate that the rotor 12 could comprise any number of rotor poles and the stator 14 could comprise any number of stator poles. In other words, in examples, the rotor 12 comprises a plurality of rotor poles and the stator 14 comprises a plurality of stator poles. In some examples, the number of stator poles is greater than the number of rotor poles. In examples, each of the stator poles is arranged to be driven by a substantially sinusoidal alternating current (AC) drive current and a direct current (DC) bias current. Such arrangements may help improve available torque and reduce noise with respect to known switched reluctance motors. Further details and advantages of this arrangement will be discussed in more detail later below.

In the example motor 10 shown in Figure 1 , each stator pole comprises a first winding and a second winding. In examples, the first winding and the second winding are so called "concentrated" windings. In this case, a concentrated winding is taken to mean a winding wound around a single stator pole (e.g. where the stator pole comprises a single stator tooth). For example, stator pole 18b comprises a first winding 20a and a second winding 20b. Example directions of current flowing in each winding are illustrated according to the usual circle/dot (current direction is out of page) and circle/cross convention (current direction is into page) although it will be appreciated that where an alternating current is used, the direction of the current will vary.

In examples, each of the stator poles is arranged to be driven by the substantially sinusoidal alternating current (AC) drive current and the direct current (DC) bias current so as to cause the rotor to rotate about the axis 15, for example using a 3 -phase AC drive current to drive pole pairs A/A', B/B' and C/C, where the AC current for a pole (or pole pair) has a 120 degree phase shift (phase difference) with respect to the current for an adjacent pole (or pole pair). However, it will be appreciated that other appropriate phase shifts could be used.

Figure 2 is a schematic representation of the stator pole 18b and the rotor pole 16b of the reluctance motor 10. As shown in Figure 2, the stator pole 18b comprises the first winding 20a and the second winding 20b. The first winding 20a and the second winding 20b are arranged to apply a magnetic field in a radial direction with respect to the stator 14 when driven with a current. In examples, the first winding and the second winding are coaxial with each other.

In examples, the first winding 20a is arranged with respect to the stator pole 18b so as to be driven by the AC drive current, and the second winding 20b is arranged with respect to the stator pole 18b so as to be driven by the DC drive current. However, it will be appreciated that the first winding 20a could be driven by the DC bias current and the second winding could be driven by the AC drive current. As illustrated in the example of Figure 2, the first winding 20a is different from the second winding 20b. In other words, the first winding is separate from the second winding so that they can be driven independently, e.g. with different currents. In examples, each stator pole comprises a pair of windings, the pair of windings being arranged so that they can be driven independently from each other.

Although in Figure 2 the rotor pole 16b is shown as rotating anti-clockwise, it will be appreciated that the rotor 12 could rotate in either direction about the axis 15.

In other words, the stator 14 according to examples of the disclosure comprises a first set of first windings (e.g. including the first winding 20a), and a second set of second windings (e.g. including the second winding 20b). Each first winding is associated with a respective stator pole, and each second winding is associated with the respective stator pole associated with the first winding.

As mentioned above, in examples, the first winding 20a is arranged with respect to the stator pole 18b so as to be driven by the AC drive current, and the second winding 20b is arranged with respect to the stator pole 18b so as to be driven by the DC drive current. In other words, one of the windings of the pair of windings is arranged to be driven with an AC drive current and the other one of the windings of the pair of windings is arranged to be driven with a DC bias current. The magnetic field generated by the first and second winding (pair of windings) of each stator pole can be considered to be substantially equivalent to that generated by one winding when driven by a combined AC drive current and DC bias current.

Figure 3 schematically illustrates an example of drive current for applying to the reluctance motor. In particular, Figure 3 shows a graph illustrating current (i) against time (t) for an example drive current 20 for driving the reluctance motor 10. The drive current 20 can be considered to comprise two components, a sinusoidal AC drive current component and a DC bias component. In other words, the drive current 20 comprises a substantially sinusoidal AC drive current and a DC bias current. In the example of Figure 3, the magnitude of the DC bias current is I₀ and the peak AC current value with respect to the DC bias current is refen-ed to as Ii. The period of the sinusoidal AC drive current is T.

An apparatus for driving a reluctance motor according to examples of the disclosure will now be described with reference to Figure 4.

Figure 4 is a schematic diagram of a motor control apparatus 30 for controlling the reluctance motor. The apparatus 30 comprises an AC generator 32, a DC generator 34, and AC driver 36, a DC driver 38, a controller 40, and a motor speed detector 42. In the example of Figure 4, the AC generator is operable to generate a substantially sinusoidal AC drive current (for example as shown in Figure 3) and supply the AC drive current to the AC driver 36. The AC driver is operable to apply the AC drive current to one or more stator poles of the stator 14 of the reluctance motor 10, for example to each of the first windings (e.g. including winding 20a). In the example of Figure 4, the DC generator 34 is operable to generate a DC bias current (for example I₀ in Figure 3) and supply the DC bias current to the DC driver 38. The DC driver 38 is operable to apply the DC bias current to one or more of the stator poles of the reluctance motor 10, for example to each of the second windings (e.g. including winding 20b).

In some examples, the AC driver 36 and the DC driver 38 comprise wiring for directly communicating the AC drive current and DC bias current to the respective windings of the stator 14. In other words, for example, the AC driver 36 can be thought of as AC driving means for applying the AC drive current to a stator pole (e.g. applied to winding 20a of stator pole 18b) and the DC driver can be thought of as DC driving means for applying the DC bias current to the stator pole (e.g. applied to winding 20b of stator pole 18b).

In examples, the controller 40 is operable to control one or more of: the AC generator 32; the DC generator 34; the AC driver 36; and the DC driver 38.

In some examples, the AC driver 36 and DC driver 38 act under control of the controller 40 so as to apply the AC drive current and DC drive current to achieve a desired rotation as determined by the controller. In examples, the controller is operable to select a subset of the stator poles to which the AC drive current and/or the DC bias current should be applied. This can help improve the flexibility of operation of the motor 10. However, it will be appreciated that other arrangements for applying the AC drive current and the DC bias current may be used.

In the example shown in Figure 4, the controller 40 is operable to control the generation of the AC drive current by the AC generator 32 and generation of the DC bias current by the DC generator 34. In some examples, the controller is operable to generate control signals based on motor speed data generated by the motor speed detector 42.

The motor speed detector is operable to detect a rotational speed of the rotor and generate motor speed data indicative of the rotational speed of the rotor. In an example, the motor speed detector 42 is operable to detect the rotational speed of the rotor 12 using known hall effect sensor techniques. In another example, the motor speed detector 42 is operable to detect the rotational speed of the rotor 12 based on back emf (electromotive force) in one or more of the stator windings using known techniques. However, it will be appreciated that other suitable techniques could be used for detecting the rotational speed of the rotor 12.

An example of circuitry for driving the reluctance motor will now be described with reference to Figures 5A to 5C.

Figures 5A-5C are schematic representations of an example of motor drive circuitry for driving the reluctance motor 10.

Figure 5A illustrates example circuitry for generating an AC drive current and a DC bias current. In examples, the AC generator 32 comprises AC generating circuitry 52 and the DC generator 34 comprises DC generating circuitry 54.

The AC generating circuitry 52 comprises a 3 -phase AC inverter comprising a power source U_d, capacitors CI and C2, and a plurality of switching elements S1-S6. In examples, the power source U_d comprises a battery although it will be appreciated that other power sources may be used, such as an array of batteries, a rectified AC signal, fuel cell, photovoltaic cell and the like.

Each switching element comprises a bipolar transistor (e.g. NPN) and a diode in parallel to the emitter/collector of the transistor. The base connection of each transistor is controllable by the controller 40 so as to generate a 3 phase sinusoidal current and output the current to output terminals P, Q, R. Although in examples the circuitry 52 is driven by the controller 40 so as to generate a sinusoidal output, it will be appreciated that the output of the AC circuitry 52 could be driven to produce other waveforms such as a substantially sinusoidal wavefomi, triangular waveform and the like. It will be appreciated that other circuit arrangements could be used to generate the AC drive current.

The DC generating circuitry 54 comprises a variable DC power source 56. In an example, the DC power source 56 comprises a DC rectifier for generating a DC current source from mains line voltage. In other examples, the DC power source 56 comprises a battery and a variable DC-DC converter. In some examples, where the DC power source 56 is powered by a battery, the battery may be the same as the battery for providing power to the AC generating circuitry (e.g. power source U_d). The DC generating circuitry is operable to output a bipolar DC current via DC output terminals S, T. It will be appreciated that other circuit arrangements could be used to generate the DC bias current.

In examples, the AC driver 36 is operable to connect the output terminals P, Q, R to input terminals P', Q', R' of the stator first windings (e.g. winding 20a) as schematically illustrated in Figure 5B. In other words, the AC drive current is applied to poles A, B, and C (18f, 18b, and 18a respectively). In an example, the first windings of the poles A, B, and C have a common connection as illustrated in Figure 5B, although it will be appreciated that other wiring configurations could be used.

In examples, the DC driver 38 is operable to connect the output terminals S, T to respective input terminals S', T' of the stator second windings (e.g. winding 20b) as schematically illustrated in Figure 5C. In other words, the DC bias current is applied to poles A, B, and C (18f, 18b, and 18a respectively). In an example, the second windings of the poles A, B, and C are connected in series as illustrated in Figure 5C, although it will be appreciated that other wiring configurations could be used.

Some advantageous features of the disclosure will now be described with reference to Figures 6 and 7 which show data generated from a computer generated model. In other words, Figures 6 and 7 illustrate data obtained from simulating operation of the motor.

Figure 6 is a graph illustrating an example of torque against current density for different types of reluctance motor. As can be seen in Figure 6, a conventional unipolar switched reluctance motor (labelled as "Unipolar" in Figures 6 and 7) of the simulation can provide more torque for a given current density than a synchronous AC bipolar reluctance motor (labelled as "AC bipolar" in Figures 6 and 7) as simulated. In the example of Figure 6, the torque which can be provided by a reluctance motor according to the present disclosure as simulated (labelled as "DC-Bias+AC" in Figures 6 and 7) is between that of a conventional unipolar switched reluctance motor and a synchronous AC bipolar reluctance motor.

Figure 7 is a graph illustrating noise level (sound level) against motor speed for different types of reluctance motor as simulated. In the example of Figure 7, it can be seen that the noise level (sound level e.g. in dBA) of the reluctance motor of the present disclosure is very similar to that of a synchronous AC bipolar reluctance motor and less than that of a conventional unipolar switched reluctance motor. In other words, the techniques and arrangements described in the present disclosure can help provide a reluctance motor with increased torque output and lower noise levels.

In examples, the controller 40 is operable to control a ratio of magnitudes of the AC drive current and the DC drive current with respect to each other, for example by sending suitable control signals to the AC driver 36 and the DC driver 38. In other examples, the controller 40 is operable to control the ratio of magnitudes of the AC drive current and the DC bias current by sending suitable control signals to the AC generator 32 and the DC generator 34. This allows the controller to control the torque which can be provided by the motor 10. An example of variation of torque for different AC drive current/DC bias current ratios (AC/DC ratio) is illustrated in Figure 8. Figure 8 is a graph illustrating torque against current angle for different simulated AC/DC drive current ratios. In the disclosure 'current angle' is a control parameter by which the phase angle between generated back-EMF and applied current is controlled by the inverter-controller (such as that described herein with reference to Figure 4), expressed in electrical degrees (deg. elec).

Typically, the low-speed torque (for example, where the rotation speed is less than a low speed threshold) is maximised when the current angle is 0 deg. elec. However, in order to operate at higher speeds with constant power, the current angle value typically needs to deviate from 0 deg. elec. to some other value depending on the operating conditions of the motor.

As can be seen from Figure 8, a higher AC/DC ratio increases the torque which can be provided by the motor 10. Therefore, in examples, the controller 40 can control the amount of torque which the motor 10 can provide depending on operational conditions of the motor. For example for starting from zero rotational speed and for acceleration of rotational speed when the rotational speed of the motor is less than the low speed threshold (e.g. low speed), higher torque at lower rotational speeds is typically required. Therefore, in examples, where the rotational speed is less than the low speed threshold, the controller 40 is operable to control the amount of torque so as to provide a relatively high torque (e.g. greater than a low speed torque threshold). As another example, where the motor is being driven at relatively high speeds (for example rotational speed is above a high speed threshold) lower torque at higher rotational speeds is typically required. Therefore, in examples, where the rotational speed is greater than a high speed threshold, the controller 40 is operable to control the amount of torque so as to provide a relatively low torque (e.g. less than a high speed torque threshold).

In an example, the controller 40 is operable to control the magnitudes of the AC drive current and the DC bias current in dependence upon rotational speed of the motor so as to control torque applied by the motor. This can help increase a speed range over which the motor can operate. As mentioned above, in examples, the rotational speed of the motor is detected by the motor speed detector 42, although it will be appreciated that other techniques for measuring the rotational speed could be used. In examples, the controller 40 comprises a memory for storing a look up table (LUT) and AC/DC ratios and corresponding torque values for different current angles are stored in the look-up table. In examples, the controller 40 is operable to control the magnitudes of the AC drive current and DC bias current by reference to the LUT. However, it will be appreciated that other techniques could be used, such as calculation of appropriate AC/DC ratio from a suitable equation comprising current angle, torque, and AC/DC ratio as parameters.

Figure 9 is a graph illustrating line voltage against current angle for different simulated AC/DC drive current ratios. As shown in Figure 9, lower AC/DC ratios provide lower line voltage requirements for the power supply. This can be important when the power source for driving the motor is a battery, with a limited maximum line voltage (such as indicated by the solid line in Figure 9). In examples, the controller 40 is operable to control the ratio of magnitudes of the AC drive current and the DC bias current so as to limit a line voltage value to less than a line voltage threshold value. In the example shown in Figure 9, the line voltage threshold value corresponds with the voltage limitation value (e.g. 100V), although it will be appreciated that other line voltage threshold values could be used. In examples, the controller 40 comprises a memory for storing a look up table (LUT) and AC/DC ratios and corresponding line voltage values for different current angles are stored in the look-up table. In examples, the controller 40 is operable to control the magnitudes of the AC drive current and DC bias current by reference to the LUT. However, it will be appreciated that other techniques could be used, such as calculation of appropriate AC/DC ratio from a suitable equation comprising current angle, line voltage, and AC/DC ratio as parameters.

In some examples, the apparatus 30 is operable to control the effective number of poles of the motor 10, for example as described below with respect to Figures 10 and 11.

Figure 10 is a graph illustrating current density against rotor position for a simulated reluctance motor (such as reluctance motor 10) driven with an AC bipolar current waveform (e.g. simulated from a suitable computer model). In examples, the controller 40 is operable to control the DC generator to generate a zero current bias current (i.e. I₀ = 0). Therefore, the applied current through the second windings will be zero. In this example, the effective number of pole-pairs in this mode of operation is 2, as illustrated in Figure 10.

Figure 11 is a graph illustrating current density against rotor position for a simulated reluctance motor driven with a combined AC + DC current waveform (e.g. simulated from a suitable computer model). For example, Figure 11 illustrates driving of the reluctance motor 10 with an AC drive current applied to the first windings and a DC bias current applied to the second windings. As can be seen in Figure 11, the effective number of poles in this mode of operation is 4.

In other words, the effective number of pole pairs in this mode of operation is 4. For example, in a bipolar excited reluctance motor or a permanent magnet motor, the number of pole pairs is commonly quoted, for example 2 pole pairs in the example shown in Figure 10. As another example, in a unipolar excited reluctance machine (e.g. motor), the number of reluctance poles is often quoted. For example, Figure 1 1 illustrates a mode of operation in which the number of reluctance poles is 4. In examples, one reluctance pole in a unipolar excited motor is equivalent to one pole-pair in a bipolar excited motor.

In other words, in examples, the controller 40 is operable, in a first mode of operation, to cause only the AC drive current to be applied to the stator pole and, in a second mode of operation, to cause both the AC drive current and the DC bias current to be applied to the stator pole(s). In this example, the first mode of operation corresponds to operation of the motor 10 as a bipolar AC motor and the second mode of operation coiTesponds to operation of the motor 10 for example as described above with reference to Figures 1 to 9. Therefore, the controller can cause the motor to switch between modes of operation.

In examples, the first mode of operation corresponds with operation of the motor as having a first number of effective poles and the second mode of operation corresponds with operation of the motor as having a second number of effective poles, with the second number of effective poles being greater than the first number of effective poles. For the examples illustrated in Figures 10 and 1 1, the second number of effective poles is twice the first number of effective poles. In other words, for example, switching from the first mode to the second mode can be considered to be a pole-doubling operation, although it will be appreciated that this may vary depending on the number of stator poles and rotor poles.

As mentioned above with respect to Figures 6 and 7, an AC bipolar reluctance motor has different operational qualities form the motor 10 when the motor 10 is operated with a combined AC + DC drive. The AC+DC drive (i.e. second mode of operation) may provide higher torque, higher number of poles, and higher frequency (higher rate of switching of drive transistors such as SI to S6 described with respect to Figure 5 A) than a bipolar AC reluctance motor (first mode of operation). Thus the second mode of operation tends to be more suited to situations where high torque (e.g. above a torque threshold) is required at low speed (below a speed threshold). Bipolar AC drive (first mode of operation) typically has lower torque, lower number of poles and lower frequency (lower rate of switching of drive transistors such as S 1 to S6 described with respect to Figure 5 A) than the second mode of operation and so is typically more suitable for use in higher speed, low torque requirements.

Therefore, in some examples, the controller 40 is operable to control the motor 10 in the first mode of operation when the rotational speed of the motor is less than a motor speed threshold, and to control the motor 10 in the second mode of operation when the rotational speed of the motor is equal to or greater than the motor speed threshold. This can help optimise operation of the motor 10 as well as help improve the speed range over which the motor 10 can operate.

Operation of the reluctance motor 10 in an asymmetric winding connections configuration and a symmetric winding connections configuration will now be described with reference to Figures 12 and 13.

Figure 12 is a schematic representation of a reluctance motor with asymmetric winding connections. Figure 13 is a schematic representation of a reluctance motor with symmetric winding connections. In particular, Figures 12 and 13 show the reluctance motor 10. The first windings (for AC drive current) are shown but the second windings (for DC bias current) have been omitted for clarity in understanding the Figures.

Asymmetric configuration is typically used with unipolar switched reluctance motors, whereas symmetric configuration is typically used with bipolar AC reluctance motors.

In some examples, the second windings are omitted from the motor 10. In these examples, the motor 10 comprises the first windings but not the second windings, and the motor is driven with the AC generator 32 and the AC driver 36 under control of the controller 40.

In an asymmetric configuration (illustrated in Figure 12), the windings of the stator poles are configured and driven so that the magnetic fields for stator poles around a first portion (e.g. a first half of the stator) of the circumference of the stator 14 are directed axially inwards and the magnetic fields for stator poles around a second portion of the circumference of the stator 14 (e.g. a second half of the stator) are directed axially outwards.

In a symmetric configuration, the windings of the stator poles are configured and driven so that the magnetic field for the stator poles alternates between being directed inwards and being directed outwards for each stator pole in a circumferential direction (for example as illustrated in Figure 13).

In other words, in examples, each of the stator poles has an associated first winding and second winding, and the controller 40 is arranged to apply the AC drive current to the first winding in an asymmetric connection configuration. In other examples, each of the stator poles has an associated first winding and second winding, and the controller 40 is arranged to apply the AC drive current to the first winding in a symmetric connection configuration.

In examples, the torque which can be provided by the motor is substantially the same for both configurations. However, the line voltage requirements for the asymmetric configuration are different from the line voltage requirements of the symmetric configuration. Figure 14 is a graph illustrating line voltage against current angle of a reluctance motor with asymmetric winding connections and a reluctance motor with symmetric winding connections. As shown in Figure 14, the line voltage requirements in the symmetric configuration are less than those in the asymmetric configuration.

Therefore, in examples, the controller 40 is operable to control the AC driver 36 to cause the motor 10 to operate in either the symmetric or asymmetric configuration, depending on operating conditions of the motor 10. This may help improve the speed range over which the motor can operate.

As mentioned above, for example for starting from zero rotational speed and for acceleration of rotational speed when the rotational speed of the motor is less than the low speed threshold (e.g. low speed), higher torque at lower rotational speeds is typically required. Therefore, in examples, the controller is operable to cause the motor 10 to operate in the asymmetric configuration when the rotational speed of the motor 10 is below a motor speed threshold, so as to provide greater torque than when in the symmetric configuration. In examples, the motor speed threshold is the same as the low speed threshold, although it will be appreciated they could be different from each other.

As mentioned above, as another example, where the motor is being driven at relatively high speeds (for example rotational speed is above a high speed threshold) lower torque at higher rotational speeds is typically required. Therefore, in examples, where the rotational speed is greater than the motor speed threshold the controller is operable to cause the motor 10 to operate in the symmetric configuration. In examples, the motor speed threshold is the same as the high speed threshold, although it will be appreciated they could be different from each other.

In other words, for example, a reluctance motor (such as that described with respect to Figures 12 to 14 comprises a rotor comprising a plurality of rotor poles and a stator comprising a plurality of stator poles, each stator pole comprising a stator winding arranged to be driven by an alternating current (AC) drive current so as to cause the rotor to rotate about a rotation axis of the motor. In the examples of Figures 12 to 14 each stator winding is arranged to be driven in one of a symmetric connection configuration and an asymmetric connection configuration.

More generally, in examples, the motor control apparatus 30 can be used to drive a reluctance motor comprising a rotor comprising a plurality of rotor poles and a stator comprising a plurality of stator poles, each stator pole comprising a stator winding arranged to be driven by a substantially sinusoidal alternating current (AC) drive current so as to cause the rotor to rotate about a rotation axis of the motor (for example, the motor described with respect to Figures 12 to 14). In this example, the motor control apparatus comprises AC generating means (for example AC generator 32) for generating the substantially sinusoidal alternating current (AC) drive current and AC driving means (for example AC driver 36) for applying the AC drive current to the stator poles. As mentioned above, the controller 40 is operable to control the AC driver 36. In examples, the controller 40 is arranged to apply the AC drive current to the respective windings of the stator poles in one of a symmetric connection configuration and an asymmetric connection configuration.

In some examples, the controller 40 means is operable to apply the AC drive current to the respective windings of the stator poles in the asymmetric connection configuration when the rotational speed of the motor 10 is less than a motor speed threshold (for example as mentioned above). In some examples, the controller 40 is operable to apply the AC drive current to the respective windings of the stator poles in the symmetric connection configuration when the rotational speed of the motor 10 is greater than the motor speed threshold (for example as mentioned above).

In other words, in examples, the motor control apparatus is operable to implement a method for driving a reluctance motor comprising a rotor comprising a plurality of rotor poles and a stator comprising a plurality of stator poles, each stator pole comprising a stator winding arranged to be driven by a substantially sinusoidal alternating current (AC) drive current so as to cause the rotor to rotate about a rotation axis of the motor. In examples, the method comprises generating the substantially sinusoidal alternating cun'ent (AC) drive current, applying the AC drive current to the stator poles, and controlling the AC drive current so as to apply the AC drive current to the respective windings of the stator poles in one of a symmetric connection configuration and an asymmetric connection configuration.

The examples described above have referred to a configuration in which the motor 10 comprises concentrated windings. However, the drive apparatus according to the examples above can also be used to drive a synchronous reluctance motor with distributed windings.

Figure 15 is a schematic representation of a synchronous reluctance motor according to examples of the disclosure. In particular, Figure 15 shows a synchronous reluctance motor 100 which comprises a rotor 102 and a stator 104 arranged circumferentially around the rotor 102. The rotor 102 is arranged with respect to the stator 104 so that, in operation, the rotor 102 can rotate with respect to the stator 104 about an axis 105. In examples, the stator comprises a plurality of stator teeth 106. In examples, a first winding 108 is arranged around a set of stator teeth comprising a subset of stator teeth 112. A second winding 110 is arranged around the set of stator teeth. In other words, the windings of the reluctance motor 100 are distributed windings. In examples, the reluctance motor 100 is operated using the techniques described above with reference to Figures 1 to 11. It is believed that the motor 100 may provide a similar functionality in terms of torque performance to that provided by the motor control apparatus 30 and motor 10.

In examples, the first winding 108 is arranged with respect to a first stator pole (for example comprising the subset of stator teeth 1 12) so as to be driven by the AC drive current. In examples, the second winding 110 is different from the first winding and the second winding 110 is arranged around the set of stator teeth (e.g. subset of stator teeth 112) and arranged with respect to the first stator pole so as to be driven by the DC bias current. In other words, in examples, each stator pole comprises a plurality of stator teeth (e.g. stator teeth 112) and each of the stator poles is arranged to be driven by a substantially sinusoidal alternating current (AC) drive current and a direct current (DC) bias current. However, it will be appreciated that the first winding 108 could be arranged to be driven by the DC bias current and the second winding 110 could be arranged to be driven by the AC drive current.

In examples, the subset of stator teeth comprises more than one stator tooth, although it will be appreciated that the subset of stator teeth could comprise only one stator tooth. In the case of the subset comprising one stator tooth, each winding can be thought of as corresponding to a concentrated winding, for example as described herein with reference to the motor 10.

Figure 16 is a schematic representation of a motor control apparatus for controlling a reluctance motor. In particular, the example illustrated in Figure 16 shows a motor control apparatus 200. The motor control apparatus comprises an AC generator 32, a DC generator 34, a controller 40 and a motor speed detector 42, which operate in a similar manner to that described above with reference to Figures 1 to 11. The motor control apparatus comprises an adder 202 and an AC + DC driver 204.

The controller 40 is operable to communicate with the adder 202 and the AC+DC driver 204 so as to control operation of the adder 202 and the AC+DC driver. The adder 202 is operable to add AC drive current generated by the AC generator 32 to a DC bias current generated by the DC generator 34 so as to generate a combined AC plus DC drive current. The adder 202 is operable to communicate the combined AC plus DC drive current to the AC+DC driver 204. The AC+DC driver 204 is arranged to apply the combined AC plus DC drive current to the same winding of the stator pole, for example, winding 20a, although it will be appreciated that the combined AC plus DC drive current could be applied to any of the other windings of the motor (e.g. motor 10). It is believed that the motor control apparatus 200 may provide a similar functionality in terms of torque performance to that provided by the motor control apparatus 30 and motor 10.

Methods for driving a motor according to examples of the disclosure will now be described with reference to Figures 17 and 18.

Figure 17 is a flowchart of a method of driving a motor such as the motor 10. At a step sl O, the AC generator 32 generates a substantially sinusoidal AC drive current, for example as described above. At a step sl2, the DC generator 34 generates a DC bias current, for example as described above. At a step sl4, the AC driver 36 applies the substantially sinusoidal drive current to a stator pole (e.g. stator pole 18b although it will be appreciated that the AC drive current could be applied to any of the stator poles) for example as described above. At a step si 6, the DC driver 38 applies the DC bias current to the stator pole (e.g. stator pole 18b although it will be appreciated thai the AC drive current could be applied to any of the stator poles) for example as described above. In examples, the steps slO and sl2 are earned out simultaneously with each other and the steps sl4 and si 6 are carried out simultaneously with each other. However, it will be appreciated that the steps slO to sl6 could be carried out in any suitable order.

Figure 18 is a flowchart of a method of driving a motor such as motor 10 described with respect to Figures 12 to 14, although it will be appreciated that the methods of Figure 17 and Figure 18 could be used to drive other configurations of reluctance motor. At a step s20, the AC generator 32 generates a substantially sinusoidal AC drive current, for example as described above. At a step s22, the AC driver 36 applies the substantially sinusoidal drive current to a stator pole (e.g. stator pole 18b although it will be appreciated that the AC drive current could be applied to any of the stator poles) for example as described above. At a step s24, the controller 40 applies the AC drive current to the respective windings of the stator poles in one of a symmetric connection configuration and an asymmetric connection configuration, for example as described above with reference to Figures 12 to 14.

Although the description above has generally been given with respect to applying current to one pole, this has been for simplicity in presenting the disclosure. It will be appreciated that the AC drive current and the DC bias current may be applied to any number of the poles so as to cause the rotor to rotate, for example using the current described with respect to Figures 3 and 4 applied with appropriate phase shifts between poles. For example, for a 6 stator pole, 4 rotor pole motor (e.g. the motor 10), a phase shift of 120 degrees between each AC drive current could be used so that the AC drive current would be considered to a be a 3 -phase drive current, although it will be appreciated that any other phase shift and number of phases could be used for the AC drive current for each pole (or pair of poles).

Additionally, it will be appreciated that other appropriate stator pole and rotor poles arrangements could be used, with one or more of the stator poles being driven as described in the examples of the disclosure so as to cause the rotor to rotate about the motor axis.

It will be appreciated that the motor 10 or motor 100 could be a synchronous reluctance motor or a variable reluctance motor as appropriate. Although the examples relate to a substantially sinusoidal AC drive current, it will be appreciated that other alternating waveforms could be used.

The various methods set out above may be implemented by a motor control apparatus, such as the motor control apparatus 30 or motor control apparatus 200, for example by using a computer program product comprising processor implementable instructions stored on a data carrier (removable storage medium) such as a floppy disk, optical disk, hard disk, PROM, RAM, flash memory or any combination of these or other storage media, or transmitted via data signals on a network such as an Ethernet, a wireless network, the Internet, or any combination of these of other networks, or realised in hardware as an ASIC (application specific integrated circuit) or an FPGA (field programmable gate array) or other configurable circuit or bespoke circuit suitable to use in adapting the existing equivalent device.

In conclusion, although a variety of examples have been described herein, these are provided by way of example only, and many variations and modifications on such examples will be apparent to the skilled person and fall within the spirit and scope of the present invention, which is defined by the appended claims and their equivalents.

Claims

1. An apparatus for driving a reluctance motor comprising a stator having a plurality of stator poles, the apparatus comprising:

AC generating means for generating a substantially sinusoidal alternating current (AC) drive current;

DC generating means for generating a direct current (DC) bias current;

AC driving means for applying the AC drive current to a stator pole of the stator of the reluctance motor; and

DC driving means for applying the DC bias current to the stator pole of the reluctance motor.

2. An apparatus according to claim 1, in which:

the stator pole comprises a first winding and the AC driving means is arranged to apply the AC drive current to the first winding of the stator pole;

the stator pole comprises a second winding and the DC driving means is arranged to apply the DC bias current to the second winding, the second winding being different from the first winding.

3. An apparatus according to claim 1 or claim 2, comprising:

controlling means for controlling the AC driving means and the DC driving means.

4. An apparatus according to claim 3, in which the controlling means is operable to control a ratio of magnitudes of the AC drive current and the DC bias current with respect to each other.

5. An apparatus according to claim 4, in which the controlling means is arranged to control the ratio of magnitudes of the AC drive current and the DC bias current so as to limit a line voltage value to less than a line voltage threshold value.

6. An apparatus according to claim 4 or claim 5, in which the controlling means is arranged to control the ratio of magnitudes of the AC drive current and the DC bias current in dependence upon rotational speed of the motor so as to control torque applied by the motor.

7. An apparatus according to any of claims 3 to 6, in which:

the controlling means is operable, in a first mode of operation, to cause only the AC drive current to be applied to the stator pole; and

the controlling means is operable, in a second mode of operation, to cause both the AC drive current and the DC bias current to be applied to the stator pole.

8. An apparatus according to claim 7, in which:

the controlling means is operable to control the motor in the first mode of operation when the rotational speed of the motor is less than a motor speed threshold; and

the controlling means is operable to control the motor in the second mode of operation when the rotational speed of the motor is equal to or greater than the motor speed threshold.

9. An apparatus according to claim 7 or claim 8, in which:

the first mode of operation corresponds with operation of the motor as having a first number of effective poles;

the second mode of operation corresponds with operation of the motor as having a second number of effective poles; and

the second number of effective poles is twice the first number of effective poles.

10. An apparatus according to any of claims 3 to 9, in which:

each of the stator poles has an associated first winding and second winding; and the controlling means is arranged to apply the AC drive current to the first winding in an asymmetric connection configuration.

1 1. An apparatus according to any of claims 3 to 9, in which:

each of the stator poles has an associated first winding and second winding; and the controlling means is arranged to apply the AC drive current to the first winding in a symmetric connection configuration.

12. An apparatus according to claim 1, comprising:

adding means for adding the AC drive current to the DC bias current so as to generate a combined AC plus DC drive current,

in which the AC driving means and the DC driving means are arranged to apply the AC plus DC drive current to the same winding of the stator pole.

13. A reluctance motor comprising:

a rotor comprising a plurality of rotor poles; and

a stator comprising a plurality of stator poles;

in which each of the stator poles is arranged to be driven by a substantially sinusoidal alternating current (AC) drive current and a direct current (DC) bias current.

14. A motor according to claim 13, comprising:

a first winding arranged with respect to a first stator pole so as to be driven by the AC drive current; and

a second winding arranged with respect to the first stator pole so as to be driven by the DC bias current, the second winding being different from the first winding.

15. A motor according to claim 14, in which:

the stator comprises a plurality of stator teeth;

the first winding is arranged around a set of stator teeth comprising a subset of the plurality of stator teeth;

the second winding is arranged around the set of stator teeth.

16. A motor according to claim 15, in which the subset of stator teeth comprises more than one stator tooth.

17. A motor according to any of claims 13 to 16, in which the motor is a synchronous reluctance motor.

18. A motor according to any of claims 13 to 16, in which the motor is a variable reluctance motor.

19. A motor according to any of claims 13 to 18, in which:

the plurality of rotor poles comprises 4 rotor poles; and

the plurality of stator poles comprises 6 stator poles.

20. A reluctance motor system comprising:

an apparatus according to any of claims 1 to 12; and a motor according to any of claims 13 to 19.

21. A method for driving a reluctance motor comprising a plurality of stator poles, the method comprising:

generating a substantially sinusoidal alternating current (AC) drive current;

generating a direct current (DC) bias current;

applying the AC drive current to a stator pole of the stator of the reluctance motor; and applying the DC bias current to the stator pole of the reluctance motor.

22. A reluctance motor comprising:

a rotor comprising a plurality of rotor poles;

a stator comprising a plurality of stator poles, the stator comprising a plurality of stator teeth;

a first winding arranged around a set of stator teeth comprising a subset of the plurality of stator teeth, the first winding being arranged with respect to a first stator pole so as to be driven by the AC drive current; and

a second winding different from the first winding, the second winding being arranged around the set of stator teeth and arranged with respect to the first stator pole so as to be driven by the DC bias current,

in which:

each of the stator poles is arranged to be driven by a substantially sinusoidal alternating current (AC) drive current and a direct current (DC) bias current; and

the subset of stator teeth comprises more than one stator tooth.

23. A reluctance motor comprising:

a rotor comprising a plurality of rotor poles; and

a stator comprising a plurality of stator poles, each stator pole comprising a stator winding arranged to be driven by a substantially sinusoidal alternating current (AC) drive current so as to cause the rotor to rotate about a rotation axis of the motor;

in which each stator winding is arranged to be driven in one of a symmetric connection configuration and an asymmetric connection configuration.

24. An apparatus for driving a reluctance motor comprising a rotor comprising a plurality of rotor poles and a stator comprising a plurality of stator poles, each stator pole comprising a stator winding arranged to be driven by a substantially sinusoidal alternating current (AC) drive current so as to cause the rotor to rotate about a rotation axis of the motor, the apparatus comprising:

AC generating means for generating the substantially sinusoidal alternating current (AC) drive current;

AC driving means for applying the AC drive current to the stator poles; and controlling means for controlling the AC driving means,

in which:

the controlling means is arranged to apply the AC drive current to the respective windings of the stator poles in one of a symmetric connection configuration and an asymmetric connection configuration.

25. An apparatus according to claim 24, in which:

the controlling means is operable to apply the AC drive current to the respective windings of the stator poles in the asymmetric connection configuration when the rotational speed of the motor 10 is less than a motor speed threshold; and

the controlling means is operable to apply the AC drive current to the respective windings of the stator poles in the symmetric connection configuration when the rotational speed of the motor 10 is greater than the motor speed threshold.

26. A method for driving a reluctance motor comprising a rotor comprising a plurality of rotor poles and a stator comprising a plurality of stator poles, each stator pole comprising a stator winding arranged to be driven by a substantially sinusoidal alternating current (AC) drive current so as to cause the rotor to rotate about a rotation axis of the motor, the method comprising:

generating the substantially sinusoidal alternating current (AC) drive current;

applying the AC drive current to the stator poles; and

controlling the AC drive current so as to apply the AC drive current to the respective windings of the stator poles in one of a symmetric connection configuration and an asymmetric connection configuration.

27. An apparatus substantially as hereinbefore described.

28. A motor substantially as hereinbefore described. A method substantially as hereinbefore described.

A motor system substantially as hereinbefore described.