Control of an electric motor

Abstract

An electric motor includes first, second, and third phase windings driven by an inverter having first, second, and third legs. A method 100 of controlling the electric motor comprises electrically iso

Description

B ACKGROUND There is a general desire to improve electric machines, such as electric motors, in a number of ways. For example, improvements may be desired in terms of size, weight, power density, manufacturing cost, efficiency, reliability, and noise. SUMMARY A first aspect of the present invention provides a method of controlling an electric motor, the electric motor comprising first, second, and third phase windings driven by an inverter comprising first, second, and third legs, wherein the method comprises: electrically isolating the third leg of the inverter, such that current cannot flow through the third leg of the inverter; applying, whilst the third leg of the inverter is electrically isolated, a voltage to the inverter such that current flows through the first and second legs of the inverter; controlling a state of the inverter to allow current flowing through the first and second legs of the inverter to decay toward zero; and when the current flowing through the first and second legs of the inverter reaches zero, electrically isolating the first and second legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter. By electrically isolating the third leg of the inverter, and applying a voltage to the inverter such that current flows through the first and second legs of the inverter whilst the third leg of the inverter is electrically isolated, the multi-phase inverter may effectively be utilised as a single-phase inverter to cause current to flow through the first and second phase windings of the electric motor. It has been found that utilising the inverter in such a manner may allow for production of a positive average torque from the electric motor, even where a magnitude of an input voltage of the inverter is lower than a magnitude of a peak of a line-to-line back EMF induced in the electric motor. This can increase overall power delivery capability of the electric motor compared to alternative control schemes. The method may be performed when a magnitude of an input voltage of the inverter is lower than a magnitude of a peak of a line-to-line back EMF induced in the electric motor. An input voltage to the inverter may be a mains input voltage, for example a voltage having a magnitude that varies sinusoidally across an electrical cycle. The method may be performed for that part of the electrical cycle in which the magnitude of the input voltage to the inverter is lower than a magnitude of a peak of a line-to-line back EMF induced in the electric motor. Each of the first, second, and third, legs of the inverter may comprise a respective high-side switch and a respective low-side switch, the inverter may comprise a plurality of inverter states, each inverter state corresponding to a different configuration of the high-side and low-side switches of the first, second, and third, legs of the inverter, and electrically isolating the third leg of the inverter may comprise controlling the inverter to be in an inverter state in which current cannot pass through the high-side switch and the low-side switch of the third leg of the inverter. Electrically isolating the third leg of the inverter may comprise controlling the inverter to be in an inverter state in which the high-side switch and the low-side switch of the third leg of the inverter are turned off. Electrically isolating the first and second legs of the inverter may comprise controlling the inverter to be in an inverter state in which current cannot pass through the high-side switches and the low-side switches of the first and second legs of the inverter. Electrically isolating the first and second legs of the inverter may comprise controlling the inverter to be in an inverter state in which the high-side switches and the low-side switches of the first and second legs of the inverter are turned off. Applying, whilst the third leg of the inverter is electrically isolated, the voltage to the inverter such that current flows through the first and second legs of the inverter, may comprise controlling the inverter to be in an inverter state in which current cannot pass through the high-side switches and the low-side switches of the first and second legs of the inverter, and in which current can pass through the high-side switch of the first leg of the inverter and through the low-side switch of the second leg of the inverter, or vice versa. Applying, whilst the third leg of the inverter is electrically isolated, the voltage to the inverter such that current flows through the first and second legs of the inverter, may comprise controlling the inverter to be in an inverter state in which the high-side switch and the low-side switch of the third leg of the inverter are turned off, and in which one of: the high-side switch of the first leg of the inverter and the low-side switch of the second leg of the inverter are turned on, whilst the low-side switch of the first leg of the inverter and the high-side switch of the second leg of the inverter are turned off; and the high-side switch of the first leg of the inverter and the low-side switch of the second leg of the inverter are turned off, whilst the low-side switch of the first leg of the inverter and the high-side switch of the second leg of the inverter are turned on. Controlling a state of the inverter to allow current flowing through the first and second legs of the inverter to decay toward zero may comprise one of: controlling the inverter to be an inverter state in which current can flow through the first and second legs of the inverter in one direction only; and freewheeling the inverter. Controlling a state of the inverter to allow current flowing through the first and second legs of the inverter to decay toward zero may comprise controlling the inverter to be in an inverter state in which one of: the high-side switch of the first leg of the inverter is in a switch state that allows conduction in one direction only, whilst the low-side switch of the first leg of the inverter is turned off, and in which the low-side switch of the second leg of the inverter is in a switch state that allows conduction in one direction only, whilst the high-side switch of the second leg of the inverter is turned off; and the low-side switch of the first leg of the inverter is in a switch state that allows conduction in one direction only, whilst the high-side switch of the first leg of the inverter is turned off, and in which the high-side switch of the second leg of the inverter is in a switch state that allows conduction in one direction only, whilst the low-side switch of the second leg of the inverter is turned off. Controlling a state of the inverter to allow current flowing through the first and second legs of the inverter to decay toward zero may comprise controlling the inverter to be in an inverter state in which the high-side and low-side switches of the first and second legs of the inverter are in switch states that enable freewheeling of the inverter. The high-side and low-side switches of the first, second, and third, legs of the inverter may comprise bi-directional switches. Use of bi-directional switches may facilitate electrical isolation of legs of the inverter, as bidirectional switches typically comprise a fully off state, in contrast to conventional FETs which allow for regenerative current flow via their anti-parallel diodes. A bi-directional switch may comprise an on state, in which current flow is allowed in both directions between source and drain terminals of the bi-directional switch. A bi-directional switch may comprise an anti-parallel diode state, in which current flow is allowed from the source terminal to the drain terminal of the bi-directional switch. A bi-directional switch may comprise a parallel diode state, in which current flow is allowed from the drain terminal to the source terminal of the bi-directional switch. A bidirectional switch may comprise an off state, in which an open circuit condition is created. The high-side and low-side switches of the first, second, and third, legs of the inverter may comprise gallium nitride switches. The method may comprise: subsequently to electrically isolating the first and second legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter, modifying a state of the inverter such that the second leg remains electrically isolated and the first and third legs are no longer electrically isolated; applying, whilst the second leg of the inverter is electrically isolated, a voltage to the inverter such that current flows through the first and third legs of the inverter; controlling a state of the inverter to allow current flowing through the first and third legs of the inverter to decay toward zero; and when the current flowing through the first and third legs of the inverter reaches zero, electrically isolating the first and third legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter. The method may comprise: subsequently to electrically isolating the first and third legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter, modifying a state of the inverter such that the first leg remains electrically isolated and the second and third legs are no longer electrically isolated; applying, whilst the first leg of the inverter is electrically isolated, a voltage to the inverter such that current flows through the second and third legs of the inverter; controlling a state of the inverter to allow current flowing through the second and third legs of the inverter to decay toward zero; and when the current flowing through the second and third legs of the inverter reaches zero, electrically isolating the second and third legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter. In such a manner a different one of the third, second, and first legs of the inverter can be sequentially isolated whilst current flows through the remaining non-isolated legs of the inverter. Each such excitation of the non-isolated legs of the inverter may be referred to herein as a drive cycle. The method may comprise applying the voltage to the inverter in response to determination of a back EMF zero crossing of a line-to-line back EMF between the phase windings that are to be driven by the voltage applied to the inverter. The method may comprise offsetting a start time of applying the voltage to the inverter relative to the determined back EMF zero crossing. The offset may comprise a predetermined value, for example a value stored in a memory accessible by a controller of the electric motor, or the offset may comprise a value calculated in real-time. The method may comprise advancing the start time of applying the voltage to the inverter relative to the determined back EMF zero crossing. This may enable an amount of time in which current flows through the phase windings to be increased relative to applying the voltage at the determined back EMF zero crossing, which may provide an increase in positive average torque. The offset may also be referred to as an advance angle. The method may comprise: over a time period in which a magnitude of an input voltage to the inverter varies, performing a sequence of drive cycles in which: one of the first, second, and third legs of the inverter is electrically isolated whilst the remaining legs of the inverter are not electrically isolated; a voltage is applied to the inverter such that current flows through the remaining legs of the inverter which are not electrically isolated; current flowing through the remaining legs of the inverter which are not electrically isolated is allowed to decay towards zero; and when the current flowing through the remaining legs of the inverter which are not electrically isolated reaches zero, electrically isolating the remaining legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter; and wherein the offset of the start time of applying the voltage to the inverter relative to the determined back EMF zero crossing is fixed for each drive cycle. In a drive cycle there may be multiple instances of applying a voltage to the inverter such that current flows through the remaining legs of the inverter which are not electrically isolated, and allowing the current flowing through the remaining legs of the inverter which are not electrically isolated to decay towards zero, before the current flowing through the remaining legs of the inverter which are not electrically isolated reaches zero. The method may comprise: over a time period in which a magnitude of an input voltage to the inverter varies, performing a sequence of drive cycles in which: one of the first, second, and third legs of the inverter is electrically isolated whilst the remaining legs of the inverter are not electrically isolated; a voltage is applied to the inverter such that current flows through the remaining legs of the inverter which are not electrically isolated; current flowing through the remaining legs of the inverter which are not electrically isolated is allowed to decay towards zero; and when the current flowing through the remaining legs of the inverter which are not electrically isolated reaches zero, electrically isolating the remaining legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter; and wherein the offset of the start time of applying the voltage to the inverter relative to the determined back EMF zero crossing varies across the time period, and is based on the magnitude of the input voltage to the inverter at the time at which the voltage is to be applied. If a fixed offset is applied, then, as the inverter input voltage varies across a sequence of drive cycles, current pulse peaks can be higher, requiring a longer time period for current flowing through legs of the inverter to decay to zero. This can lead to current in the phase windings being unable to fully decay to zero ahead of a next drive cycle, resulting in the next drive cycle being skipped. Applying a variable offset may mitigate for this. In a drive cycle there may be multiple instances of applying a voltage to the inverter such that current flows through the remaining legs of the inverter which are not electrically isolated, and allowing the current flowing through the remaining legs of the inverter which are not electrically isolated to decay towards zero, before the current flowing through the remaining legs of the inverter which are not electrically isolated reaches zero. The offset may vary inversely with respect to the magnitude of the input voltage to the inverter, for example with the offset decreasing as the magnitude of the input voltage to the inverter increases. The method may comprise allowing, when a gradient of current flowing through the first and second legs of the inverter becomes negative, current flowing through the second and third legs of the inverter to decay toward zero. Controlling a state of the inverter to allow current flowing through legs of the inverter to decay toward zero may comprise at least one of: controlling the inverter to be in an inverter state in which current can flow through the legs of the inverter in one direction only; and freewheeling the inverter. Controlling the inverter to be in such an inverter state may be in response to a peak of current flowing through the first and / or second leg of the inverter. Controlling the inverter to be in such an inverter state may be in response to detection of a negative gradient of current flowing through the first and / or second leg of the inverter. The method may comprise: over a time period in which a magnitude of an input voltage to the inverter varies, performing a sequence of drive cycles in which: one of the first, second, and third legs of the inverter is electrically isolated whilst the remaining legs of the inverter are not electrically isolated; a voltage is applied to the inverter, for a conduction period, such that current flows through the remaining legs of the inverter which are not electrically isolated; current flowing through the remaining legs of the inverter which are not electrically isolated is allowed to decay towards zero; and when the current flowing through the remaining legs of the inverter which are not electrically isolated reaches zero, electrically isolating the remaining legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter; and at least one of: the conduction period is fixed for each drive cycle within the time period; a length of time between controlling the inverter to be an inverter state in which current can decay toward zero, and a start of a conduction period of a next subsequent drive cycle within the time period, is fixed; and a length of time between freewheeling the inverter and a start of a conduction period of a next subsequent drive cycle within the time period, is fixed. This may provide for increased positive torque production relative to, for example, allowing the current to decay in response to detection of a negative current gradient. The method may comprise: over a time period in which a magnitude of an input voltage to the inverter varies, performing a sequence of drive cycles in which: one of the first, second, and third legs of the inverter is electrically isolated whilst the remaining legs of the inverter are not electrically isolated; a voltage is applied to the inverter, for a conduction period, such that current flows through the remaining legs of the inverter which are not electrically isolated; current flowing through the remaining legs of the inverter which are not electrically isolated is allowed to decay towards zero; and when the current flowing through the remaining legs of the inverter which are not electrically isolated reaches zero, electrically isolating the remaining legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter; and at least one of: the conduction period varies within the time period; a length of time between controlling the inverter to be an inverter state in which current can decay toward zero, and a start of a conduction period of a next subsequent drive cycle within the time period, varies within the time period; and a length of time between freewheeling the inverter and a start of a conduction period of a next subsequent drive cycle within the time period, varies within the time period. If a fixed conduction period, or length of time, is applied, then, as the inverter input voltage varies across a sequence of drive cycles, current pulse peaks can be higher, requiring a longer time period for current flowing through legs of the inverter to decay to zero. This can lead to current in the phase windings being unable to fully decay to zero ahead of a next drive cycle, resulting in the next drive cycle being skipped. Applying a variable conduction period or length of time may mitigate for this. The length of time between controlling the inverter to be an inverter state in which current can decay toward zero, and a start of a conduction period of a next subsequent drive cycle within the time period, may comprise a fixed component and a variable component. The variable component may vary with a magnitude of the input voltage to the inverter, for example with the variable component comprising a repeating pattern with an amplitude that changes at a frequency twice that of the input voltage to the inverter. The method may comprise a first sub-control method and a second sub-control method different to the first sub-control method, wherein: the first sub-control method comprises: performing a sequence of drive cycles in which: one of the first, second, and third legs of the inverter is electrically isolated whilst the remaining legs of the inverter are not electrically isolated; a voltage is applied to the inverter, for a conduction period, such that current flows through the remaining legs of the inverter which are not electrically isolated; current flowing through the remaining legs of the inverter which are not electrically isolated is allowed to decay towards zero; and when the current flowing through the remaining legs of the inverter which are not electrically isolated reaches zero, electrically isolating the remaining legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter; the first sub-control method is performed during a first portion of an electrical cycle of an input voltage of the inverter; and the second subcontrol method is performed during a second portion of the electrical cycle of the input voltage of the inverter different to the first portion of the electrical cycle of the input voltage of the inverter. The first sub-control method may be performed when a magnitude of the input voltage of the inverter is lower than a magnitude of a peak of a line-to-line back EMF induced in the electric motor; and the second sub-control method may be performed when a magnitude of the input voltage of the inverter is higher than the magnitude of the peak of a line-to-line back EMF induced in the electric motor. The method may comprise detecting a transition of the magnitude of the input voltage of the inverter from below the magnitude of the peak of the line-to-line back EMF induced in the electric motor to above the magnitude of the peak of the line-to-line back EMF induced in the electric motor, or vice-versa, and performing one of the first and second sub-control methods in response to the detection. The method may comprise transitioning from the first sub-control method to the second sub-control method in response to peaks in current flowing through the remaining legs of the inverter which are not electrically isolated, for example when the magnitude of the input voltage to the inverter transitions from below the magnitude of the peak of the line-to-line back EMF induced in the electric motor to above the magnitude of the peak of the line-to-line back EMF induced in the electric motor. The second sub-control method may comprise performing any of space vector pulse width modulation, field oriented control, direct torque control, sinusoidal pulse width modulation control, and sinusoidal hysteresis control. The method may comprise transitioning from the second sub-control method to the first sub-control method by: determining a magnitude of current flowing through the respective first, second, and third legs of the inverter; for the leg of the inverter having the lowest determined magnitude of current: controlling a state of the inverter to allow current flowing through the leg of the inverter to decay to zero; and where the current flowing through the leg of the inverter reaches zero, electrically isolating the leg of the inverter; and performing the first sub-control method. For the leg of the inverter having the lowest determined magnitude of current, controlling a state of the inverter to allow current flowing through the leg of the inverter to decay to zero may be performed whilst controlling a state of the inverter to allow current to flow through the remaining two legs of the inverter. Controlling a state of the inverter to allow current flowing through the leg of the inverter to decay to zero may comprise one of: controlling the inverter to be an inverter state in which current can flow through the leg of the inverter in one direction only; and freewheeling the inverter. Electrically isolating the leg of the inverter may comprise turning off the high-side and the low-side switches of the leg of the inverter. A second aspect of the present invention provides a controller for an electric motor system, the controller configured to perform a method as claimed in any one of the preceding claims. A third aspect of the present invention provides an electric motor system comprising an electric motor, an inverter, and a controller according to the second aspect of the present invention. The electric motor may be a three-phase electric motor, and the inverter may be a three-phase inverter. The electric motor may comprise a star connected electric motor, for example with a separated neutral point. In some examples, the electric motor may comprise n phase windings, where n is a positive integer greater than or equal to three, the inverter may comprise n legs, and a method may comprise electrically isolating n-2 legs of the inverter, such that current cannot flow through the n-2 legs of the inverter; applying, whilst the n-2 legs of the inverter are electrically isolated, a voltage to the inverter such that current flows through the remaining two legs of the inverter; controlling a state of the inverter to allow current flowing through the remaining two legs of the inverter to decay toward zero; and when the current flowing through the remaining two legs of the inverter reaches zero, electrically isolating the remaining two legs of the inverter, such that current cannot flow through the legs of the inverter. A fourth aspect of the present invention provides a method of controlling an electric motor driven by an inverter, wherein the method comprises determining when a magnitude of an input voltage to the inverter transitions from below a magnitude of a peak of the line-to-line back EMF induced in the electric motor to above the magnitude of the peak of the line-to-line back EMF induced in the electric motor, or vice-versa, and performing one of a first sub-control method and a second sub-control method different to the first sub-control method in response to the detection. The first sub-control method may comprise: electrically isolating a third leg of the inverter, such that current cannot flow through the third leg of the inverter; applying, whilst the third leg of the inverter is electrically isolated, a voltage to the inverter such that current flows through first and second legs of the inverter; allowing current flowing through the first and second legs of the inverter to decay toward zero; and when the current flowing through the first and second legs of the inverter reaches zero, electrically isolating the first and second legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter. The second sub-control method may comprise performing space vector pulse width modulation. The method may comprise performing the first sub-control method where the magnitude of the input voltage to the inverter is below a magnitude of the peak of the line-to-line back EMF induced in the electric motor. The method may comprise performing the second sub-control method where the magnitude of the input voltage to the inverter is above a magnitude of the peak of the line-to-line back EMF induced in the electric motor. Optional features of aspects of the present invention may be equally applied to other aspects of the present invention, where appropriate. BRIEF DESCRIPTION OF THE DRAWINGS Figure lisa schematic illustration of a motor system; Figure 2 is a table illustrating switch states of a bi-directional switch of an inverter of the motor system of Figure 1; Figure 3(a) is an example voltage waveform diagram for the motor system of Figure 1; Figure 3(b) is an example current waveform diagram for the motor system of Figure 1; Figures 4(a)-(d) are example inverter states of the inverter that produce the example current waveforms of Figure 3(b); Figure 5 is a flow diagram illustrating a first example method; Figures 6(a)-(c) are further example inverter states of the inverter that produce the example current waveforms of Figure 3(b); Figure 7 is an example current waveform diagram illustrating a constant pre-commutation time; Figure 8 is an example torque plot obtained using the constant pre-commutation time of Figure 7; Figure 9(a) is an example diagram illustrating a variable pre-commutation time; Figure 9(b) is an example current waveform diagram illustrating the variable precommutation time; Figure 10 is an example torque plot obtained using the variable pre-commutation time of Figures 9(a) and 9(b); Figure 11 is an example current waveform diagram illustrating a constant advance angle; Figure 12 is an example torque plot obtained using the constant advance angle of Figure ii; Figure 13 is an example diagram illustrating a variable advance angle; Figure 14 is an example torque plot obtained using the variable advance angle of Figure 13; Figure 15 is a flow diagram illustrating a second example method; Figure 16 is an example current waveform diagram illustrating a transition from the method of Figure 5 to space vector pulse width modulation; Figure 17 is an example current waveform diagram illustrating a transition from space vector pulse width modulation to the method of Figure 5; and Figure 18 is an example diagram illustrating switching states of the inverter used in the transition of Figure 17. DETAILED DESCRIPTION An electric motor system 10 is shown schematically in Figure 1 connected to a mains power source 18, and comprises an electric motor 12, an inverter 14, and a controller 16. The electric motor 12 is a three phase electric motor having three phase windings (nominally denoted A, B, and C), with each phase modelled in Figure 1 by a resistor RPH, and inductance LPH, and a back EMF BEMF. The phase windings are star connected, with a separated neutral point. The inverter 14 comprises a first leg 20, a second leg 22, a third leg 24, and a DC shunt resistor R4. Each of the first 20, second 22, and third 24 legs comprises a respective high-side switch S1,S3,S5, a respective low-side switch S2,S4,S6, and a respective shunt resistor R1,R2,R3. It will be appreciated that the first 20, second 22, and third 24 legs of the inverter 14 can be considered to correspond to a respective one of the phase windings A,B,C of the electric motor 12. Each of the switches S1-S6 is bidirectional and comprises four switch states: ON, DI, D2, and OFF. When the state of a switch S1-S6 is ON, the switch S1-S6 is conductive in both a first direction and a second direction. When the state of the switch S1-S6 is DI, the switch S1-S6 is conductive in the first direction and non-conductive in the second direction. Conversely, when the state of the switch S1-S6 is D2, the switch S1-S6 is non-conductive in the first direction and conductive in the second direction. DI and D2 may therefore be regarded as diode states. With the particular arrangement of switches S1-S6 shown in Figure 1, the first direction may be regarded as downward (i.e. DI = downward conducting), and the second direction may be regarded as upward (i.e. D2 = upward conducting). Finally, when the state of the switch S1-S6 is OFF, the switch S1-S6 is non-conductive in both the first direction and the second direction. Figure 2 illustrates the different states of each switch S1-S6, along with the equivalent circuit. In comparison to a MOFSET having a body diode or an IGBT having an equivalent antiparallel diode, the bidirectional switches S1-S6 have two additional switch states. For example, when a MOSFET is turned ON, the switch conducts in both directions. When the MOSFET is turned OFF, the switch continues to conduct in one direction owing to the inherent body diode. In contrast to the bidirectional switch S1-S6 described above, the MOSFET does not have an open-circuit state in which the switch is non-conductive in both directions. Additionally, whilst the MOSFET is capable of conducting in a first direction only when turned OFF (i.e. through the body diode), the switch is not capable of conducting in a second opposite direction only. The provision of the inverter 14 having bidirectional switches S1-S6 has the advantage that, irrespective of the polarity of the voltage on the power lines, the switches S1-S6 may be controlled such that a voltage of either polarity may be applied to the phase winding. As a result, motor system 10 may be used with an AC power supply without the need for a rectifier. Each switch S1-S6 may comprise a gallium nitride switch, which has a relatively high breakdown voltage and is thus well-suited for operation at mains voltages. Nevertheless, other types of bidirectional switch capable of being controlled in both directions may be used. The shunt resistors R1-R3 and the DC shunt resistor R4 are utilised to provide voltage and / or current readings to the controller 16 during operation of the electric motor system 10. In some examples, any of the shunt resistors R1-R3 and the DC shunt resistor R4 may be omitted. The controller 16 is configured to, using the received voltage and / or current values alongside other determined variables like motor speed, control a state of the inverter 14 by controlling switch states of the switches S1-S6, to apply a voltage to the phase windings of the electric motor 12. This is achieved by sending control signals CS1-CS12 to the gates of the switches S1-S6. The mains power source 18 provides an AC voltage to the inverter 14, with the AC voltage varying in magnitude across an electrical cycle. A three-phase electric motor can be controlled either by having two phases conducting at a time, or all three phases conducting at the same time. In the case where two phases are conducting at the same time and MOSFET switches connected to the non-conducting phase are turned OFF, positive torque (motoring mode) can only be produced at the instance when the inverter input voltage is greater than the magnitude of the line-to-line back emf of the two conducting phases. In particular, the presence of the permanently fixed anti-parallel diodes on MOSFET switches will prevent the constant production of positive torque (continuous motoring mode), as current can flow through the MOSFET switches connected to the non-conducting phase even though they are turned OFF. If all phases are conducting at the same time while using traditional MOSFET switches and the inverter supply voltage magnitude is less than a peak of the magnitude of line-to-line back emf, reactive power is generated and consequently negative torque, resulting in unintended braking of the motor, leading to increased torque ripple and reduced utilization of the motor drive to deliver positive (i.e. motoring) power. The motor system 10 of Figure 1 mitigates for this by use of the bidirectional GaN switches S1-S6, and appropriate control of the switch states of those switches S1-S6. In particular, where a magnitude of an input voltage of the inverter 14 is lower than a magnitude of a peak of a line-to-line back EMF induced in the electric motor 12, the controller 16 electrically isolates one leg of the inverter 14, with the remaining two legs of the inverter 14 controlled in a similar manner to a single-phase inverter, as will be described in more detail hereinafter. Taking conduction in phase A and B as an example, and with reference to Figures 3a-b and Figures 4a-d, when a zero-crossing of the line-to-line back EMF AB is detected at time T1 in Figure 3b, the controller 16 controls the high-side switch SI of the first leg 20 and the low-side switch S4 of the second leg 22 to be ON, whilst the high-side switches S3,S5 of the second 22 and third 24 legs, and the low-side switches S2,S6 of the first 20 and third 24 legs are controlled to be OFF. Such an inverter state is shown in Figure 4a, where it will be appreciated that switch state OFF is indicated by use of a lighter colour for switches and their corresponding conduction paths. As the switches S5,S6 of the third leg 24 are in switch state OFF, the third leg 24 of the inverter 14 can be considered to be electrically isolated, and current cannot flow through the third leg of the inverter 14, and hence cannot flow through phase winding C of the electric motor 12. With such an inverter state set, current flows through the first 20 and second 22 legs of the inverter 14, with the magnitude of the current flowing across phase windings A and B increasing. Prior to commencement of current regeneration, the controller 16 controls the low-side switch S2 of the first leg 20 and the high-side switch S3 of the second leg 22 to move from switch state OFF to switch state D2 (as the applied voltage is positive), whilst the high-side switch SI of the first leg 20 and the low-side switch S4 of the second leg 22 remain ON, and the high-side S5 and low-side S6 switches of the third leg 24 remain OFF. Such an inverter state is shown in Figure 4b. At time T2 in Figure 3b, a gradient of current flowing through phase winding A becomes negative. In response to this, the controller 16 controls the high-side switch SI of the first leg 20 and the low-side switch S4 of the second leg 22 to move from switch state ON to switch state OFF, whilst the low-side switch S2 of the first leg 20 and the high-side switch S3 of the second leg 22 remain in switch state D2, and the high-side S5 and low-side S6 switches of the third leg 24 remain in switch state OFF. Such an inverter state is shown in Figure 4c. Here, as the low-side switch S2 of the first leg 20 and the high-side switch S3 of the second leg 22 are in switch state D2 whilst the high-side switch SI of the first leg 20, the low-side switch S4 of the second leg 22, and the high-side S5 and low-side S6 switches of the third leg 24 are in switch state OFF, current regeneration is enabled, and current flowing through the first 20 and second 22 legs of the inverter 14 is allowed to decay towards zero. At time T3 in Figure 3b, when the current flowing through the first 20 and second 22 legs of the inverter 14 reaches zero, the controller 16 controls the low-side switch S2 of the first leg 20 and the high-side switch S3 of the second leg 22 to move from switch state D2 to switch state OFF, whilst the high-side switch SI of the first leg 20, the low-side switch S4 of the second leg 22, and the high-side S5 and low-side S6 switches of the third leg 24 remain in switch state OFF. In such a case, all switches S1-S6 are in switch state OFF, and the first 20, second 22, and third 24 legs of the inverter 14 can be considered to be electrically isolated, such that current cannot flow through the first 20, second 22, and third 24 legs of the inverter 14. Such an inverter state is illustrated in Figure 4d. This effectively clamps the current at zero. The sequence can then be repeated in response to subsequent detections of zero-crossings of the line-to-line back EMF, for example with the zero-crossing of the line-to-line back EMF between phases C and A occurring next in Figures 3a and 3b. Each application of voltage, i.e. each conduction, is referred to as a drive cycle herein. By electrically isolating the third leg 24 of the inverter 14, and applying a voltage to the inverter 14 such that current flows through the first 20 and second 22 legs of the inverter 14 whilst the third leg 24 of the inverter 14 is electrically isolated, the inverter 14 may effectively be utilised as a single-phase inverter to cause current to flow through the first A and second B phase windings of the electric motor 12. It has been found that utilising the inverter 14 in such a manner may allow for production of a positive average torque from the electric motor 12, even where a magnitude of an input voltage of the inverter 14 is lower than a magnitude of a peak of a line-to-line back EMF induced in the electric motor 12. This can increase overall power delivery capability of the electric motor 12 compared to alternative control schemes. A method 100 in accordance with the above is illustrated in the flow diagram of Figure 5. The method 100 comprises electrically isolating 102 the third leg 24 of the inverter 14, such that current cannot flow through the third leg 24 of the inverter 14. The method 100 comprises applying 104 applying, whilst the third leg 24 of the inverter 14 is electrically isolated, a voltage to the inverter 14 such that current flows through the first 20 and second 22 legs of the inverter 14. The method 100 comprises controlling 106 a state of the inverter 14 to allow current flowing through the first 20 and second 22 legs of the inverter 14 to decay toward zero. The method 100 comprises, when the current flowing through the first 20 and second 22 legs of the inverter 14 reaches zero, electrically isolating 108 the first 20 and second 22 legs of the inverter 14, such that current cannot flow through the first 20, second 22, and third 24 legs of the inverter 14. Switch states of the inverter 14 that allow for conduction through phases AB, BC, and CA, are shown in Figures 6a-c respectively. It will be appreciated that there may be multiple instances of applied voltage, each followed by allowing the current to decay towards zero, before the current reaches zero. In the discussion above, conduction through the conducting phases A and B takes place at detection of a zero-crossing crossing of the line-to-line back EMF AB, and a transition to current regeneration takes place at a time when a gradient of current flowing through phase winding A becomes negative. Whilst this has been found to enable production of unidirectional positive average torque when the magnitude of the input voltage of the inverter 14 is lower than the magnitude of the peak of a line-to-line back EMF induced in the electric motor 12, it has been found that the magnitude of positive average torque can be increased by modifying timing parameters relating to conduction. The controller 16 can make use of several control schemes to provide an increase in production of positive average torque, as will be discussed in further detail hereinafter. One such control scheme provides a constant pre-commutation time, or electrical angle, across the time period in which the method 100 of Figure 5 is implemented. Here, the precommutation time is the length of time, or electrical angle, between the start of current regeneration (end of conduction) and the start of a conduction of a next subsequent drive cycle. The pre-commutation time is denoted X in the schematic current waveform diagram of Figure 7, where a constant pre-commutation time is utilised (Xi=X2). By utilising a constant pre-commutation time, more torque per drive cycle can be produced when compared to a method where a transition to current regeneration takes place at a time when a gradient of current flowing through phase winding A becomes negative. This is illustrated in Figure 8. It will be appreciated that, instead of a pre-commutation time, a conduction period can be defined, with the conduction period being the length of time, or electrical angle, between the start and end of conduction for a given drive cycle. A fixed conduction period may then be utilised in the same manner as the fixed pre-commutation time. Another control scheme provides a controlled variable pre-commutation time, or electrical angle, across the time period in which the method 100 of Figure 5 is implemented. In particular, as the input voltage to the inverter 14 increases across an electrical cycle, a peak of the current pulse also increases, leading to a longer time needed to allow the current to decay to zero. This then reduces the time period in which current is clamped at zero before the next drive cycle in the sequence. Applying a variable pre-commutation time enables this to be accounted for. A suitable form for a variable pre-commutation time is illustrated in Figure 9a. As can be seen, the variable pre-commutation time comprises a fixed DC offset component, and a variable component that varies with the input voltage to the inverter 14. The variable component varies at a frequency twice that of the input voltage to the inverter 14. Implementation of such a variable pre-commutation time is illustrated schematically in Figure 9b, where By use of such a variable pre-commutation time, a larger precommutation time is applied for larger input voltages when compared to the control scheme that utilises the fixed pre-commutation time. This leads to a greater average torque per drive cycle when compared to the control scheme that utilises the fixed precommutation time. This is illustrated in Figure 10. It will be appreciated that alternative relationships between the pre-commutation time and the input voltage to the inverter 14 can be utilised. In another control scheme implemented by the controller 16, the start of the conduction period is advanced relative to the determined zero-crossing of the line-to-line back EMF. This can either be by an advance angle, or an advance time, as desired. It will be appreciated that negative advance angles or advance times can be utilised, and that these may be referred to as retard angles or retard times, as desired. When commutating, i.e. beginning a conduction period of a new drive cycle, at the determined zero-crossing, current build-up time is limited to between the zero-crossing and the start of current regeneration, which can limit the amount of torque produced. To mitigate for this, the controller 16 can apply a constant positive non-zero advance angle relative to the determined zero-crossing across the time period in which the method 100 of Figure 5 is implemented. This is illustrated schematically in Figure 11, where the advance angle is constant (<|>i=c|>2). Use of the constant positive non-zero advance angle can widen the current build-up period, leading to larger current peaks being attained before regeneration and subsequent switch turn-off takes place, thereby translating to higher average positive torque per drive cycle. This is illustrated in Figure 12 in combination with applying a variable pre-commutation time, relative to a control scheme where no advance angle is applied alongside applying a variable pre-commutation time. It will be appreciated that a constant positive non-zero advance angle can also be utilised alongside application of a constant pre-commutation time, or indeed a method where a transition to current regeneration takes place at a time when a gradient of current flowing through a phase winding becomes negative. Instead of a constant advance angle, the controller 16 can apply a variable advance angle. In particular, and similar to the discussion above in relation to pre-commutation time, as the input voltage to the inverter 14 increases across an electrical cycle, a peak of the current pulse also increases, leading to a longer time needed to allow the current to decay to zero. This then reduces the time period in which current is clamped at zero before the next drive cycle in the sequence. In some instances, current in a phase winding may be unable to be fully discharged before a next drive cycle is due to take place, resulting in the next drive cycle being skipped. Use of a variable advance angle can mitigate for this, for example with the advance angle varying inversely with the input voltage to the inverter 14. Such a variable advance angle is illustrated schematically in Figure 13, where the advance angle decreases linearly as the input voltage to the inverter 14 increases. Varying the advance angle as the input voltage to the inverter 14 varies can provide increased average torque per drive cycle relative to use of a constant advance angle. This is illustrated in Figure 14, relative to a control scheme that utilises a constant advance angle in combination with a varying pre-commutation time. It will be appreciated that a varying advance angle can also be utilised alongside application of a constant pre-commutation time, or indeed a method where a transition to current regeneration takes place at a time when a gradient of current flowing through a phase winding becomes negative. In the above, the method 100 of Figure 5 has been discussed as being applied when the magnitude of the input voltage of the inverter 14 is lower than the magnitude of a peak of a line-to-line back EMF induced in the electric motor 12. The motor could equally be applied when the magnitude of the input voltage of the inverter 14 is higher than the magnitude of a peak of a line-to-line back EMF induced in the electric motor 12, for example with a single control scheme applied across an electrical cycle. It has been found, however, that there may be improvements in efficiency by implementing different control schemes at different points of an electrical cycle. In the present case, the controller 16 implements the method 100 of Figure 5 in response to a determination that the magnitude of the input voltage of the inverter 14 is lower than the magnitude of a peak of a line-to-line back EMF induced in the electric motor 12, and implements a three-phase control scheme, such as space vector pulse width modulation (SVPWM), in response to a determination that the magnitude of the input voltage of the inverter 14 is higher than the magnitude of a peak of a line-to-line back EMF induced in the electric motor 12. A method 200 in accordance with the above is illustrated in Figure 15. The method 200 comprises determining 202 when a magnitude of an input voltage to the inverter transitions from below a magnitude of a peak of the line-to-line back EMF induced in the electric motor to above the magnitude of the peak of the line-to-line back EMF induced in the electric motor, or vice-versa, and performing 204 one of a first sub-control method and a second sub-control method different to the first sub-control method in response to the detection. It will be appreciated that the method 100 of Figure 5 can be considered to be a first subcontrol method, whilst a three-phase control scheme such as SVPWM can be considered to be a second sub-control method. It will further be appreciated that SVPWM is one example of a three-phase control scheme, and that other three-phase control schemes are also envisaged. Without actively controlling and optimizing the transition from SVPWM to the method 100 of Figure 5, and vice versa, it has been found that there may be an increased distortion in the AC mains voltage when compared to simply using the method 100 of Figure 5 across a full electrical cycle. In some cases, the level of distortion to the AC mains voltage must be assessed in accordance with IEC 61000-3-2 Electromagnetic compatibility (EMC) - Part 3-2: Limits - Limits for harmonic current emissions (equipment input current <16 A per phase), which limits mains voltage distortion by prescribing the maximum value for harmonic currents from the second harmonic up to and including the 40th harmonic current. To mitigate for distortion in the AC mains voltage, the controller 16 can control the transition from SVPWM to the method 100 of Figure 5, and vice versa. Firstly considering the transition from the method 100 of Figure 5 to SVPWM, for example when the magnitude of the input voltage of the inverter 14 transitions from lower than the magnitude of a peak of a line-to-line back EMF induced in the electric motor 12 to higher than the magnitude of a peak of a line-to-line back EMF induced in the electric motor 12, the controller 16 initiates the transition at the peak of the phase current in the two conducting phases, as illustrated in Figure 16. This is achieved by monitoring the gradient of the current, and initiating the transition when the gradient of the current becomes negative. Alternatively, the transition could take place at a given current level, or at a given time interval, from the start of conduction. Then considering the transition from SVPWM to the method 100 of Figure 5, for example when the magnitude of the input voltage of the inverter 14 transitions from higher than the magnitude of a peak of a line-to-line back EMF induced in the electric motor 12 to lower than the magnitude of a peak of a line-to-line back EMF induced in the electric motor 12, the controller 16 initiates the transition by observing the magnitude of the phase currents flowing through the first 20, second 22, and third 24 legs of the inverter 14, and transitioning between methods when the phase current with the smallest magnitude reaches zero. This is illustrated in Figure 17, which appropriate switch states of the switches S1-S6 to achieve this illustrated in Figure 18. In particular, by observing the magnitude and polarity of the phase current during the three-phase control (SVPWM), the controller 16 sets the inverter to a state (state 1 in Figure 18) in which the high-side switch SI of the first leg 20, and the low-side switches S4,S6 of the second 22 and third 24 legs are in the ON switch state, and the low-side switch S2 of the first leg, and the high-side switches S3,S5 of the second 22 and third 24 legs, are in the OFF switch state. This will ensure continuous current flow in the same direction, after which the controller 16 transfers from SVPWM to the method 100 of Figure 5. After a deadtime period has passed, the controller 16 sets the inverter 14 to a transient state (state 2 in Figure 18) in which the high-side switch SI of the first leg 200, and the low-side switches S4, S6 of the second 20 and third 22 legs, are in the ON switch state, the low-side switch S2 of the first leg 20 and the high-side switch S5 of the third leg 24 are in the OFF switch state, and the high-side switch S3 of the second leg 22 is in the D2 switch state. This prepares the inverter 14 for initiation of freewheeling of the current loop with the smallest magnitude (which is phase B, in Figure 17), which takes place at time T1 in Figure 17. Freewheeling involves the controller 16 setting the inverter 14 to a state (state 3 in Figure 18) in which the high-side switch SI of the first leg 20 and the low-side switch S6 of the third leg 24 are in the ON switch state, the low-side switches S2,S4 of the first 20 and second 22 legs and the high-side switch S5 of the third leg 24 are in the OFF switch state, and the high-side switch S3 of the second leg 22 is in the D2 switch state. Freewheeling occurs until the current reaches zero. When this occurs (at time T2 in Figure 17), the controller 16 sets the inverter 14 to a state (state 4 in Figure 18) in which the high-side switch SI of the first leg 20 and the low-side switch S6 of the third leg 24 are in the ON switch state, the high-side S3 and low-side S4 switches of the second leg 22 are in the OFF switch state, and the low-side switch S2 of the first leg 20 and the high-side switch S5 of the third leg 24 are in the D2 switch state. This means the second leg 22 of the inverter 14 is electrically isolated, such that no current can flow through the second leg 22 of the inverter 14, and the method 100 of Figure 5 is being implemented. The inverter 14 is subsequently (at time T3 in Figure 17), set by the controller 16 to an inverter state (state 5 in Figure 18) in which the high-side switches SI,S3 of the first 20 and second 22 legs, and the low-side switches S4,S6 of the second 22 and third 24 legs, are in the OFF switch state, and the low-side switch S2 of the first leg 20 and the high-side switch S5 of the third leg 24 are in the D2 switch state. This enables current flowing in the first 20 and second 22 legs of the inverter 14 to decay to zero in readiness for the next drive cycle. It will be appreciated that the above is described in the context of phase B being the phase with lowest current magnitude at the point at which transition from SVPWM to the method 100 of Figure 5 is desired, and that different switches S1-S6 will be in different states depending on which phase current is lowest at point of the desired transition, and what polarity the input voltage of the inverter 16 is. The general principle of the different inverter states above apply irrespective of which phase has the lowest current magnitude at the point at which transition from SVPWM to the method 100 of Figure 5 is desired. By appropriately controlling the transition from SVPWM to the method 100 of Figure 5, and vice versa, it has been found that torque ripple may be reduced, and that distortion in the AC mains voltage may be reduced, relative to methods where the transition is uncontrolled. In general, the methods disclosed herein enable production of unidirectional positive average torque when the magnitude of the input voltage of the inverter 14 is lower than the magnitude of the peak of a line-to-line back EMF induced in the electric motor 12, whilst also providing for reduced torque ripple, and reduced distortion in the AC mains voltage. Whilst described above in the context of an inverter having bidirectional switches, it will be appreciated by a person skilled in the art that the methods disclosed herein could be implemented by alternative inverters. For example, an inverter utilising MOSFETS can be used, alongside appropriate further switches that enable electrical isolation of the first, second, and third legs of such an inverter. Furthermore, whilst described above in the context of a three-phase motor and a three-5 phase inverter, it will be appreciated that the teachings above can be generalised to motors comprising at least three phase windings, and inverters comprising at least three legs. In some such examples, the motor may have a positive integer number of phase windings, n, greater than three, the inverter may have n legs, and n-2 legs of the inverter may be electrically isolated whilst current flows through the remaining two legs of the inverter. In 10 such examples, the motor can be controlled as a single phase motor, in spite of having multiple phase windings.

Claims

1. A method of controlling an electric motor, the electric motor comprising first, second, and third phase windings driven by an inverter comprising first, second, and third legs, wherein the method comprises:electrically isolating the third leg of the inverter, such that current cannot flow through the third leg of the inverter;applying, whilst the third leg of the inverter is electrically isolated, a voltage to the inverter such that current flows through the first and second legs of the inverter;controlling a state of the inverter to allow current flowing through the first and second legs of the inverter to decay toward zero; andwhen the current flowing through the first and second legs of the inverter reaches zero, electrically isolating the first and second legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter.

2. A method as claimed in Claim 1, wherein the method is performed when a magnitude of an input voltage of the inverter is lower than a magnitude of a peak of a line-to-line back EMF induced in the electric motor.

3. A method as claimed in Claim 1 or Claim 2, wherein each of the first, second, and third, legs of the inverter comprises a respective high-side switch and a respective low-side switch, the inverter comprises a plurality of inverter states, each inverter state corresponding to a different configuration of the high-side and low-side switches of the first, second, and third, legs of the inverter, and electrically isolating the third leg of the inverter comprises controlling the inverter to be in an inverter state in which current cannot pass through the high-side switch and the low-side switch of the third leg of the inverter.

4. A method as claimed in Claim 3, wherein electrically isolating the third leg of the inverter comprises controlling the inverter to be in an inverter state in which the high-side switch and the low-side switch of the third leg of the inverter are turned off.

5. A method as claimed in Claim 3 or Claim 4, wherein electrically isolating the first and second legs of the inverter comprises controlling the inverter to be in an inverter state in which current cannot pass through the high-side switches and the low-side switches of the first and second legs of the inverter.

6. A method as claimed in any one of Claims 3 to 5, wherein the high-side and low-side switches of the first, second, and third, legs of the inverter comprise bi-directional switches.

7. A method as claimed in any one of the preceding claims, wherein the method comprises applying the voltage to the inverter in response to determination of a back EMF zero crossing of a line-to-line back EMF between the phase windings that are to be driven by the voltage applied to the inverter.

8. A method as claimed in Claim 7, wherein the method comprises offsetting a start time of applying the voltage to the inverter relative to the determined back EMF zero crossing.

9. A method as claimed in Claim 8, wherein the method comprises advancing the start time of applying the voltage to the inverter relative to the determined back EMF zero crossing.

10. A method as claimed in Claim 8 or Claim 9, wherein the method comprises: over a time period in which a magnitude of an input voltage to the inverter varies, performing a sequence of drive cycles in which:one of the first, second, and third legs of the inverter is electrically isolated whilst the remaining legs of the inverter are not electrically isolated;a voltage is applied to the inverter such that current flows through the remaining legs of the inverter which are not electrically isolated;current flowing through the remaining legs of the inverter which are not electrically isolated is allowed to decay towards zero; andwhen the current flowing through the remaining legs of the inverter which are not electrically isolated reaches zero, electrically isolating the remaining legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter; andwherein the offset of the start time of applying the voltage to the inverter relative to the determined back EMF zero crossing is fixed for each drive cycle.

11. A method as claimed in Claim 8 or Claim 9, wherein the method comprises:over a time period in which a magnitude of an input voltage to the inverter varies, performing a sequence of drive cycles in which:one of the first, second, and third legs of the inverter is electrically isolated whilst the remaining legs of the inverter are not electrically isolated;a voltage is applied to the inverter such that current flows through the remaining legs of the inverter which are not electrically isolated;current flowing through the remaining legs of the inverter which are not electrically isolated is allowed to decay towards zero; andwhen the current flowing through the remaining legs of the inverter which are not electrically isolated reaches zero, electrically isolating the remaining legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter; andwherein the offset of the start time of applying the voltage to the inverter relative to the determined back EMF zero crossing varies across the time period, and is based on the magnitude of the input voltage to the inverter at the time at which the voltage is to be applied.

12. A method as claimed in any one of the preceding claims, wherein controlling a state of the inverter to allow current flowing through legs of the inverter to decay toward zero comprises at least one of:controlling the inverter to be an inverter state in which current can flow through the legs of the inverter in one direction only; andfreewheeling the inverter.

13. A method as claimed in Claim 12, wherein the method comprises:over a time period in which a magnitude of an input voltage to the inverter varies, performing a sequence of drive cycles in which:one of the first, second, and third legs of the inverter is electrically isolated whilst the remaining legs of the inverter are not electrically isolated;a voltage is applied to the inverter, for a conduction period, such that current flows through the remaining legs of the inverter which are not electrically isolated;current flowing through the remaining legs of the inverter which are not electrically isolated is allowed to decay towards zero; andwhen the current flowing through the remaining legs of the inverter which are not electrically isolated reaches zero, electrically isolating the remaining legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter; andat least one of:the conduction period is fixed for each drive cycle within the time period;a length of time between controlling the inverter to be an inverter state in which current can decay toward zero, and a start of a conduction period of a next subsequent drive cycle within the time period, is fixed; anda length of time between freewheeling the inverter and a start of a conduction period of a next subsequent drive cycle within the time period, is fixed.

14. A method as claimed in Claim 13, wherein the method comprises:over a time period in which a magnitude of an input voltage to the inverter varies, performing a sequence of drive cycles in which:one of the first, second, and third legs of the inverter is electrically isolated whilst the remaining legs of the inverter are not electrically isolated;a voltage is applied to the inverter, for a conduction period, such that current flows through the remaining legs of the inverter which are not electrically isolated;current flowing through the remaining legs of the inverter which are not electrically isolated is allowed to decay towards zero; andwhen the current flowing through the remaining legs of the inverter which are not electrically isolated reaches zero, electrically isolating the remaining legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter; andat least one ofthe conduction period varies within the time period;a length of time between controlling the inverter to be an inverter state in which current can decay toward zero, and a start of a conduction period of a next subsequent drive cycle within the time period, varies within the time period; anda length of time between freewheeling the inverter and a start of a conduction period of a next subsequent drive cycle within the time period, varies within the time period.

15. A method as claimed in any one of the preceding claims, wherein the method comprises a first sub-control method and a second sub-control method different to the first sub-control method, wherein:the first sub-control method comprises:performing a sequence of drive cycles in which:one of the first, second, and third legs of the inverter is electrically isolated whilst the remaining legs of the inverter are not electrically isolated;a voltage is applied to the inverter, for a conduction period, such that current flows through the remaining legs of the inverter which are not electrically isolated;current flowing through the remaining legs of the inverter which are not electrically isolated is allowed to decay towards zero; andwhen the current flowing through the remaining legs of the inverter which are not electrically isolated reaches zero, electrically isolating the remaining legs of the inverter, such that current cannot flow through the first, second, and third legs of the inverter;the first sub-control method is performed during a first portion of an electrical cycle of an input voltage of the inverter; andthe second sub-control method is performed during a second portion of the electrical cycle of the input voltage of the inverter different to the first portion of the electrical cycle of the input voltage of the inverter.

16. A method as claimed in Claim 15, wherein:the first sub-control method is performed when a magnitude of the input voltage of the inverter is lower than a magnitude of a peak of a line-to-line back EMF induced in the electric motor; andthe second sub-control method is performed when a magnitude of the input voltage of the inverter is higher than the magnitude of the peak of a line-to-line back EMF induced in the electric motor.

17. A method as claimed in Claim 15 or Claim 16, wherein the method comprises transitioning from the first sub-control method to the second sub-control method in response to peaks in current flowing through the remaining legs of the inverter which are not electrically isolated.

18. A method as claimed in any of Claims 15 to 17, wherein the method comprises transitioning from the second sub-control method to the first sub-control method by:determining a magnitude of current flowing through the respective first, second, and third legs of the inverter;for the leg of the inverter having the lowest determined magnitude of current:controlling a state of the inverter to allow current flowing through the leg of the inverter to decay to zero; andwhere the current flowing through the leg of the inverter reaches zero, electrically isolating the leg of the inverter; andperforming the first sub-control method.

19. A controller for an electric motor system, the controller configured to perform a method as claimed in any one of the preceding claims.

20. An electric motor system comprising an electric motor, an inverter, and a controller as claimed in Claim 19.

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