GB2540602A - A controller for an electric machine - Google Patents

Abstract

A controller 400 for placing an electric motor in a non-operative, or disabled, condition places an inverter 410 in: a first configuration if a rotor of the motor is rotating above, or equal to, a predetermined velocity (e.g. a predetermined angular velocity); and in a second configuration if the rotor is rotating below the predetermined velocity. The first configuration allows coil windings 60 of the motor to be short-circuited and re-directed back to the motor, while the second configuration allows voltage generated by the motor to be connected across a voltage source, e.g. battery, in an open circuit configuration. The predetermined velocity may be equal to, or less than, a velocity that results in the motor generating a back emf that is equal to, or less than, the voltage source. The inverter 410 is switched from the first configuration to the second configuration when the rotor velocity drops from above the predetermined velocity to below the predetermined velocity. The inverter 410 includes a plurality of switches, each switch having a reverse diode, wherein the plurality of switches and associated reverse diodes place the motor in an open circuit configuration by coupling a plurality of coil windings across the voltage source.

Description

A CONTROLLER FOR AN ELECTRIC MACHINE

The present invention relates to a controller, in particular a controller for an electric machine.

Electric motors work on the principle that a current carrying wire will experience a force when in the presence of a magnetic field. When the current carrying wire is placed perpendicular to the magnetic field the force on the current carrying wire is proportional to the flux density of the magnetic field. Typically, in an electric motor the force on a current carrying wire is formed as a rotational torque . A three phase electric motor typically includes three coil sets, where each coil set is arranged to generate a magnetic field associated with one of the three phases of an alternating voltage.

To increase the number of magnetic poles formed within an electric motor, each coil set will typically have a number of coil sub-sets that are distributed around the periphery of the electric motor, which are driven to produce a rotating magnetic field.

By way of illustration, Figure 1 shows a typical three phase electric motor 10 having three coil sets 14, 16, 18. Each coil set consists of four coil sub-sets that are connected in series, where for a given coil set the magnetic field generated by the respective coil sub-sets will have a common phase .

The three coil sets of a three phase electric motor are typically configured in either a delta or wye configuration. A control unit for a three phase electric motor having a DC power supply will typically include a three phase bridge inverter that generates a three phase voltage supply for driving the electric motor. Each of the respective voltage phases is applied to a respective coil set of the electric motor . A three phase bridge inverter includes a number of switching devices, for example power electronic switches such as Insulated Gate Bipolar Transistor (IGBT) switches, which are used to generate an alternating voltage from a DC voltage supply.

Examples of known types of electric motor include the induction motor, brushless permanent magnet motor, switched reluctance motor and synchronous slip ring motor, which have a rotor and a stator, as is well known to a person skilled in the art.

The rotor for a permanent magnet motor typically includes a plurality of permanent magnets, where the plurality of permanent magnets is mounted on or in a rotor back-iron such that the magnetic field alternates in polarity around the circumference of the rotor. As the rotor rotates relative to the stator the plurality of permanent magnets are arranged to sweep across the ends of coil windings mounted on the stator. Appropriate switching of currents in the coil windings allows synchronized attraction and repulsion of the poles of the permanent magnets to produce rotation or the rotor .

As the force on the current carrying wire, and consequently the torque for an electric motor, is proportional to the flux density of a magnetic field it is desirable for a synchronous permanent magnet traction motor, where torque is an important criterion for the motor, to use magnets with as high a flux density as possible.

However, as the rotor rotates relative to the coil windings a back electromotive force, otherwise known as a back EMF, is generated that opposes the original applied voltage and consequently acts against the current flow that causes the rotor to rotate, thereby limiting the maximum rotation velocity of the rotor.

For a given battery voltage, to increase the maximum rotation velocity of the rotor flux weakening is typically employed, thereby allowing the back EMF to increase above battery voltage.

However, if an inverter used for providing current to an electric motor is placed in a first non-switching mode, where the inverter is configured to place the electric motor in an open circuit configuration, and the rotor of the electric motor is rotating at high speed, if the back EMF is higher than the battery voltage, current can flow from the electric motor into the battery, causing a braking torque, as illustrated in Figure 7, which may cause vehicle stability issues.

Alternatively, if the inverter used for providing current to an electric motor is placed in a second non-switching mode, where the inverter is configured to placed the electric motor in a short circuit configuration, current induced in the electric motors coil windings as a result of the back EMF can create a drag torque, as illustrated in Figure 8, which may also affect vehicle stability.

It is desirable to improve this situation.

In accordance with an aspect of the present invention there is provided a controller and method according to the accompanying claims.

The present invention as claimed has the advantage of placing an inverter in a configuration that minimises drag torque for a disabled or non-operational electric motor or an electric motor not creating a drive torque, where the inverter configuration is based on the rotor velocity of the electric motor. Additionally, it is possible to place an electric motor in a non-drive mode of operation without drawing any power from the electric motor's power source while also minimising drag torque.

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

Figure 1 illustrates a prior art electric motor;

Figure 2 illustrates an exploded view of a rotor according to an embodiment of the present invention;

Figure 3 illustrates a rotor according to an embodiment of the present invention;

Figure 4 illustrates a control device according to an embodiment of the present invention;

Figure 5 illustrates an exploded view of a control device according to an embodiment of the present invention;

Figures 6 illustrates a prior art inverter;

Figure 7 illustrates a first drag torque graph according to an embodiment of the present invention;

Figure 8 illustrates a second drag torque graph according to an embodiment of the present invention;

Figure 9 illustrates a third drag torque graph according to an embodiment of the present invention.

The embodiment of the invention described is for a controller for controlling the configuration and operation of an inverter, where the inverter is arranged to control current within coil windings of an electric motor. For the purposes of the present embodiment the electric motor is for use in a wheel of a vehicle, however the electric motor may be located anywhere within the vehicle. The motor is of the type having a set of coils being part of the stator for attachment to a vehicle, radially surrounded by a rotor carrying a set of magnets for attachment to a wheel. For the avoidance of doubt, the various aspects of the invention are equally applicable to an electric generator having the same arrangement. As such, the definition of electric motor is intended to include electric generator. In addition, some of the aspects of the invention are applicable to an arrangement having the rotor centrally mounted within radially surrounding coils. As would be appreciated by a person skilled in the art, the present invention is applicable for use with other types of electric motors.

For the purposes of the present embodiment, as illustrated in Figure 2 and Figure 3, the in-wheel electric motor includes a stator 252 comprising a heat sink 253, multiple coils 254, two control devices 400 mounted on the heat sink 253 on a rear portion of the stator for driving the coils, and an annular capacitor, otherwise known as a DC link capacitor, mounted on the stator within the inner radius of the control devices 400. The coils 254 are formed on stator tooth laminations to form coil windings. A stator cover 256 is mounted on the rear portion of the stator 252, enclosing the control devices 400 to form the stator 252, which may then be fixed to a vehicle and does not rotate relative to the vehicle during use.

Each control device 400 includes two inverters 410 and control logic 420, which in the present embodiment includes a processor, for controlling the operation of the inverters 410, which is schematically represented in Figure 4.

Although for the purposes of the present embodiment the inwheel electric motor includes two control devices, where each control device includes control logic, in other words a controller, for controlling the operation of an inverter, any configuration of control logic and inverter combination may be used, including placing the control logic and/or inverters remote to the electric motor.

The annular capacitor is coupled across the inverters 410 and the electric motor's DC power source for reducing voltage ripple on the electric motor's power supply line, otherwise known as the DC busbar, and for reducing voltage overshoots during operation of the electric motor. For reduced inductance the capacitor is mounted adjacent to the control devices 400. A rotor 240 comprises a front portion 220 and a cylindrical portion 221 forming a cover, which substantially surrounds the stator 252. The rotor includes a plurality of permanent magnets 242 arranged around the inside of the cylindrical portion 221. For the purposes of the present embodiment 32 magnet pairs are mounted on the inside of the cylindrical portion 221. However, any number of magnet pairs may be used.

The magnets are in close proximity to the coil windings on the stator 252 so that magnetic fields generated by the coils interact with the magnets 242 arranged around the inside of the cylindrical portion 221 of the rotor 240 to cause the rotor 240 to rotate. As the permanent magnets 242 are utilized to generate a drive torque for driving the electric motor, the permanent magnets are typically called drive magnets.

The rotor 240 is attached to the stator 252 by a bearing block 223. The bearing block 223 can be a standard bearing block as would be used in a vehicle to which this motor assembly is to be fitted. The bearing block comprises two parts, a first part fixed to the stator and a second part fixed to the rotor. The bearing block is fixed to a central portion 253 of the wall of the stator 252 and also to a central portion 225 of the housing wall 220 of the rotor 240. The rotor 240 is thus rotationally fixed to the vehicle with which it is to be used via the bearing block 223 at the central portion 225 of the rotor 240. This has an advantage in that a wheel rim and tyre can then be fixed to the rotor 240 at the central portion 225 using the normal wheel bolts to fix the wheel rim to the central portion of the rotor and consequently firmly onto the rotatable side of the bearing block 223. The wheel bolts may be fitted through the central portion 225 of the rotor through into the bearing block itself. With both the rotor 240 and the wheel being mounted to the bearing block 223 there is a one to one correspondence between the angle of rotation of the rotor and the wheel.

Figure 3 shows an exploded view of the same motor assembly illustrated in Figure 2 from the opposite side. The rotor 240 comprises the outer rotor wall 220 and circumferential wall 221 within which magnets 242 are circumferentially arranged. As previously described, the stator 252 is connected to the rotor 240 via the bearing block at the central portions of the rotor and stator walls. A V shaped seal is provided between the circumferential wall 221 of the rotor and the outer edge of the stator.

The rotor also includes a set of magnets 227 for position sensing, otherwise known as commutation magnets, which in conjunction with sensors mounted on the stator allows for a rotor flux angle to be estimated. The rotor flux angle defines the positional relationship of the drive magnets to the coil windings. Alternatively, in place of a set of separate magnets the rotor may include a ring of magnetic material that has multiple poles that act as a set of separate magnets.

To allow the commutation magnets to be used to calculate a rotor flux angle, preferably each drive magnet has an associated commutation magnet, where the rotor flux angle is derived from the flux angle associated with the set of commutation magnets by calibrating the measured commutation magnet flux angle. To simplify the correlation between the commutation magnet flux angle and the rotor flux angle, preferably the set of commutation magnets has the same number of magnets or magnet pole pairs as the set of drive magnet pairs, where the commutation magnets and associated drive magnets are approximately radially aligned with each other. Accordingly, for the purposes of the present embodiment the set of commutation magnets has 32 magnet pairs, where each magnet pair is approximately radially aligned with a respective drive magnet pair. A sensor, which in this embodiment is a Hall sensor, is mounted on the stator. The sensor is positioned so that as the rotor rotates each of the commutation magnets that form the commutation magnet ring respectively rotates past the sensor .

As the rotor rotates relative to the stator the commutation magnets correspondingly rotate past the sensor with the Hall sensor outputting an AC voltage signal, where the sensor outputs a complete voltage cycle of 360 electrical degrees for each magnet pair that passes the sensor.

For improved position detection, preferably the sensor includes an associated second sensor placed 90 electrical degrees displaced from the first sensor.

In the present embodiment the electric motor includes four coil sets with each coil set having three coil sub-sets that are coupled in a wye configuration to form a three phase sub-motor, resulting in the motor having four three phase sub-motors. The operation of the respective sub-motors is controlled via one of the two control devices 400, as described below. However, although the present embodiment describes an electric motor having four coil sets (i.e. four sub motors) the motor may equally have one or more coil sets with associated control devices. In a preferred embodiment the motor includes eight coil sets 60 with each coil set having three coil sub-sets that are coupled in a wye configuration to form a three phase sub-motor, resulting in the motor having eight three phase sub-motors. Similarly, each coil set may have any number of coil sub-sets, thereby allowing each sub-motor to have two or more phases.

Figure 4 illustrates the connections between the respective coil sets 60 and the control devices 400, where a respective coil set 60 is connected to a respective three phase inverter 410 included on a control device 400 for controlling current flow within the respective coil sets. As is well known to a person skilled in the art, a three phase inverter contains six switches, where a three phase alternating voltage may be generated by the controlled operation of the six switches. However, the number of switches will depend upon the number of voltage phases to be applied to the respective sub motors, where the sub motors can be constructed to have any number of phases.

Preferably, the control devices 400 are of a modular construction. Figure 5 illustrates an exploded view of a preferred embodiment, where each control device 400, otherwise known as a power module, includes a power printed circuit board 500 in which are mounted two power substrate assemblies 510, a control printed circuit board 520, four power source busbars 530 for connecting to a DC battery, and six phase winding busbars 540 for connecting to respective coil windings. Each of the control device components are mounted within a control device housing 550 with the four power source busbars 530 being mounted on an opposite side of the control device housing 550 to the phase winding busbars 540.

Each power substrate 510 is arranged to be mounted in a respective aperture formed in the power printed circuit board 500.

The power printed circuit board 500 includes a variety of components that include drivers for the inverter switches formed on the power substrate assemblies 510, where the drivers are typically used to convert control signals into a suitable form to turn the inverter switches on and off.

The control printed circuit board 520 includes a processor for controlling the operation of the inverter switches. Additionally, each control printed circuit board 520 includes an interface arrangement to allow communication between the respective control devices 400 via a communication bus with one control device 400 being arranged to communicate with a vehicle controller mounted external to the electric motor. The processor 420 on each control device 400 is arranged to handle communication over the interface arrangement.

As stated above, the processors 420 on the respective control devices 400 are arranged to control the operation of the inverter switches mounted on the respective power substrates 520 within the control housing 550, thereby allowing each of the electric motor coil sets 60 to be supplied with a three phase voltage supply resulting in the respective coil sub-sets generating a rotating magnetic field. As stated above, although the present embodiment describes each coil set 60 as having three coil sub-sets the present invention is not limited by this and it would be appreciated that each coil set 60 may have one or more coil sub-sets .

Under the control of the respective processors 420, each three phase bridge inverter 410 is arranged to provide pulse width modulation PWM voltage control across the respective coil sub-sets, thereby generating a current flow in the respective coil sub-sets for providing a required torque by the respective sub-motors. PWM control works by using the motor inductance to average out an applied pulse voltage to drive the required current into the motor coils. Using PWM control an applied voltage is switched across the motor windings. During the period when voltage is switched across the motor coils, the current rises in the motor coils at a rate dictated by their inductance and the applied voltage. The PWM voltage control is switched off before the current has increased beyond a required value, thereby allowing precise control of the current to be achieved.

For a given coil set 60 the three phase bridge inverter 410 switches are arranged to apply a single voltage phase across each of the coil sub-sets 61, 62, 63.

Using PWM switching, the plurality of switches are arranged to apply an alternating voltage across the respective coil sub-sets. The voltage envelope and phase angle of the electrical signals is determined by the modulating voltage pulses .

The inverter switches can include semiconductor devices such as MOSFETs or IGBTs. In the present example, the switches comprise IGBTs. However, any suitable known switching circuit can be employed for controlling the current. For a three phase inverter having six switches configured to drive a three phase electric motor, the six switches are configured as three parallel sets of two switches, where each pair of switches is placed in series and form a leg 600 of the three phase bridge circuit, with a fly-back diode 610, otherwise known as a reverse diode, coupled in antiparallel across each switch 620, as illustrated in Figure 6. A single phase inverter will have two pairs of switches 620 arranged in series to form two legs 600 of an inverter.

As stated above, each of the inverter legs 600 are electrically coupled between a pair of power source busbars.

As stated above, PWM switching is used to apply an alternating voltage to the electric motors coil windings, where the rotor speed is dependent upon the amplitude of the voltage applied across the coil windings, where the torque applied to the rotor results from drive current within the coil windings.

If a condition occurs that results in the electric motor becoming non-operational or needs to be placed in a mode where no substantive drive torque is generated by the electric motor (i.e. the electric motor is placed in a disabled mode), the controllers within the respective control devices, or alternatively a single controller, is arranged to place the inverter switches in one of two configurations based on the velocity that the rotor is rotating at. As described below, the inverter configuration is selected to avoid any substantial current flowing back into the battery and to limit back-emf.

In a first configuration the inverter switches 620 are placed in an open circuit configuration resulting in any current generated by the electric motor flowing through the fly-back diodes 610 that are coupled in anti-parallel across each of the inverter switches 620 and onto the power source busbars .

In the first configuration, if the back EMF generated by the electric motor is higher than the power supply voltage for the electric motor, current will flow, via the fly-back diodes 610, onto the power source busbar and into the relevant electric motor power source, for example a battery, resulting in a drag torque being generated by the electric motor .

By way of illustration, Figure 7 illustrates an example of the variation in drag torque, line A, and electric motor generated current, line B, verse angular rotational speed of the rotor when the electric motor inverters are placed in the first configuration for a battery voltage of approximately 320V.

As can be seen in Figure 7, for rotor velocities above 900 rpm, the back EMF generated by the electric motor exceeds the battery voltage resulting in current flow to the battery, thereby causing a drag torque.

For illustrative purposes, Figure 8 provides a graph showing back EMF voltage generated by an electric motor relative to rotor angular velocity for the electric motor. As illustrated, for a rotor velocity of approximately 900rpm the back EMF generated by the electric motor corresponds to approximately 320V.

In a second configuration the inverter switches 620 are placed in a configuration that results in the short circuiting of the electric motor coil windings. For example, by closing either the low side or high side inverter switches 620, thereby preventing voltage resulting from the back emf generated by the electric motors being placed on the power source busbars.

By way of illustration, Figure 9 illustrates an example of the variation in drag torque, line C, and electric motor generated current, line D, verse angular rotational speed of the rotor when the electric motor inverters are placed in the second configuration for a battery voltage of approximately 320V.

As can be seen in Figure 9, at low rotor velocities current flowing through the electric motors coil windings create a drag torque, which reduces as the rotor velocity increases as a result of increased inductance.

In a preferred embodiment, when placing the electric motor in a disabled condition the controller is arranged to place the inverter, or inverters, in the first configuration (i.e. in an open circuit configuration) if the velocity of the rotor results in the electric motor generating a back emf that is less than voltage of the power source supplying the electric motor.

However, if the velocity of the rotor results in the electric motor generating a back emf that is greater than the voltage of the power source supplying the electric motor, the controller is arranged to place the inverter, or inverters, in the second configuration (i.e. in a short circuited configuration), thereby minimising the drag torque generated by the electric motor when disabled.

For example, as illustrated in Figure 10, if the rotor velocity is greater than value E (e.g. approximately 950rpm when the power source across the inverter switches is 320V), upon a disable condition being entered, the controller is arranged to place the respective inverter switches 620 in the second configuration.

Correspondingly, if the rotor velocity is less than value E (e.g. approximately 950rpm when the power source across the inverter switches is 320V), upon a disable condition being entered, the controller is arranged to place the respective inverter switches 620 in the first configuration.

Additionally, the controller can be configured to use rotor speed information, for example as generated by the commutation sensors, to switch between the first configuration and the second configuration based on the velocity of the rotor. For example, at a rotor velocity that results in the back EMF generated by the electric motor being above the voltage of the electric motors power supply, upon a disable condition for the electric motor being entered the controller is arranged to place the electric motor inverters in the second configuration. However, upon the rotor velocity falling below a first predetermined velocity value, where the back EMF generated by the electric motor is below the voltage of the electric motor power supply but above a second predetermined velocity value, the controller is arranged to automatically switch the inverter configuration from the second configuration to the first configuration. The second predetermined velocity value is selected to minimise drag torque resulting from resistive loads associated with the electric motors coil windings. Accordingly, as described above, the predetermined velocity will be supply voltage dependent.

Although the present embodiment describes the use of commutation sensors for determining rotor velocity, any suitable means may be used to determine whether the rotor is rotating above or below a predetermined velocity. For example, by measuring drag torque for a given inverter configuration or the electric motor phase current.

For example, as illustrated in Figure 10, if the rotor is rotating at a value greater than E, upon the controller placing the motor in a disabled condition the respective inverter switches are placed in the second configuration. Upon the rotor velocity reducing below E (i.e. a first predetermined value) but before drag torque resulting from resistive loads starts to appreciably increase (e.g. value F, in other words a second predetermined value), the respective inverter switches are placed in the first configuration. Accordingly, if the controller is arranged to switch between the second configuration and the first configuration at rotor velocity E, the drag torque experienced by the electric motor as the rotor velocity reduces from 1600rpm to zero is illustrated by line G. Line H illustrates the drag torque that the electric motor would experience between 1600rpm to 950rpm if the respective inverter switches were placed in the first configuration at these velocities. Line I illustrates the drag torque that the electric motor would experience between 950rpm to Orpm if the respective inverter switches are placed in the second configuration at these velocities.

Any suitable mechanism may be used for selecting the first predetermined value and/or the second predetermined value. For example, the values may be set manually by a user of the vehicle or during a calibration phase of the vehicle where the first predetermined value and second predetermined value are selected to avoid any substantial current flowing to the electric motors power source or a drag torque occurring due to a drag torque occurring due to resistive loads associated with the electric motor coil windings.

Consequently, by appropriate selection of the first predetermined value this ensures that when the electric motor is in a non-operation/disabled condition or mode where no drive torque is being generated, the electric motor does not generating a back EMF greater than the power source/battery used for powering the electric motor.

Claims (10)

1. A controller for an electric motor, the controller comprising means arranged to place an inverter in a first configuration if a rotor of the electric motor is rotating above or egual to a predetermined velocity, and means arranged to place the inverter in a second configuration if the rotor of the electric motor is rotating below the predetermined velocity, wherein the first configuration of the inverter allows coil windings of the electric motor to be placed in a short circuit configuration and the second configuration of the inverter allows voltage generated by the electric motor to be connected across a voltage source.

2. A controller according to claim 1, wherein the voltage source is a battery.

3. A controller according to claim 1 or 2, wherein the predetermined velocity is a predetermined angular velocity.

4. A controller according to any one of the preceding claims, further comprising means for determining that an electric motor should be placed in a first operating mode, wherein in response to a determination that the electric motor should be placed in the first operating mode the inverter is placed in the first condition or the second condition based on the velocity of the rotor.

5. A controller according to claim 4, wherein when the electric motor is in the first operating mode the electric motor is arranged to generate no substantive drive torque.

5. A controller according to any one of the preceding claims, wherein the second configuration of the inverter corresponds to an open circuit configuration.

6. A controller according to any one of the preceding claims, wherein the predetermined velocity corresponds to a velocity of the rotor that is egual to or less than a velocity that would result in the electric motor generating a back emf equal or less than the voltage source.

7. A controller according to any one of the preceding claims, further comprising means arranged to switch the inverter from the first configuration to the second configuration upon the rotor velocity dropping from above the predetermined velocity to below the predetermined velocity.

8. A controller according to any one of the preceding claims, wherein the inverter includes a plurality of switches that are arranged to place the electric motor in a short circuit configuration by coupling together a plurality of electric motor coil windings.

9. A controller according to any one of the preceding claims, wherein the inverter includes a plurality of switches with each switch having a reverse diode, wherein the plurality of switches and associated reverse diodes are arranged to place the electric motor in an open circuit configuration by coupling a plurality of electric motor coil windings across the voltage source.

10. A method for an electric motor, the method comprising placing an inverter in a first configuration if a rotor of the electric motor is rotating above or equal to a predetermined velocity, and placing the inverter in a second configuration if the rotor of the electric motor is rotating below the predetermined velocity, wherein the first configuration of the inverter allows coil windings of the electric motor to placed in a short circuit configuration and the second configuration of the inverter allows voltage generated by the electric motor to be connected across a voltage source.