WO2012131506A2

WO2012131506A2 - An electric motor arrangement and method of controlling thereof

Info

Publication number: WO2012131506A2
Application number: PCT/IB2012/051042
Authority: WO
Inventors: Tim Martin
Original assignee: Protean Electric Limited
Priority date: 2011-03-25
Filing date: 2012-03-06
Publication date: 2012-10-04
Also published as: GB201105033D0; CN102780442B; GB2477229A; CN102780442A; WO2012131506A3; GB2477229B

Abstract

An electric motor arrangement comprising an electric motor and a controller, wherein the electric motor includes a stator having a plurality of coil windings, and a rotor having a plurality of permanent magnets, and the controller is arranged to control current flow through the plurality of coil windings that result from rotation of the permanent magnets relative to the coil windings, wherein the current is defined by a stator current vector represented in a two co-ordinate time invariant system having a rotating quadrature and direct axis by a quadrature current component and a direct current component, wherein the controller is arranged to vary the quadrature current component and the direct current component so that the input electrical power of the electric motor is greater than or equal to zero.

Description

AN ELECTRIC MOTOR ARRANGEMENT AND METHOD OF CONTROLLING

THEREOF

The present invention relates to an electric motor arrangement, in particular an electric motor arrangement that can be configured to reduce current flow to an energy storage device during regenerative braking of an electric motor.

With increased interest being placed in environmentally friendly vehicles there has, perhaps unsurprisingly, been a corresponding increase in interest in the use of electric vehicles.

Electric vehicles typically use an electric motor to provide both drive for the vehicle and regenerative braking for stopping the vehicle. To effect regenerative braking rotary motion of a drive wheel connected to an electric motor is converted into electric energy. This involves consumption of kinetic energy that provides a braking force to the drive wheels by applying a braking torque in an opposite direction to the rotation of the drive wheels.

The electric energy generated during regenerative braking results in current flow within coil windings of the electric motor, where the current is typically directed to an energy storage device. The recovered energy can then be used, when required, to drive the electric motor, thereby increasing the operational efficiency of the electric motor.

However, if a condition should occur that prevents regenerative current being stored within the energy storage device this can result in a reduction in regenerative braking torque.

One solution to this problem has been to use dump resistors, which are used within a vehicle to absorb regenerative currents that cannot be stored within an energy storage device, thereby ensuring that regenerative braking torque is not compromised should a condition occur that prevents regenerative current being stored within the energy storage device. However, dump resistors can be bulky and expensive.

It is desirable to improve this situation. In accordance with an aspect of the present invention there is provided an electric motor arrangement and method according to the accompanying claims. This provides the advantage of allowing a regenerative braking torque to be generated without the need for the associated regenerative current to be stored in an energy storage device or the need for a dump resistor.

By controlling the phase voltage applied to coil windings of an electric motor so that regenerative current is arranged to flow through the motor coils and power electronics of an electric motor, rather than back to the power source, the present invention allows effectively for a controlled short-circuit of the stator coils, thereby dissipating

regenerative energy as heat in the stator windings. If the power supply is capable of sourcing current to the motor, then there will be a speed below which the motor can enter a powered braking mode that will take current from the power source in order to provide a braking torque.

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

Figure 1 illustrates an exploded view of an electric motor as used in an embodiment of the present invention; Figure 2 illustrates an exploded view of the electric motor shown in figure 2 from an alternative angle;

Figure 3 illustrates an example arrangement of coil sets for an electric motor according to an embodiment of the present invention;

Figure 4 illustrates a three phase stator current complex space vector;

Figure 5 illustrates a three phase stator current reference frame with a d, q rotating reference frame; Figure 6 illustrates a set of plots of power against Iq for a number of different positive values of coe;

Figure 7 illustrates a comparison of braking current and rotational speed of an electric motor rotor according to an embodiment of the present invention;

Figure 8 graphically illustrates a comparison of current in a quadrature axis and current in a direct axis for an electric motor according to an embodiment of the present invention;

The embodiment of the invention described is a permanent magnet synchronous electric motor for use in a wheel of a vehicle, that is to say an in-wheel electric motor. However, as would be appreciated by a person skilled in the art, the invention is applicable to other types of permanent magnet synchronous electric motors. 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. In addition, some of the aspects of the invention are applicable to an arrangement having the rotor centrally mounted within radially surrounding coils.

The physical arrangement of the embodying assembly is best understood with respect to Figures 1 and 2. The assembly can be described as a motor with built in electronics and bearing, or could also be described as a hub motor or hub drive as it is built to accommodate a separate wheel. However, the described permanent magnet synchronous electric motor configuration is merely for illustrative purposes, as such other permanent magnet synchronous electric motor configurations may be utilized.

Referring first to Figure 1, the assembly comprises a stator 252 comprising a rear portion

230 forming a first part of the housing of the assembly, and a heat sink and drive arrangement 231 comprising multiple coils and electronics to drive the coils as well as a heat sink. The coil drive arrangement 231 is fixed to the rear portion 230 to form the stator 252 which may then be fixed to a vehicle and does not rotate during use. The coils themselves are formed on tooth laminations which together with the drive arrangement

231 and rear portion 230 form the stator 252. Although not shown, also mounted to the stator are a plurality of capacitor circuit boards for providing capacitance between the electric motor and the voltage supply to reduce voltage line drop.

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 magnets 242 arranged around the inside of the cylindrical portion 221. The magnets are thus in close proximity to the coils on the assembly 231 so that magnetic fields generated by the coils in the assembly 231 generate a force on the magnets 242 arranged around the inside of the cylindrical portion 221 of the rotor 240 thereby causing the rotor 240 to rotate.

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 233 of the wall 230 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 a significant 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. The existing bearing block can then fitted inside the assembly and the whole arrangement fitted to the vehicle on the stator side and the normal rim and wheel fitted to the rotor so that the rim and wheel surrounds the whole motor assembly.

Figure 2 shows an exploded view of the same assembly as Figure 1 from the opposite side showing the stator 252 comprising the rear stator wall 230 and coil and electronics assembly 231. 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 223 at the central portions of the rotor and stator walls. Additionally shown in Figure 1 are control devices 80, otherwise known as motor drive circuits, which, as described below, includes an inverter. Additionally in Figures 1 and 2 a V shaped seal 350 is provided between the circumferential wall 221 of the rotor and the outer edge of the stator housing 230. Further, in Figure 2, the magnetic ring 227 comprising a commutation focusing ring and a plurality of magnets is provided for the purpose of indicating the position of the rotor with respect to the stator to a series of sensors arranged on the control devices 80 of the stator 252. Figure 3 schematically shows an example of an electric motor 40 in accordance with an embodiment of this invention. In this example, the motor is generally circular. However, it will be appreciated that embodiments of this invention can employ other topologies. For example a linear arrangement of coils for producing linear movement is envisaged.

The motor 40 in this example includes eight coil sets 60 with each coil set 60 having three coil sub-sets 61, 62, 63 that are coupled to a respective control device 64, where each control device 64 and respective coil sub-sets form a three phase logical or sub electric motor that can be controlled independently of the other sub motors. The control devices 64 drive their respective sub motor with a three phase voltage supply, thereby allowing the respective coil sub-sets to generate a rotating magnetic field. Although the present embodiment describes each coil set 60 as having three coil sub-sets 61, 62, 63, the present invention is not limited by this and it would be appreciated that each coil set 60 could have two or more coil sub-sets. Equally, although the present embodiment describes an electric motor having eight coil sets 60 (i.e. eight sub motors) the motor could have two or more coil sets with associated control devices (i.e. two or more sub motors).

The motor 40 can include a rotor (not shown in Figure 3) positioned in the centre of the circle defined by the positioning of the various coils of the motor, thereby to allow rotation of the rotor within the rotating magnetic field produced by the coils. Preferably, though, the rotor is arranged around the coils as previously disclosed in Figures 1 and 2. The rotor may typically comprise one or more permanent magnets arranged to rotate such that their poles sweep across the ends of the coils of the motor 40. Appropriate switching of currents in the coils of the coil sub-sets 61, 62, 63 allows synchronized attraction and repulsion of the poles of the permanent magnet of the rotor to produce the rotating action of the motor 40. It will be appreciated that Figure 3 is highly schematic and, in practice, the coil sub-sets will be arranged at the outer periphery of the stator with the rotor magnets surrounding the coils. As stated above, each control device includes a three phase bridge inverter which, as is well known to a person skilled in the art, contains six switches. The three phase bridge inverter is coupled to the three subset coils of a coil set 60 to form a three phase electric motor configuration. Accordingly, as stated above, the motor includes eight three phase sub-motors, where each three phase sub-motor includes a control device 64 coupled to the three sub-set coils of a coil set 60.

Each three phase bridge inverter is arranged to provide PWM voltage control across the respective coil sub-sets 61, 62, 63 to generate a current and provide a required drive or braking torque for the respective sub-motors, as described below. Each three phase bridge inverter can be controlled to provide PWM voltage control via any suitable form of controller. The controller can be located locally to the electric motor or centrally within the vehicle.

For a given coil set the three phase bridge switches of a control device 64 are arranged to apply a single voltage phase across each of the coil sub-sets 61, 62, 63.

The phase angle of the resulting current flow in each coil sub-set is separated by 120 degrees, as represented in Figure 4 by the three axis A, B, C. The sinusoidal voltage waveforms generated by the control devices 64 under the control of the controller are created using a Space Vector modulation technique known as Field Orientation Control, where the rotor flux and stator currents are represented by respective vectors. As illustrated in Figure 4, currents ia, ib, ic represent the instantaneous current in the respective stator coils in the A, B, and C axis of a three phase current reference frame, where the stator current vector is defined by ∑s ⁼ ½ + t * ^a' ⁱc , where a = e(i*2*n/3). Field Oriented Control is based on projections that transform a three phase time and speed dependent system into a two co-ordinate time invariant system, where a stator current component is aligned with a quadrature axis q and a magnetic flux component is aligned with a direct axis d.

Field Orientated Control algorithms utilize Clarke and Park transforms to transform the three phase voltage and current of a three phase motor into a two dimension co-ordinate system.

The Clarke transforms use the three phase current ia, ib, and ic to calculate currents in the two phase orthogonal stator axis ia and ίβ. A Park transformation is then used to transform the two fixed co-ordinate stator axis ia and ίβ to a two co-ordinate time invariant system id and iq, which defines a d, q rotating reference frame. Figure 5 illustrates the relationship of the stator current in the d,q rotating reference frame with respect to the two phase orthogonal stator axis ia and ίβ and the a, b and c stationary reference frame.

Under normal drive conditions the rotor phase angle 9r, which is defined by the rotor magnetic flux vector ΨΡν, and the stator electrical phase angle 9e are aligned with the d- axis, thereby maintaining synchronization between the rotor phase angle 9r and the stator electrical phase angle 9e. For the purposes of the present embodiment, the rotor phase angle 9r is measured using the rotor commutation magnets and position sensors mounted on the control devices 80, as is well known to a person skilled in the art.

Current in the quadrature (q) axis results in motor torque, current in the direct (d) axis results a magnetic field variation of the rotor magnets.

Typically, as is well known to a person skilled in the art, during regenerative braking the electric motor is sourcing current back into an energy storage device (e.g. a battery), hence the input power is negative. However, to ensure that regenerative current does not flow back to a power source associated with the electric motor, for example an energy storage device, the controller is arranged so that during regenerative braking it can vary the current in the quadrature axis and the direct axis so that the input electrical power of the electric motor is greater than or equal to zero. By controlling the d-axis current, that is to say the field current Id, the generated current in the q-axis can be contained within the motor coils and power electronics. This ensures that the regenerative current is contained within the electric motor coil windings and associated control devices.

The controller can be arranged to vary the current in the quadrature axis and the direct axis so that the input electrical power of the electric motor is greater than or equal to zero in response to a predetermined criteria, for example if the energy storage device voltage level exceeds a predetermined threshold, or in response to a user selection.

The determination of current values in the quadrature axis and direct axis for containing the regenerative current within the electric motor coil windings and associated control devices will now be described.

The equations that describe the components of applied voltage for a permanent magnet synchronous electric motor are:

where Vd and Vq are the applied phase voltages in the d-q reference frame, Id and Iq are the phase currents in the d-q reference frame, Ld and Lq are the phase inductances in the d-q reference frame, R is the phase DC resistance, is the back-emf constant in V/rads^" ^l, and is the electrical angular speed in rads^"1 whose sign denotes direction of rotation.

In a steady-state condition, Eqn. 1 and Eqn. 2 can be simplified to

Eqn. 3

Eqn. 4

The input power of the motor is given Eqn. 5 therefore, for input power to be greater than or equal to zero

Eqn. 6

For ease of calculation, the following assumptions have been made:

1) the quiescent current from the motor electronics, such as the control devices, is drawn from a power source separate from the DC link (i.e. the electric motor power source), and so is ignored;

2) the power losses in the control devices are also ignored.

These components become additional power terms to be considered in Eqn. 5, but for now will not be considered.

During normal operating conditions of a permanent magnet synchronous electric motor typically ½ ^{= 0} .

Combining Eqn. 4 and Eqn. 6 to obtain an expression for electrical power ¾ at the limit where ¾ ^{= δ} :

Eqn. 7 is a quadratic equation with roots at:

Figure 6 is a set of plots of Eqn. 7 with different positive values of .

Therefore, for input power to the electric motor to be greater than or equal to zero, the following holds true:

Since, for braking, must be of opposite sign to ^ω _* , Eqn. 8 becomes:

A comparison of braking current and rotational speed of the electric motor rotor based on Eqn. 9 is illustrated in Figure 7, where it can be seen that there is a boundary between powered and regenerative braking. At the boundary between powered and regenerative braking the electrical input power is zero - there is no current flowing into or out of the motor via the dc link. The 'powered braking regions', where the input power to the electric motor is greater than zero as illustrated in Figure 7, are regions where there will always be current drawn from the DC link (e.g. from an energy storage device) used for powering the electric motor in order to provide braking torque. That is to say, in a 'powered braking region' during braking the electric motor draws a current from an energy storage device and does not create a regenerative current.

The line of 'zero power braking' is when the input power to the motor becomes zero, since there is no current drawn from the DC link to provide the braking torque. As stated above, this line is defined by Eqn. 9, where:

The 'regen braking regions', as illustrated in Figure 7, are regions where the electric motor back-emf results in the generation of braking current and the input power of the electric motor becomes negative.

The following calculations illustrate the required d-axis current control required to prevent current flow to the DC link by ensuring that the electrical power of the electric motor during regenerative braking is greater than or equal to zero:

By substituting Eqn. 3 and Eqn. 6 this provides for the limiting condition, which defines quadratic equation:

Eqn. 9

The resulting quadratic equation has two solutions. The solution that provides the lowest required d-axis current will be:

Eqn.

This expression is for a salient motor where ,

However, for simplification purposes a non-salient motor will be considered, where:

Eqn. 11

Accordingly, for a non-salient electric motor, from Eqn. 9, Eqn. 10 becomes:

To avoid maximum coil current being exceeded, the following limiting condition must also be considered:

.V.-i.V Eqn. 13

Where IqMAx is the maximum allowed coil current. Substituting Eqn. 11 and Eqn. 13 into Eqn. 9:

Therefore, for a non-salient motor, the maximum available braking current can be described as:

where

. , , f 1 for i¾

Therefore, as torque direction is opposing motor direction of rotation for a motor braking condition where the sign of I_q does not equal the sign of ω_ε, in order to prevent regenerated current flowing back into the DC link, and to prevent the total coil current from exceeding its maximum value, the d-axis current Id is calculated as follows: here

then

Based on these equations, according to an embodiment of the present invention, for a non-salient electric motor having the parameter values listed below, Figure 8 illustrates the maximum braking current available for a motor with a maximum coil current of 75 Amps and corresponding d-axis current so that the electrical power of the electric motor during regenerative braking is greater than or equal to zero.

Parameter Value Description

R 0.240 Ohms Coil dc resistance

L 0.650 mH Coil inductance

¥m 1.232 V/rads^"1 Motor back-EMF constant

n 32 Number of magnetic pole-pairs on the rotor

IMAX 75 Amps Peak coil current rating

Figure 8 includes three plots defined by line OB, line ABC and line ODE. Line OB is defined by Eqn. 10. Line AB corresponds to Iq = Iq_max.

Line BC is defined by Eqn. 14

Line ABC is the curve of maximum available braking current Iq, such that the peak coil current does not exceed its maximum rating of 75A. Within the region OABO the input power is positive, meaning the electric motor draws current from the DC link in order to provide a braking torque. The line ODE is required Id current corresponding to the maximum Iq current as defined by Eqn. 15.

The line OBC is the curve of maximum available braking current such that the input power, and thus the input current from the DC link, is zero. Line ODE is the required d- axis current to provide this condition, that is to say the d-axis current required to prevent current being provided back to the DC link.

For any Iq current demand up to the limit defined by Eqn. 14, Id is calculated with Eqn. 15, thereby avoiding regenerative current being generated.

As stated above, the controller is configured to control Iq and Id in response to a predetermined criteria so that during regenerative braking the electrical power of the electric motor is greater than or equal to zero. By way of illustration three different controller topologies will be described.

In a first embodiment of a controller that is schematically illustrated in Figure 9, an open loop configuration is adopted, where the components of the output voltage in the d-q reference frame are calculated based on the electric motor model defined by Eqn. 3 and 4.

Depending on the processing power of the electric motor controller, Id and Iq demand can be calculated from Eqn. 14 and Eqn. 15. These values are then used in Eqn. 3 and Eqn. 4 to obtain Vd and Vq, or alternatively Vd and Vq can be interpolated from lookup-tables generated externally, the inputs of which will be torque demand and speed of rotation.

From Vd and Vq, the applied phase voltages Va, Vb and Vc can be derived from an inverse Park-Clarke transformation. In a second embodiment of a controller that is schematically illustrated in Figure 10, a closed loop configuration with an Id calculation is adopted. In this embodiment the required Iq and Id currents are calculated from Eqn. 14 and Eqn. 15, and are used as inputs for a proportional integral (PI) current control loop, as is well known to a person skilled in the art, in order to obtain the applied phase voltages.

Alternatively, the required values can be stored in lookup tables generated externally, the inputs of which are torque demand and speed of rotation.

In a third embodiment of a controller that is schematically illustrated in Figure 11 , a closed loop configuration with dynamic Id is adopted.

In this embodiment Eqn. 14 is used to prevent the coil current exceeding its maximum rating. The calculated Iq is used as the input for the proportional integral current control loop, as is well known to a person skilled in the art.

The required Id current is derived dynamically from the outputs of the P-I control loop using Eqn. 6, re-arranged to become:

Eqn. 16

Claims

1. An electric motor arrangement comprising an electric motor and a controller, wherein the electric motor includes a stator having a plurality of coil windings, and a rotor having a plurality of permanent magnets, and the controller is arranged to control current flow through the plurality of coil windings to generate a braking torque on the rotor, wherein the current is defined by a stator current vector represented in a two coordinate time invariant system having a rotating quadrature axis and a direct axis by a quadrature current component and a direct current component, wherein the controller is arranged to vary the quadrature current component and the direct current component so that the input electrical power of the electric motor is greater than or equal to zero.

2. An electric motor arrangement according to claim 1, wherein the controller is arranged to calculate the direct current component using

and the quadrature current component using

where R corresponds to coil winding series resistance in ohms, ω_ε is the electrical frequency in radians per second, and Wm is the motor back-emf constant in volts per radian per second.

3. An electric motor arrangement according to claim 1, further comprising a memory for storing a table of quadrature current components and corresponding direct current components.

4. An electric motor arrangement according to any one of the preceding claims, wherein the electric motor is an in- wheel electric motor.

5. An electric motor arrangement according to any one of the preceding claims, wherein the electric motor has a plurality of sub motors, wherein each sub motor has a plurality of coil windings.

6. A vehicle having an electric motor arrangement according to any one of the preceding claims.

7. A method of regenerative braking for an electric motor arrangement having an electric motor and a controller, wherein the electric motor includes a stator having a plurality of coil windings, and a rotor having a plurality of permanent magnets, and the controller is arranged to control current flow through the plurality of coil windings to generate a braking torque on the rotor, wherein the current is defined by a stator current vector represented in a two co-ordinate time invariant system having a rotating quadrature axis and a direct axis by a quadrature current component and a direct current component, the method comprising varying the quadrature current component and the direct current component so that the input electrical power of the electric motor is greater than or equal to zero.