WO2018109465A1

WO2018109465A1 - A stator for an electric motor or generator

Info

Publication number: WO2018109465A1
Application number: PCT/GB2017/053728
Authority: WO
Inventors: Gareth Roberts
Original assignee: Protean Electric Limited
Priority date: 2016-12-14
Filing date: 2017-12-13
Publication date: 2018-06-21
Also published as: GB2557635A; GB201621219D0; CN109155547A

Abstract

A stator for an electric motor or generator, the stator including a first circumferential portion and a second circumferential portion, wherein the first circumferential portion is oriented substantially perpendicular to the second circumferential portion, wherein a first cooling channel is formed in the first circumferential portion and a second cooling channel is formed in the second circumferential portion, wherein the outlet of the first cooling channel is coupled to the inlet of the second cooling channel.

Description

A STATOR FOR AN ELECTRIC MOTOR OR GENERATOR

The present invention relates to a stator for an electric motor or generator, in particular a stator having a cooling channel.

With increased interest being placed in environmentally friendly vehicles there has been a corresponding increase in interest in the use of electric motors for providing drive torque for electric vehicles.

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 .

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.

In the commercial arena three phase electric motors are the most common kind of electric motor available. 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.

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.

Typically, the three phase bridge inverter will generate a three phase voltage supply using a form of pulse width modulation (PWM) voltage control. 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 coils. During this on period, the current rises in the motor coils at a rate dictated by its inductance and the applied voltage. The PWM control is then required to switch off before the current has changed too much so that precise control of the current is achieved.

A three phase bridge inverter includes a number of switching devices, for example power electronic switches such as

Insulated Gate Bipolar Transistor (IGBT) switches.

In the context of an electric vehicle motor, a drive design that is becoming increasing popular is an integrated in- wheel electric motor design in which an electric motor and its associated control system are integrated within a wheel of a vehicle. However, the integration of an electric motor, and its associated control system, within a wheel of a vehicle can impose increased thermal management considerations upon the electric motor, which can result in a reduction in

efficiency and power generating capabilities of an electric motor.

In accordance with an aspect of the present invention there is provided a stator for an electric motor or generator according to the accompanying claims.

The invention as claimed provides the advantage of allowing a cooling arrangement within a stator for an electric motor or generator to be used to provide cooling to a plurality of components mounted on stator, where the cooling arrangement can be configured to provide optimised cooling based upon the cooling requirements of the different components.

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 a motor embodying the present invention; Figure 2 is an exploded view of the motor of Figure 1 from an alternative angle;

Figure 3 illustrates a stator according to an embodiment of the present invention; Figure 4 illustrates an exploded view of a control device;

Figure 5 illustrates a cooling channel according to an embodiment of present invention;

Figure 6 illustrates a cooling channel according to an embodiment of present invention. Figure 1 and Figure 2 illustrate an electric motor assembly incorporating an electric motor having a cooling arrangement according to the present invention where the electric motor assembly includes built in electronics and is configured for use as a hub motor or in-wheel electric motor built to be accommodated within a wheel. However, the present invention could be incorporated in any form of electric motor. The electric motor can also be configured as a generator.

For the purposes of the present embodiment, as illustrated in Figure 1 and Figure 2, the in-wheel electric motor includes a stator assembly 252 and a rotor assembly 240. The stator assembly 252 comprising a heat sink 253 having a cooling arrangement, as described below, multiple coils, an electronics module mounted in a rear portion of the stator for driving the coils, and a capacitor 257 mounted on the stator within a recess 257 formed on the rear portion of the stator. In a preferred embodiment the capacitor is an annular capacitor element. The coils are mounted on a plurality of stator teeth to form coil windings. The stator teeth are preferably formed from laminations of electrical steel, where the stator teeth are preferably configured as a back-iron attached or mounted to the heat sink 253, as is well known to a person skilled in the art. A stator cover 256 is mounted on the rear portion of the stator 252, enclosing the electronics module to form the stator assembly 252, which may then be fixed to a vehicle and does not rotate relative to the vehicle during use .

The electronics module includes two control devices 400, where each control device 400 includes two inverters and control logic, which in the present embodiment includes a processor, for controlling the operation of the inverter, as described below.

To reduce the effects of inductance on the inverters, housed in the respective control devices 400, when switching current, the capacitors mounted on the stator is used as a local voltage source for the electric motor inverters. By placing a capacitor close to an inverter the inductance associated with the voltage source is minimised. A rotor 240 comprises a front portion 220 and a cylindrical portion 221 forming a cover, which substantially surrounds the stator assembly 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 assembly 240 to cause the rotor assembly 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 2 shows an exploded view of the same assembly as Figure 1 from the opposite side showing the stator assembly 252 and rotor assembly 240. The rotor assembly 240

comprises the outer rotor wall 220 and circumferential wall 221 within which magnets 242 are circumferentially arranged. As previously described, the stator assembly 252 is

connected to the rotor assembly 240 via the bearing block at the central portions of the rotor and stator walls. 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.

The motor in this embodiment 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. 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. For example in a preferred embodiment the motor includes eight coil sets with each coil set 60 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.

Figure 3 illustrates a transparent three dimensional image of the heat sink 253. The heat sink 253 includes a first circumferential portion 310 oriented substantially perpendicular to a second

circumferential portion 320, where the first circumferential portion 310 includes a first surface 330 oriented

substantially perpendicular to a second surface 340 on the second circumferential portion 320.

A first cooling channel 350 is formed in the first

circumferential portion 310 and a second cooling channel 360 is formed in the second circumferential portion 320. As described above, the second circumferential portion 320 is arranged to receive a back-iron, preferably formed from laminations of electrical steel, where the back-iron

includes stator teeth having coil windings. The back-iron is arranged to be mounted on the second surface 340 of the second circumferential portion 320.

The control devices 400, which form the electronic module 255, are arranged to be mounted on the first surface 330 of the first circumferential portion 310 for controlling current in the electrical coil windings mounted on the stator teeth. Figure 4 illustrates a modular construction of the control module 400 with an exploded view of a preferred embodiment of a control module 400, where each control module 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 (not shown) for connecting to the capacitor component, six phase winding busbars (not shown) for connecting to respective coil windings, two insert modules 560 and six current sensors. Each current sensor includes a Hall sensor and a section of soft ferromagnetic material 530 arranged to be mounted adjacent to the Hall sensor, where preferably each Hall sensor is arranged to be mounted in a cutout section of a piece of soft ferromagnetic material fashioned in a toroid shape. Each power substrate assemblies 510 include an inverter for controlling current in a respective coil set.

Each of the control module components are mounted within a control module housing 550 with the four power source busbars and the six phase winding busbars being mounted, via the respective insert modules, on the power printed circuit board 500 on opposite sides of the control device housing 550.

Each power substrate 510 is arranged to be mounted in a respective aperture formed in the power printed circuit board 500, where each of the power substrates 510 has a 3mm copper base plate 600 upon which is formed a three phase inverter 410. The copper base plate 600 is provided to allow heat to be extracted from the power substrate 510.

Accordingly, any suitable heat conducting material may be used, for example a metal matrix composite consisting of aluminum matrix with silicon carbide particles such as

AlSiC.

A corresponding aperture 511 is also formed in the control module housing 550 to allow the copper base plate for each of the power substrates 510 is placed in an aperture formed in the stator heat sink 253 when the control device housing 550 is mounted to the stator. The aperture allows the respective copper base plate to extend into the first cooling channel, as described below, thereby allowing for cooling to be applied directly to the base of each of the power substrates 510.

Mounted on the underside of the power printed circuit board 500, adjacent to the copper base plate of the power

substrate assemblies 510, are the six Hall sensors (not shown) for measuring the current in the respective coil windings associated with two of the four coil sets. The Hall sensor readings are provided to the control printed circuit board 520. The power printed circuit board 500 includes a variety of other components that include drivers for the inverter switches formed on the power substrate assemblies 510, where the drivers are used to convert control signals from the control printed circuit board 520 into a suitable form for operating switches mounted on the power printed circuit board 500, however these components will not be discussed in any further detail. The insert modules 560 are arranged to be mounted over the power printed circuit board 500 when the power printed circuit board 500 is mounted in the control module housing 550. Each insert module 560 is arranged to be mounted over a respective power substrate assembly 510 mounted on the power printed circuit board 500, with each insert module 560 having an aperture arranged to extend around inverter switches formed on a respective power substrate assembly 510.

Each insert module 560 is arranged to carry two power source busbars and three phase windings busbars for coupling the inverter formed on the power substrate assembly 510, over which the insert module 560 is mounted, to the capacitor component and to the phase windings of a coil set,

respectively .

The insert module 560 also acts as a spacer for separating the control printed circuit board 520 from the power printed circuit board 500 when both the power printed circuit board 500 and the control printed circuit board 520 are mounted in the control module housing 550. A first pair of the power source busbars mounted on one of the insert modules 560 is for providing a voltage source to a first inverter 410 formed on one of the power substrates assemblies 510. A second pair of the power source busbars mounted on a second insert module 560 is for providing a voltage source to a second inverter 410 formed on the other power substrate assembly 510.

For each pair of power source busbars, one of the power source busbars is located in a first plane formed above the plane of the power circuit board 500. The other power source busbar is located in a second plane above the first plane. Preferably, each pair of power source busbars are arranged to be substantially co-planar.

Located in the control module housing 550 on the opposite side of the respective power substrate assemblies 510 to the power source busbars are the six phase winding busbars. A phase winding busbar is coupled to each inverter leg for coupling to a respective coil winding, as is well known to a person skilled in the art (i.e. a phase winding busbar is coupled to each leg of the three phase inverter formed on one of the power substrate assemblies 510 and a phase winding busbar is coupled to each leg of the three phase inverter formed on the other power substrate assembly 510) .

The control printed circuit board 520 is arranged to be mounted in the control module housing 550 above the power printed circuit board 500.

The control printed circuit board 520 includes a processor 420 for controlling the operation of the respective inverter switches to allow each of the electric motor coil sets 60 to be supplied with a three phase voltage supply using PWM voltage control across the respective coil sub-sets 61, 62, 63. For a given torque requirement, the three phase voltage applied across the respective coil sets is determined using field oriented control FOC, which is performed by the processor on the control printed circuit board using the current sensors mounted within the control module housing 550 for measuring the generated current.

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. One well known example of such a switching circuit is the three phase bridge circuit 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 of the three phase bridge circuit. A DC power source is coupled across the legs of the inverter, with the respective coil windings of an electric motor being coupled between a respective pair of switches, as is well known to a person skilled in the art. A single phase inverter will have two pairs of switches arranged in series to form two legs of an inverter .

The three phase voltage supply results in the generation of current flow in the respective coil sub-sets and a

corresponding rotating magnetic field for providing a required torque by the respective sub-motors.

Additionally, each control printed circuit board 520 includes an interface arrangement to allow communication between the respective control modules 400 via a

communication bus with one control module 400 being arranged to communicate with a vehicle controller mounted external to the electric motor, where the externally mounted controller will typically provide a required torque value to the control module 400. The processor 420 on each control modules 400 is arranged to handle communication over the interface arrangement.

As stated above, 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 may have one or more coil sub-sets .

Returning to Figure 3, preferably the first surface 330 of the first circumferential portion 310 includes a plurality of apertures 370 arranged to extend into the first cooling channel 350, wherein the respective apertures 370 are arranged to receive a copper base plate from one of the control devices 400, thereby allowing the copper base plate to extend into the first cooling channel 350.

The first cooling channel 350 includes an inlet port 380 for allowing a cooling fluid to enter the first cooling channel 350. Any suitable cooling fluid may be used for providing cooling to the heat sink 253, for example a liquid such as water. From the inlet port 380, preferably the first cooling channel 350 extends substantially 360 degrees around the first circumferential portion 310 with a coupling member 385 extending from the outlet 386 of the first cooling channel 350 to an inlet 390 of the second cooling channel 360, where preferably the second cooling channel 360 extends

substantially 360 degrees around the second circumferential portion 320 with an outlet port 395 formed at the end of the second cooling channel to allow the cooling fluid to exit the heat sink 253.

Preferably the first cooling channel 350 is arranged to extend in a radial direction in the first circumferential portion 310 and the second cooling channel 360 is arranged to extend in an axial direction in the second

circumferential portion 320, where the cross sectional area of the first cooling channel 350 and the cross sectional area of the second cooling channel 360 are selected based on the relative cooling requirements of the control devices 400 and the stator teeth respectively.

For example, although the coil windings will typically generate more heat than the control devices 400 mounted on the heat sink 253, the cooling provided by the first cooling channel 350 can be balanced, for example by adjusting the cross sectional area, to provide a predetermined cooling offset compared to the second cooling channel 360.

Additionally, by having a first cooling channel arranged to provide cooling to the control devices 400 and a second cooling channel arranged to provide cooling to the coil windings, this allows for cooling to be specifically

targeted at the control devices 400 and the coil windings respectively, where the cooling channels can be configured to provide optimum cooling to the control devices 400 and the coil windings. For example, to further optimise the cooling performance of the first cooling channel 350 and/or the second cooling channel 360, preferably the cross sectional area of the first cooling channel 350 and/or the second cooling channel 360 is arranged to vary at different locations around the first circumferential portion 310 and second circumferential portion 320, respectively, to modify the amount of cooling provided by the first cooling channel 350 and/or second cooling channel 360 at different locations around the stator.

For example, by increasing the cross sectional area for the first cooling channel 350 in the region of the apertures 370 will provide a bypass route around the relevant base plate so that subsequent base plates receive a similar flow and back pressure is not excessively large as a result of the copper plates partially blocking the cooling channel.

Accordingly, the configuration of the first cooling channel and the second cooling channel can be independently designed to allow the flow of the cooling fluid through the

respective cooling channels to have optimum back pressure and flow rate without the risk that cooling targeted to the control devices could be diverted to the coil windings and vice versa.

Figure 5 illustrates a cross sectional view of the stator heat sink 253, where the first cooling channel 350 is formed in the first circumferential portion 310 of the stator heat sink 253 and the second cooling channel 360 is formed in the second circumferential portion 320 of the stator heat sink 253. Coolant is arranged to flow around the first cooling channel 350 and the second cooling channel 360, as described above . Preferably, to further aid in the balancing of the cooling provided by the first cooling channel 350 and the second cooling channel 360, the first cooling channel 350 is arranged to extend into the second circumferential portion 320 of the stator, as illustrated in Figure 5. Extending the first cooling channel 350 into the second circumferential portion 320 of the stator allows the first cooling channel 350 to provide cooling to both the control devices 400 and the coil windings. The extent that the first cooling channel 350 is arranged to axially extend into the second

circumferential portion 320 can be adjusted based on the cooling needs of the control devices 400 and the coil windings . As illustrated in Figure 6, the first cooling channel 350 is arranged to extend adjacent to the first surface 330 of the first circumferential portion 310 of the stator heat sink 253 upon which the control devices 400 are arranged to be mounted .

The second cooling channel 360 is arranged to extend

adjacent to the second surface 340 of the stator heat sink 253 upon which the stator teeth and coil windings 254 are arranged to be mounted.

Additionally, the second cooling channel 360 is arranged to extend adjacent to a third surface 610 that is substantially parallel to the second surface 340, where the third surface 610 forms one side of the recess 257 for housing the

capacitor 620.

Accordingly, the present invention provides a cooling arrangement arranged to cool the electrical coils 254, a first electrical device (i.e. the control devices 400) and a second electrical device (i.e. the capacitor) .

As illustrated in Figure 6, as described above, in a

preferred embodiment the first cooling channel 350 is arranged to extend into the second circumferential portion iron, stator teeth and coil windings 630 and/or the capacitor 620. Although the present embodiments, describe a first cooling channel 350 and a second cooling channel 360 having

orthogonally oriented portions that are configured to provide optimum cooling to separate elements of the electric motor and associated control system, the electric motor may also include additional cooling channels for cooling other components, where the additional cooling channels may be of conventional design or as described above.

Claims

1. A stator for an electric motor or generator, the stator including a first circumferential portion and a second circumferential portion, wherein the first circumferential portion is oriented substantially perpendicular to the second circumferential portion, wherein a first cooling channel is formed in the first circumferential portion and a second cooling channel is formed in the second

circumferential portion, wherein the outlet of the first cooling channel is coupled to the inlet of the second cooling channel.

2. A stator according to claim 1, wherein a first surface of the first circumferential portion is oriented

substantially perpendicular to a second surface of the second circumferential portion.

3. A stator according to claims 1 or 2, wherein the first cooling channel is oriented substantially perpendicularly to the second cooling channel.

4. A stator according to claim 2 or claim 3 when dependent upon claim 2, wherein stator teeth having electrical coil windings are formed or mounted on the second surface of the second circumferential portion and a first electrical device is arranged to control current in the electrical coil windings is arranged to be mounted on the first surface of the first circumferential portion.

5. A stator according to claim 4, wherein the first cooling channel is arranged to provide cooling to the first electrical device and the second cooling channel is arranged to provide cooling to the stator teeth.

6. A stator according to any one of the preceding claims, wherein coolant fluid is arranged to flow through the first cooling channel and through the second cooling channel.

7. A stator according to any one of claims 4 to 6, wherein the cross section of the first cooling channel and second cooling channel are selected based on the relative cooling requirements of the first electrical device and the stator teeth respectively.

8. A stator according to any one of the preceding claims, wherein the first cooling channel and the second cooling channel are arranged to extend in a circumferential

direction around the stator.

9. A stator according to any one of claims 4 to 8, wherein the first electrical device is an IGBT or other power control device

10. A stator according to any one of the preceding claims, wherein the cross sectional area of the first cooling channel and/or the second cooling channel varies at

different locations around the stator to modify the amount of cooling provided by the cooling channel at the different locations around the stator.