Patent ID: 12191792

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The embodiment of the invention described is for a circuit for use with an electric motor system, where the circuit is used to control the configuration and operation of an inverter following the power source/battery for the electric motor system becoming disconnected from the electric motor system's high voltage busbar. The inverter is arranged to control current within coil windings of an electric motor. In particular, the embodiment of the invention is arranged to place the electric motor system into a short circuit configuration upon the power source/battery for the electric motor system becoming disconnected from the electric motor system's high voltage busbar.

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 an 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 inFIG.2andFIG.3, the in-wheel electric motor includes a stator252comprising a heat sink253, multiple coils254, two control devices400mounted on the heat sink253on 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 devices400. The coils254are formed on stator tooth laminations to form coil windings. A stator cover256is mounted on the rear portion of the stator252, enclosing the control devices400to form the stator252, which may then be fixed to a vehicle and does not rotate relative to the vehicle during use.

Each control device400includes two inverters410and control logic420, which in the present embodiment includes a processor, for controlling the operation of the inverters410, which is schematically represented inFIG.4.

Although for purposes of the present embodiment the in-wheel 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 inverters410and 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 devices400.

A rotor240comprises a front portion220and a cylindrical portion221forming a cover, which substantially surrounds the stator252. The rotor includes a plurality of permanent magnets242arranged around the inside of the cylindrical portion221. For the purposes of the present embodiment 32 magnet pairs are mounted on the inside of the cylindrical portion221. However, any number of magnet pairs may be used.

The magnets are in close proximity to the coil windings on the stator252so that magnetic fields generated by the coils interact with the magnets242arranged around the inside of the cylindrical portion221of the rotor240to cause the rotor240to rotate. As the permanent magnets242are utilized to generate a drive torque for driving the electric motor, the permanent magnets are typically called drive magnets.

The rotor240is attached to the stator252by a bearing block223. The bearing block223can 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 portion253of the wall of the stator252and also to a central portion225of the housing wall220of the rotor240. The rotor240is thus rotationally fixed to the vehicle with which it is to be used via the bearing block223at the central portion225of the rotor240. This has an advantage in that a wheel rim and tire can then be fixed to the rotor240at the central portion225using 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 block223. The wheel bolts may be fitted through the central portion225of the rotor through into the bearing block itself. With both the rotor240and the wheel being mounted to the bearing block223there is a one to one correspondence between the angle of rotation of the rotor and wheel.

FIG.3shows an exploded view of the same motor assembly illustrated inFIG.2from the opposite side. The rotor240comprises the outer rotor wall220and circumferential wall221within which magnets242are circumferentially arranged. As previously described, the stator252is connected to the rotor240via the bearing block at the central portions of the rotor and stator walls.

The rotor also includes a set of magnets227for 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 placed90electrical 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 devices400, 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 sets60with 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.

FIG.4illustrates the connections between the respective coil sets60and the control devices400, where a respective coil set60is connected to a respective three phase inverter410included on a control device400for controlling current flow within the respective coil sets. Each of the respective three phase inverters contain six switches configured in a three phase arrangement having three high side switches and three low side switches, as described below, 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 devices400are of a modular construction.FIG.5illustrates an exploded view of a preferred embodiment, where each control device400, otherwise known as a power module, includes a power printed circuit board500in which are mounted two power substrate assemblies510, a control printed circuit board520, four power source busbars530for connecting to a DC battery, and six phase winding busbars540for connecting to respective coil windings. Each of the control device components are mounted within a control device housing550with the four power source busbars530being mounted on an opposite side of the control device housing550to the phase winding busbars540.

Each power substrate510is arranged to be mounted in a respective aperture formed in the power printed circuit board500.

The power printed circuit board500includes a variety of components that include drivers for the inverter switches formed on the power substrate assemblies510, 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 board520includes a processor for controlling the operation of the inverter switches. Additionally, each control printed circuit board520includes an interface arrangement to allow communication between the respective control devices400via a communication bus with one control device400being arranged to communicate with a vehicle controller mounted external to the electric motor. The processor420on each control device400is arranged to handle communication over the interface arrangement.

As stated above, the processors420on the respective control devices400are arranged to control the operation of the inverter switches mounted on the respective power substrates520within the control housing550, thereby allowing each of the electric motor coil sets60to 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 set60as having three coil sub-sets the present invention is not limited by this and it would be appreciated that each coil set60may have one or more coils sub-sets.

Under the control of the respective processors420, each three phase bridge inverter410is 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 set60the three phase bridge inverter410switches are arranged to apply a single voltage phase across each of the coil sub-sets.

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, as illustrated inFIG.1, where each pair of switches is placed in series and form a leg600of the three phase bridge circuit to form a three phase bridge inverter configuration. A fly-back diode610, otherwise known as a reverse diode, is coupled in anti-parallel across each switch620, as illustrated inFIG.6. A single phase inverter will have two pairs of switches620arranged in series to form two legs600of an inverter.

As stated above, each of the inverter legs600are 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 form the drive current within the coil windings.

Should a fault occur in the electric motor system that causes the electric motor to become non-operational, for example the power source/battery used for driving the electric motor system becomes disconnected from the electric motor systems high voltage busbar, and there is a fault with the low voltage supply for the electric motor system, the electric motor system includes a circuit for placing the inverter into an active short circuit mode. The circuit utilizes the back EMF generated by the electric motor for placing the inverter into the active short circuit mode, thereby avoiding the need for a separate low voltage power rail for the inverter switches.

A first embodiment of the circuit incorporated into an electric motor system is illustrated inFIG.7, where the same reference numerals are used to reference the same features as those shown inFIG.1.

FIG.7shows an electric motor system having a battery12coupled to a first busbar11and a second busbar12, which in turn are coupled to a DC link capacitor13and a three phase inverter having three high side switches14and three low side switches15. As illustrated inFIG.7, the high side inverter switches14are coupled to the first busbar11(i.e. the positive busbar), while the low side inverter switches15are coupled to the second busbar12(i.e. the negative busbar). Each high side and low side switch combination form a leg of the inverter, with each leg of the inverter being coupled to a coil winding of a three phase electric motor.

Additionally, a circuit comprising a first impedance70and a second impedance71are coupled in series between the first power source busbar11and the second power source busbar12. The first impedance70is selected to such that a large percentage of the voltage drop across the first power source busbar11and the second power source busbar12is formed across the first impedance70, for example 400V. The second impedance71is selected to allow low voltage to be formed across the second impedance71such that the voltage at a point between the first impedance70and the second impedance71is a low voltage, for example 12V or 24V, which is suitable for controlling the operation of the low side inverter switches15. As shown inFIG.7, this voltage point is connected to the plurality of the low side switches15, where a controller72is preferably utilized to couple the voltage to the plurality of low side inverter switches15upon the occurrence of a first predetermined condition, for example a fault in the electric motor and/or associated control system that causes the electric motor to become non-operational. The voltage applied to the low side inverter switch causes the low side inverter switches15to close. Similarly, the high side inverter switches14are placed in an open configuration, where this may be achieved by any suitable means, for example as a result of no voltage being applied to the respective switches when enhancement mode MOSFET's are used.

As a result of the plurality of low side inverter switches15being placed in a close circuit and the high side inverter switches14being placed in an open configuration the coil windings of an electric motor are placed in a short circuit configuration. Although the present embodiment describes the short circuit configuration as having the low side inverter switches15closed and the high side inverter switches14open, equally this configuration may be reversed where the voltage from the back EMF is used to keep the low side inverter switches15open and, if necessary, the high side inverter switches closed.

For example, if a fault in the electric motor and/or associated control system is identified as being a result of one of the low side inverter switches staying open, then the back EMF will be used to keep the high side switches closed, as described above, with the remaining low side inverter switches being kept open. Equally, if this scenario were to be reversed, the low side inverter switches would be closed and the remaining operation high side switches would be allowed to go open.

A second embodiment of the circuit incorporated into an electric motor system is illustrated inFIG.8, where the same reference numerals are used to reference the same features as those shown inFIG.1. This circuit provides greater control over the circuit illustrated inFIG.7, for example this circuit provides greater control on the timing as to when the low side inverter switches are closed for placing the coil windings of the electric motor in a short circuit configuration, while avoiding the need for a separate low voltage power rail for the inverter switches.

FIG.8shows a electric motor system having a battery10coupled to a first busbar11and a second busbar12, which in turn are coupled to a DC link capacitor13and a three phase inverter having three high side switches14and three low side switches15. As illustrated inFIG.8, the high side inverter switches14are coupled to the first busbar11(i.e. the positive busbar), while the low side inverter15switches are coupled to the second busbar12(i.e. the negative busbar). Each high side and low side switch combination form a leg of the inverter, with each leg of the inverter being coupled to a coil winding of a three phase electric motor.

Additionally, a circuit comprising a first switch80, a first capacitor81and a second capacitor82are coupled in series between the first power source busbar11and the second power source busbar12, where the low side inverter switches15are coupled to a point between the first capacitor81and the second capacitor82.

Preferably, a first voltage clamp device83is coupled across the first capacitor81and a second voltage clamp device84is coupled across the second capacitor82, as described below.

As shown inFIG.8, the voltage point between the first capacitor and the second capacitor is coupled to the plurality of low side switches.

In a preferred embodiment, the first capacitor81is a high voltage capacitor, the second capacitor82is a low voltage capacitor, the first switch80is a depletion mode semiconductor switch, the first voltage clamp device83is a high voltage clamp device, for example a metal oxide varistor, resistor or Zener diode, for voltage clamping, and the second voltage clamp device84is a low voltage clamp device, for example a metal oxide varistor, resistor or Zener diode, for voltage clamping.

During normal operation of the electric motor, the first switch80receives a control signal, for example via a controller or logic device85, which keeps the switch open, thereby electrically isolating the first capacitor81and the second capacitor82from the voltage across the first power source busbar11and the second power source busbar12. Consequently, the circuit does not introduce any power loss during the normal operation of the electric motor.

Upon the occurrence of a predetermined condition associated with a fault within the electric motor system, the first switch80is closed resulting in the first capacitor81and the second capacitor82being charged by the DC link capacitor via the first busbar11and the second busbar12. If the fault within the electric motor system results in a loss of low voltage, by using a depletion mode for semiconductor the loss of low voltage will automatically result in the first switch closing.

Upon the first switch80being closed, the ratio of the steady state voltages across the first capacitor81and the second capacitor82will be dependent upon their respective impedance, and the clamping devices used, where the respective values are chosen so that the majority of the voltage drop is formed across the first capacitor81. As a consequence of the first capacitor81being a high voltage capacitor, the voltage at the point between the first capacitor81and the second capacitor82is a low voltage, for example 12V or 24V. However the circuit component values may be chosen to provide any suitable voltage value to the low side inverter switches.

The low voltage is provided to the low side inverter switches15causing them to close. Similarly, the high side inverter switches14are placed in an open configuration, where this may be achieved by any suitable means, for example as a result of no voltage being applied to the respective switches when enhancement mode MOSFET's are used.

As a consequence, upon the occurrence of the predetermined condition, the low side inverter switches15are closed and the high side inverter switches14are opened resulting in the electric motor coil windings being placed in a short circuit configuration.

In a preferred embodiment, an additional passive device (not shown) may be used to condition the voltage provided to the low side inverter switches for improved operation of the low side inverter switches.

Although the present embodiment describes the short circuit configuration as having the low side inverter switches15closed and the high side inverter switches14open, equally this configuration may be reversed where the voltage from the back EMF is used to keep the low side inverter switches15open and, if necessary, the high side inverter switches closed.

For example, if a fault in the electric motor and/or associated control system is identified as being a result of one of the low side inverter switches staying open, then the back EMF will be used to keep the high side switches closed, as described above, with the remaining low side inverter switches being kept open. Equally, if this scenario were to be reversed, the low side inverter switches would be closed and the remaining operation high side switches would be allowed to go open.

FIGS.9(a),9(b),9(c), and9(d)illustrate, the change in voltage/current following a fault condition within the electric motor system that results in the back EMF from the electric motor forming across the DC link capacitor, whereFIG.9(a)illustrates the change in voltage in the DC link capacitor13,FIG.9(b)illustrates the change in charge current across the first capacitor81and the second capacitor82,FIG.9(c)illustrates the change in voltage across the first capacitor81,FIG.9(d)illustrates the change in voltage across the second capacitor82.

With respect toFIG.9(a), at zero seconds, as a result of a fault within the electric motor system, the voltage across the DC link capacitor13starts to rise as a result of the back EMF from the electric motor. Once the DC link capacitor voltage exceeds a predefined value, which within the present embodiment is 450V, the first switch is closed resulting in a current pulse charging the first capacitor81and second capacitor82. The charge time and current amplitude will be determined by the lumped inductance and lumped resistance of the circuit.

As illustrated inFIGS.9(c) and9(d), the first capacitor81is charged to approximately 400V, while the second capacitor82is charged to approximately 40V with a small voltage drop across the switch80, where in a preferred embodiment the energy stored on the second capacitor82is used to drive a linear regulator, or similar switching mode power supply, to produce a 15V turn on voltage for the low side inverter switches15, thereby ensuring that the electric motor is placed in a short circuit configuration even if a low voltage power rail fails, thereby protecting the inverter from an over voltage condition.

Although the above embodiment describes the predetermined condition for placing the electric motor in a short circuit configuration being a result of the DC link capacitor voltage exceeding a threshold value, any predetermined condition associated with a fault with the electric motor system may be used. For example, a fault identified with a low power rail used for controlling the operation of the inverter, a fault with one or more of the inverter switches, or identification that the battery10becoming disconnected from the first power source busbar11and/or second power source busbar12.