Circuit for an inverter

Abstract

A circuit for a multiphase bridge inverter, the multiphase bridge inverter having a plurality of high-side switches arranged to be connected to a first power bus and a plurality of low-side switches arranged to be connected to a second power bus, wherein the multiphase inverter is arranged to control the current in the coil windings of an electric motor, and wherein a DC link capacitor is connected between the first power bus and the second power bus, the circuit including a first impedance and a second impedance connected in series between the first power bus and the second power bus and in parallel with the DC link capacitor, and means connected between the first impedance and the second impedance for providing a voltage from the connection between the first impedance and the second impedance to the plurality of low-side switches when a first predetermined condition occurs, so as to place the plurality of low-side switches in a closed circuit configuration, thereby allowing the coil windings of the electric motor to be placed in a short-circuit configuration.

Description

Technical Field

[0001] This invention relates to a circuit, and more particularly to a circuit for an inverter used to control the current in the coil windings of an electric motor. Background Technology

[0002] Electric motors operate on the principle that a current-carrying conductor experiences a force when a magnetic field is present. When the current-carrying conductor is placed perpendicular to the magnetic field, the force on the conductor is proportional to the flux density of the magnetic field. Typically, in an electric motor, the force on the current-carrying conductor is expressed as rotational torque.

[0003] Examples of known types of electric motors include induction motors, brushless permanent magnet motors, switched reluctance motors, and synchronous slip ring motors, which have a rotor and a stator, as are known to those skilled in the art.

[0004] A rotor used in a permanent magnet motor typically comprises multiple permanent magnets mounted on or within a rotor guard such that the magnetic field alternates in polarity around the rotor's circumference. As the rotor rotates relative to the stator, the permanent magnets are arranged to sweep across the ends of coil windings mounted on the stator. Appropriate switching of the current in the coil windings allows for the synchronized attraction and repulsion of the permanent magnet poles, generating rotor rotation.

[0005] However, when the rotor rotates relative to the coil windings, a back electromotive force, also known as back EMF, is generated. This back EMF acts opposite to the initially applied voltage and thus affects the current that causes the rotor to rotate, thereby limiting the maximum rotational speed of the rotor. To increase the maximum rotational speed of the permanent magnet electric motor, magnetic field attenuation is used. However, magnetic field attenuation can cause the back EMF generated by the rotor rotating relative to the coil windings to increase to a level higher than the DC bus level used to generate current in the coil windings.

[0006] If a fault occurs in the electric motor system, such as one in the electric motor and / or its power supply that causes a power source like a battery to be disconnected to avoid a high reverse EMF voltage on the positive power bus, the electric motor can be placed in a short-circuit operating mode, also known as an active short-circuit mode. In this mode, all coil windings are connected to the negative power bus using a switch in the inverter, or alternatively, all coil windings are connected to the positive power bus. As a result, a reverse EMF is applied across the coil windings, protecting the inverter and any other components connected to the power bus (also known as the high-voltage bus), such as DC link capacitors, from overvoltage.

[0007] For the purpose of explanation, Figure 1A known electric motor system is shown, comprising a battery 10 connected to a first bus 11 and a second bus 12, which in turn are connected to a DC link capacitor 13 and a three-phase inverter with three high-side inverter switches 14 and three low-side inverter switches 15. Figure 1 As shown, the high-side inverter switch 14 is connected to the first bus 11 (i.e., the positive bus), while the low-side inverter switch 15 is connected to the second bus 12 (i.e., the negative bus). Each high-side and low-side switch combination forms a branch of the inverter, wherein each branch of the inverter is connected to a coil winding 16 of a three-phase electric motor.

[0008] Each of the low-side inverter switches is connected to, for example, a 12-volt or 24-volt low-voltage power rail (not shown), and is arranged to close the low-side inverter switch when a fault is detected in the electric motor system, thereby putting the inverter into active short-circuit mode.

[0009] However, if the power supply / battery is disconnected and the low-voltage power rail fails, the inverter cannot be put into active short-circuit mode, thus exposing the inverter, DC link capacitors, and battery to potential overvoltage conditions caused by reverse EMF from the electric motor.

[0010] To address this issue, a backup low-voltage power rail is typically used for critical drive systems. However, a backup low-voltage power rail has two potential drawbacks: it takes some time to "wake up" and it increases the overall cost of the drive system.

[0011] We hope to improve this situation. Summary of the Invention

[0012] According to one aspect of the invention, a circuit is provided according to the appended claims.

[0013] The claimed invention has the advantage of ensuring operation in active short-circuit mode without the need for an additional low-voltage source, wherein the circuit is compact, low-cost and easy to integrate into an electric motor system. Attached Figure Description

[0014] The invention will now be described by way of example with reference to the accompanying drawings, in which:

[0015] Figure 1 This illustrates a prior art electric motor system;

[0016] Figure 2 An exploded view of a rotor according to an embodiment of the present invention is shown;

[0017] Figure 3 A rotor according to an embodiment of the present invention is shown;

[0018] Figure 4 A control device according to an embodiment of the present invention is shown;

[0019] Figure 5 An exploded view of a control device according to an embodiment of the present invention is shown;

[0020] Figure 6 An inverter of the prior art is shown;

[0021] Figure 7 An electric motor system according to a first embodiment of the present invention is shown;

[0022] Figure 8 An electric motor system according to a second embodiment of the present invention is shown;

[0023] Figure 9 illustrates the electrical characteristics associated with embodiments of the present invention. Detailed Implementation

[0024] The described embodiments of the invention are circuits for use with electric motor systems, wherein the circuitry controls the configuration and operation of an inverter after the power supply / battery for the electric motor system is disconnected from the high-voltage bus of the electric motor system. The inverter is arranged to control the current in the coil windings of the electric motor. Specifically, embodiments of the invention are arranged to place the electric motor system in a short-circuit configuration when the power supply / battery for the electric motor system is disconnected from the high-voltage bus of the electric motor system.

[0025] For the purposes of this embodiment, the electric motor is used in the wheels of a vehicle; however, the electric motor can be located anywhere within the vehicle. The motor is of the type having a set of coils that are part of the stator attached to the vehicle and radially surrounded by a rotor carrying a set of magnets attached to the wheels. To avoid confusion, various aspects of the invention are equally applicable to generators having the same arrangement. Therefore, the definition of an electric motor is intended to include generators. Furthermore, some aspects of the invention are applicable to arrangements having a rotor centrally mounted within radially surrounding coils. As those skilled in the art will understand, the invention can be applied to other types of electric motors.

[0026] For the purposes of this embodiment, such as Figure 2 and Figure 3As shown, the in-wheel electric motor includes a stator 252, which includes a radiator 253, a plurality of coils 254, two control devices 400 mounted on the radiator 253 at the rear of the stator to drive the coils, and an annular capacitor mounted on the stator within the inner diameter of the control devices 400, the annular capacitor also referred to as a DC link capacitor. The coils 254 are formed on stator laminations to form coil windings. A stator cover 256 is mounted on the rear of the stator 252, surrounding the control devices 400 to form the stator 252, allowing the stator to be fixed to the vehicle and not rotate relative to the vehicle during use.

[0027] Each control device 400 includes two inverters 410 and control logic 420. In this embodiment, the control logic includes a processor for controlling the operation of the inverters 410. Figure 4 It is shown schematically in the diagram.

[0028] Although for the purposes of this embodiment, the in-wheel electric motor includes two control devices, each of which includes control logic for controlling the operation of the inverter, in other words, a controller, any configuration of the combination of control logic and inverter can be used, including placing the control logic and / or inverter away from the electric motor.

[0029] A toroidal capacitor is connected between the inverter 410 and the DC power supply of the electric motor to reduce voltage ripple on the electric motor's power supply line (also known as the DC bus) and to reduce voltage overshoot during electric motor operation. To reduce inductance, the capacitor is mounted adjacent to the control unit 400.

[0030] The rotor 240 includes a front portion 220 and a cylindrical portion 221 forming a cover that substantially surrounds the stator 252. The rotor includes a plurality of permanent magnets 242 arranged around the inner side of the cylindrical portion 221. For the purposes of this embodiment, 32 pairs of magnets are mounted on the inner side of the cylindrical portion 221. However, any number of magnet pairs can be used.

[0031] The magnet is positioned very close to the coil windings on the stator 252, such that the magnetic field generated by the coils interacts with the magnet 242 arranged inside the cylindrical portion 221 of the rotor 240, causing the rotor 240 to rotate. Because a permanent magnet 242 is used to generate the drive torque for driving the electric motor, the permanent magnet is often referred to as the drive magnet.

[0032] The rotor 240 is attached to the stator 252 via a bearing housing 223. The bearing housing 223 can be a standard bearing housing, as used in the vehicle to which this motor assembly will be mounted. The bearing housing comprises two parts, a first part fixed to the stator and a second part fixed to the rotor. The bearing housing is fixed to the center portion 253 of the wall of the stator 252 and also to the center portion 225 of the housing wall 220 of the rotor 240. The rotor 240 is thus rotatably fixed to the vehicle to which it will be used via the bearing housing 223 at the center portion 225 of the rotor 240. This has the advantage that the rim and tire can then be fixed to the rotor 240 at the center portion 225 using normal wheel bolts, thus securing the rim to the center portion of the rotor and therefore firmly to the rotatable side of the bearing housing 223. The wheel bolts can pass through the center portion 225 of the rotor and be fixed into the bearing housing itself. Since both the rotor 240 and the wheel are mounted to the bearing housing 223, there is a one-to-one correspondence between the rotation angles of the rotor and the wheel.

[0033] Figure 3 Shown from the opposite side Figure 2 An exploded view of the same motor assembly is shown. Rotor 240 includes an outer rotor wall 220 and a circumferential wall 221, with magnets 242 arranged circumferentially within the circumferential wall. As previously described, stator 252 is connected to rotor 240 via bearing housings at the center of the rotor wall and stator wall.

[0034] The rotor also includes a set of magnets 227 for position sensing, also known as commutating magnets, which, in conjunction with sensors mounted on the stator, allow estimation of the rotor flux angle. The rotor flux angle defines the positional relationship between the drive magnets and the coil windings. Alternatively, instead of a separate set of magnets, the rotor may include a ring of magnetic material having multiple magnetic poles that act as a separate set of magnets.

[0035] To allow the use of commutating magnets to calculate the rotor flux angle, preferably, each drive magnet has an associated commutating magnet, wherein the rotor flux angle is derived from the flux angle associated with the set of commutating magnets by calibrating the measured commutating magnet flux angle. To simplify the correlation between the commutating magnet flux angle and the rotor flux angle, preferably, the set of commutating magnets has the same number of magnets or pole pairs as the set of drive magnets, wherein the commutating magnets and associated drive magnets are substantially radially aligned with each other. Therefore, for the purposes of this embodiment, the set of commutating magnets has 32 magnet pairs, wherein each magnet pair is substantially radially aligned with a corresponding drive magnet pair.

[0036] In this embodiment, the Hall sensor is mounted on the stator. The sensor is positioned such that as the rotor rotates, each of the commutating magnets forming the commutating magnetic ring rotates past the sensor.

[0037] As the rotor rotates relative to the stator, the commutating magnets rotate accordingly past the sensor, and the Hall sensor outputs an AC voltage signal, wherein the sensor outputs a complete voltage cycle of 360 electrical degrees for each magnet pair passing through the sensor.

[0038] For improved position detection, preferably, the sensor includes an associated second sensor positioned 90 electrical degrees offset from the first sensor.

[0039] In this embodiment, the electric motor includes four coil groups, each coil group having three coil subgroups connected in a Y-configuration to form a three-phase sub-motor, resulting in the motor having four three-phase sub-motors. As described below, the operation of each sub-motor is controlled via one of two control devices 400. However, although this embodiment describes an electric motor with four coil groups (i.e., four sub-motors), the motor can similarly have one or more coil groups with associated control devices. In a preferred embodiment, the motor includes eight coil groups 60, each coil group having three coil subgroups connected in a Y-configuration to form a three-phase sub-motor, resulting in the motor having eight three-phase sub-motors. Similarly, each coil group can have any number of coil subgroups, allowing each sub-motor to have two or more phases.

[0040] Figure 4 The connection between the respective coil group 60 and the control device 400 is shown, wherein the respective coil group 60 is connected to a respective three-phase inverter 410 included in the control device 400 to control the current within the respective coil group. Each of the individual three-phase inverters contains six switches configured in a three-phase arrangement with three high-side switches and three low-side switches, as described below, wherein a three-phase alternating voltage can be generated by the controlled operation of the six switches. However, the number of switches will depend on the number of voltage phases to be applied to the respective sub-motor, wherein the sub-motor can be constructed with any number of phases.

[0041] Preferably, the control device 400 has a modular structure. Figure 5 An exploded view of a preferred embodiment is shown, wherein each control device 400, also referred to as a power module, includes a power printed circuit board 500 in which two power substrate assemblies 510 are mounted, a control printed circuit board 520, four power buses 530 for connection to a DC battery, and six phase winding buses 540 for connection to corresponding coil windings. Each control device component is mounted within a control device housing 550, wherein the four power buses 530 are mounted on the side of the control device housing 550 opposite to the phase winding buses 540.

[0042] Each power board 510 is arranged to be mounted in a corresponding hole formed in the power printed circuit board 500.

[0043] The power printed circuit board 500 includes various components, including drivers for inverter switches formed on the power substrate assembly 510, wherein the drivers are typically used to convert control signals into appropriate forms to turn the inverter switches on and off.

[0044] The control printed circuit board 520 includes a processor for controlling the operation of the inverter switch. Additionally, each control printed circuit board 520 includes an interface device to allow communication between corresponding control devices 400 via a communication bus, one of which is arranged to communicate with a vehicle controller mounted externally to the electric motor. A processor 420 on each control device 400 is arranged to process the communication via the interface device.

[0045] As described above, the processor 420 on the corresponding control device 400 is arranged to control the operation of the inverter switch mounted on the corresponding power base 520 within the control housing 550, thereby allowing a three-phase voltage supply to be supplied to each electric motor coil group 60, causing the corresponding coil subgroup to generate a rotating magnetic field. As described above, although this embodiment describes each coil group 60 as having three coil subgroups, the invention is not limited thereto, and it should be understood that each coil group 60 may have one or more coil subgroups.

[0046] Under the control of the corresponding processor 420, each three-phase bridge inverter 410 is arranged to provide pulse width modulation (PWM) voltage control across the corresponding coil subgroup, thereby generating current in the corresponding coil subgroup to provide the required torque by the corresponding sub-motor.

[0047] PWM control operates by using the average of the applied pulsed voltage through the motor inductor to drive the desired current into the motor coils. With PWM control, the applied voltage is switched across the motor windings. During the time the voltage is switched across the motor coils, the current rises in the motor coils at a rate determined by its inductance and the applied voltage. The PWM voltage control is cut off before the current has increased to a value exceeding the desired value, thus allowing for precise current control.

[0048] For a given coil group 60, each three-phase bridge inverter 410 is switched to apply a single voltage phase across each of the coil subgroups.

[0049] Using PWM switching, the plurality of switches are arranged to apply alternating voltages across corresponding coil subgroups. The voltage envelope and phase angle of the electrical signal are determined by the modulated voltage pulses.

[0050] Inverter switches can include semiconductor devices such as MOSFETs or IGBTs. In this example, the switch includes an IGBT. However, any suitable known switching circuit can be used to control the current. For a three-phase inverter with six switches configured to drive a three-phase electric motor, the six switches are configured as three groups of two switches in parallel, such as... Figure 1 As shown, each pair of switches is placed in series and forms a branch 600 of a three-phase bridge circuit to form a three-phase bridge inverter configuration. Figure 6 As shown, flyback diodes 610, also known as reverse diodes, are connected in anti-parallel connection across each switch 620. A single-phase inverter will have two pairs of switches 620 arranged in series to form two branches 600 of the inverter.

[0051] As described above, each inverter foot 600 is electrically connected between a pair of power buses.

[0052] As described above, PWM switching is used to apply an alternating voltage to the coil windings of an electric motor, where the rotor speed depends on the magnitude of the voltage applied across the coil windings, and the torque applied to the rotor is generated by the drive current within the coil windings.

[0053] If a fault occurs in the electric motor system that renders the electric motor inoperable, such as a disconnection between the power supply / battery driving the electric motor system and the high-voltage bus of the electric motor system, and a fault in the low-voltage supply of the electric motor system, the electric motor system includes circuitry for placing the inverter in an active short-circuit mode. This circuitry uses the back electromotive force generated by the electric motor to place the inverter in an active short-circuit mode, thereby eliminating the need for a separate low-voltage power rail for the inverter switching.

[0054] Figure 7 A first embodiment of a circuit incorporated into an electric motor system is shown, wherein the same reference numerals are used to indicate... Figure 1 The features shown are the same.

[0055] Figure 7 An electric motor system with a battery 12 connected to a first bus 11 and a second bus 12 is shown. The first and second buses are in turn connected to a DC link capacitor 13 and a three-phase inverter with three high-side switches 14 and three low-side switches 15. Figure 7 As shown, the high-side inverter switch 14 is connected to the first bus 11 (i.e., the positive bus), while the low-side inverter switch 15 is connected to the second bus 12 (i.e., the negative bus). Each high-side and low-side switch combination forms a branch of the inverter, wherein each branch of the inverter is connected to a coil winding of a three-phase electric motor.

[0056] Additionally, a circuit comprising a first impedance 70 and a second impedance 71 is connected in series between the first power bus 11 and the second power bus 12. The first impedance 70 is selected such that a large percentage of the voltage drop across the first power bus 11 and the second power bus 12 occurs across the first impedance 70, for example, 400V. The second impedance 71 is selected to allow a low voltage to occur across the second impedance 71, such that the voltage at a point between the first impedance 70 and the second impedance 71 is low, for example, 12V or 24V, suitable for controlling the operation of the low-side inverter switch 15. Figure 7 As shown, this voltage point is connected to the plurality of low-side switches 15, wherein, preferably, in the event of a first predetermined condition, such as a fault in the electric motor and / or related control system that renders the electric motor inoperable, the controller 72 connects the voltage to the plurality of low-side inverter switches 15. The voltage applied to the low-side inverter switches causes the low-side inverter switches 15 to close. Similarly, the high-side inverter switch 14 is placed in an open configuration, which can be achieved by any suitable means, for example, as a result of no voltage being applied to the corresponding switch when using an enhancement-mode MOSFET.

[0057] As a result of the plurality of low-side inverter switches 15 being placed in a closed circuit and the high-side inverter switch 14 being placed in an open configuration, the coil windings of the electric motor are placed in a short-circuit configuration. Although this embodiment describes the short-circuit configuration as closing the low-side inverter switch 15 and opening the high-side inverter switch 14, this configuration can also be reversed, wherein a voltage from the reverse EMF is used to keep the low-side inverter switch 15 open and, if necessary, to keep the high-side inverter switch closed.

[0058] For example, if a fault in the electric motor and / or related control system is identified as a result of one of the plurality of low-side inverter switches remaining open, then reverse EMF is used to keep the high-side switch closed, as described above, while the remaining low-side inverter switches remain open. Similarly, if this is reversed, the low-side inverter switch will be closed, and the remaining operating high-side switches will be allowed to open.

[0059] Figure 8 A second embodiment of the circuitry incorporated into an electric motor system is shown, wherein the same reference numerals are used to denote the circuitry. Figure 1 The features shown are the same as those described. This circuit provides the same features as... Figure 7 The circuit shown provides better control, for example, by offering better timing control over when the low-side inverter switch closes to put the electric motor's coil windings in a short-circuit configuration, while eliminating the need for a separate low-voltage power rail for the inverter switch.

[0060] Figure 8 An electric motor system with a battery 10 connected to a first bus 11 and a second bus 12 is shown. The first and second buses are in turn connected to a DC link capacitor 13 and a three-phase inverter with three high-side switches 14 and three low-side switches 15. Figure 8 As shown, the high-side inverter switch 14 is connected to the first bus 11 (i.e., the positive bus), while the low-side inverter switch 15 is connected to the second bus 12 (i.e., the negative bus). Each high-side and low-side switch combination forms a branch of the inverter, wherein each branch of the inverter is connected to a coil winding of a three-phase electric motor.

[0061] Additionally, a circuit including a first switch 80, a first capacitor 81, and a second capacitor 82 is connected in series between a first power bus 11 and a second power bus 12, wherein a low-side inverter switch 15 is connected to the point between the first capacitor 81 and the second capacitor 82.

[0062] Preferably, the first voltage clamping device 83 is connected across the first capacitor 81, and the second voltage clamping device 84 is connected across the second capacitor 82, as described below.

[0063] like Figure 8 As shown, the voltage point between the first capacitor and the second capacitor is connected to the plurality of low-side switches.

[0064] In a preferred embodiment, the first capacitor 81 is a high-voltage capacitor, the second capacitor 82 is a low-voltage capacitor, the first switch 80 is a depletion-type semiconductor switch, the first voltage clamping device 83 is a high-voltage clamping device for voltage clamping, such as a metal oxide rheostat, resistor, or Zener diode, and the second voltage clamping device 84 is a low-voltage clamping device for voltage clamping, such as a metal oxide rheostat, resistor, or Zener diode.

[0065] During normal operation of the electric motor, the first switch 80 receives a control signal, for example via a controller or logic device 85, to keep the switch open, thereby electrically isolating the first capacitor 81 and the second capacitor 82 from the voltage across the first power bus 11 and the second power bus 12. Therefore, the circuit does not introduce any power loss during normal operation of the electric motor.

[0066] When a predetermined condition associated with a fault within the electric motor system occurs, the first switch 80 closes, causing the first capacitor 81 and the second capacitor 82 to be charged by the DC link capacitors via the first bus 11 and the second bus 12. If a fault within the electric motor system results in a low voltage loss, the low voltage loss will automatically cause the first switch to close by using a depletion-type semiconductor.

[0067] When the first switch 80 is closed, the ratio of the steady-state voltages across the first capacitor 81 and the second capacitor 82 will depend on their respective impedances and the clamping devices used, where the values ​​are chosen such that a large portion of the voltage drop is formed across the first capacitor 81. Since the first capacitor 81 is a high-voltage capacitor, the voltage at the point between the first capacitor 81 and the second capacitor 82 is a low voltage, such as 12V or 24V. However, circuit component values ​​can be selected to provide any suitable voltage value to the low-side inverter switch.

[0068] A low voltage is supplied to the low-side inverter switches 15, causing them to close. Similarly, the high-side inverter switches 14 are placed in an open configuration, which can be achieved by any suitable means, such as as a result of no voltage being applied to the respective switches when using enhancement-mode MOSFETs.

[0069] As a result, when the predetermined conditions occur, the low-side inverter switch 15 closes and the high-side inverter switch 14 opens, causing the electric motor coil winding to be placed in a short-circuit configuration.

[0070] In a preferred embodiment, for improved operation of the low-side inverter switch, an additional passive device (not shown) can be used to regulate the voltage supplied to the low-side inverter switch.

[0071] Although this embodiment describes the short-circuit configuration as closing the low-side inverter switch 15 and opening the high-side inverter switch 14, this configuration can also be reversed, where a voltage from the reverse EMF is used to keep the low-side inverter switch 15 open and, if necessary, to keep the high-side inverter switch closed.

[0072] For example, if a fault in the electric motor and / or associated control system is identified as a result of one of the plurality of low-side inverter switches remaining open, then reverse EMF is used to keep the high-side switch closed, as described above, while the remaining low-side inverter switches remain open. Similarly, if this is reversed, the low-side inverter switches will be closed, and the remaining operating high-side switches will be allowed to open.

[0073] Figure 9a , 9b Figures 9c and 9d illustrate the voltage / current changes following a fault condition within an electric motor system, which causes a reverse EMF from the electric motor to form across the DC link capacitor, wherein... Figure 9a The voltage change in capacitor 13 of the DC link is shown. Figure 9b The changes in charging current across the first capacitor 81 and the second capacitor 82 are shown. Figure 9c The voltage change across the first capacitor 81 is shown. Figure 9dThe voltage change across the second capacitor 82 is shown.

[0074] about Figure 9a At zero seconds, as a result of a fault within the electric motor system, the voltage across DC link capacitor 13 begins to rise due to the back EMF from the electric motor. Once the DC link capacitor voltage exceeds a predetermined value, 450V in this embodiment, the first switch closes, causing a current pulse to charge the first capacitor 81 and the second capacitor 82. The charging time and current amplitude are determined by the lumped inductance and lumped resistance of the circuit.

[0075] like Figure 9c and 9d As shown, the first capacitor 81 is charged to approximately 400V, while the second capacitor 82 is charged to approximately 40V, resulting in a small voltage drop across the switch 80. In a preferred embodiment, the energy stored in the second capacitor 82 is used to drive a linear regulator or similar switch-mode power supply to generate a 15V turn-on voltage for the low-side inverter switch 15, thereby ensuring that the electric motor is placed in a short-circuit configuration even if the low-voltage power rail fails, thus protecting the inverter from overvoltage conditions.

[0076] Although the above embodiments describe a predetermined condition for placing the electric motor in a short-circuit configuration as a result of the DC link capacitor voltage exceeding a threshold, any predetermined condition associated with a fault in the electric motor system can be used. For example, an identified fault in the low power rail used to control inverter operation, a fault in one or more inverter switches, or identification of battery 10 being disconnected from the first power bus 11 and / or the second power bus 12.

Claims

1. A circuit for a multiphase bridge inverter, the multiphase bridge inverter having a plurality of high-side switches arranged to be connected to a first power bus and a plurality of low-side switches arranged to be connected to a second power bus, wherein the multiphase bridge inverter is arranged to control current in the coil windings of an electric motor, and wherein a DC link capacitor is connected between the first power bus and the second power bus, the circuit including a first impedance and a second impedance connected in series between the first power bus and the second power bus and in parallel with the DC link capacitor, and means connected between the first impedance and the second impedance, the means being configured to provide a voltage from the connection between the first impedance and the second impedance to the plurality of low-side switches upon the occurrence of a first predetermined condition, to place the plurality of low-side switches in a closed circuit configuration to allow the coil windings of the electric motor to be short-circuited, or to provide a voltage to the plurality of high-side switches upon the occurrence of the first predetermined condition, to place the plurality of high-side switches in a closed circuit configuration to allow the coil windings of the electric motor to be short-circuited; the circuit further including a controller and a first switch, wherein, The first switch is connected between the first power bus and the first impedance, and the controller is arranged to close the first switch under a second predetermined condition.

2. The circuit according to claim 1, wherein the first impedance is a first capacitor and the second impedance is a second capacitor.

3. The circuit according to claim 2, wherein the first capacitor is a high-voltage capacitor and the second capacitor is a low-voltage capacitor.

4. The circuit according to claim 1, wherein the first predetermined condition is the closing of the first switch.

5. The circuit according to claim 1, wherein the second predetermined condition is that the voltage difference between the first power bus and the second power bus increases to a predetermined value or higher.

6. The circuit of claim 1, wherein the second predetermined condition is that the voltage drop from the low-voltage power rail falls below a predetermined value.

7. The circuit of claim 1, wherein the first predetermined condition is that the voltage difference between the first power bus and the second power bus increases to above a predetermined value or the voltage drop from the low-voltage power rail decreases to below a predetermined value.

8. The circuit according to claim 2 further includes a high-voltage clamp connected in parallel with the first capacitor and a low-voltage clamp connected in parallel with the second capacitor, the high-voltage clamp being used to clamp the voltage across the first capacitor to a predetermined value, and the low-voltage clamp being used to clamp the voltage across the second capacitor to a predetermined value.

9. The circuit of claim 1, wherein the first switch is a depletion-type semiconductor switch.

10. The circuit of claim 1, further comprising means arranged to determine whether one of the plurality of low-side switches remains open, wherein, When determining whether one of the plurality of low-side switches remains open, the means connected between the first impedance and the second impedance are arranged to place the plurality of high-side switches in a closed-circuit configuration to allow the coil windings of the electric motor to be placed in a short-circuit configuration.

11. The circuit of claim 1, further comprising means arranged to determine whether one of the plurality of high-side switches remains open, wherein, When determining whether one of the plurality of high-side switches remains open, the means connected between the first impedance and the second impedance is arranged to place the plurality of low-side switches in a closed-circuit configuration to allow the coil windings of the electric motor to be placed in a short-circuit configuration.

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