Control system for a motor drive

Abstract

Control system for a motor drive configured to supply energy to an electromotor (1) from a battery, said motor drive (22) comprising a set of power switches and being configured to convert a continuous voltage (Vdc1) into an alternative voltage, and also to supply a first back generated direct voltage (Vdc2) to the battery when the motor works as a generator, the control system comprising: - a driver unit (5) for controlling the power switches with an active short circuit (ASC) function to put the phases of the motor in a short-circuit configuration and - an input of the driver unit (5) for engaging the ASC function, - an independent auxiliary inverter (71) giving a signal representative of the back electromotive force of the motor, wherein the signal given by the auxiliary inverter can be processed and linked to the input of the driver unit for activating the ASC function.

Description

DescriptionCONTROL SYSTEM FOR A MOTOR DRIVE

[0001] This disclosure concerns a control system for a motor drive.Technical Field

[0002] This disclosure pertains to the field of the vehicles which are motorised by an electromotor and for which energy is recovered during deceleration. It concerns all kinds of vehicles with an electromotor: two-wheelers or three-wheelers and also carsBackground Art

[0003] It is known to have an electrical motor, a battery supplying the motor and a motor drive between the battery and the motor. The motor drive works as a converter: it delivers the direct current coming from the battery alternatively to the phases of the motor and it rectifies a back electromotive force coming from the motor when the motor works as a generator so as it can load the battery. The motor drive comprises power switches (for example mosfet transistors) which are controlled by a driver unit. This driver unit includes in most cases a security operating mode which prevents damages due to a high voltage. If a high back electromotive force is detected, this security operating mode, also called active short circuit (ASC) mode is engaged by a control unit and the driver unit puts the switches so that the phases of the motor are short circuited.

[0004] However, if the control unit is out of operation for some reasons, it can no more send instructions to the power switch driver unit and the motor drive does not longer work properly. In this case, the back electromotive force generated by the motor, if it becomes too high, can damage components on the vehicle because the driver unit is no more controlled, and the ASC mode or function is not activated. This can for example happen when the vehicle is driving key-off, especially downhill.Summary

[0005] This disclosure improves the situation. It is a general object of this disclosure to provide an auxiliary protection circuit which also works when the driver unit of the power switches is out of operation.

[0006] It is proposed a control system for a motor drive, the motor drive is configured be connected on one hand to an electrical traction motor of a vehicle on the other hand, can be connected to a battery, said motor drive comprising a set of power switches, the control system comprising:- a driver unit for controlling the power switches of the motor drive with an active short circuitfunction to put at least one phase of the motor in a short-circuit configuration and- an input of the driver unit for engaging the active short circuit function.

[0007] According to this disclosure, the control system further comprises an independent auxiliary circuit giving a signal representative of the back electromotive force of the motor, and the signal given by the auxiliary circuit can be processed and linked to the input of the driver unit for activating the active short circuit function.

[0008] In this new control system, the implementation of an auxiliary circuit allows to measure the back electromotive force (EMF), or a signal equivalent to this back EMF, in an independent way so that the signal given by this auxiliary circuit can be used in all conditions and can control the driver unit also if a motor control unit which controls the pawer switch driver unit in a “normal” mode is no more operative. From another point of view, whether or not the battery is connected to the motor drive, the motor is protected, and this prevents the back EMF voltage from being too high. A back EMF (or equivalent) can also be measured when the battery is connected to the motor drive.

[0009] The following features, can be optionally implemented, separately or in combination one with the others:

[0010] - the auxiliary circuit is an auxiliary inverter with a low pass filter and in a preferred embodiment,

[0011] - the independent auxiliary inverter is a inverter diode bridge linked to at least one phase of the motor, and preferably all phases of the motor,

[0012] - the signal given by the auxiliary circuit is processed by a logic circuit for activating the active short circuit function and the logic circuit is configured so that the active short circuit function is engaged when the signal given by the auxiliary circuit exceeds a first threshold, then the auxiliary circuit measures back EMF voltage independently from DC link voltage,

[0013] - the logic circuit is configured so that the active short circuit function is disengaged when the signal given by the auxiliary circuit goes below a second threshold which is lower than the first threshold,

[0014] - the logic circuit comprises a comparator with hysteresis,

[0015] - the motor drive is connected to a battery, control system characterized in that said motor drive is able to supply a first back generated direct voltage to the battery when the motor works as a generator, and in that the active short circuit function is disengaged when first back generated direct voltage goes below a third threshold,

[0016] - the first back generated direct voltage and the signal given by the auxiliary circuit are electronically linked by a circuit, specially by a diode.

[0017] - the control system also includes switch means controlled by a motor control unit to inhibit the action of the independent auxiliary circuit, i.e. to inhibit engaging the active short circuit function; or the control system also includes switch means controlled by a motor control unit to inhibit the processed signal of the auxiliary circuit;

[0018] - the independent auxiliary circuit and the logic circuit are hardware implemented,

[0019] - the independent auxiliary circuit also comprises a discharge resistor;

[0020] In another aspect, it is proposed a propulsion system for a vehicle comprising an electrical motor and a battery, characterised in that it further comprises a control system as described here above.

[0021] In another aspect, it is proposed a vehicle characterised in that it comprises a propulsion system as described here above.Brief Description of Drawings

[0022] Other features, details and advantages will be shown in the following detailed description and on the figures, on which:Fig. 1

[0023] [Fig. 1] is a schematic view of an electric motor and a control system of this motor according to an embodiment.Fig. 2

[0024] [Fig. 2] is a first diagram showing variations of signals over time for a first working mode.Fig. 3

[0025] [Fig. 3] is a second diagram showing variation of a signal over time for a second working mode.Fig. 4

[0026] [Fig. 4] is schematic view of an auxiliary inverter according to an embodiment.Fig. 5

[0027] [Fig. 5] is schematic view of an advanced logic circuit according to an embodiment.Description of embodiments

[0028] It is now referred to figure 1 . This figure first shows an electromotor 1 having three electric phases U, V, and W. This electromotor 1 is controlled by a motor drive comprising a power module 2, and a motor control unit 3. A battery 4 with an integrated battery management module supplies energy to the electromotor 1 through the motor drive.

[0029] The power module 2 includes a set of switches forming an inverter 21 which on one side can supply alternatively each phase of the electromotor 1 with current coming from the battery 4 when the electromotor 1 furnishes a work and on another side can rectify an alternative current generated by the electromotor 1 into a direct current which can load the battery 4 when the electromotor 1 works as a generator. The set of switches is driven by a power switch driver unit 5 which is controlled by the motor control unit 3. In the embodiment shown on figure 1 , the switches are MOSFET switches, but other switches could work.

[0030] The battery 4 is connected to the power module 2 and a battery relay SW1 managed by the battery management system of the battery 4 is foreseen between the battery 4 and the power module 2. A DC-link capacitor C1 is mounted in parallel to the inverter 21 (and to the battery 4). The battery terminal voltage is Vdc1 and the capacitor (C1) terminal DC- link voltage is Vdc2.

[0031] This system is a known system that works well. Sensors give information to the motor control unit 3 and to the battery 4 and the switches are driven automatically both when the motor is operating as a motor and when it is operating as a generator.

[0032] In order to protect the power module 2, the motor control unit 3 measures namely the DC-link voltage Vdc2. If this voltage exceeds a predefined high threshold, an active short circuit (ASC) function (or mode) is then activated by the motor control unit 3 so that the phases (preferably all phases) of the motor are short circuited. If this DC-link voltage goes below a predefined low threshold, the motor control unit 3 disengages the active short circuit (ASC) function. The system can oscillate between these both states: ASC engaged / disengaged.

[0033] For this configuration (with only motor control unit 3 and no other auxiliary circuit as explained hereafter) the ASC function can only be engaged (and disengaged) when the battery relay SW1 is open, i.e. the battery 4 is disconnected. Otherwise, the DC-link voltage Vdc2 stays equal to the battery voltage. In this last case (battery relay closed), if a back electromotive force (EMF) is generated, and if this back EMF becomes high, it will cause a current flow from the electromotor 1 to the battery 4 without significant DC-link voltage Vdc2 increase. This current flows first through the inverter 21 and can damage the power switches and it can also damage the battery 4 due to uncontrolled battery charging. This can lead in turn even in fire.

[0034] When the battery relay SW1 is open (battery 4 disconnected), if the ASC function cannot be activated, for example because the motor control unit 3 is not powered (the vehicle is key off), the DC-link voltage Vdc2 will follow the back electromotive force (back EMF) or voltage and can exceed the voltage rating of components linked to DC-link Vdc2. The consequences of such a situation can be a component destruction, for example of the power switches of the inverter 21 and / or damages on auxiliaries linked to the system.

[0035] For both cases i.e., battery relay SW1 open or closed, figure 1 proposes an auxiliary protection circuit 6 which allows an activation of the ASC function also when the motor control unit 3 is no longer powered.

[0036] The proposed auxiliary protection circuit 6 comprises an auxiliary inverter 7 and a triggering system 8.

[0037] The auxiliary inverter 7 is configured to measure the back electromotive force directly. In the proposed embodiment, it comprises an auxiliary inverter 71 and an RC filter 72.

[0038] The auxiliary inverter 71 proposed here is a diode bridge with three pairs of diodes, one pair of diodes for each phase of electromotor 1. In a simplified embodiment, the inverter could have only one (or two) pair of diodes. RC filter 72 is parallel to the auxiliary inverter 71 and forms with it a peak value inverter which gives a signal VBEMF representative of the back electromotive force generated by the electromotor 1 . The auxiliary inverter 7 also includes a discharge resistor R3 (figure 1).

[0039] This proposed auxiliary inverter 7 is completely passive. No supply is necessary: the signal VBEMF is furnished in all conditions. This signal supplies triggering system 8.

[0040] Figure 1 together with figure 4 give an example of a triggering system 8. This system comprises a comparator 81 as a central element which presents a hysteresis. The signal BEMF is divided with help of two resistors and gives a new signal VASC_TRIG which is also representative of the back electromotive force of electromotor 1. This new signal is compared inside comparator 81 to a reference signal V12 given by an independent voltage generator (not shown). Comparator 81 gives an ASC signal (0 or 1) which is intended to engage or disengage the active short circuit function in the power switch driver unit 5. Therefore, the ASC signal is linked to an input of this driver unit 5.

[0041] This proposed auxiliary system works both when the battery 4 is connected (battery relay SW1 closed) and when it is disconnected (battery relay SW1 open).

[0042] An improved embodiment foresees an inhibit function controlled by the motor control unit 3 and which renders the auxiliary protection circuit 6 inoperative. In predefined conditions, namely when the motor control unit 3 is operative (key on), this unit can deliveran INHIBIT signal. In this case, the motor control unit 3 controls directly the power switches driver unit 5 (line COMM on figure 1). Figure 4 shows an example of an electrical assembly intended to render the auxiliary protection circuit 6 inoperative. When an “active” INHIBIT signal is sent (figure 4) the signal VASC_TRIG is forced so that the output signal of the comparator (ASC signal) stays at 0.

[0043] Figure 5 proposes a further improvement of the system described here above. Figure 5 gives an example of the implementation of following idea: when the auxiliary protection circuit 6 works, the ASC function is engaged based on the signal delivered by the auxiliary inverter 7 (VBEMF or VASC_TRIG), but the ASC function is disengaged based on the DC-link voltage (Vdc2). The assembly shown on figure 5 is only a proposed solution to achieve this idea but a man with ordinary skill in the art knows other ways to implement this working mode. The assembly of figure 5 can be easily hardware implemented and is therefore an advantageous embodiment among another.

[0044] It is clear for a man with ordinary skill in the art that the electronic assembly on figure 5 engages the ASC function when VBEMF exceeds a higher threshold and disengages the ASC function based when VDC (=Vdc2) goes below a lower threshold.

[0045] Figure 2 shows the working of the triggering system 8 when only BEMF is considered to engage and to disengage the ASC function and figure 3 shows the working of the triggering system 8 when the ASC function is engaged and disengaged based on VBEMF and on Vdc2 (figure 5).

[0046] On figure 2 (corresponding to figure 1), Vdc1 is the terminal voltage of battery 4. Two thresholds VTh1 and VTh2 are defined so that Vdc1 > VTh1 > VTh2. The higher threshold VTh1 needs to be below the minimum possible battery voltage. When the electromotor 1 works as a generator, VBEMF increases and can reach VTh1 (depends on the speed of the motor). In this case, the ASC function is engaged and automatically VBEMF will decrease until VTh2. The ASC function will then be disengaged. If electromotor 1 further works as a generator, VBEMF will again increase if the speed of the motor is still high enough. In this example, the ASC function will be engaged and disengaged as long as electromotor 1 works as a generator with a high speed (enough to reach VTh1).

[0047] On figure 3 (together with figure 5), in the same conditions, for example a vehicle going downhill, there are less oscillations between the engagements and disengagements of the ASC function. For an increasing back electromotive force, VBEMF is measured directly by the auxiliary inverter 7. When the upper threshold (for example the same than on figure 2, i.e. VTh1 is crossed from below, the ASC function is activated. As a consequence, the back electromotive force begins to fall (and VBEMF too). For decreasing VBEMF, the DC-linkvoltage Vdc2 is measured. When the lower threshold (VTh3 which can be equal or not to VTh2) is crossed from above, the ASC function is disengaged. The back electromotive force begins then to rise. There is here also a cycling operation between the higher threshold and the lower threshold.

[0048] If the battery relay SW1 stays closed (and the battery 4 connected), ASC function stays engaged until the system (motor control unit 3) sets an INHIBIT signal, for example at next vehicle start.Industrial Applicability

[0049] The technical solutions presented here can be used to protect a motor drive of an electromotor also in a situation where the motor is driven and an inverter of the motor drive is not controlled in a defined way, for example when the corresponding vehicle is driving key off. The proposed solution works both, when the battery is connected and when the battery is disconnected. So, the protection can be active in all situations.

[0050] The present disclosure proposes an auxiliary inverter which measures a back electromotive force in all situations with passive components so that an active short circuit function can be engaged although the control system is not powered.

[0051] In an advanced implementation, an inhibit signal can be delivered by the control system when it is active so that the motor drive is controlled by the control system in a normal way and the auxiliary protection system is not inhibited in a limp mode (for example motor driven key off).

[0052] Another advanced implementation, independent from the preceding one, concerns the triggering of the active short circuit function. In a preferred embodiment, the control system presents an asymmetric way to engage and disengage the ASC function (or mode). Advantageously, the ASC function engages when the back electromotive force, i.e. the voltage measured by the auxiliary inverter exceeds a higher threshold and disengages when a DC link voltage goes below a lower threshold, wherein the DC link voltage is a voltage measured at the terminals of a capacitor which is parallel to the “main” inverter, i.e. the inverter of the electromotor.

[0053] This disclosure is not limited to the embodiments described above and the alternatives described here, which are only examples. The invention encompasses every alternative that a person skilled in the art would envisage when reading this text.

Claims

Claims

1. Control system for a motor drive (2), the motor drive (2) is configured be connected on one hand to an electrical traction motor (1) of a vehicle on the other hand, can be connected to a battery, said motor drive (2) comprising a set of power switches, the control system comprising:- a driver unit (5) for controlling the power switches of the motor drive (2) with an active short circuit function to put at least one phase of the motor in a short-circuit configuration and- an input of the driver unit (5) for engaging the active short circuit function, characterised in that the control system further comprises an independent auxiliary circuit (71 , 72) giving a signal representative of the back electromotive force of the motor (1), and in that the signal (VBEMF) given by the auxiliary circuit (71 , 72) can be processed and linked to the input of the driver unit (5) for activating the active short circuit function.

2. Control system according to claim 1 , characterised in that the auxiliary circuit is an auxiliary inverter (71) with a low pass filter (72).

3. Control system according to claim 1 to 2, characterised in that the independent auxiliary inverter is an inverter diode bridge linked to at least one phase of the motor, and preferably all phases of the motor.

4. Control system according to claim 1 to 3, characterised in that the signal given by the auxiliary circuit is processed by a logic circuit for activating the active short circuit function and in that the logic circuit is configured so that the active short circuit function is engaged when the signal (VBEMF) given by the auxiliary circuit (71 ,72) exceeds a first threshold (Vth1).

5. Control system according to claim 1 to 4, characterised in that the logic circuit is configured so that the active short circuit function is disengaged when the signal ( BEMF) given by the auxiliary circuit (71 ,72) goes below a second threshold (Vth2) which is lower than the first threshold (Vth1).

6. Control system according to one of claims 1 to 5, characterised in that said logic circuit comprises a comparator with hysteresis.

7. Control system according to one of claims 1 to 6, where the motor drive (2) is connected to a battery (4), control system characterized in that said motor drive (2) is able to supply a first back generated direct voltage (Vdc2) to the battery (4) when the motor works as a generator, and in that the active short circuit function is disengaged when first back generated direct voltage (Vdc2) goes below a third threshold (Vth3).

8. Control system according to claim 7, wherein the first back generated direct voltage (Vdc2) and the signal (VBEMF) given by the auxiliary circuit (71 , 72) are electronically linked by a circuit, specially by a diode.

9. Control system according to one of claims 1 to 7, characterised in that the control system also includes switch means controlled by a motor control unit to inhibit the action of the independent auxiliary circuit (71 , 72).

10. Propulsion system for a vehicle comprising an electrical motor and a battery, characterised in that it further comprises a control system according to one of claims 1 to 9.

11. Vehicle characterised in that it comprises a propulsion system according to claim 10.

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