WO2016041601A1

WO2016041601A1 - An electric machine assembly

Info

Publication number: WO2016041601A1
Application number: PCT/EP2014/069952
Authority: WO
Inventors: Lars Thomasson; Rickard Persson
Original assignee: Höganäs Ab
Priority date: 2014-09-19
Filing date: 2014-09-19
Publication date: 2016-03-24

Abstract

The present invention relates to an electric machine assembly comprising an electric DC or AC machine having a plurality of power terminals for supply of a DC or AC generator voltage and receipt of a DC or AC input drive voltage. A motor control unit is operatively coupled to the electric machine and comprises a bi-directional AC/DC power converter which is configured to operate in a first conversion mode for conversion of the DC or AC generator voltage into a rectified DC voltage at first and second battery connections of the motor control unit and a second conversion mode for conversion of a DC battery voltage applied at the first and second battery connections into the DC or AC input drive voltage. A controller of the motor control unit is coupled to the bi-directional AC/DC power converter for selection of one of the first and second conversion modes. The motor control unit comprises an overvoltage protection circuit operatively coupled between the first and second battery connections. The overvoltage protection circuit comprises a first controllable semiconductor device configured to selectively electrically connect or disconnect the first and second battery connections in accordance with a first trigger signal applied to a control terminal of the controllable semiconductor device. A trigger voltage generator coupled to the first and second battery connections is configured to generate the first trigger signal in response to the rectified DC voltage exceeding a predetermined trigger voltage level.

Description

AN ELECTRIC MACHINE ASSEMBLY

The present invention relates to an electric machine assembly comprising an electric DC or AC machine having a plurality of power terminals for supply of a DC or AC generator voltage and receipt of a DC or AC input drive voltage. A motor control unit is operatively coupled to the electric machine and comprises a bi-directional AC/DC power converter which is configured to operate in a first conversion mode for conversion of the DC or AC generator voltage into a rectified DC voltage at first and second battery connections of the motor control unit and a second conversion mode for conversion of a DC battery voltage, applied at the first and second battery connections, into the DC or AC input drive voltage. A controller of the motor control unit is coupled to the bi-directional AC/DC power converter for selection of one of the first and second conversion modes. The motor control unit comprises an overvolt- age protection circuit operatively coupled between the first and second battery con- nections. The overvoltage protection circuit comprises a first controllable semiconductor device configured to selectively electrically connect or disconnect the first and second battery connections in accordance with a first trigger signal applied to a control terminal of the controllable semiconductor device. A trigger voltage generator coupled to the first and second battery connections is configured to generate the first trigger signal in response to the rectified DC voltage exceeding a predetermined trigger voltage level.

BACKGROUND OF THE INVENTION

Electric machine assemblies, or electric motor and generator assemblies, for driving electric vehicles like cars and bicycles are in wide-spread use today not least due to the increasing popularity of electric bicycles. The electric machine assembly is normally powered by a battery pack for example comprising a plurality of rechargeable Li-Ion battery cells with a certain energy storage capacity. To drive the electric vehicle, a DC voltage supplied by the battery pack is typically converted into an appro- priate single or multi-phase AC drive voltage applied to the electric machine, operating in a motor mode, by a DC/AC power converter or inverter housed in a general motor controller unit (MCU). However, the electric machine has in addition to the motor operation an inherent ability to function as an electric AC generator, i.e. it pro- duces electrical power, if the electric machine is driven by an external force. The external force may be supplied by a bicyclist pedalling the electric bike. Furthermore, it is generally desirable to allow the power generated by the electric machine in its generator mode to charge the rechargeable battery cells for various reasons such as power conservation. This type of operation is often designated "regenerative" and may include so-called regenerative braking where the moving electric vehicle's momentum (kinetic energy) is converted into electricity that recharges (regenerates) the rechargeable battery cells as the vehicle is slowing down and/or stopping.

Despite the advantages of operating the electric machine assembly in the regenerative mode, it entails certain yet unresolved technical challenges. One of these is related to the potential generation of harmful overvoltage conditions within the MCU. When the rectified DC voltage generated by the bi-directional DC/AC power con- verter exceeds an instantaneous DC battery voltage supplied by the battery pack, the MCU will supply a charging current to the rechargeable battery cells to charge these. The regenerative mode may be selected by the MCU in response to a brake lever or pedal of the electric vehicle being pulled or activated. If the rechargeable battery cells are already fully charged and cannot admit any further charge, a battery management system of the battery pack will typically disconnect the rechargeable battery cells to prevent damage, destruction or overheating of the battery cells. However, without the battery cells acting as an energy reservoir absorbing the charging current supplied from the rectified DC voltage, MCU circuitry now becomes more vulnerable to the generated electrical power supplied by the electric machine operating in the motor mode. In particular, certain passive components of the MCU such as electrolytic smoothing capacitor(s) of the bi-directional DC/AC power converter will be continuously charged by the current generated by the electric machine in its generator mode leading to a constantly rising level of the rectified DC voltage. Hence, if the regenerative operation mode persists for a prolonged time period while the battery pack is disconnected from the electric machine assembly, e.g. because the battery cells already are fully charged as discussed above, the rectified DC voltage may rise to harmful voltage levels where the passive or active components of the MCU are destroyed or impaired by overvoltage. In addition, the overvoltage situation may lead to a violation of applicable electrical safety regulations. For electric bicycle applications of electric machine assemblies, the regenerative mode may last for a prolonged time period for example if the bicyclist pedals down a long steep slope or hill or loses control over the bicycle on the steep slope. This problem is solved by the present invention by adding an overvoltage protection circuit to the MCU that is configured to temporarily short-circuit the rectified DC voltage through a first controllable semiconductor device if a dangerously high rectified DC voltage level is detected, i.e. if the rectified DC voltage exceeds a predetermined trigger or threshold voltage level. The use of a controllable semiconductor device allows flexible control of operational parameters of the overvoltage protection circuit such as threshold voltage level, power loss, resistance of the short-circuit connection, duration of the short-circuit connection etc. The use of a controllable semiconductor device to establish the short-circuiting of the rectified DC voltage also facilitate fully autonomous or automatic operation of the overvoltage protection circuit requiring no user intervention to re-establish normal operation of the MCU after the overvoltage protection circuit has been activated or triggered.

The short circuit connection established by the first controllable semiconductor device serves as an electric brake to the electric machine, which will contribute to bring down the speed and the short circuit will naturally remove the overvoltage situation in a substantially instantaneous manner. This is of course provided that the on-state or conducting state resistance of the first controllable semiconductor device is sufficiently small and that the current handling capability of the first controllable semiconductor device is sufficiently large to absorb extremely large transient current peaks as explained in detail below with reference to the various embodiments of the present invention. While the short-circuiting of the rectified DC voltage may appear as a rather drastic measure, the internal electrical impedance of the electric machine will tend to limit at least a quasi-steady state level of the short circuit current to a level comparable to normal motor mode operation.

SUMMARY OF THE INVENTION

A first aspect of the invention relates to an electric machine assembly comprising an electric DC or AC machine having a plurality of power terminals for supply of a DC or AC generator voltage and receipt of a DC or AC input drive voltage or motor volt- age. A motor control unit is operatively coupled to the electric machine, through the power terminals, and comprises a bi-directional power converter, such as a bidirectional AC/DC converter, which is configured to operate in a first conversion mode for conversion of the DC or AC generator voltage into a rectified DC voltage at first and second battery connections of the motor control unit and a second conversion mode for conversion of a DC battery voltage applied at the first and second battery connections into the DC or AC input drive voltage. A controller of the motor control unit (MCU) is coupled to the bi-directional AC/DC power converter for selection of one of the first and second conversion modes. The motor control unit com- prises an overvoltage protection circuit operatively coupled between the first and second battery connections. The overvoltage protection circuit comprises a first controllable semiconductor device configured to selectively electrically connect or disconnect the first and second battery connections in accordance with a first trigger signal applied to a control terminal of the controllable semiconductor device. A trig- ger voltage generator, coupled to the first and second battery connections, is configured to generate the first trigger signal in response to the rectified DC voltage exceeding a predetermined trigger voltage level.

The electric machine preferably comprises a multi-phase AC machine, but may in the alternative comprise a DC machine. The electric machine assembly will normally predominantly operate in the second conversion mode, i.e. electric motor operation, where power from the DC battery voltage is converted by the bi-directional AC/DC power converter into an appropriate DC or AC input drive voltage, or motor voltage, for the electric machine to drive the electric vehicle. However, when the electric ma- chine assembly operates in the first conversion mode, i.e. electric generator operation, the electric machine behaves as a generator with the bi-directional AC/DC power converter converting the generated DC or AC generator voltage into the rectified DC voltage at the first and second battery connections. Hence, when a rechargeable battery pack is coupled to the first and second battery connections a charging current is supplied to the rechargeable battery. The electric machine assembly is thus operating in the previously-discussed regenerative mode or state where for example braking energy of the electric vehicle is converted to into charging current for the battery pack. Under normal regenerative operating conditions of the electric machine assembly, the regenerative operation mode or state is unprob- lematic and the supply of charging current to the battery pack controlled in an appropriate fashion by the controller or processor of the MCU. However, certain anomalous operating events beyond control of the MCU controller may occur during the regenerative operation mode of the electric machine assembly, such as the pre- viously discussed disconnection of the battery pack either by the battery management system or possibly accidentally. These anomalous operating events may cause the rectified DC voltage to rise to harmful voltage levels where passive or active components of the MCU are destroyed or impaired for the reasons discussed above unless precautionary measures are taken. The overvoltage protection circuit protects the MCU circuitry for such harmful voltage levels by establishing the electrical connection, or short circuit, between the first and second battery connections reducing the rectified DC voltage to a very small level - for example below 1 volt or another acceptable voltage level within safe operating limits of the MCU circuitry. The first and second battery connections may comprise respective externally accessible electrical contacts that may protrude from a housing structure of the electric machine assembly such as a coupling plug or coupling surface. Hence, the electric machine assembly may be electrically connected to a battery pack by mechanical engagement between the first and second externally accessible battery connections and a mating pair of externally accessible battery terminals of the battery pack.

The controllable semiconductor device allows flexible control of operational parameters of the overvoltage protection circuit such as threshold voltage level, power loss, resistance of the electrical connection, or short circuit, between the first and second battery connections or short-circuit connection, duration of the short-circuit connection etc. The use of the controllable semiconductor device for establishing the short- circuiting operation also facilitate fully autonomous or automatic operation of the overvoltage protection circuit requiring no user intervention to re-establish normal operation of the MCU after triggering or activating the overvoltage protection circuit. In a preferred embodiment of the invention the first controllable semiconductor device comprises a thyristor, such as a Silicon Controlled Rectifier (SCR). The thyristor comprising an anode and a cathode connected to the first and second battery connections, respectively, to electrically connect these in a forward conducting mode of the thyristor and disconnect these in a forward blocking mode of the thyristor. A gate terminal of the thyristor is coupled to the trigger voltage generator for receipt of the first trigger signal. The thyristor is a particularly well-suited semiconductor device for providing the desired electrical connection between the first and second battery connections because thyristors offer an extremely large current conduction capabil- ity and low forward voltage. Hence, the thyristor is capable of effectively short- circuiting the first and second battery connections and reduce the rectified DC voltage to a non-harmful level, thereby removing the ensuing, or already encountered, overvoltage condition by rapidly pulling the rectified DC voltage down to the non- harmful voltage level. The extremely large maximum current capability of the thyris- tor is advantageous because the bi-directional AC/DC power converter typically comprises one or more electrolytic smoothing or filtering capacitor(s) charged to the rectified DC voltage. These electrolytic smoothing capacitor(s) are instantly discharged through the first semiconductor device when it starts to conduct and this leads to an extremely large initial discharge current transients or peak due to a low internal impedance of such capacitors. As evidenced by the experimental measurement results discussed in detail below in connection with the appended drawings, the discharge current transient may reach a peak level of 800 - 1000 A for a typical electric bicycle adapted embodiment of the present electric machine assembly. Another advantage of the thyristor device is the automatic return to its off-state/non- conducting state once the current through the device drops below the so-called holding current. Hence, once the cause of the harmful overvoltage situation is removed or eliminated, the short-circuit current through the thyristor drops and the thyristor automatically returns to the off-state. In the off-state of the thyristor, the rectified DC voltage returns to a normal or target level and normal operation of the electric machine assembly is re-established including normal operation of the controller. The controller may have been left without appropriate power supply voltage during the short-circuiting of the rectified DC voltage and hence non-functional.

A preferred embodiment of the present overvoltage protection circuit comprises a second controllable semiconductor device configured to selectively electrically connect or disconnect the first and second battery connections in accordance with a second trigger signal applied to a control terminal of the second controllable semiconductor device by a control circuit. A time delay circuit is furthermore coupled to the first trigger signal for delaying the second trigger signal relative to the first trigger signal with a predetermined time period such as a time period between 0.1 ms and 1 0 ms. The delay of the second trigger signal has the effect that the second controllable semiconductor device starts to conduct at a delay of the predetermined time period relative to the first controllable semiconductor device thereby directing the previously discussed transient short-circuit current, now at a reduced level, mainly through the latter device. This split of short-circuit current over time has several advantages, in particular when the first controllable semiconductor device comprises a thyristor device. In that embodiment, the second controllable semiconductor device preferably comprises a transistor such as a bipolar transistor, MOSFET or IGBT transistor. The two latter transistor devices may exhibit a very small on-resistance or voltage drop, respectively, despite being unable to withstand a current of the magnitude of the above-discussed initial short-circuit current peak. Hence, in the latter embodiment, the MOSFET transistor comprises a drain and a source, or the IGBT transistor a collector and emitter, connected to the first and second battery connec- tions to electrically connect these through the transistor in an on-state and disconnect these in an off-state. A control terminal of the transistor, e.g. gate terminal of the MOSFET or IGBT transistor, is coupled to the control circuit for receipt of the second trigger signal. The characteristics of the transistor may be selected such that a voltage drop across the transistor in its on-state is markedly smaller than a mini- mum voltage drop across the PN-junction of the thyristor. This may for example be accomplished if the on-resistance of the MOSFET or IGBT transistor is less than 1 00 ΓΠΩ, more preferably less than 1 0 ΓΠΩ as explained in additional detail in connection with the appended drawings. Therefore, the short circuit current drawn from the rectified DC voltage may be completely redirected from the thyristor to the tran- sistor after the predetermined delay time period where the transistor starts to conduct. This redirection of the short circuit current causes the current through thyristor to drop below a so-called holding current of the thyristor automatically switching the latter device back to its non-conducting state. Thereafter, the transistor is left alone to maintain the short-circuiting of the rectified DC voltage. This is advantageous be- cause power losses in the transistor, in particular MOSFET or IGBT transistors, are smaller than the power loss in the thyristor as explained below in further detail. Furthermore, the initial conducting state of the thyristor only will pull the rectified DC voltage down to a small level such as about 1 volt before the transistor starts to conduct such that the latter device is switched on at nearly zero drain-source or collector-emitter voltage thereby minimizing power losses.

The present overvoltage protection circuit is preferably adapted to automatically remove the short circuit connection between the first and second battery connections through the transistor after a preset time period. According to this embodiment, the control circuit of the transistor comprises a discharge circuit, coupled to the control terminal of the transistor, such as the gate of the MOSFET, configured to discharge the control terminal in accordance with a first time constant. In this manner, the transistor automatically switches from the on-state to the off-state after a preset on time period. The time constant is preferably selected such that the preset on time period lies between 1 and 20 seconds such about 5 -10 seconds. The first time constant of the discharge circuit may for example be determined by a combination of resistances and capacitances of resistors and capacitors, respectively. Once, the transistor has been switched to the off-state so as to remove the short-circuiting of the rectified DC voltage (because the thyristor was already earlier switched off), the rectified DC voltage rapidly increases reverting to the normal DC voltage level.

However, if the previously discussed anomalous operating condition, which triggered the overvoltage protection circuit in the first place, persists, the rectified DC voltage level will rapidly increase to a level exceeding the predetermined trigger voltage level once again. Consequently, leading to subsequent cycles, each lasting for the above-mentioned preset on time period, of activation and subsequent deactivation of the overvoltage protection circuit until the abnormal operating condition is eliminated.

The control circuit of the transistor is preferably autonomously powered by a DC power supply operating independently of the rectified DC voltage when the first and second battery connections are connected or short-circuited via the thyristor and/or the transistor. This may be accomplished by a capacitor and diode based DC power supply circuit arrangement as explained in detail below with reference to the appended drawings.

The skilled person will appreciate that the predetermined trigger voltage level of the trigger voltage generator may be defined in numerous ways, for example by using a traditional comparator arrangement with a pair of inputs connected to the rectified DC voltage and a DC reference voltage setting the desired trigger voltage level, respectively. The comparator output can be used to provide the first trigger signal applied to the control terminal of the controllable semiconductor device. According to a preferred embodiment of the invention, the trigger voltage generator comprises one or more cascaded Diode(s) for alternating current, i.e. so-called DIAC(s) configured for setting the predetermined trigger voltage level. The DIACs may be coupled across the rectified DC voltage via a resistor network and a connection node between the DIACs and the resistor network supplying the first trigger signal.

A preferred embodiment of the discharge circuit comprises a semiconductor switch configured to accelerate the discharge of the control terminal of the transistor, e.g. MOSFET or IGBT, in response to the rectified DC voltage reaches a preset threshold voltage. The semiconductor switch preferably comprises a bipolar transistor or a FET transistor, e.g. a MOSFET, with a base or gate, respectively, connected to the first battery connection. The preset threshold voltage comprises a base-emitter or gate-source voltage drop of the bipolar transistor or FET transistor, respectively. This forward voltage drop is accordingly about 0.6 - 0.7 volt for a bipolar transistor and about one threshold voltage for a MOSFET. The connection of the gate or base terminal to the first battery connection ensures the gate or base terminal senses and responds to an increase of the rectified DC voltage level. Rapidly discharging the gate terminal of the MOSFET or IGBT transistor when the latter is switched from its on-state to its off-state, minimizes the time period during which the MOSFET or IGBT operates in the so-called linear operation region. This reduces in turn power losses in the MOSFET or IGBT transistor as explained in additional detail below with reference to the appended drawings. The discharge acceleration is caused by a state switching of the semiconductor switch of the discharge circuit, from an off-state to an on-state, such that the gate of the MOSFET or IGBT transistor is actively discharged through the bipolar or FET transistor rather than passively discharged by resistors.

A second aspect of the invention relates to an electric machine system for electric bicycles comprising an electric machine assembly according to any of the above described embodiments thereof and a battery pack comprising first and second ex- ternally accessible battery terminals electrically connected to the first and second battery connections, respectively, of the electric machine assembly. The battery pack comprising a plurality of rechargeable battery cells coupled to the first and second externally accessible battery terminals and a battery management system for controlling charging and discharging of the rechargeable battery cells. The battery management system may be configured to disconnect the plurality of rechargeable battery cells from at least one of the first and second externally accessible battery terminals in response to a predetermined charging condition of the plurality of rechargeable battery cells. The rechargeable battery cells may comprise various popular battery technologies such as Nickel Metal Hydride (NiMH), Nickel Cadmium (NiCD), Lithium, Lithium-Ion (Li-Ion), Lithium Phosphate etc. The ability of the battery management system to disconnect the plurality of rechargeable battery cells from the externally accessible battery terminal(s) for protection purposes may lead to the previously discussed abnormal operating conditions of the electric machine assembly with the accompanying and potentially destructive overvoltage conditions. These destructive overvoltage conditions are prevented by the presence of the overvoltage protection circuit in the electric machine assembly as described in detail above. A third aspect of the invention relates to an electric drive system for light electric vehicles, comprising an electric machine assembly according to any of the above described embodiments thereof and a user interface unit connected to the controller of the assembly through a wired or wireless data communication link for setting a motor assistance level of the electric machine in accordance with a user entry. The skilled person will understand that the user interface unit preferably comprises a display for displaying operational parameters of the electric drive system and associated battery for example vehicle speed, battery charging level etc. The user interface unit may comprise one or more user operable control buttons for setting operational parameters of light electric vehicle such as motor assistance level. The light electric vehicle may comprise bicycle, moped, golf cart, scooter, lawn mower etc.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will be described in more detail in connection with the appended drawings, in which: FIG. 1 is an electrical circuit diagram of an electric machine system for electric bicycles comprising an electric machine assembly in accordance with a first embodiment of the invention,

FIG. 2 is an electrical circuit diagram of an overvoltage protection circuit in accord- ance with a preferred embodiment thereof integrated with the electric machine assembly; and

FIGS. 3 and 4 show respective graphs of experimentally measured voltage and current waveforms of the prototype overvoltage protection circuit coupled to a 3-phase electric machine during generator mode operation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 is an electrical circuit diagram of an electric machine system 100 for electric bicycles comprising an electric machine assembly in accordance with a first embodiment of the invention. The electric machine assembly comprises a 3-phase electric AC machine 102 and a motor control unit 101 electrically coupled to the 3-phase electric AC machine 102 via three power terminals of the AC machine 102. The 3- phase electric AC machine 102 is capable of operating in either an AC generator mode or an AC motor mode. In the AC motor mode, a 3-phase AC input drive voltage, or motor voltage, is supplied to the AC machine from a bi-directional AC/DC power converter 103 to rotate or drive the motor at a desired speed. In the AC generator mode, the 3-phase electric machine 102 is driven by an external force and supplies in response a 3-phase AC drive voltage to the bi-directional AC/DC power converter 103. A processor 105, such as a programmable microprocessor, controls conversion modes of the bi-directional AC/DC power converter 103 and may be set to a first conversion mode where the generated 3-phase AC generator voltage is converted into a rectified DC voltage V_DC across a smoothing or filtering capacitor C4. The processor 105 may comprise a custom or industry standard type of microprocessor or DSP. The rectified DC voltage V_DC is supplied at the first and second battery connections 1 15, 1 16 for coupling of the rectified DC voltage to a mating pair of externally accessible battery terminals of a battery pack 109 of the electric machine system 100. The first and second battery connections 1 15, 1 16 may comprise respective externally accessible electrical contacts that may protrude from a housing structure of the electric machine assembly such as a coupling plug or coupling sur- face for mechanical engagement to the mating pair of externally accessible battery terminals of the battery pack 109.

The battery pack 109 comprises a plurality of rechargeable battery cells 1 13a, 1 13b, 1 13c coupled to the pair of externally accessible battery terminals. The battery pack 109 furthermore comprises a battery management system (BMS) for controlling charging and discharging of the rechargeable battery cells. The battery management system is inter alia configured to disconnect the plurality of rechargeable battery cells 1 13a, 1 13b, 1 13c from the first and/or second externally accessible battery terminal(s) for protection purposes as described in additional detail below.

When the 3-phase electric AC machine 102 is operating in the AC generator mode and gives rise to a rectified DC voltage level that exceeds the instantaneous DC voltage of the battery pack 109, a battery charging current is forced into the re- chargeable battery cells 1 13a, 1 13b, 1 13c and the electric machine system 100 is operating in the previously-discussed regenerative mode or state. On the other hand, if the rectified DC voltage level is smaller than the instantaneous DC voltage supplied by the battery pack 109, the processor 105 detects this condition and selects the normal motor mode of operation of the bicycle and controls the power flow direction through the bi-directional AC/DC power converter 103 accordingly. The bidirectional AC/DC power converter 103 generates the 3-phase AC input drive voltage under processor control which is applied to the 3-phase electric AC machine 102 through three suitable power terminals on the AC machine 102. In this manner, a 3-phase drive current is derived from the DC current supplied by battery pack 109 and flows into the 3-phase electric AC machine 102 and discharging the rechargeable batteries in the process. The amount of drive current to the 3-phase electric AC machine 102 and hence motor power is naturally controlled by the processor 105 for example in accordance with a pedal speed or bicyclist controlled lever or handle setting.

The skilled person will appreciate that the bi-directional AC/DC power converter 103 may include a PWM circuit that generates the 3-phase AC input drive voltage to the 3-phase machine 102 as respective pulse width modulated motor coil signals of appropriate frequency and amplitude. The MCU 101 comprises an overvoltage protection circuit 107 that is configured to temporarily short-circuit the rectified DC voltage V_DC supplied at the first externally accessible connection 1 15 to the ground potential existing on the second externally accessible connection 1 16 in the present embodiment of the invention, thereby pro- tecting passive and active components of the MCU 101 from dangerously large voltage levels of V_DC that may exceed their respective maximum safe operating limits. The short-circuiting of the rectified DC voltage V_DC is activated if the latter voltage exceeds a predetermined trigger or threshold voltage level. The design, components and operation of the overvoltage protection circuit 107 is discussed below in detail in connection with the circuit diagram on FIG. 2.

FIG. 2 is an electrical circuit diagram of the overvoltage protection circuit 107 in accordance with a preferred embodiment thereof. The design of the present overvoltage protection circuit 107 is suitable for electric bicycle applications that may utilize a Hoganas A6 3-phase AC motor. The overall operation of the overvoltage protection circuit 107 comprises at least five successive and distinct events or steps as explained below. As previously mentioned, the dangerous overvoltage conditions may occur under the previously discussed "regenerative" mode of operation where the 3-phase electric AC machine 102 functions as an AC generator and the occur- rence of certain undesired system events where the processor 105 for various reasons has lost control over the charging of the battery pack etc.

In a first distinct step, an overvoltage condition of the rectified DC voltage V_DC is detected by breakdown of the two cascaded Diode(s) for alternating current (DIACs) DIAC1 and DIAC2 coupled to the rectified DC voltage V_DC. Two DIACs are connected in series/cascade in order to match a desired threshold voltage of approximately 65 Volts because DIACs are only available with certain distinct breakdown voltages. Since, a DIAC breakdown voltage of 32 volts is commonly available two such cascaded devices are used to obtain a threshold at approximately 64 volts which ac- cordingly corresponds to a predetermined trigger voltage level of the overvoltage protection circuit 107. Once, the DIACs are turned on, or forward conducting, by a rectified DC voltage V_DC that exceeds the selected predetermined trigger voltage level, the DIACs continue to conduct as long as the forward current is higher than a break-over current level, IBO, of the DIACs. The IBO of the DIACs may be about 15 μΑ for the preferred DIAC types. After breakdown of the DIACs, forward current through the DIACs is largely limited by resistor R1 which may have a value around 68 Ω. Furthermore, after breakdown the DIACs will conduct current from the rectified DC voltage and charge capacitor C1 until the current drops below the break-over current level, or until the rectified DC voltage drops as a result of the short-circuiting action of a first controllable semiconductor device in form of the Silicon Controlled Rectifier SCR1 , or until the rectified DC voltage drops for any other reason, e.g. a reduction of the bicycle speed. Since the DIACs conduct a small leakage current before break-down, R2 is inserted to create a leakage path to ground and thereby avoid charging C1 with this leakage current. Once, the DIACs conduct forward current after break-down, C1 becomes rapidly charged to about 20 volts which will cause zener diode D2 to start conducting and thereby generate a trigger signal at a gate or control terminal of SCR1 , i.e. in effect jumping to a second distinct step of the operation of the overvoltage protection circuit 107 by switching the SCR1 to a forward conducting mode. The SCR1 is arranged with its anode connected to the rectified DC voltage V_DC and its cathode connected to ground such that these are short-circuited by electrically connecting the first and second battery connections 1 15, 1 16 through the conducting thyristor with an extremely low resistance. The charge on electrolytic smoothing or filtering capacitor C4, which is connected to the first battery connection 1 15 and therefore charged to the rectified DC voltage V_DC, is in response instantly discharged through SCR1 leading to an extremely high discharge current peak that may reach a level of 800 - 1000 A. This initial discharge event is depicted on graph 301 of FIG. 3 where the waveform 304 is the measured rectified DC voltage V_DC and waveform 306 is the measured discharge or short-circuiting current through SCR1 . The graph is a screen-shot from a digital storage oscilloscope and the x-axis depicts time in steps of 40 με per division and the y-axis depicts either voltage in steps of 10 volts per division or current in 250 A per division. It is evident that rectified DC voltage V_DC drops from a steady state level of 66 volts to about 1 volt at approximately 100 s after the firing instant of SCR1. The short-circuiting current through SCR1 rises rapidly to a peak value of about 800 A at approximately 25 s of the firing instant of SCR1 . Based on the above explanations of the operation of the present overvoltage protection circuit 107, the skilled person will appreciate that components DIAC1 , DIAC2, R1 , R2, C1 , D1 , D2 and R3 form a trigger voltage generator for SCR1 with a well- defined threshold voltage governed by the series connected DIAC1 and DIAC2.

In a third distinct step, a small time delay is added to the operation of the overvoltage protection circuit 107 before a second controllable semiconductor device in form of MOSFET T1 is switched to an on-state or conducting state by an associated control circuit. The small time delay is created by a delay network of the control circuit comprising capacitors by C1 , C2, C3 and resistor R4. The voltage across C2 is equal to gate-source voltage U_Gs of MOSFET T1 such that the latter will start to conduct when the voltage across C2 reaches a threshold voltage of T1. This threshold voltage may lie around 4 volts for the preferred type of MOSFET T1 such as a FDB035N10A. D1 prevents discharge of capacitors C1 , C2 and C3 through R2. R3 is inserted to limit gate current of SCR1.

As mentioned above, C1 is charged through the two cascaded DIACs once these are fired. Diode D1 prevents the charge on C1 , C2 and C3 from bleeding off through R2 as mentioned above. The charge of C1 will be shared with C2 and C3, as a cur- rent running through R4 charges C2 and C3. Hence, once the voltage across C2 reaches about 4 volts, MOSFET T1 (item 205) will start to conduct and the voltage on C2 therefore function as a trigger signal for MOSFET T1 . The component values of the a delay network are preferably selected such that the voltage across C2 reaches 4 volts with a predetermined time delay for example a delay between 0.1 ms and 10 ms, more preferably between 0.5 ms and 2.0 ms, relative to the triggering instant of SCR1 . At the latter point in time, a drain-source voltage (U_Ds) of MOSFET T1 has dropped to a few volts because of the already conducting state of SCR1 as evidenced by the plot of the waveform 304 of the rectified DC voltage V_DC on graph 301. The relatively small drain-source voltage (U_Ds) of MOSFET T1 is ad- vantageous since turn-on of T1 will be effected at almost at zero device voltage causing relatively low switching losses within MOSFET T1 .

The skilled person will appreciate that the control circuit of the MOSFET T1 is autonomously powered by a local DC power supply provided by the combined charge on C1 and C2. Hence, the control circuit is capable of operating independently of the rectified DC voltage V_DC when the first and second battery connections are connected or short-circuit via the SCR1 and the rectified DC voltage therefore close to zero. This is important because the short-circuiting of the rectified DC voltage V_DC will for example interrupt the operation of the processor 105, or any other control circuitry of the MCU, which is typically supplied with power from the rectified DC voltage V_DC.

The switching of the MOSFET T1 to its on-state in response to the gate voltage rises above the threshold voltage starts the fourth distinct step of operation of the pre- sent overvoltage protection circuit 107. The MOSFET T1 comprises a drain terminal connected to the first battery connection 1 15 and hence to the rectified DC voltage thereon. A source terminal of the MOSFET T1 is coupled or connected to the second battery connection 1 16 and therefore ground. Hence, the rectified DC voltage is short-circuited to ground by the small drain source resistance of T1 in its conducting state or on-state. Hence, for a certain time period, both T1 and SCR1 will be conducting simultaneously or in parallel and jointly operative to short-circuit the rectified DC voltage. However, once the MOSFET T1 has entered its on-state short-circuit current will be largely removed from SCR1 and redirected to MOSFET T1 because the minimum voltage drop across SCR1 is determined by a PN-junction voltage re- suiting in a minimum voltage drop of approximately 1 volt between the anode and cathode of SCR1. In contrast, MOSFET T1 behaves more like a resistor with small or low resistance in its on-state. For the preferred type of MOSFET the on-state resistance may be less than 10 mQ. The static short circuit current of about 25 A, as dictated by the impedance of the motor windings of the utilized Hoganas A6 3-phase AC motor, will therefore cause a voltage drop of less than 10^*25 mV = 250 mV across the MOSFET T1 . Since this voltage drop across the MOSFET T1 is markedly smaller than the minimum voltage drop across the PN-junction of SCR1 , the short circuit current drawn from the rectified DC voltage V_DC will be completely redirected to T1 after a short time period. The redirection of the short circuit current from SCR1 to the MOSFET T1 causes the current through SCR1 to drop below a so-called holding current of the device such that SCR1 automatically switches back to its nonconducting state and leaves MOSFET1 to alone maintain the short-circuiting of the rectified DC voltage. These discharge events are depicted on graph 31 1 of FIG. 3 wherein the waveform 314 is the measured gate-source voltage U_Gs of MOSFET T1 and waveform 316 is the measured drain-source voltage (U_Ds) of MOSFET T1 . The latter corresponds to the rectified DC voltage. The waveform 318 depicts the measured drain current of MOSFET T1 . As above, the graph is a screen-shot from a digital storage oscilloscope. The x-axis depicts time in steps of 1 ms per division and the y-axis depicts either voltage in steps of either 10 or 5 volts per division or current in 10 A per division. It is evident, that MOSFET T1 does not conduct any noticeable current during an initial time period of about 1.5 ms after SCR1 is triggered. Thereafter, MOSFET T1 turns on once the gate control voltage, or trigger signal, reaches a value of about 4 volts and the current though T1 rapidly reaches a peak value of about 40 A before decreasing to the previously discussed steady average level of about 25 A with a noticeable amount of ripple. The peak current through the MOSFET T1 is accordingly significantly smaller than the 800 A through SCR1 leading to advantageously re- laxed power handling capacity of the MOSFET and therefore markedly smaller size than device that would be capable of handling the 800 A.

In a fifth distinct step of the operation of the present overvoltage protection circuit 107, the MOSFET T1 is automatically switched back to its off-state or non- conducting state after a preset on time period by a discharge circuit. The discharge circuit comprises gate resistor R5 which is a relatively high ohmic resistance that will discharge the gate terminal of T1 with a certain time constant because of the fixed amount of charge on C2 and C1 and the zero-voltage condition of rectified DC voltage. Hence, once the gate voltage has been discharged to a value below the threshold voltage of T1 , the latter switches back to its off-state defining the duration of the preset on time period. The time constant is preferably selected such that the preset on time period lies between 1 and 20 seconds such about 5 -10 seconds. However, the discharge of the gate terminal of T1 caused by the "bleeding" type of discharge through resistor R5 places MOSFET T1 in its so-called linear operation region for a disadvantageously long period of time. This linear region operation time has been markedly reduced in accordance with a particularly advantageous embodiment of the discharge circuit which is configured to accelerate the discharge of the gate terminal of the MOSFET T1 in response to the rectified DC voltage at the drain of MOSFET T1 reaches a preset threshold voltage. The acceleration of the dis- charge of T1 is carried out by the circuit arrangement comprising resistors R6 and R7, capacitor C3 and bipolar transistor T2 which extracts charge from the gate of T1 . When the level of the rectified DC voltage V_DC, which equals the drain voltage of T1 , starts to increase due to T1 entering its linear operating region, T2 will switch on causing a portion of the gate charge of T1 to be nearly instantly transferred to C3. This causes a rapid decrease of the gate-source voltage U_Gs of T1 which accelerates the turn-off process of T1 . This accelerated turn-off process minimizes switching losses of T1 by reducing the duration of the time period T1 operates in its linear region. The skilled person will appreciate that the preset threshold voltage, which triggers the accelerated discharge process, is set to approximately 0.7 volt which is the voltage required to turn-on bipolar transistor T1. However, in other embodiments other types of semiconductor transistor may be used for example a MOSFET or JFET providing higher or lower preset threshold voltages. Finally, the skilled person will understand that R7 allows for discharging of C3 between consecutive operation- al cycles of the overvoltage protection circuit 107.

This accelerated discharge of T1 is depicted on graph 401 of FIG. 4 where the waveform 404 is the measured discharge or short-circuiting current through

MOSFET T1 . Waveform 406 is the rectified DC voltage and drain voltage of T1 and waveform 408 is the measured of the gate-source voltage U_Gs of T1. The graph is a screen-shot from a digital storage oscilloscope and the x-axis depicts time in steps of 1 ms per division and the y-axis depicts either voltage in steps of 1.0 or 5.0 volts per division or current in 10 A per division. The rectified DC voltage V_DC abruptly increases from an initial level of about zero volts due to short-circuiting through T1 to a level about 65 volts (outside the range of the graph). In response to the rise of the drain voltage of T1 , the gate-source voltage U_Gs of T1 on waveform 408 abruptly falls from an initial value of about 5 volts, which is above the threshold voltage of T1 rending the device in a conducting state, to about 3.5 volts. The 3.5 volts is below the threshold voltage of the utilized sample of the FDB035N10A MOSFET as dis- cussed above such that the device switches to its non-conducting state or off-state. In response, the short-circuit current through MOSFET T1 , as depicted on waveform 404, drops abruptly from the previously discussed quasi-stationary value of about 25 A to about zero. Consequently, after completion of the fifth distinct operational step, the MOSFET T1 has been returned to its off-state. In addition, SCR1 has already automatically been returned to its off-state or non-conducting state in response to the above discussed redirection of short-circuit current to T1 . This event caused the forward current through SCR1 to drop below the so-called holding current level of SCR1 which caused the latter to automatically switch to the off-state. Hence, once the overvolt- age protection circuit 107 returns to its non-conducting and non-short-circuiting state, the rectified DC voltage is recharged by current delivered by the 3-phase AC machine operating in generator mode if the regenerative working conditions of the electric machine system 100 persist. If the rectified DC voltage therefore once again rises to a level above the predetermined trigger voltage level, i.e. the over-voltage condition, the overvoltage protection circuit is re-activated and steps 1 to 5 repeats. On the other hand, if the overvoltage condition has ceased normal operation of the MCU is re-established in either of the motor mode or generator mode.

The repeated activation of the overvoltage protection circuit during a persisting overvoltage situation is illustrated on graph 41 1 of FIG. 4 wherein waveform 414 is the measured rectified DC voltage and waveform 416 is the measured short-circuit current through the overvoltage protection circuit. As previously, the graph 41 1 is digital storage oscilloscope screen-shot. The x-axis depicts time in steps of 4 seconds per division and the y-axis depicts either voltage in 10 volts per division or current in 100 A per division. The peaks of the rectified DC voltage waveform 414 illustrate how the overvoltage protection circuit is promptly activated each time the rectified DC voltage exceeds the predetermined trigger voltage level of about 64 volts. Upon activation, the rectified DC voltage rapidly falls down to nearly zero voltage effectively protecting the MCU components against overvoltage conditions. Furthermore, the time between successive peaks of the rectified DC voltage waveform 414 illustrates the previously discussed duration of the preset on time period of about 6 seconds defined by the discharge circuit coupled to the gate of MOSFET T1 . Finally, the short-circuit current waveform 416 displays the previously discussed steady level of about 25 A. The short 800 A short-circuit currents pulses running through SCR1 are not depicted on the present short-circuit current waveform 416 of graph 41 1 due to the selected sampling rate and time-scale of the digital storage scope.

Claims

1 . An electric machine assembly comprising:

an electric DC or AC machine comprising a plurality of power terminals for supply of a DC or AC generator voltage and receipt of a DC or AC input drive voltage, a motor control unit operatively coupled to the electric machine through the power terminals, wherein the motor control unit comprises:

a bi-directional AC/DC power converter configured to operate in a first conversion mode for conversion of the DC or AC generator voltage into a rectified DC voltage at first and second battery connections of the motor control unit and a second conversion mode for conversion of a DC battery voltage applied at the first and second battery connections into the DC or AC input drive voltage,

a controller coupled to the bi-directional AC/DC power converter for selection of one of the first and second conversion modes,

an overvoltage protection circuit operatively coupled between the first and second battery connections,

said overvoltage protection circuit comprising a first controllable semiconductor device configured to selectively electrically connect or disconnect the first and second battery connections in accordance with a first trigger signal applied to a control ter- minal of the controllable semiconductor device,

a trigger voltage generator coupled to the first and second battery connections and configured to generate the first trigger signal in response to the rectified DC voltage exceeding a predetermined trigger voltage level.

2. An electric machine assembly according to claim 1 , wherein the first controllable semiconductor device comprises a thyristor, such as a Silicon Controlled Rectifier (SCR), said thyristor comprising:

an anode and a cathode connected to the first and second battery connections, respectively, to electrically connect these in a forward conducting mode of the thyristor and disconnect these in a forward blocking mode of the thyristor,

a gate terminal of the thyristor coupled to the trigger voltage generator for receipt of the first trigger signal.

3. An electric machine assembly according to claim 2, wherein the overvoltage protection circuit comprises a second controllable semiconductor device configured to selectively electrically connect or disconnect the first and second battery connections in accordance with a second trigger signal applied to a control terminal of the second controllable semiconductor device by a control circuit;

a time delay circuit coupled to the first trigger signal for delaying the second trigger signal relative to the first trigger signal with a predetermined time period such as a time period between 0.1 ms and 10 ms.

4. An electric machine assembly according to claim 3, wherein the second controllable semiconductor device comprises a transistor such as a MOSFET or IGBT tran- sistor;

said transistor comprising a drain and a source, or collector and emitter, connected to the first and second battery connections to electrically connect these through the transistor in an on-state and disconnect these in an off-state,

a gate or base terminal of the transistor being coupled to the control circuit for re- ceipt of the second trigger signal.

5. An electric machine assembly according to claim 4, wherein the control circuit of the MOSFET or IGBT transistor comprises a discharge circuit, coupled to the gate terminal of the MOSFET or IGBT transistor, configured to discharge the gate in ac- cordance with a first time constant such that the MOSFET or IGBT transistor automatically switches from the on-state to the off-state after a preset on time period.

6. An electric machine assembly according to claim 4 or 5, wherein the control circuit of the MOSFET or IGBT transistor is autonomously powered by a DC power supply operating independently of the rectified DC voltage when the first and second battery connections are connected via the thyristor and/or the MOSFET or IGBT transistor.

7. An electric machine assembly according to claim 5 or 6, wherein the discharge circuit comprises a semiconductor switch configured to accelerate the discharge of the gate terminal of the MOSFET or IGBT transistor in response to the rectified DC voltage reaches a preset threshold voltage.

8. An electric machine assembly according to any of claims 5 to 7, wherein the first time constant of the discharge circuit is determined by a combination of resistances and capacitances of resistors and capacitors, respectively.

9. An electric machine assembly according to claim 7 or 8, wherein the semiconductor switch comprises a bipolar transistor or a FET transistor with a base or gate, respectively, connected to the first battery connection;

the preset threshold voltage comprising a base-emitter or gate source voltage of the bipolar transistor or FET transistor, respectively.

1 0. An electric machine assembly according to any of the preceding claims, wherein the trigger voltage generator comprises one or more cascaded

Diode(s) for alternating current (DIACs) configured for determining the predetermined trigger voltage level.

1 1 . An electric machine assembly according to any of claims 4-1 0, wherein the control circuit of the MOSFET or IGBT transistor is configured to provide a duration of the preset on time period between 1 .0 s and 20 s.

12. An electric machine assembly according to any of claims 4-1 0, wherein the on- resistance of the MOSFET or IGBT transistor in the on-state is less than 1 00 ΓΠΩ, more preferably less than 10 ΓΠΩ.

1 3. An electric machine assembly according to any of the preceding claims, wherein the electric machine comprises a multi-phase AC machine.

14. An electric machine system for electric bicycles, comprising:

an electric machine assembly according to any of the preceding claims,

a battery pack comprising first and second externally accessible battery terminals electrically connected to the first and second battery connections, respectively, of the electric machine assembly,

the battery pack comprising a plurality of rechargeable battery cells coupled to the first and second externally accessible battery terminals and a battery management system for controlling charging and discharging of the rechargeable battery cells, wherein the battery management system is configured to disconnect the plurality of rechargeable battery cells from at least one of the first and second externally accessible battery terminals in response to a predetermined charging condition of the plurality of rechargeable battery cells.

15. An electric drive system for light electric vehicles, comprising:

an electric machine assembly according to any of claims 1 -13,

a user interface unit connected to the controller of the assembly through a wired or wireless data communication link for setting a motor assistance level of the electric machine in accordance with a user entry.