Reduced ripple inverter for hybrid drive systems

A powertrain for a vehicle includes a wye wound electric machine and a controller. The electric machine is coupled with an inverter. The controller is configured to, in response to an electrical connection between the vehicle and an AC grid, couple a capacitor between a neutral terminal of the electric machine and a negative terminal of the inverter to absorb reactive power from the AC grid.

TECHNICAL FIELD

This application is generally related to an electric machine and inverter system configured to reduce a current ripple during charging of the electric vehicle from an AC grid.

BACKGROUND

Electrified vehicles including hybrid-electric vehicles (HEVs) and battery electric vehicles (BEVs) rely on a traction battery to provide power to a traction motor for propulsion and a power inverter therebetween to convert direct current (DC) power to alternating current (AC) power. The typical AC traction motor is a 3-phase motor that may be powered by 3 sinusoidal signals each driven with 120 degrees phase separation. The traction battery is configured to operate in a particular voltage range and provide a maximum current. The traction battery is alternatively referred to as a high-voltage battery. However, improved performance of electric machines may be achieved by operating in a different voltage range, typically at voltages greater than the traction battery terminal voltage. Likewise, the current requirements to drive a vehicular electric machine are commonly referred to as high current.

Also, many electrified vehicles include a DC-DC converter, also referred to as a variable voltage converter (VVC), to convert the voltage of the traction battery to an operational voltage level of the electric machine. The electric machine, that may include a traction motor and a generator, may require high voltage and high current. Due to the voltage and current requirements, a battery module and a power electronics module are typically in continuous communication.

SUMMARY

A powertrain for a vehicle includes a wye wound electric machine and a controller. The electric machine is coupled with an inverter. The controller is configured to, in response to an electrical connection between the vehicle and an AC grid, couple a capacitor between a neutral terminal of the electric machine and a negative terminal of the inverter to absorb reactive power from the AC grid.

A method of controlling a powertrain includes, in response to an electrical connection between an AC grid and an electric vehicle containing the powertrain, modulating switches of an inverter according to reactive power from the AC grid to induce a field in a wye wound electric machine of the powertrain to absorb a portion of the reactive power.

A powertrain for a vehicle includes a wye wound electric machine and a controller. The electric machine is coupled with an inverter. The controller is configured to, in response to an electrical connection between the vehicle and an AC grid, modulate switches of the inverter to flow a current in the electric machine to absorb reactive power from the AC grid in at least one winding of the electric machine.

DETAILED DESCRIPTION

A single phase alternate current (AC) charger for electric vehicles typically passes an AC ripple at various frequencies. Typically, the most noticeable frequency component is the component at twice the grid or line frequency, and the grid input power has a pulsing shape with a dc offset (Pin_dc), a large AC component at twice the line frequency and a peak to peak value of 2(Pin_dc). This power ripple causes a current ripple at twice the grid frequency on the battery side such that extra circuitry may be required to protect the battery. Also, there are current ripples at other frequencies, such as different orders of harmonics due to grid distortion and the switching frequency of a charger's semiconductor switches. To filter these ripples, a large dc-link capacitor is required in the charger. This bulk capacitor increases the cost, volume and weight of the charger.

As the electric vehicle is not in motion during AC grid charging, its electrical drive system (e.g., powertrain or E-drive system), which includes a traction drive inverter and an electric machine, is typically disconnected from the battery. Here, an apparatus and method is presented that utilizes the E-drive system and operates it as a low frequency current compensator during charging. A powertrain is disclosed that includes a capacitor that is selectively coupled between a neutral terminal of a wye wound electric machine and a negative terminal of the motor inverter. In another embodiment, the controller for the powertrain is configured to modulate switches of the inverter during AC grid charging, at a frequency greater than the line frequency, to flow a current through one phase winding of the electric machine such that the current returns via a different phase winding of the electric machine.

FIG. 1depicts a hybrid electric vehicle illustrating internal electric powertrain components configured to flow a current through windings of an electric machine2by operating the electric machine inverter4while charging a high voltage traction battery6via a battery charger8that is coupled with an AC grid10. In one embodiment, a controller (e.g., the electric machine inverter controller) may selectively engage or couple a capacitor12between a neutral terminal of the electric machine and a negative terminal of the electric machine inverter. Also, the controller may modulate the switches of the electric machine inverter at a frequency greater than the line frequency to reduce a ripple current based on the line frequency and harmonics of the line frequency. Here, a controller (e.g., the electric machine inverter controller) may selectively modulate switches of the inverter4to absorb reactive energy from the AC grid10. The modulation of the switches of the inverter4may be done to absorb the reactive energy in the capacitor12and/or in the inductive windings of the electric machine2.

FIG. 2depicts an electrified vehicle112that may be referred to as a plug-in hybrid-electric vehicle (PHEV). A plug-in hybrid-electric vehicle112may comprise one or more electric machines114mechanically coupled to a hybrid transmission116. The electric machines114may be capable of operating as a motor or a generator. In addition, the hybrid transmission116is mechanically coupled to an engine118. The hybrid transmission116is also mechanically coupled to a drive shaft120that is mechanically coupled to the wheels122. The electric machines114can provide propulsion and deceleration capability when the engine118is turned on or off. The electric machines114may also act as generators and can provide fuel economy benefits by recovering energy that would normally be lost as heat in a friction braking system. The electric machines114may also reduce vehicle emissions by allowing the engine118to operate at more efficient speeds and allowing the hybrid-electric vehicle112to be operated in electric mode with the engine118off under certain conditions. An electrified vehicle112may also be a battery electric vehicle (BEV). In a BEV configuration, the engine118may not be present. In other configurations, the electrified vehicle112may be a full hybrid-electric vehicle (FHEV) without plug-in capability.

A traction battery or battery pack124stores energy that can be used by the electric machines114. The vehicle battery pack124may provide a high-voltage direct current (DC) output. The traction battery124may be electrically coupled to one or more power electronics modules126. One or more contactors142may isolate the traction battery124from other components when opened and connect the traction battery124to other components when closed. The power electronics module126is also electrically coupled to the electric machines114and provides the ability to bi-directionally transfer energy between the traction battery124and the electric machines114. For example, a traction battery124may provide a DC voltage while the electric machines114may operate with a three-phase alternating current (AC) to function. The power electronics module126may convert the DC voltage to a three-phase AC current to operate the electric machines114. In a regenerative mode, the power electronics module126may convert the three-phase AC current from the electric machines114acting as generators to the DC voltage compatible with the traction battery124.

The vehicle112may include a variable-voltage converter (VVC)152electrically coupled between the traction battery124and the power electronics module126. The VVC152may be a DC/DC boost converter configured to increase or boost the voltage provided by the traction battery124. By increasing the voltage, current requirements may be decreased leading to a reduction in wiring size for the power electronics module126and the electric machines114. Further, the electric machines114may be operated with better efficiency and lower losses.

In addition to providing energy for propulsion, the traction battery124may provide energy for other vehicle electrical systems. The vehicle112may include a DC/DC converter module128that converts the high-voltage DC output of the traction battery124to a low voltage DC supply that is compatible with low-voltage vehicle loads. An output of the DC/DC converter module128may be electrically coupled to an auxiliary battery130(e.g., 12V battery) for charging the auxiliary battery130. The low-voltage systems may be electrically coupled to the auxiliary battery130. One or more electrical loads146may be coupled to the high-voltage bus. The electrical loads146may have an associated controller that operates and controls the electrical loads146when appropriate. Examples of electrical loads146may be a fan, an electric heating element and/or an air-conditioning compressor.

The electrified vehicle112may be configured to recharge the traction battery124from an external power source136. The external power source136may be a connection to an electrical outlet. The external power source136may be electrically coupled to a charger or electric vehicle supply equipment (EVSE)138. The external power source136may be an electrical power distribution network or grid as provided by an electric utility company. The EVSE138may provide circuitry and controls to regulate and manage the transfer of energy between the power source136and the vehicle112. The external power source136may provide DC or AC electric power to the EVSE138. The EVSE138may have a charge connector140for plugging into a charge port134of the vehicle112. The charge port134may be any type of port configured to transfer power from the EVSE138to the vehicle112. The charge port134may be electrically coupled to a charger or on-board power conversion module132. The power conversion module132may condition the power supplied from the EVSE138to provide the proper voltage and current levels to the traction battery124. The power conversion module132may interface with the EVSE138to coordinate the delivery of power to the vehicle112. The EVSE connector140may have pins that mate with corresponding recesses of the charge port134. Alternatively, various components described as being electrically coupled or connected may transfer power using a wireless inductive coupling.

One or more wheel brakes144may be provided for decelerating the vehicle112and preventing motion of the vehicle112. The wheel brakes144may be hydraulically actuated, electrically actuated, or some combination thereof. The wheel brakes144may be a part of a brake system150. The brake system150may include other components to operate the wheel brakes144. For simplicity, the figure depicts a single connection between the brake system150and one of the wheel brakes144. A connection between the brake system150and the other wheel brakes144is implied. The brake system150may include a controller to monitor and coordinate the brake system150. The brake system150may monitor the brake components and control the wheel brakes144for vehicle deceleration. The brake system150may respond to driver commands and may also operate autonomously to implement features such as stability control. The controller of the brake system150may implement a method of applying a requested brake force when requested by another controller or sub-function.

Electronic modules in the vehicle112may communicate via one or more vehicle networks. The vehicle network may include a plurality of channels for communication. One channel of the vehicle network may be a serial bus such as a Controller Area Network (CAN). One of the channels of the vehicle network may include an Ethernet network defined by Institute of Electrical and Electronics Engineers (IEEE) 802 family of standards. Additional channels of the vehicle network may include discrete connections between modules and may include power signals from the auxiliary battery130. Different signals may be transferred over different channels of the vehicle network. For example, video signals may be transferred over a high-speed channel (e.g., Ethernet) while control signals may be transferred over CAN or discrete signals. The vehicle network may include any hardware and software components that aid in transferring signals and data between modules. The vehicle network is not shown inFIG. 1but it may be implied that the vehicle network may connect to any electronic module that is present in the vehicle112. A vehicle system controller (VSC)148may be present to coordinate the operation of the various components.

Often the VVC152is configured as a boost converter. The VVC152may include input terminals that may be coupled to terminals of the traction battery124through the contactors142. The VVC152may include output terminals coupled to terminals of the power electronics module126. The VVC152may be operated to cause a voltage at the output terminals to be greater than a voltage at the input terminals. The vehicle112may include a VVC controller that monitors and controls electrical parameters (e.g., voltage and current) at various locations within the VVC152. In some configurations, the VVC controller may be included as part of the VVC152. The VVC controller may determine an output voltage reference, Vdc*. The VVC controller may determine, based on the electrical parameters and the voltage reference, Vdc*, a control signal sufficient to cause the VVC152to achieve the desired output voltage. In some configurations, the control signal may be implemented as a pulse-width modulated (PWM) signal in which a duty cycle of the PWM signal is varied. The control signal may be operated at a predetermined switching frequency. The VVC controller may command the VVC152to provide the desired output voltage using the control signal. The particular control signal at which the VVC152is operated may be directly related to the amount of voltage boost to be provided by the VVC152.

With reference toFIG. 2, the VVC152may boost or “step up” the voltage potential of the electrical power provided by the traction battery124. The traction battery124may provide high-voltage (HV) DC power. In some configurations, the traction battery124may provide a voltage between 150 and 400 Volts. The contactor142may be electrically coupled in series between the traction battery124and the VVC152. When the contactor142is closed, the HV DC power may be transferred from the traction battery124to the VVC152. An input capacitor202may be electrically coupled in parallel to the traction battery124. The input capacitor202may stabilize the bus voltage and reduce any voltage and current ripple. The VVC152may receive the HV DC power and boost or “step up” the voltage potential of the input voltage according to the duty cycle. Often an output capacitor is electrically coupled between the output terminals of the VVC152and the input of the power electronics module126to stabilize the bus voltage and reduce voltage and current ripple at the output of the VVC152.

With reference toFIG. 3, a system300is provided for controlling a power electronics module (PEM)126. The PEM126ofFIG. 3is shown to include a plurality of switches302(e.g., IGBTs) configured to collectively operate as an inverter with first, second, and third phase legs316,318,320. While the inverter is shown as a three-phase converter, the inverter may include additional phase legs. For example, the inverter may be a four-phase converter, a five-phase converter, a six-phase converter, etc. In addition, the PEM126may include multiple converters with each inverter in the PEM126including three or more phase legs. For example, the system300may control two or more inverters in the PEM126. The PEM126may further include a DC to DC converter having high power switches (e.g., IGBTs) to convert a power electronics module input voltage to a power electronics module output voltage via boost, buck or a combination thereof.

As shown inFIG. 3, the inverter may be a DC-to-AC converter. In operation, the DC-to-AC converter receives DC power from a DC power link306through a DC bus304and converts the DC power to AC power. The AC power is transmitted via the phase currents ia, ib, and ic to drive an AC machine also referred to as an electric machine114, such as a three-phase permanent-magnet synchronous motor (PMSM) as depicted inFIG. 3. In such an example, the DC power link306may include a DC storage battery to provide DC power to the DC bus304. In another example, the inverter may operate as an AC-to-DC converter that converts AC power from the AC machine114(e.g., generator) to DC power, which the DC bus304can provide to the DC power link306. Furthermore, the system300may control the PEM126in other power electronic topologies.

With continuing reference toFIG. 3, each of the phase legs316,318,320in the inverter includes power switches302, which may be implemented by various types of controllable switches. In one embodiment, each power switch302may include a diode and a transistor, (e.g., an IGBT). The diodes ofFIG. 3are labeled Da1, Da2, Db1, Db2, Dc1, and Dc2while the IGBTs ofFIG. 3are respectively labeled Sa1, Sa2, Sb1, Sb2, Sc1, and Sc2. The power switches Sa1, Sa2, Da1, and Da2are part of phase leg A of the three-phase converter, which is labeled as the first phase leg A316inFIG. 3. Similarly, the power switches Sb1, Sb2, Db1, and Db2are part of phase leg B318and the power switches Sc1, Sc2, Dc1, and Dc2are part of phase leg C320of the three-phase converter. The inverter may include any number of the power switches302or circuit elements depending on the particular configuration of the inverter. The diodes (Dxx) are connected in parallel with the IGBTs (Sxx) however, as the polarities are reversed for proper operation, this configuration is often referred to as being connected anti-parallel. A diode in this anti-parallel configuration is also called a freewheeling diode.

As illustrated inFIG. 3, current sensors CSa, CSb, and CScare provided to sense current flow in the respective phase legs316,318,320.FIG. 3shows the current sensors CSa, CSb, and CScseparate from the PEM126. However, current sensors CSa, CSb, and CScmay be integrated as part of the PEM126depending on its configuration. Current sensors CSa, CSb, and CScofFIG. 3are installed in series with each of phase legs A, B and C (i.e., phase legs316,318,320inFIG. 3) and provide the respective feedback signals ias, ibs, and ics(also illustrated inFIG. 3) for the system300. The feedback signals ias, ibs, and icsmay be raw current signals processed by logic device (LD)310or may be embedded or encoded with data or information about the current flow through the respective phase legs316,318,320. Also, the power switches302(e.g., IGBTs) may include current sensing capability. The current sensing capability may include being configured with a current mirror output, which may provide data/signals representative of ias, ibs, and ics. The data/signals may indicate a direction of current flow, a magnitude of current flow, or both the direction and magnitude of current flow through the respective phase legs A, B, and C.

Referring again toFIG. 3, the system300includes a logic device (LD) or controller310. The controller or LD310can be implemented by various types or combinations of electronic devices and/or microprocessor-based computers or controllers. To implement a method of controlling the PEM126, the controller310may execute a computer program or algorithm embedded or encoded with the method and stored in volatile and/or persistent memory312. Alternatively, logic may be encoded in discrete logic, a microprocessor, a microcontroller, or a logic or gate array stored on one or more integrated circuit chips. As shown in the embodiment ofFIG. 3, the controller310receives and processes the feedback signals ias, ibs, and icsto control the phase currents ia, ib, and ic, such that the phase currents ia, ib, and icflow through the phase legs316,318,320and into the respective windings of the electric machine114according to various current or voltage patterns. For example, current patterns can include patterns of phase currents ia, ib, and icflowing into and away from the DC-bus304or a DC-bus capacitor308. The DC-bus capacitor308ofFIG. 3is shown separate from the PEM126. However, the DC-bus capacitor308may be integrated as part of the PEM126.

As shown inFIG. 3, a storage medium312(hereinafter “memory”), such as computer-readable memory may store the computer program or algorithm embedded or encoded with the method. In addition, the memory312may store data or information about the various operating conditions or components in the PEM126. For example, the memory312may store data or information about current flow through the respective phase legs316,318,320. The memory312can be part of the controller310as shown inFIG. 3. However, the memory312may be positioned in any suitable location accessible by the controller310.

As illustrated inFIG. 3, the controller310transmits at least one control signal236to the power converter system126. The power converter system126receives the control signal322to control the switching configuration of the inverter and therefore the current flow through the respective phase legs316,318, and320. The switching configuration is a set of switching states of the power switches302in the inverter. In general, the switching configuration of the inverter determines how the inverter converts power between the DC power link306and the electric machine114.

To control the switching configuration of the inverter, the inverter changes the switching state of each power switch302in the inverter to either an ON state or an OFF state based on the control signal322. In the illustrated embodiment, to switch the power switch302to either ON or OFF states, the controller/LD310provides the gate voltage (Vg) to each power switch302and therefore drives the switching state of each power switch302. Gate voltages Vga1, Vga2, Vgb1, Vgb2, Vgc1, and Vgc2(shown inFIG. 3) control the switching state and characteristics of the respective power switches302. While the inverter is shown as a voltage-driven device inFIG. 3, the inverter may be a current-driven device or controlled by other strategies that switch the power switch302between ON and OFF states. The controller310may change the gate drive for each IGBT based on the rotational speed of the electric machine114, the mirror current, or a temperature of the IGBT switch. The change in gate drive may be selected from a plurality of gate drive currents in which the change gate drive current is proportional to a change in IGBT switching speed.

As also shown inFIG. 3, each phase leg316,318, and320includes two switches302. However, only one switch in each of the legs316,318,320can be in the ON state without shorting the DC power link306. Thus, in each phase leg, the switching state of the lower switch is typically opposite the switching state of the corresponding upper switch. The top switches are typically referred to as high-side switches (i.e.,302A,302B,302C) and the lower switches are typically referred to as low-side switches (i.e.,302D,302E,302F). Consequently, a HIGH state of a phase leg refers to the upper switch in the leg in the ON state with the lower switch in the OFF state. Likewise, a LOW state of the phase leg refers to the upper switch in the leg in the OFF state with the lower switch in the ON state. As a result, IGBTs with current mirror capability may be on all IGBTs, a subset of IGBTs (e.g., Sa1, Sb1, Sc1) or a single IGBT.

Two situations can occur during an active state of the three-phase converter example illustrated inFIG. 2: (1) two phase legs are in the HIGH state while the third phase leg is in the LOW state, or (2) one phase leg is in the HIGH state while the other two phase legs are in the LOW state. Thus, one phase leg in the three-phase converter, which may be defined as the “reference” phase for a specific active state of the inverter, is in a state opposite to the other two phase legs, or “non-reference” phases, that have the same state. Consequently, the non-reference phases are either both in the HIGH state or both in the LOW state during an active state of the inverter.

Solid state devices (SSD), such as Insulated Gate Bipolar junction Transistors (IGBTs), Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), or Bipolar Junction Transistors (BJTs) are widely used in a variety of automotive and industrial applications, such as electric motor drives, power inverters, DC-DC converters, and power modules. Operation of an IGBT and a MOSFET is voltage controlled, in which the operation is based on a voltage applied to a gate of the IGBT or MOSFET, while operation of a BJT is current controlled, in which the operation is based on a current applied to a base of the BJT. Here, the use of SSDs or high-power relays may be used to control, alter, or modulate a current between a battery and an electric machine of a vehicle.

FIG. 4is a diagram of a hybrid vehicle powertrain400including electric machine402that may be configured to provide torque to drive a wheel of the vehicle or generate a current by utilizing rotational force of the wheel to rotate the electric machine402. The electric machine is coupled with an electric machine inverter404that converts an AC current to a direct current (DC) current. During operation of the vehicle, a high voltage traction battery406is used to provide a propulsive force to rotate the electric machine and store energy captured by the electric machine from rotational energy of the wheel. When the vehicle is not in motion it may be desirable to increase the battery406state of charge (SOC) by coupling the battery406with an AC power grid410via a battery charger408. One artifact of the use of the AC grid is that harmonics of the grid frequency may result in peak voltages propagated to the battery and components of the powertrain. Here, a capacitor412is selectively coupled with the neutral terminal of the electric machine402via switch414and the switches (418A,418B,418C,420A,420B, and420C) of the inverter404are modulated to pass reactive power through the inductive windings416A,416B, and416C of the electric machine402. This allows a current flowing through the windings of the electric machine416A,416B, and416C to flow to be absorbed by the capacitor412when engaged by the switch414. The current is controlled via pull-up switches418A,418B, and418C and pull-down switches420A,420B, and420C, these switches are also referred to as high-side switches418A,418B, and418C and low-side switches420A,420B, and420C.

The battery charger may be an AC Level 1, Level 2 or Level 3 charger as defined by the Society of Automotive engineers (SAE) such as described in SAE J1772 and other SAE specifications. Here, the switch R1414, which may be a relay, IGBT, MOSFET, or other solid state switch, selectively couples the capacitor C1412between the neutral terminal of the elected machine402and the negative bus of the inverter404. While the vehicle is coupled with the AC grid and not in motion, the electric machine stator windings (i.e., inductors L1, L2, and L3). The inverter modulated the switches (416and418) at a frequency fsw that is greater than the line frequency of the ac grid410. The frequency fsw may be greater than 20 times (e.g., 1 KHz, 1.2 KHz, 2 KHz, 2.4 KHz, 5 KHz, or 6 KHz) the line frequency (e.g., 50 Hz or 60 Hz).

The smoothing capacitor C1412is used as energy storage device to absorb ripple power. The inductor Lm represents the winding inductance of the electric machine402. The inductance Lm is used to transfer the reactive energy to the capacitor412and not typically used as an energy storage device. Depending upon the value of the inductance of the windings (416A,416B, and416C), the switching frequency and a low frequency ripple magnitude, the inductor may operate in a discontinuous mode or continuous mode.

The switches in the three phase legs are divided into two groups: the upper three switches (418A,418B, and418C) and the lower three switches (420A,420B, and420C). Within each group, the switches can operate in different modes. For example, in a first “parallel” mode, the three switches (e.g., the upper switches or the lower switches) act simultaneously such that all upper switches are activated equally and all lower switches are activated equally. Another mode is an “interleaving” mode in which the three switches (e.g., the upper switches or the lower switches) operate in ⅓ of switching cycle apart. A third “selective” mode is such that only one or two switches operates at a given time. Although this has been illustrated using 3-phase electric machines, this invention is not limited to a 3-phase electric machine as it may also be implemented in a 6-phase, 9-phase, or other poly-phase electric machine in which the balancing capacitor is coupled between a neutral terminal of the poly-phase electric machine and a negative terminal of the inverter for the electric machine. The operation of the switches is such that no steady state rotational torque is applied to the electric machine as any transient torques produced by the fields induced will generally be equal and opposite in some embodiments or will be balanced such that the rotational torque is substantially zero. For example, flowing the same (balanced) current through all phases of an electric machine will generate a balanced uniform field in the electric machine such that minimal or no rotational torque results. Further, the switches may be modulated to compensate for the rotor position and differences in characteristics of the electrical components of the inverter (e.g., switches, diodes, and connections) and phases of the electric machine (e.g.,416A,416A, and416A).

FIG. 5is a block diagram of inverter signal flow500for a hybrid vehicle during AC charging. The control of the inverter is performed to direct the reactive power of the charging operation from the AC grid to the capacitor. Here, sensing items on the grid side may include voltage/current (e.g., Iac and Vac) which may be calculated by a first control block502(e.g., an AC charger controller) to produce phase information and input power data. The phase information and input power data may be communicated to a second control block504that may be located within the vehicle and used along with sensing information which may include current of the inductor Lm (Ilm) and a voltage of the capacitor (Vc1). The output of this control flow is an inverter gating signal that is used to couple the smoothing capacitor (e.g. smoothing capacitor412) between the neutral terminal of the electric machine and the negative terminal of the inverter. Further, this inverter gating signal may also be used to control switches of the inverter (418A,418B,418C,420A,420B, and420C).

FIG. 6is a graphical representation of AC characteristics600of charging and powertrain components during AC charging of a hybrid vehicle with respect to time602. An input power604, inductor current606and bulk DC capacitor voltage608are graphically shown with respect to time602. At time610, the current is zero when the input DC power is equal to the grid power, and at time612at ½ the grid frequency, the input power is zero when the current is at a minimum. These waveforms of the input power, inductor current and capacitor voltage (only DC and twice the grid frequency components are shown). During this measurement, the battery charger is operated such that an input voltage and current to the battery charger achieves unity power factor, but in most cases, a typical commercial charger may not achieve a unity power factor. However, in non-unity power factor cases and systems having other frequency components, these control methods and circuits may be applied.

For calculation purposes, assume that all AC side low frequency ripples are passed to the battery side through the charger. The input power may then be based on:
Pin=Pin_dc×(1+cos(2fgrid×2πt))  (1)

The reactive energy to be absorbed by the capacitor C1may be calculated based on:

The voltage swing of the capacitor may be calculated based on:

Equation (4) provides a guideline for the capacitor selection. For example, consider a 3.3 KW charger with a 60 Hz grid frequency, a 400V capacitor dc voltage and 50 V capacitor voltage ripple, (3,300/(4*π*60*50*400)) thus a 200 uF capacitor may be used to satisfy the requirement of equation 4.

FIG. 7is a diagram of a hybrid vehicle powertrain700including electric machine702that may be configured to provide torque to drive a wheel of the vehicle or generate a current by utilizing rotational force of the wheel to rotate the electric machine702. The electric machine is coupled with an electric machine inverter704that converts an AC current to a direct current (DC) current. During operation of the vehicle, a high voltage traction battery706is used to provide a propulsive force to rotate the electric machine and store energy captured by the electric machine from rotational energy of the wheel. When the vehicle is not in motion it may be desirable to increase the battery706state of charge (SOC) by coupling the battery706with an AC power grid710via a battery charger708. One artifact of the use of the AC grid is that harmonics of the grid frequency may result in peak voltages propagated to the powertrain. Here, a current may be directed through at least one of the phases of the electric machine702via the inverter704. A current may be directed to flow through at least one winding of the electric machine (e.g.,716A,716B, and716C) and then return via a different winding of the electric machine. The current is controlled via pull-up switches718A,718B, and718C and pull-down switches720A,720B, and720C. For example, a current may be directed to flow through a first phase716A via turning-on a first switch718A and the current may then return via a second phase716B via turning-on a second switch720B. In an alternative embodiment, the current may return via a second phase716B and a third phase716C via turning-on a second switch720B and a third switch720C.

An equivalent circuit forFIG. 7is basically an H-bridge with an inductor across the bridge. The inductor Lm represents the equivalent winding inductance of the phases in either series or parallel dependent upon the switch configuration, which may have different values depending on different circuit configurations. The pull-up switches718A,718B, and718C and pull-down switches720A,720B, and720C form a full bridge inverter and are controlled to generate the inductor current ILm. The pull-up switches718A,718B, and718C and pull-down switches720A,720B, and720C may be operated at a frequency fsw that may be much higher (>20 times) than the line frequency of the AC grid. And the pull-up switches718A,718B, and718C and pull-down switches720A,720B, and720C may be controlled such that the inductor current ILm tracks the input power to compensate for the reactive power components thereof. Although this has been illustrated using 3-phase electric machines, this embodiment is not limited to a 3-phase electric machine as it may also be implemented in a 6-phase, 9-phase, or other poly-phase electric machine in which a current flows out at least one phase and returns via at least one different phase. The operation of the switches is such that no steady state rotational torque is applied to the electric machine as transient torques produced by the fields induced will generally be equal and opposite in some embodiments or will be balanced such that the rotational torque is substantially zero. For example, flowing the same (balanced) current through one phase of an electric machine and returning the current via a separate different phase to generate a balanced uniform field in the electric machine such that minimal or no rotational torque results. Another example is flowing a current through one phase of an electric machine and returning the current via the two remaining phases to generate a balanced uniform field in the electric machine such that minimal or no rotational torque results. Further, the switches may be modulated to compensate for the rotor position and differences in characteristics of the electrical components of the inverter (e.g., switches, diodes, and connections) and phases of the electric machine (e.g.,716A,716A, and716A).

In the following analysis, it is also assumed that the input voltage and current have unity power factor, which is the case for most commercial chargers. However, for non-unity power factor case, and for other frequency component, the analysis will be similar. Here, two cases are investigated, first when ILmhas a large DC value plus AC ripples, and second when ILmhas no or small DC value plus AC ripples.

In the case in which ILmhas a large DC value plus AC ripples, the inductor current is always positive and the AC component of the inductor current ILmtracks the input reactive power to compensate for the grid side ripple. Here, the current ripple on the inductor may be calculated based on:

In which the required inductance of the motor stator windings may be based on:

Equation 6 provides a guideline for a recommended inductance. For example, for a 3.3 KW charger with 60 Hz grid frequency, 50 A current ripple and 400 A inductor DC current, a 200 uH equivalent inductance of the stator windings may be used to satisfy the requirement.

The control of the inverter is to direct the low frequency reactive power to the inductor. The sensing items on the grid side may include voltage/current (e.g., Iac and Vac) that may be used to produce phase information and input power data. The phase information and input power data may be used along with vehicle sensing information which may include current of the inductor Lm (Ilm).

The second case when ILmhas no or small dc value plus ac ripples, the inductor current may go negative. And the AC component of the inductor current tracks the input reactive power to compensate for the grid side ripple. Unity power factor input power may be based on the following equations:

Equation 9 provides the inductor current value used to compensate for the power at twice the grid frequency ripple at unity power factor conditions. The polarity of the inductor current may be selected as desired to minimize the conduction loss of the circuit (e.g., through the switches and components of the powertrain).FIG. 8shows waveforms associated with characteristics of the circuit including an inductor current that is illustrative of one embodiment, in which the polarity switches each time the inductor current goes to zero.

FIG. 8is a graphical representation of AC characteristics800of charging and powertrain components during AC charging of a hybrid vehicle with respect to time802. An input power804, inductor power806, inductor energy808, and inductor current810are graphically shown with respect to time802. During this measurement, the battery charger was operated such that an input voltage and current to the battery charger achieved unity power factor, however in most cases, a typical commercial charger may not achieve a unity power factor. However, for non-unity power factor cases and systems with other frequency components, the control methods and circuits may be applied.

Here, the peak inductor current is based on:

For example, for a 3.3 KW charger with 60 Hz grid frequency and a 200 uH equivalent inductance of the stator windings, the peak inductor current is around 200 A.

To fully utilize the three phases of the electric machine and balance their thermal performance, the following modulation phase utilization scheme could be used. For the three intervals shown inFIG. 8, each phase may be operated in two of them, which has one ‘out’ and one ‘in’ current. One example of a modulation scheme is tabulated in Table 1.

TABLE 1The direction of current flow of each phase at different intervalsPhase123AOut0InBInOut0C0InOut

It should be noted that it is also possible for a two phase modulation scheme, which is shown in Table 2.

TABLE 2The direction of current flow of each phase at different intervalsPhase123AOutOutInBInOutOutCOutInOut

FIG. 9is a flow diagram900of a control system for an inverter to channel reactive power to a balancing capacitor. In operation902a controller branches based on an operational mode of a hybrid vehicle. If the vehicle is not in a charging mode, the controller will branch back to operation902. If the operational mode is a charging mode, the controller branches to operation904.

In operation904, the controller calculates the input power ripple (e.g., Pin as shown inFIG. 6andFIG. 8) and an operating loss of the inverter and proceeds to operation906. In operation906, the controller branches based on the input power ripple exceeding a threshold. The threshold may be based on a predetermined value, for example the predetermined value may include the maximum allowable ripple value to the battery or the predetermined value may be determined by the charging efficiency requirement and the inverter circuit loss. If the input power ripple is less than the threshold, the controller will exit, and if the input power ripple is greater than the threshold, then the controller will branch to operation908.

In operation908, the controller branches based on the inverter switching loss exceeding a threshold. The threshold may be based on a predetermined value, for example the predetermined value may include the maximum allowable ripple value to the battery or the predetermined value may be determined by the charging efficiency requirement and the inverter circuit loss. If the inverter switching loss is less than the threshold, the controller will proceed to operation910in which the controller will operate all phases of the electric machine and proceed to operation914. If the inverter switching loss is greater than the threshold, the controller will proceed to operation912in which the controller will operate a limited number phases of the electric machine and proceed to operation914. The limited number may be a single phase of a three phase electric machine, or 2 phases of a three phase electric machine. For example, a system as shown inFIG. 4may engage all three phases (416A,416B, and416C) if the switching loss is below the threshold thereby minimizing the inductance to the balancing capacitor. And if the loss is greater than the threshold, the system as shown inFIG. 4may engage a limited number of phases such as a single phase (i.e.,416A,416B, or416C) or a combination of two phases (i.e.,416A+416B,416A+416C, or416B+416C) thereby minimizing power that needs to be dissipated in the inverter or in the inductance(s) prior flowing to the balancing capacitor. This flow diagram may also be used in a system without a balancing capacitor (e.g.,FIG. 7). In that embodiment for example, the system as shown inFIG. 7may engage all three phases (e.g. flowing a current out a single phase and returning the current via the remaining two phases). This can be visualized as flowing out416A and returning on416B+416C, or flowing out416B and returning on416A+416C, or flowing out416C and returning on416A+416B. Likewise, if the loss is greater than the threshold, the system as shown inFIG. 7may engage a limited number of phases such as a single phase for both source and return (i.e., out on416A and return on416B, or out on416A and return on416C, or out on416B and return on416C) thereby minimizing power that needs to be dissipated in the inverter or in the inductance(s). If the system has a balancing capacitor, the controller proceeds to operation914flows, however in embodiments of the system that do not have a balancing capacitor, the system will proceed to operation916.

In operation914the controller couples a balancing capacitor between a neutral terminal of the wye wound electric machine and a negative terminal of the inverter for the electric machine. Although this has been illustrated using 3-phase electric machines, this invention is not limited to a 3-phase electric machine as it may also be implemented in a 6-phase, 9-phase, or other poly-phase electric machine in which the balancing capacitor is coupled between a neutral terminal of the poly-phase electric machine and a negative terminal of the inverter for the electric machine. After the balancing capacitor is coupled, the controller proceeds to operation916.

In operation916, the controller operates the inverter to channel reactive power from the charging operation to the balancing capacitor via the switches of the inverter. In an embodiment without the balancing capacitor, the controller operates the inverter to channel reactive power through phases of the electric machine via the switches of the inverter. After the controller proceeds to operation918. In operation918, the controller branches in response to a status of the charging, if the charging is not complete, the controller will branch to operation916and continue to operate the inverter. If the charging is complete, the controller will exit.