Electric drive system and energy management method

An electric drive system includes an energy storage system (ESS), a power conversion system, and an alternating current (AC) traction system. The ESS provides or receives electric power. The ESS includes a first energy storage unit and a second energy storage unit. The power conversion system is electrically coupled to the ESS for converting an input power to an output power. The AC traction system is electrically coupled to the power conversion system for converting the output power of the power conversion system to mechanical torques. The AC traction system includes a first AC drive device and a second AC drive device. An energy management system (EMS) is in electrical communication with the ESS, the AC traction system, and the power conversion system for providing control signals.

BACKGROUND

Embodiments of the disclosure relate generally to electric drive systems and energy management methods, useful in a variety of propulsion systems such as an electric vehicle (EV) system.

With the increased concern about energy crisis and environmental pollution caused by the fossil fuel exhaust, there is a growing interest in developing electric-powered vehicles, in which conventional fossil energy may be replaced by energy sources such as lead-acid batteries, fuel cells, flywheel batteries, etc. Electric drive system is one of the key components in the electric-powered vehicles. In the electric drive system, several kinds of AC motors such as induction motor (IM), interior permanent motor (IPM) and switched reluctance motor (SRC) are widely used for converting electric power into mechanical torques. It is desired that the propulsion system may have features of high efficiency, and high performance with low cost. These features may be achieved by choosing appropriate energy sources, appropriate AC motors, and then combining them together in an appropriate structure.

Currently, most of the electric drive systems include a single energy storage system (ESS) or a single motor, or a dual ESS with a single motor. However, on the one hand, a single ESS may not satisfy both the energy and power requirements such as low energy, slow charging rate, and a shorter life due to its own limitation of charge/discharge characteristics. On the other hand, a single motor may not work with a high efficiency across the entire operating range. It will cause problems of oversize, high cost, and low efficiency in order to meet high performances such as a short acceleration time and a long running mileage of the propulsion system.

Therefore, it is desirable to provide systems and methods to address the above-mentioned problems.

BRIEF DESCRIPTION

In accordance with one embodiment disclosed herein, an electric drive system is provided. The electric drive system includes an energy storage system (ESS), a power conversion system, an AC traction system, and an energy management system (EMS). The ESS includes a first energy storage unit for providing or receiving a first power and a second energy storage unit for providing or receiving a second power. The power conversion system is electrically coupled to the ESS for converting an input power to an output power. The power conversion system includes a power conversion device. The power conversion device includes a first terminal coupled to the first energy storage unit, a second terminal coupled to the second energy storage unit, a third terminal, and a fourth terminal. The AC traction system is electrically coupled to the power conversion system for converting the output power of the power conversion system to mechanical torques. The AC traction system includes a first AC drive device and a second AC drive device. The first AC drive device is coupled to the third terminal of the power conversion device for providing or receiving a first mechanical torque. The second AC drive device is coupled to the fourth terminal of the power conversion device for providing or receiving a second mechanical torque. The EMS is in communication with the ESS, the AC traction system, and the power conversion system for providing control signals.

In accordance with another embodiment disclosed herein, an energy management method is provided. The energy management method includes enabling at least one of the first AC drive device and the second AC drive device based at least in part on an electric drive system speed signal. The energy management method includes enabling at least one of a first energy storage unit and a second energy storage unit. The energy management method includes sending power control signals to the power conversion system for controlling power flow paths in the electric drive system. The controlling of the power flow paths includes receiving or providing an input power by at least one of a first terminal and a second terminal of the power conversion system and providing or receiving an output power by at least one of a third terminal and a fourth terminal of the power conversion system.

DETAILED DESCRIPTION

FIG. 1illustrates a block diagram of an electric drive system10applied in a propulsion system (not shown) for driving two loads in accordance with an exemplary embodiment of the present disclosure. The propulsion system may include a forklift system and a crane system, for example. The electric drive system10includes an ESS11, a power conversion system13, an AC traction system15, and an energy management system (EMS)17. In some embodiments, the power conversion system13is configured for converting a DC power generated from the ESS11into another DC power provided to the AC traction system15. In some embodiments, the power conversion system13is configured for converting a DC power generated from the AC traction system15into another DC power for charging the ESS11.

The EMS17is arranged to be in electrical communication with the ESS11, the power conversion system13, and the AC drive system15. In some embodiments, the EMS17may be configured to send power control signals171to enable the power conversion system13to provide necessary DC power for the AC traction system15. In some embodiments, the EMS17may be configured to send power control signals171to enable the power conversion system18to provide necessary DC power for charging the ESS11. In some embodiments, the EMS17may be configured to send torque control signals173to the AC traction system15according to one or more command signals (e.g., electric drive system speed signal25) to enable the AC traction system15to provide necessary mechanical torques to the load system31.

In some embodiments, the ESS11includes a first energy storage unit12for providing or receiving a first power and a second energy storage unit14for providing or receiving a second power. In some embodiments, the ESS11may include more than two energy storage units. In some embodiments, the AC traction system15includes a first AC drive device20for providing or receiving a first mechanical torque and a second AC drive device22for providing or receiving a second mechanical torque. In some embodiments, the AC traction system15may include more than two AC drive devices.

The power conversion system13includes a power conversion device18. The power conversion device18includes a first terminal24electrically coupled to the first energy storage unit12, a second terminal26electrically coupled to the second energy storage unit14, a third terminal28electrically coupled to the first AC drive device20, and a fourth terminal30electrically coupled to the second AC drive device22. In some embodiments, the power conversion device18may include more than four terminals.

When the electric drive system10is operated in an electric driven mode, in some embodiments, the first terminal24may be configured to receive a first input power from the first energy storage unit12, and the second terminal26may be configured to receive a second input power from the second energy storage unit14. In some embodiments, the power conversion device18is configured to perform power conversion with respect to the received first input power and second input power, provide a first output power at the third terminal28, and provide a second output power at the fourth terminal30. The first output power may be provided to the first AC drive device20, and the second output power may be provided to the second AC drive device22.

When the electric drive system10is operated in a regenerative or braking mode, in some embodiments, the third terminal28may be configured to receive a first input power from the first AC drive device20, and the fourth terminal30may be configured to receive a second input power from the second AC drive device22. In some embodiments, the power conversion device18is configured to perform power conversion with respect to the received first input power and the second input power, provide first output power at the first terminal24for charging the ESS11(e.g., first energy storage unit12), and provide second output power at the second terminal26for charging the ESS11(e.g., second energy storage unit14).

The power conversion system13may include a pre-charging switching device16electrically coupled between the first energy storage unit12and the second energy storage unit14for selectively providing a power flow path between the first energy storage unit12and the second energy storage unit14. In some embodiments, the power flow path may be unidirectional, so that the first energy storage unit12may deliver electric power through the unidirectional power flow path for charging the second energy storage unit14. In some embodiments, the power flow path may be bi-directional, so that the electric power can be delivered between the first energy storage unit12and the second energy storage unit14through the bi-directional power flow path.

A load system31may include a first load27coupled to the first AC drive device20and a second load29coupled to the second AC drive device22. In some embodiments, the first load27and the second load29may receive a first mechanical torque from the first AC drive device20and a second mechanical torque from the second AC drive device22respectively. In some embodiments, when the electric drive system10is operated in the regenerative or braking mode, the first load27and the second load29may provide a first mechanical torque and a second mechanical torque to the first AC drive device20and the second AC drive device22respectively. Then the first and second mechanical torques may be converted by the first and second AC drive devices20,22, respectively, into electric power for charging the ESS11.

With a combination of at least two energy storage units, at least one power providing or receiving action may be taken in an appropriate manner to improve the power utilization and extend the life of ESS. With a combination of at least two AC drive devices, at least one power conversion action may be taken in a flexible muffler to improve the efficiency of the electric drive system.

FIG. 2illustrates a block diagram of an electric drive system50applied in a propulsion system (not shown) for driving a single load23in accordance with an exemplary embodiment of the present disclosure. The propulsion system may include an EV system and an elevator system. The electric drive system50is similar to the electric drive system10shown inFIG. 1and includes the ESS11, the power conversion system13, the AC traction system15, and the EMS17. Thus, detail description about the ESS11, the power conversion system13, the AC traction system15, and the EMS17are omitted herein. In some embodiments, the electric drive system50includes a power split device19mechanically coupled between the AC traction system15and the load23.

The power split device19includes a first mechanical terminal32, a second mechanical terminal34, and a third mechanical terminal36. The first mechanical terminal32is mechanically coupled to the first AC drive device20for receiving or providing a first mechanical torque. The second mechanical terminal34is mechanically coupled to the second AC drive device22for receiving or providing a second mechanical torque. The third mechanical terminal36is mechanically coupled to the load23for providing or receiving a third mechanical torque. In some embodiments, the power split device19includes one or more gears35for transmitting mechanical torques provided by the AC traction system15to the load23.

In some embodiments, when the propulsion system is driving a heavy load, the first mechanical terminal32receives a first mechanical torque from the first AC drive device20, the second mechanical terminal34receives a second mechanical torque from the second AC drive device22, and the third mechanical terminal36provides a third mechanical torque to the load23. The third mechanical torque is a combination of the first mechanical torque and the second mechanical torque by the power split device19.

In some embodiments, when the propulsion system is operated in a regenerative or braking mode, the third mechanical terminal36receives a third mechanical torque from the load23, the first mechanical terminal32provides a first mechanical torque to the first AC drive device20, and the second mechanical terminal34provides a second mechanical torque to the second AC drive device22. The first mechanical torque and the second mechanical torque are split from the third mechanical torque by the power split device19. Then the first mechanical torque and the second mechanical torque may be converted into electric power by the AC traction system15for charging the ESS11.

In some embodiments, when the propulsion system is driving a light load, the first mechanical terminal32receives a first mechanical torque from the first AC drive device20, the second mechanical terminal34provides a second mechanical torque to the second AC drive device22, and the third mechanical terminal36provides a third mechanical torque to the load23. The second mechanical torque and the third mechanical torque are split from the first mechanical torque by the power split device19. Then the second mechanical torque may be converted into electric power by the second AC drive device22for charging the ESS11.

In some embodiments, the electric drive system50may include an optional transmission device21. The transmission device21may be mechanically coupled between the AC, traction system15and the power split device19for matching the output speed of the AC traction system15with the electric drive system speed.

In some embodiments, the transmission device21may be mechanically coupled between the first AC drive device20and the first mechanical terminal32of the power split device19. In some embodiments, the transmission device21may be mechanically coupled between the second AC drive device22and the second mechanical terminal34of the power split device19. In some embodiments, the transmission device21may be eliminated when the output speed of the AC traction system15meets requirements of the propulsion system.

The weight, size, and cost of the electric drive system may be decreased by selecting appropriate energy storage units, appropriate AC drive devices, and an appropriate combination of the energy storage units and the AC drive devices.

In some embodiments, the ESS11may include a battery and another battery used as the first energy storage unit12and the second energy storage unit14, respectively. In some embodiments, the ESS11may include a battery and an ultra-capacitor used as the first energy storage unit12and the second energy storage unit14, respectively. In some embodiments, the ESS11may include other kinds of energy source such as a flywheel battery.

In some embodiments, the AC traction system15may include an induction motor (IM) and another IM used as the first AC drive device20and the second AC drive device22, respectively. In some embodiments, the AC traction system15may include an IM and an interior permanent motor (IPM) used as the first AC drive device20and the second AC drive device22, respectively. In some embodiments, the AC traction system15may include other kinds of AC motor.

FIG. 3illustrates a diagram of an electric drive system100applied in an EV system (not shown) in accordance with an exemplary embodiment of the present disclosure. The electric drive system100includes an ESS102, a power conversion system104, an AC traction system106, an EMS108, a power split device122, and a transmission device119.

In some embodiments, the ESS102includes a battery101and an ultra-capacitor103. The AC traction system106includes a first AC drive device116and a second AC drive device118. The first AC drive device116includes a first DC/AC inverter111and an IM115. The second AC drive device118includes a second DC/AC inverter113and an IPM117. In some embodiments, a single power conversion device107is used which has been described in detail in the power conversion device18shown inFIG. 1.

Several power flow paths may be formed in the electric drive system100. In some embodiments, the battery101may be enabled to deliver electric power through the power conversion system104, which in turn provides converted electric power to at least one of the IM115and the IPM117, thereby at least a first power flow path is formed. In some embodiments, the ultra-capacitor103may be enabled to deliver electric power through the power conversion system104, which in turn provides converted electric power to at least one of the IM115and the IPM117, thereby at least a second power flow path is formed.

In some embodiments, the mechanical torques of at least one of the IM115and the IPM117firstly are converted into electric power and then the electric power may be delivered through the power conversion system104for charging at least one of the battery101and the ultra-capacitor103, thereby at least a third power flow path is formed. In some embodiments, the battery101may deliver electric power through the pre-charging switching device105for charging the ultra-capacitor103, thereby at least a fourth power path is formed.

A flexible charging strategy can be implemented to charge at least one of the battery101and the ultra-capacitor103. For example, in some embodiments, the ultra-capacitor103can be charged with electric power generated from the IM115and converted by the single power conversion device107. This is different from the prior charging strategy that charging the ultra-capacitor103through the use of pre-charging switching device105. In some embodiments, the EMS108may be configured to implement a decoupling algorithm for controlling the IM115and the IPM117separately.

FIG. 4illustrates a diagram of an electric drive system200with a first power conversion device207and a second power conversion device209coupled in parallel applied in an EV system (not shown) in accordance with an exemplary embodiment of the present disclosure. In some embodiments, the power split device121may include one or more planetary sets37for transmitting mechanical torques provided by the AC traction system106to the load123.

In some embodiments, the first power conversion device207includes a first terminal231and a third terminal235. The second power conversion device209includes a second terminal233and a fourth terminal237.

In some embodiments, electric power from IM115may be delivered to the ultra-capacitor103through the power conversion system204indirectly via the pre-charging switching device105, and electric power from the IPM117may be delivered to the ultra-capacitor103through the power conversion system204.

A decoupling control of the two AC motors115,117may be managed by controlling the first power conversion device207and the second power conversion device209separately due to different output DC voltages at the third terminal235and the fourth terminal237.

Several charging paths may be implemented for charging the ultra-capacitor103. In some embodiments, the battery101may deliver electric power through the pre-charging switching device105for charging the ultra-capacitor103. In some embodiments, the IM115may deliver electric power through the first power conversion device207and the pre-charging switching device105for charging the ultra-capacitor103. In some embodiments, the IPM117may deliver electric power through the second power conversion device209for charging the ultra-capacitor103. In some embodiments, at least a part of the mechanical torque provided from the IM115can be converted into electric power through the IPM117and the second power conversion device209, so that the ultra-capacitor103can be charged.

FIG. 5illustrates a diagram of an electric drive system300with a first power conversion device307and a second power conversion device309coupled in series applied in an EV system (not shown) in accordance with an exemplary embodiment of the present disclosure.

The first power conversion device307includes a first terminal331, a second terminal333, and a third terminal335. In some embodiments, the third terminal335is electrically coupled to the second AC drive device118and the second power conversion device309. The second power conversion device309includes an input terminal337electrically coupled to the third terminal335of the first power conversion device307, and an output terminal339electrically coupled to the first AC drive device116. In some embodiments, the power flow paths may also be implemented in a flexible manner similar to what has been described with reference toFIG. 3.

A decoupling control of the IM115and the IPM117may be achieved by controlling the first power conversion device307and the second power conversion device309separately due to different output DC voltages at the third terminal335and the output terminal339.

In some embodiments, the second power conversion device309is coupled to the second AC drive device118. In some embodiments, the power conversion system304may include more separate power conversion devices and a flexible manner in connecting these separate power conversion devices.

FIG. 6illustrates a flowchart of an energy management method600for controlling power flow in an electric drive system in accordance with an exemplary embodiment. At least some blocks of the method600may be programmed with software instructions stored in a computer-readable storage medium which, when executed by a processor, perform various steps of the method600. The computer-readable storage medium may include volatile and nonvolatile, removable and non-removable media implemented in any method or technology. The computer-readable storage medium includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other non-transitory medium which may be used to store the desired information and which may be accessed by an instruction execution system.

In some embodiments, the method600may start at block601, when a measured electric drive system speed Vspeedis received. Then the process goes to block603, in some embodiments, a required power Preqis calculated by a total torque request signal Ttotaland Vspeed. Then the process goes to block605.

At block605, a determination is made to enable at least one of the first AC drive device20and the second AC drive device22based at least in part on Vspeed. In some embodiments, the propulsion system may be operated in a normal mode based at least in part on Vspeed. The normal mode may include a starting mode, an accelerating or uphill mode, a cruising mode, and a regenerative or braking mode. In some embodiments, the propulsion system may be operated in a fault-tolerate mode according to measured current signals or voltage signals.

In some embodiments, the operating AC drive device and an operating manner (e.g., motor, generator) of the AC motor in the AC drive device are determined based at least in part on the operating mode and Preq. If both of the first AC drive device20and the second AC drive device22are enabled, the process goes to607, if not, the process goes to609.

At block607, several optimization algorithms may be implemented in a torque distribution unit for distributing the total torque request signal Ttotalinto a first torque command signal and a second torque command signal. The first torque command signal is provided to the first AC drive device20and the second torque command signal is provided to the second AC drive device22. Then the process goes to block609.

At block609, a determination is made to enable at least one of the first energy storage unit12and the second energy storage unit14based at least in part on the operating AC drive device and the structure of the power conversion system. In some embodiments, the operating manner (e.g., charger, discharger) of the first and second energy storage units is determined.

In some embodiments, a principle for determining the operating energy storage unit is that the first energy storage unit12is operated as a primary power supplier, the second energy storage unit14is operated as an assistant. In some embodiments, the first energy storage unit12provides power to the first AC drive device20and the second energy storage unit14provides power to the second AC drive device22.

In some embodiments, the second energy storage unit14provides an additional power when Preqrapidly that the first energy storage unit12alone may fail to provide the required power Preqdue to a limitation of its charge/discharge characteristic. In some embodiments, when the second energy storage unit14fails to provide the additional power, the first energy storage unit12can be relied on to provide the needed power.

In some embodiments, the power provided by the first AC drive device20and the second AC drive device22may be partly used to charge the second energy storage unit14when the propulsion system is operated in a regenerative or braking mode. Then the process goes to block611.

At block611, power control signals171based at least in part on the enabled AC drive device and the enabled energy storage unit is determined for controlling power converting process. The power control signals171may be obtained by the description below.

The electric power may be generated from the ESS11and provided to the AC traction system15. In some embodiments, a first input power received by the first terminal24may be converted into a first output power at the third terminal28and a second output power at the fourth terminal30.

In some embodiments, a second input power received by the second terminal26may be converted into another first output power at the third terminal28and another second output power at the fourth terminal30.

In some embodiments, a first input power received by the first terminal24and a second input power received by the second terminal26are converted into another first output power at the third terminal28and another second power at the fourth terminal30.

Then electric power received by the third terminal28and the fourth terminal30are selectively provided to the AC traction system15based on the operating AC drive device.

The electric power may be generated from the AC traction system15and be used to charge the ESS11, in some embodiments, when the first AC motor in the first AC drive device20is operated as a generator, the electric power generated from the first AC drive device20may be used to charge the ESS11. The electric power can be delivered to the second energy storage unit14through the power conversion device18directly or through the pre-charging device16via the power conversion device18indirectly.

In some embodiments, when the second AC motor in the second AC drive device22is operated as a generator, the electric, power generated from the second AC drive device22may be delivered to the ESS11through the power conversion device18directly.

In some embodiments, when the first AC motor in the first AC drive device20is enabled as a motor and the second AC motor in the second AC drive device22is enabled as a generator, the mechanical torque from the first AC drive device20may be split by the power split device19into a first mechanical torque and a second mechanical torque. The first mechanical torque is supplied to the load23, and the second mechanical torque is converted to an electric power for charging the second storage unit14through the power conversion device18.

In some embodiments, the power generated from the first energy storage unit12may be delivered to the second energy storage unit14through the pre-charging switching device16. Then the process goes to block613. At block613, the power control signals171is used to control the power flow among the energy storage system11, the power conversion system13and the AC traction system15. Then the process goes to block615, torque control signals173are provided to the AC traction system15for outputting desired torques for driving the load23.

The process described above may be modified in a variety of ways. In some embodiment, an additional block may be included before block601, at the additional block, a determination is made to ascertain whether a pre-charging process should be enabled according to a current, a voltage or a power Pesu2of the second storage unit14. In some embodiments, when Pesu2<P2(where P2is a predetermined value which represents a threshold that the second energy storage unit needs to be charged), the pre-charging process is enabled. Otherwise, the pre-charging process is disabled.

In some embodiments, the pre-charging process function is initiated before the propulsion system is enabled. The pre-charging process includes charging the second energy storage unit14from the first energy storage unit12through the pre-charging switching device16which may ensure that the second energy storage unit14can provide enough power quickly when needed. Then the process goes to block601.

FIG. 7illustrates a flowchart of determining whether both of the first AC drive derive and the second AC drive device are enabled shown inFIG. 6in an EV system in accordance with an exemplary embodiment of the present disclosure. The detailed illustration is based on the structure shown inFIG. 3. However, a person having ordinary skills in the art may apply the method disclosed herein to other propulsion systems.

In some embodiments, the flowchart700may start at block701, if the EV system fails, at least one of the measured current signals or the voltage signals exceeds predetermined value, the EV system is operated in a fault-tolerate mode as shown at block705. Otherwise, the EV system is operated in a normal mode as shown at block703.

When the EV system is operated in the fault-tolerate mode, at least one of the battery101, ultra-capacitor103, IM115, and IPM117fails. The normal parts may be reconfigured as a simplified electric drive system.

In some embodiments, one of the battery101and ultra-capacitor103with both of the IM115and IPM117are reconfigured. In some embodiments, both of the battery101and ultra-capacitor103with one of the IM115and IPM117are reconfigured. In some embodiments, one of the battery101and ultra-capacitor103with one of the IM115and IPM117are reconfigured. Therefore, the EV system may be allowed to stop in a safe way in the fault-tolerate mode with a fault-tolerant capability and reliability.

When the EV system is operated in the normal mode, after receiving a vehicle speed Vspeed707and calculating Preqwith Vspeedand Ttotal709, the process goes to four branches702,704,706, and708. The four branches will determine the operating AC motor and the operating manner. Index {1} is used to indicate that the IM115is enabled, index {2} is used to indicate that the IPM117is enabled, and index {1, 2} is used to indicate that both of the IM115and IPM117are enabled.

At branch702, the EN system is operated in a starting mode711when Vspeedincreases from about 0. In the starting mode, a determination is made to ascertain whether the IPM117can meet Preq719, if yes, the process goes to block723, in which the IPM117is enabled as a motor for providing necessary mechanical torque to meet Preq. If not the process goes to block725, in which the IM115is enabled as a motor is enabled as a motor for providing necessary mechanical torque to meet Preq.

At branch704, the EV system is operated in an accelerating or uphill mode713when Vspeedchanges with a sudden increase over a period of time. In the accelerating or uphill mode, the process goes to block727, in which the IM115and IPM117are enabled as a motors for providing necessary mechanical torques to meet Preq.

At branch706, the EV system is operated in a cruising mode715when Vspeedfluctuates within a scope of a small range. In the cruising mode, the process goes to block729, in which at least one of the IM115and IPM117is enabled as a motor for providing necessary mechanical torque to meet Preq.

At branch708, the EV system is operated in a regenerative or braking mode717when Vspeedchanges with a sudden decrease over a period of time. In the regenerative or braking mode, the process goes to block731, in which both of the IM115and IPM117are enabled as generators.

FIG. 8illustrates a flowchart of calculating two torque control signals through a torque distribution unit shown inFIG. 6in an EV system in accordance with an exemplary embodiment of the present disclosure. At block801, at least one signal such as Ttotalis received. At block803, a first AC motor torque signal referred to as “Mtorque1” and a second AC motor torque signal referred to as “Mtorque2” are received. At block805, a first AC motor speed signal referred to as “Mspeed1” and a second AC motor speed signal referred to as “Mspeed2” are received.

At block807, a first torque command signal referred to herein as “Ttorque1” and a second torque command signal referred to herein as “Ttorque2” are calculated through a torque distribution unit. The torque distribution unit may be embodied as software which may be implemented by the EMS17described above. In some embodiments, Ttotalmay be provided by either the first AC motor (e.g., IM115) or the second AC motor (e.g., IPM117) or both the first and second AC motors with an appropriate torque distribution algorithm.

In some embodiments, Ttorque1and Ttorque2may be calculated through a fixed proportion algorithm which is based at least in part on torque providing capability of the first and second AC motors as shown inFIG. 9. Waveforms901and905show the torque providing capability of the first and second AC motors (e.g., IM115and IPM117). In an embodiment, one of the first and second AC motors can meet the requirement of Ttotal, if not, the other AC motor will work to provide an additional torque according to the following equations:
a Ttorque1=Ttotal(0≦a≦1)  (1),
b Ttorque2=Ttotal(0≦b≦1)  (2),
a+b=1  (3).

Where a and b are two variables, in some embodiments, a and b may include a group of fixed values. When a=0, b=1, the first AC motor115itself provides Ttotal, when a=1, b=0, the second AC motor117itself provides Ttotal. In some embodiments, a and b may include several groups of fixed values, so that an optimal torque distribution method can be implemented.

In some embodiments, Ttotalmay be distributed to an average torque referred herein as “Taverage” and a peak torque referred herein as “Tpeak”. Ttorque1and Ttorque2are equal to Taverageand Tpeakrespectively. For example, a peak torque is needed, in particular, with a sudden acceleration. In an embodiment, one of the first and second AC motors can provide the peak torque in a quick and stable response.

As shown inFIG. 9, the first AC motor can provide a large torque within a narrow speed range and the second AC motor can provide a smaller torque with a quick response within a wide speed range. In some embodiments, the second AC motor is used as an assistant which can work on and off. Basically, the algorithm is according to the following equations:
Ttotal=Taverage+Tpeak(4),
Ttorque1=Taverage(5),
Ttorque2=Tpeak(6).

In some embodiments, Ttorque1and Ttorque2may be calculated through an algorithm of maximizing the system efficiency according to the high efficiency areas903,907of the first and second AC motors shown inFIG. 9. In an embodiment, both the first and second AC motors can work in their own high efficiency areas by using the second AC motor to assist the first AC motor. In some embodiments, the transmission device21is used to change the operating point which can be implemented by changing Mspeed1(e.g., waveforms913,915).

In some embodiments, when Mspeed1and Mspeed2are fixed values, the operating points of the first and second AC motors may move along with waveforms909,911within the scope of the high efficiency areas903,907. Thereby the total loss can be minimized. The algorithm may be determined according to the following equations:
Ttorque1+Ttorque2=Ttotal(7)
Ploss1+Ploss2=min  (8).

Where Ploss1and Ploss2are power loss of the first and second AC motors respectively. This method functions through checking a look up table defining high efficiency areas of the two AC motors according to efficiency maps of each AC motor in the first AC drive device20and the second AC drive device22for reducing the total loss based at least in part on the motor speeds Mspeed1and Mspeed2.

FIG. 10illustrates a flowchart of enabling at least one of the first energy storage unit and the second energy storage unit is enabled shown inFIG. 6in accordance with an exemplary embodiment of the present disclosure. Preqis used to represent a required power, Pbatis used to represent a power provided by the battery101, Pucis used to represent a power provided by the ultra capacitor103, Pbat(threshold)is used to represent an upper limit power provided by the battery101, Puc(threshold)is used to represent an upper limit power provided by the ultra-capacitor103, and Puc(charging)is used to represent a required charging power of the ultra-capacitor103.

At block1001, Preqand motor index are received. Then process goes to block1003, a determination is made to ascertain which branch the process will go. The process goes to block1005when receiving index {1} or index {1, 2}. Otherwise, the process goes to block1007when receiving index {2}.

At block1005, if Preqis larger than Pbat, the process goes to block1009. Otherwise, the process goes to block1011in which the battery101provides the power. At block1009, if Puc(threshold)is larger than an additional power Preq−Pbat(threshold), the process goes to block1013that the battery101provides the upper limit power itself and the ultra-capacitor103provides the additional power.

At block1007, if Preqis larger than Puc(threshold), the process goes to block1017. Otherwise, the process goes to block1019that the ultra-capacitor103provides the power. At block1017, if the ultra-capacitor103requires to be charged, the process goes to1021that the ultra-capacitor103receives the required charging power Puc(charging), and the battery101provides both the required power Preqand the required charging power of the ultra-capacitor103Puc(charging). Otherwise, the process goes to block1023that the ultra-capacitor103provides the upper limit power itself and the battery provides the additional power.

As will be understood by those familiar with the art, the present invention may be embodied in other specific forms without depending from the spirit or essential characteristics thereof. Accordingly, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.