HIGH POWER-DENSITY, HIGH BACK EMF PERMANENT MAGNET MACHINE AND METHOD OF MAKING SAME

Abstract

An electric drive system includes a permanent magnet machine having a rotor and a stator and a power converter electrically coupled to the permanent magnet machine and configured to convert a DC link voltage to an AC output voltage to drive the permanent magnet machine. The power converter includes a plurality of silicon carbide switching devices having a voltage rating that exceeds a peak line-to-line back electromotive force of the permanent magnet machine at a maximum speed of the permanent magnet machine.

Claims

1. An electric drive system comprising: a DC link; a permanent magnet machine comprising a rotor and a stator; and a power converter coupled between the permanent magnet machine and the DC link, the power converter comprising a plurality of silicon carbide (SiC) switching devices having a voltage rating that exceeds a voltage of the DC link when the permanent magnet machine operates in an uncontrolled generation mode.

2. The electric drive system of claim 1 wherein the voltage rating of the plurality of SiC switching devices is at least three kV.

3. The electric drive system of claim 1 wherein the plurality of SiC switching devices comprise SiC MOSFETs.

4. The electric drive system of claim 1 further comprising: a power source coupled to the DC link; and a controller programmed to disconnect the power source from the DC link based on a detected line-to-line back emf of the permanent magnet machine.

5. The electric drive system of claim 1 further comprising: a DC power source; a bi-directional DC-to-DC power converter configured to boost an output voltage of the DC power source and supply the boosted output voltage to the DC link; and an input filter capacitor coupled between a positive DC rail and a negative DC rail of the DC link.

6. The electric drive system of claim 1 wherein the power converter comprises a multi-phase power converter.

7. The electric drive system of claim 1 wherein the permanent magnet machine comprises a traction motor.

8. A vehicle drive system comprising: an alternator; and a power converter coupled to the alternator, the power converter comprising a plurality of gate driven silicon carbide (SiC) switching devices having a voltage rating that exceeds a peak line-to-line back electromotive force (emf) of the alternator.

9. The vehicle drive system of claim 8 wherein the peak line-to-line back emf of the alternator is defined at a maximum speed of the alternator.

10. The vehicle drive system of claim 8 further comprising a power source coupled to the to the power converter through a DC link.

11. The vehicle drive system of claim 10 further comprising at least contactor configured to disconnect the power source from the DC link.

12. The vehicle drive system of claim 11 wherein the at least one contactor comprises a first contactor positioned on a positive rail of the DC link and a second contactor positioned on a negative rail of the DC link.

13. The vehicle drive system of claim 10 further comprising a controller programmed to disconnect the power source from the DC link if a detected line-to-line back emf of the alternator is within a threshold percentage of the voltage rating of the power source.

14. The vehicle drive system of claim 10 wherein the power source is a DC power source.

15. The vehicle drive system of claim 14 further comprising a bi-directional DC-to-DC voltage converter coupled between the DC power source and the DC link.

16. The vehicle drive system of claim 8 wherein the alternator is coupled to a heat engine.

17. The vehicle drive system of claim 8 wherein the plurality of gate driven SiC switching devices comprise SiC MOSFETs.

18. A power converter for use in a drive system of a hybrid electric vehicle, the power converter comprising a plurality of silicon carbide (SiC) switching devices having a voltage rating of at least three kV.

19. The power converter of claim 18 wherein the SiC switching devices comprise SiC MOSFETs.

20. A power converter for use in a drive system of a hybrid electric vehicle, the power converter comprising a plurality of switching devices, each of the plurality of switching devices consisting of a silicon carbide (SiC) MOSFET and an anti-parallel diode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The drawings illustrate embodiments presently contemplated for carrying out the invention.

[0013] In the drawings:

[0014] FIG. 1 illustrates a conventional permanent magnet machine drive system.

[0015] FIG. 2 illustrates a high-power density permanent magnet machine drive system, according to an embodiment of the invention.

DETAILED DESCRIPTION

[0016] FIG. 1 illustrates a conventional three-phase permanent magnet machine drive system 10. System 10 includes a DC link 12 that provides a DC input voltage that is converted or inverted to an AC waveform that powers a permanent magnet machine 14. An input filter capacitor 16 is coupled across the DC link 12 for filtering the voltage V.sub.AC on the DC link 12. A power converter 18 receives the input voltage from DC link 12 when power flows from the DC link 12 to the AC permanent magnet machine 14. This direction of power flow is often referred to operating in a motoring mode. When the direction of power flow is from the permanent magnet machine 14 to the power converter 18, the input voltage to the power converter 18 is AC from the permanent magnet machine 14, while the output from the power converter 18 is a DC voltage on the DC link 12. Operation with power flow from the AC permanent magnet machine 14 to the power converter 18 is often referred to operation in a regenerative braking mode that is useful, for example, in a vehicle where it is desirable to hold a given value of speed on a downhill grade, or while decelerating the vehicle.

[0017] Power converter 18 is a typical 3-phase inverter having two series-connected switching devices per phase leg. For example, devices 20 and 22 form a first phase leg, devices 24 and 26 form a second phase leg, and devices 28 and 30 form a third phase leg. Devices 20-30 are conventional silicon semiconductor switching devices such as, for example, silicon IGBT, MOSFET, silicon bi-polar Darlington power transistor, GTO, SCR, or IGCT type devices. Diodes 32, 34, 36, 38, 40, 42 are coupled in anti-parallel relationship across respective silicon switching devices 20-30.

[0018] FIG. 2 illustrates a permanent magnet machine drive system 44 in accordance with an embodiment of the invention. Drive system 44 includes a DC link 46 having a DC source voltage V.sub.S 48. Drive system 44 includes a power source 50 that provides DC source voltage V.sub.S 48. Drive system 44 includes preferably two contactors (C1, C2) 52, 54, or at least one contactor C1 to couple or disconnect DC link 46 from power source 50. In one embodiment, power source 50 includes an AC source 58 and a rectifier 56 configured to convert a voltage of AC source 58 to the DC link or source voltage V.sub.S. In another embodiment, power source 50 includes a DC power source 58, such as a battery, a fuel cell, or a flywheel with associated power electronic converter. In yet another embodiment, power source 50 includes a DC power source 58, such as a battery, a fuel cell, an ultracapacitor, or a flywheel with an associated power electronic control coupled to a bi-directional DC-to-DC voltage converter 56 that boosts the source voltage to the DC link or source voltage V.sub.S. DC link 46 supplies a DC output voltage V.sub.DC 60 to a power converter or inverter 62. An input filter capacitor 64 is illustrated between a positive DC rail 66 and a negative DC rail 68 and serves to provide a filter function for the high frequency currents from source 50 to ensure the power quality between positive and negative rails 66, 68.

[0019] Power converter 62 receives DC input voltage V.sub.DC 60 from DC link 46 and converts the DC input voltage to provide a suitable form of AC power for driving a permanent magnet machine 70, described in detail below. A controller 72 is also included in drive system 44 and includes means to open and close contactors Cl and C2 52, 54 based on sensed voltage inputs from V.sub.S 48, V.sub.DC 60, speed sensor inputs from machine 70, plus operator inputs as well as detected faults that may occur in power converter 62. Controller 72 also includes means to control the boost power command to the bi-directional boost converter 56.

[0020] According to one embodiment, power converter 62 is a three-phase DC to AC inverter having a plurality of switching devices 74, 76, 78, 80, 82, 84. Each switching device 74-84 includes a silicon carbide (SiC) MOSFET 86, 88, 90, 92, 94, 96 and an associated anti-parallel diode 98, 100, 102, 104, 106, 108.

[0021] SiC is a crystalline substance that has material properties that make it an attractive alternative to silicon for high voltage, and high power applications. For example, SiC has a large bandgap that provides a very low leakage current, which facilitates elevated temperature operation. In fact, semiconductor devices manufactured on a SiC substrate can withstand temperatures in excess of 200 degrees C. SiC also has a high breakdown field that is about ten times that of silicon and a thermal conductivity that is about three times that of silicon, allowing higher power densities to be accommodated with SiC circuits. Further, SiC's high electron mobility enables high-speed switching. Thus, SiC has been considered as an advantageous material for use in the manufacture of next generation power semiconductor devices. Such devices include, for example, Schottky diodes, thyristors, and MOSFETs.

[0022] Moving from left to right in FIG. 2, switching devices 74, 76 are associated with a first output phase 110, switching devices 78, 80 are associated with a second output phase 112, and switching devices 82, 84 are associated with a third output phase 114. While a three-phase power converter and three-phase permanent magnet machine 70 are illustrated in FIG. 2, one skilled in the art will understand that embodiments of the present invention are equally applicable to a single-phase or other multi-phase embodiments. For example, alternate embodiments include configurations with varying number of phases, e.g., n-phase, where n=1, 2, 4, 5, 7, or higher number, where each phase of the power converter includes a plurality of switching devices similar to devices 86, 88, each with associated anti-parallel diodes similar to diodes 98, 100.

[0023] Power converter 62 drives a permanent magnet machine 70. In one embodiment, permanent magnet machine 70 is a traction motor that includes a permanent magnet rotor 116 and a stator 118, such as, for example, a traction motor for powering an electric vehicle. Permanent magnet rotor permanent magnet rotor 116, may be configured as a surface mount, interior, or buried permanent magnet rotor, according to various embodiments. In an alternate embodiment, permanent magnet machine 70 is an alternator that includes a permanent magnet rotor 116 and a stator 118, such as, for example, a permanent magnet alternator coupled to a heat engine within an Auxiliary Power Unit (APU) for generating electrical power to aid in the operation of a hybrid-electric vehicle (HEV) or a Plug-in Hybrid-Electric Vehicle (PHEV).

[0024] The high voltage rating of SiC MOSFETs 86-96 allows permanent magnet machine 70 to be designed with a high back emf without having to worry about the uncontrolled generation mode, thereby significantly increasing the power density of permanent magnet machine 70. That is, SiC MOSFETs 86-96 have a voltage rating that exceeds the DC link voltage during an uncontrolled generation mode of permanent magnet machine 70. Conventional Si IGBT power modules used power converter circuits in commercially available on-road EV, HEV, and PHEV typically have a voltage rating of approximately 600 V or 1,200 V for some larger or high performance vehicles, including SUV's, trucks, and buses. According to one embodiment, SiC MOSFETs 86-96 are high voltage SiC MOSFETs manufactured by General Electric Company having a voltage rating of approximately three to four kV. The combined high voltage SiC power converter 62 combined with high power density multi-phase permanent magnet machine 70, allows upwards of two-to-four times power density with a substantial improvement in fault tolerance during periods of loss of control over the power converter 62 or loss of gate drive to the power modules within the power converter 62. Because SiC MOSFETs 86-96 can be manufactured to be physically smaller than conventional silicon MOSFETs, SiC MOSFETs 86-96 can be packaged in an automotive environment and can be operated at higher temperatures.

[0025] Excessive emf voltage of permanent magnet machine 70 may damage DC power source 58 of power source 50. Accordingly, in one embodiment, controller 72 is configured to detect a fault in power converter 62 and the associated gate drive circuitry of power converter 62. For example, a fault may be detected if the line-to-line back emf is within a threshold percentage of the voltage rating of DC power source 58. If a fault is detected, controller 72 may be programmed to disconnect or decouple DC power source 58 from power converter 62. Accordingly, excessive emf voltage created by permanent magnet machine 70 during a fault condition within power converter 62 will not result in overvoltage damage to DC power source 58. The high voltage rating of SiC power converter 62 and its associated components 86-96 will withstand the back emf from the high-power permanent magnet machine 70, even if a potential fault occurs while machine 70 is operating at high speed.

[0026] Therefore, according to one embodiment of the invention, an electric drive system includes a permanent magnet machine having a rotor and a stator and a power converter electrically coupled to the permanent magnet machine and configured to convert a DC link voltage to an AC output voltage to drive the permanent magnet machine. The power converter includes a plurality of SiC switching devices having a voltage rating that exceeds a peak line-to-line back emf of the permanent magnet machine at a maximum speed of the permanent magnet machine.

[0027] According to another embodiment of the invention, a method of manufacturing an electric drive system includes the step of providing a SiC power converter that has a plurality of SiC switching devices and is coupleable to a power source. The method also includes the steps of providing a permanent magnet machine having a peak line-to-line back emf at maximum speed that is greater than a DC link voltage of the power source and coupling the SiC power converter to the permanent magnet machine to drive the permanent magnet machine.

[0028] According to yet another embodiment of the invention, a vehicle drive system includes a motor that has a permanent magnet rotor and a stator. The drive system also includes a DC link and a power converter electrically coupled between the DC link and the permanent magnet motor to drive the permanent magnet motor. The power converter comprises a plurality of SiC switching devices rated for a higher operating voltage than a maximum back emf capable of being developed in the stator of the permanent magnet motor.

[0029] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.