US 7242167 B2
A permanent magnet starter/generator subsystem configured in a fault tolerant architecture is described herein for small engine applications. The system allows for lighter system weight, improved system reliability, higher performance capability and reduced maintenance.
1. A permanent magnet starter/generator system including:
a permanent magnet generator (PMG) unit;
a power conversion unit (PCU) connected to the permanent magnet generator including a plurality of converter modules connected to a power output bus; and
a filter disposed between and electrically interconnected to the plurality of converter modules and the power output bus; and
wherein the power output bus is connected to at least one of a vehicle electrical load and a vehicle electrical power distribution system;
the converter modules are connected to a control module;
the power converter modules include a plurality of bi-directional power factor corrected full wave bridge modules, a plurality of isolated bi-directional converter DC-DC modules, and a plurality of feedback control modules;
the isolated DC-DC bi-directional converter modules are each connected to a filter module, one of the plurality of bi-directional power factor corrected full wave bridge modules, and one of the plurality of feedback control modules;
the bi-directional power factor corrected full wave bridge modules are each connected to one phase of a plurality of PMG phases, and one of the plurality of bi-directional power factor corrected full wave bridge modules; and
wherein the feedback control modules are each connected to one of the plurality of isolated DC-DC bi-directional converter modules and an equalizer and phase balance control module within the PCU control module.
This patent application is a divisional application of U.S. Ser. No. 10/830,934 entitled “Fault Tolerant Architecture For Permanent Magnet Starter Generator Subsystem” filed on Apr. 23, 2004, now U.S. Pat. No. 7,064,526 the entirety of which is incorporated herein by reference.
Starter/generator subsystems for small engine applications typically utilize DC brushed motor/generators and an electronic Generator Control Unit (GCU). The DC brushed machine has a number of disadvantages in comparison to a permanent magnet machine of equivalent capacity, including increased size and weight, significantly lower reliability and higher maintenance. The DC brushed machine requires relatively frequent maintenance, including replacement and servicing of brushes and bearings. Typically, maintenance intervals are 600 to 1000 operating hours for brushed machines in small aircraft applications. The brushed starter/generator also typically exhibits significant reductions in generator capability when operating at low speeds, and small aircraft applications often require operation over relatively wide speed ranges (typically 50% to 100% operational range in the generator mode).
Currently, permanent magnet generators (PMGs) are sometimes used as starter/generator subsystems for small engine applications. However, the PMG starter/generator subsystems of the current art have several disadvantages. Since the output voltage and frequency of the PMG varies directly with driveshaft speed, a more complicated regulation and start excitation control method is required, as compared to that of a brushed starter/generator. Also, the PMG output voltage cannot be controlled by means of adjusting stator field excitation (which is a common control technique used with brushed starter/generators), and therefore the PMG output cannot be electronically disabled in the event of a fault. The inability to disable PMG output power during fault mode operation is of particular concern in high reliability applications such as aircraft. A feeder cable short to engine or aircraft structure can result in a hazardous or unsafe condition for typical PMG/Power Conversion Unit (PCU) architectures. A series contractor or circuit breaker is typically required to disable the PMG output voltage. However, the PMG is typically located on or near the engine system and must survive an extremely harsh environment, particularly in aircraft applications. That harsh environment makes it very difficult (and often not technically feasible) to locate a contactor or circuit breaker close to the PMG.
U.S. Pat. No. 5,929,537 describes a starter/generator subsystem utilizing a PMG and bi-directional PCU. However, the starter/generator architecture as described and illustrated in this patent is a non-isolated topology PMG and non-isolated topology PCU. Therefore, very few if any of the “fault tolerant” aspects are achieved. The U.S. Pat. No. 5,929,537 patent also describes an architecture requiring contactors to isolate the PCU, batter, and/or AC loads. The use of high power contactors to isolate these subsystems is necessary for practical non-isolated architectures since a single break-down in the feeder cable to aircraft structure can result in a catastrophic failure condition. The PCU described in the U.S. Pat. No. 5,929,537 patent does not provide several reliability and/or performance features, including power factor corrected generator mode operation; ability to detect ground faults within the PMG, PCU and/or feeder cables; failsafe operation capability including operation in the presence of shorted windings in the PMG, shorted feeder wires, and/or failed PCU converters; or soft start and/or motoring start capability.
U.S. Pat. Nos. 5,594,322 and 5,493,200 describe a generator design with a field winding on the rotor that receives field current for control of the generator output. The PMG described in these patents is entirely different than the PMG design described for this invention. The subsystems described in the U.S. Pat. Nos. 5,594,322 and 5,493,200 patents also do not incorporate the fault tolerant capabilities described for the subsystem described by this invention.
The present invention relates to a starter/generator subsystem for small engine applications that utilizes a permanent magnet generator (PMG) and an electronic Power Conversion Unit (PCU). Further, the permanent magnet generator (PMG) does not require a field winding. The PMG and PCU architecture of this invention incorporates isolated subsystems and fault detection capabilities that lead to a highly reliable and fault tolerant system.
The fault tolerant architecture described herein enables safe operation even in the presence of various fault scenarios, and includes failsafe and fault monitoring features to ensure that appropriate actions can be taken to preclude potentially hazardous conditions. The advantages of the fault tolerant architecture and the associated PMG/PCU subsystem include lighter system weight, improved system reliability and higher performance capability.
The architecture achieves reduced weight for typical Electrical Power Generation and Distribution Systems (EPGDS), for several reasons. The PMG is significantly lighter, smaller and produces less nacelle drag than an equivalent performance brushed starter/generator (S/G). Also, while the PCU is larger and heavier than a GCU for a brushed S/G subsystem, the PCU advantageously incorporates a number of other functions of the EPGDS that are not included in typical GCUs, including ground fault detection, high current contactor functionality, bus current monitoring, fault monitoring and reporting functionality, and intelligent loan sharing operation. Therefore, the total system weight typically favors the PCU based EPGDS. Further, the feeder cable between the PMG and PCU can be much lighter weight than the DC bus feeders (assuming the PMG/PCU architecture is designed appropriately). Additional reductions in EPGDS weight are typically applicable for the architecture since the PCU can be located remote from the engine so that DC bus cables are shorter and less complex.
The architecture also provides improvements in system reliability. The fault tolerant architecture is configured so that three (although other numbers could be used) isolated and independently operable PMG and PCU phases are utilized, which allows generator capability to be retained even in the event of various fault modes. Also, the PMG contains no brushes and hence the associated high failure rate and high incidence of required maintenance is eliminated. Additionally, the PMG can be configured as a bearingless machine, and hence the associated failure rate and high incidence of maintenance is reduced. The system, when configured with a bearingless PMG assembly, is substantially maintenance-free and achieves an estimated Mean Time Before Failure of 20,000 hours or higher. Further, the PMG is designed such that continuous operation into a shorted feeder or stator winding will not result in overheating of the machine or a hazardous condition.
The architecture also improves system reliability in other ways. The PCU is designed so that a failure in one power converter will not propagate to or degrade the generator mode performance of other power converters. Also, the PMG and the PCU are designed so that generator mode operation is achieved even in the event of a failure of up to two phases of the PMG and/or simultaneous failure of up to two phases of the PCU (for a three-phase system; note, however, that reduced output power capacity may be applicable in the event of the described fault mode operation). Additionally, the PMG and the PCU are designed so that each phase is electrically and physically isolated. Therefore, a short-circuit (short) to the engine or aircraft structure within the stator winding, PMG/PCU feeder cable or PCU power converter stage will not result in degraded generator operation. In a three-phase system, up to three simultaneous shorts can be accommodated without generator operational degradation.
The architecture additionally enhances system reliability in other manners. The PCU is designed to continuously monitor for and detect a ground fault condition within any phase of the PMG and/or PCU. The ground fault detection function is fully testable via external command so that a complete end-to-end functionality verification of the ground fault detection circuit can be performed upon command. Also, the PCU can be designed to utilize multiple, parallel solid-state switching devices so that high performance and high reliability operation is achieved, as well as fault tolerant operability within each PCU converter.
The architecture also allows for higher performance capability. The PMG and the PCU are designed so that greater output capability is achieved at low engine speeds. The output power capability of a PMG is directly proportional to drive speed whereas brushed starter/generator machines typically lose output power capability at a much greater rate with reduced drive speed. The subsystem weight penalty for full generator output power capability at engine idle conditions is much greater for brushed starter/generator machines than for PMGs. Also, the PCU static and dynamic regulation capability is superior to present state of the art starter/generator subsystems since the PCU utilizes solid-state components and a high switching frequency converter. Further, the PCU can be designed for parallel operation capability and inherent load sharing without requiring control signal interconnects between PCUs or PMGs.
Additional features of the architecture also provide higher performance capability. The PCU can incorporate smart control, built-in-test, fault reporting and diagnostic trend capabilities, including a data bus interface. Further, a soft start operational capability is easily implemented with the PMG/PCU design and results in significantly less stress to associated engine drive shaft or gear train components as well as reduced battery current demand during start mode operation. The soft start characteristic is fully controllable (and can be custom tailored) by design of the PCU and can control both applied torque and transition times. Also, the PCU is designed so that engine start can be achieved with or without “motoring” of the PMG. That is, the PMG can be at any initial rotational speed (including low speed reverse rotation) prior to start and the PCU can be designed to automatically execute an engine start function. The engine start control algorithm is fully controllable (and can be custom tailored) by design of the PCU.
One embodiment of a PMG 101 is a General Dynamics (GD) part number 1209 PMG. The GD part number 1209 PMG 101 is a brushless and bearingless machine. Further, the part number 1209 PMG 101 is designed in accordance with the fault tolerant architecture described herein and is constructed with windings that are electrically isolated from machine structure and from the other windings of the PMG 101.
One embodiment of a PCU 102 is a General Dynamics (GD) part number 1208 PCU and is described below. The GD part number 1208 PCU 102 includes three converter modules 104, 105, 106. Each converter module is connected a single phase of the PMG 101 through a single pair of wires. The converter modules each include a transformer which is connected to an output filter 107. The output filter 107 attenuates high frequency components of the voltage that leaves the PCU 102 and is provided to the aircraft loads. The voltage that leaves the part number 1208 PCU 102 is regulated to 28.5 VDC (but other generator mode output voltages are possible). The part number 1208 PCU 102 also includes a connection for the 28.5VDC output return current that is isolated from the PCU 102 chassis. The converter modules, each operate as a bi-directional converter and each provide an electrically isolated interface to the PMG 101. When the starter/generator subsystem is operated as a generator, the converter modules 104, 105, 106 convert the AC voltage from the PMG 101, to a DC voltage of the desired form. When the starter/generator subsystem is operated as a starter, the converter modules 104, 105, 106 convert a DC voltage, that is derived from a source (typically a battery) connected to the PCU 102 output, to an AC voltage of the desired form. The AC voltage is applied to the PMG 101, to operate the PMG 101 as a motor and cause the engine to rotate and thus begin an engine start operation. The converter modules 104, 105, 106 are connected to the PMG by the plurality of pairs of wires 103. Additionally, the PCU 102 typically includes a control module 108 connected to the converter modules 104, 105, 106 which provides various control and synchronization functions including monitoring of various operating conditions which enable the PCU 102 to achieve the sensorless control of the PMG 101 during engine starts, as well as performing various fault monitoring and built-in-test functions. The control module 108 is typically configured to monitor various internal PCU 102 operating functions and external system operating functions are performing correctly.
In the PCU 102 embodiment illustrated in
The Speed/Phase Detection and Control module 204 of
The EMI Filter module 209 of
The various bus voltages within the PCU 102 are typically selected to optimize the overall system performance, including weight, reliability, output voltage regulation and starter torque. In the PCU 102 embodiment of
The PCU 102 when configured in the fault tolerant architecture of this invention, can provide generate mode power even in the presence of faults on the PMG 101 and/or PCU 102 feeder cables or the faults within the Bridge modules 201 or DC-DC Converter modules 202. Since each phase of the PMG 101 is electrically isolated from the other phases and from the vehicle structure, a feeder cable that has a conductor shorted to the vehicle structure will have no impact upon PMG 101 or PCU 102 operation. In fact, up to three simultaneous conductors or components within the Bridge modules 201 or DC-DC Converter modules 202, can be tolerated without any deleterious impact upon the starter/generator subsystem performance. The PMG 101 is also designed so that a shorted phase winding or a short within a phase pair of feeder cable wires will not cause the PMG 101 to overheat or produce an unsafe operating condition. A short condition within a phase of the PMG 101 or within a phase of the PCU 102 Bridge modules 201 or DC-DC Converter modules 202, will not result in complete loss of the generator mode output power for this fault tolerant architecture. A loss of one phase will result in an approximate 33% reduction in generator mode output power capacity and a loss of two phases will result in an approximate 67% reduction in generator mode output power capacity.
A “motoring start” is when the rotational speed of the PMG is measured prior to initiating an engine start and the PCU automatically adjusts the AC control excitation to “lock-in” on the initial rotational speed of the PMG and start accelerating from that speed instead of beginning the PMG speed control from a lower speed.
The PMG 101 can also be designed for operation as an integral assembly on an engine shaft (i.e., PMG rotates at same speed as a main or auxiliary engine shaft; no bearings are required) or a shaft mounted, gearbox integrated assembly such as the GD part number 1209 PMG 101 (i.e., PMG rotor is mounted directly to driveshaft on engine accessory gearbox (AGB)). Further, the PMG may be designed for operation as an AGB mounted assembly (i.e., the PMG incorporates a spline and bearings so that the assembly interfaces to engine AGB in similar or identical fashion to conventional brushed S/Gs). Additionally, an optional contactor or circuit breaker can be added in series with the PMG output to remove power from the feeder cables and/or PCU input. The use of this contactor or circuit breaker is optional with the “fault tolerant” architecture.
Other embodiments and uses for the invention also exist. The fault tolerant architecture applications include aircraft and/or automotive applications requiring engine start operation and generation of DC electrical power. The applications also include aircraft and/or automotive applications requiring engine start operation and generation of AC electrical power. Further applications include aircraft and/or automotive applications requiring engine start operation and generation of DC and AC electrical power.
The PCU 102 can be designed for operation as a single assembly located adjacent to or remote from PMG 101. The PCU 102 may also be designed for operation as multiple assemblies located adjacent to or remote from PMG 101. Further the PCU 102 and PMG 101 may be interconnected with a low current feeder cable (i.e., the PMG 101 is designed to produce a voltage that is greater than the average PCU 102 output voltage) or interconnected with a high current feeder cable (i.e., the PMG 101 is designed to produce a voltage that is equal or less than the average PCU 102 output voltage).