Electricity: single generator systems – Automatic control of generator or driving means – Plural conditions
Reexamination Certificate
1999-12-20
2001-08-21
Ponomarenko, Nicholas (Department: 2834)
Electricity: single generator systems
Automatic control of generator or driving means
Plural conditions
C322S038000, C307S084000
Reexamination Certificate
active
06278262
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to auxiliary power units (“APU”s), and more particularly to controllable APU systems that allow an APU powering an AC generator to be brought online and off-line without imparting high shock torques.
BACKGROUND OF THE INVENTION
APU systems are typically used as adjuncts to primary engines, in aircrafts and the like. An APU, typically in the form of an independent secondary gas turbine, provides shaft power to drive interconnected equipment requiring constant speed operation. Often, an APU is used to drive an electrical generator forming part of an APU system. APUs and APU systems are often used as secondary power sources when an associated primary engine is not fully operational. For example APU systems may be used to provide back-up power to the electrical power system of an aircraft, while grounded or during an in-flight emergency.
Often APU systems include gearboxes used to match the speed and torque requirements of interconnected generators. Most typically, the gearbox hard-couples the APU to the generator. During transient conditions, such as, for example, when the APU system is brought on and off-line, the gearbox and APU may be subjected to high impact torques.
Accordingly, the use of one or more clutches to disconnect the gearbox has been suggested. Such a clutch arrangement is, for example, disclosed in U.S. Pat. No. 5,174,109.
The APU generator, however, is often used to provide power to an operational electrical system. Mere use of a clutch will not necessarily guard against mechanical shocks imparted as a result of electrical phase and frequency differences between the APU generator output and the operational electrical system.
Accordingly, an improved APU system allowing electrical generators to be brought on-line and off-line, smoothly without imparting or being the subject of unnecessary torques is desirable.
SUMMARY OF THE INVENTION
In accordance with an aspect of the present invention, an APU generator forming part of an APU system, is brought “online” smoothly in order to replace a primary generator operating at a steady-state rotational speed that is interconnected with a load. The rotational speed of the APU generator is adjusted until it is beneath the steady-state rotational speed. Then, the APU generator is interconnected to the load and the primary generator. This causes the APU generator to accelerate and motor at the steady-state rotational speed dictated by the primary generator. Then, the rotational speed of the APU generator may be increased until it provides power to the load. Thereafter, the primary generator may be disconnected from the load.
Advantageously, by allowing the APU generator to motor at the steady-state rotational speed of the primary generator, no high impact shocks are imparted to the generator. Moreover, preferably, the APU generator is coupled to an APU by an over-running clutch so that no torque is transferred from the generator to the APU, while accelerating to its steady-state speed.
Similarly, a primary generator may be re-connected to an electrical load, initially powered by an APU generator forming part of an APU system, in accordance with an aspect of the present invention. Specifically, the rotational speed of the APU generator may first be adjusted to a set-point beneath the steady-state rotational speed of the primary generator. Then, the primary generator operating at the steady-state rotational speed may be interconnected to the load and the APU generator. This again causes the APU generator to accelerate and motor at the steady-state rotational speed. Now, as the APU generator is no longer providing power to the load, it may be disconnected from the load. Again, preferably, an over-running clutch prevents the torque from being transferred to the APU.
The invention may be embodied in an APU system including an electric generator, an engine and a controller in communication with the engine, controlling the system to operate in accordance with these methods of operation.
Other aspects and features of the present invention will become apparent to those of ordinary skill in the art, upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
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Astle Jeffrey W.
Ponomarenko Nicholas
Pratt & Whitney Canada Corp.
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