Electricity: single generator systems – Automatic control of generator or driving means – Speed or frequency of generator
Reexamination Certificate
1999-08-05
2001-02-13
Mullins, Burton (Department: 2834)
Electricity: single generator systems
Automatic control of generator or driving means
Speed or frequency of generator
C322S059000, C322S061000, C322S063000, C310S159000
Reexamination Certificate
active
06188204
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to AC generators, specifically to the incorporation of a field system which does not require brushes or sliding contacts and which permits speed changes without changes in the AC frequency.
In conventional alternators the magnetic poles on the rotor have a fixed orientation with respect to the rotor. The fixed magnetic poles may be provided by having permanent magnets mounted on the rotor. The more common practice is, however to have field windings on the rotor which are supplied with DC current to create the magnetic poles with fixed orientation. The arrangement for providing this DC field current is called the excitation system of the generator. Typically the excitation system consists of a separate machine which is mechanically coupled to the shaft. Historically the exciter machines were DC generators. The DC output from the exciter was fed to the field windings on the rotor through brushes which make sliding contacts with slip rings mounted on the rotor shaft. Since the DC generator also has to have brushes on its commutator, such an excitation system involved the use of several brushes and rubbing contacts, resulting in the need for periodic brush replacements and related maintenance procedures and shut downs. Therefore brushless excitation systems came to be developed. In a typical brushless excitation system the exciter machine is an AC generator in which the AC windings are placed on its rotor. The AC output from these windings is rectified by rotor mounted diodes and fed directly to the field of the alternator without the need for any brushes or slip rings. The adjustment of the field flux is achieved by adjusting the field current of the exciter. This can be done from the stationary frame in both types of exciters. In the permanent magnet machine such a facility for adjustment does not exist. In all such AC generators with DC excitation the frequency of the generated AC is fixed by the rotational speed and the number of poles for which the generator is designed. Therefore, to ensure a constant frequency output, as is needed in most industrial power supply systems, it is essential to maintain the speed constant at the exact value determined by the number of poles of the machine.
BRIEF DESCRIPTION OF THE INVENTION
In many practical situations of electric power generation, although the need exists to maintain the frequency at a constant value, there are practical difficulties to maintain the speed at the correct value, without variations. The optimum speed of the available prime mover may be different from that dictated by the pole number and the required frequency. The objective of this invention is to achieve the generation of AC power at a speed different from that dictated by the desired frequency and the pole number of the generator as in DC excited generators and also to enable speed variations without change in the output frequency.
In contrast to the earlier schemes, the field system of the present invention, besides being a brushless scheme, totally eliminates the need for a coupled exciter machine. Using the new scheme it is possible to use a speed which is different from that of a corresponding DC excited machine. The new scheme enables stabilization of the frequency at the predetermined fixed value during speed variations. Independent control of the field is possible for adjustment of the voltage and the power factor as in conventional DC excited alternators.
In contrast to conventional DC field machines, the new scheme employs an AC field winding, which is excited by an AC source. The new scheme is applicable with single phase and poly phase excitation. For the purposes of this description we will assume that the AC is poly phase. The poly phase AC Source itself is located on the rotor in a separate winding and therefore there is no need for contact brushes or a separate exciter machine. We may designate this AC winding as the auxiliary rotor winding. For creating a poly phase voltage in the auxiliary rotor winding the new scheme employs an auxiliary stator winding which is separate from the main winding of the generator. Mutual magnetic coupling between any of the main windings and any of the auxiliary windings is totally eliminated by appropriate choice of the pole numbers for the windings, as explained in the detailed description of the invention later.
The voltage and frequency of the AC voltage induced in the auxiliary rotor winding are determined by the voltage and frequency of the input to the auxiliary winding on the stator and the speed of rotation of the machine.
Since the input to the poly phase main field winding is from an AC source, the field created by this winding will have a rotational speed relative to the rotor. Since the AC input to the main field winding is from the auxiliary rotor winding, this rotational speed will be determined by the frequency of the induced AC, which itself is determined by the frequency of the AC input to the auxiliary stator winding and the speed of rotation. The stator auxiliary winding will be fed through an inverter whose frequency will be adjustable. Therefore the speed of rotation of the main field with respect to the rotor will be adjustable at any value of the speed of rotation of the rotor by adjusting the frequency of the inverter feeding the auxiliary stator winding. The speed of rotation of the main field in space is the algebraic sum of the rotational speed of the rotor and the rotational speed of the field relative to the rotor. Therefore the rotational speed of the main field relative to the stator is adjustable by adjustment of the inverter frequency. Therefore this invention makes it possible to maintain the output frequency of the generator by the adjustment of the output frequency of the inverter feeding the auxiliary stator winding. The magnitude of the induced voltage is adjustable by adjustment of the magnitude of the AC output from the inverter. In a DC excited conventional alternator the power transferred between the prime mover and the AC source is by the process of mechanical rotation. In the present case there will be power transfer also by transformer action between the field winding and the stator winding. The ratings of the auxiliary winding and the main field winding must be adequate to handle this power also.
The main advantage of this scheme over conventional DC excited schemes is that it becomes possible to use a prime mover speed other than one of the discrete speeds that will have to be used when using a conventional generator. It is also possible to maintain the output frequency at the required value when speed changes occur. This is a brushless scheme for the field and the need for a separate exciter machine is totally eliminated. Other benefits of the scheme will become evident from a consideration of the detailed description and the drawings.
This invention can also be implemented, using a coupled exciter machine, without the need for brushes and sliding contacts. In such a case the auxiliary windings will be located in the exciter machine. The physical separation will then eliminate magnetic coupling between the main and auxiliary windings without the need to have restrictions on the choice of pole numbers for this purpose.
REFERENCES:
patent: 3801840 (1974-04-01), Kant
patent: 3930175 (1975-12-01), Chirgwin
patent: 4609862 (1986-09-01), Becker et al.
patent: 5012148 (1991-04-01), Vithayathil
patent: 5598091 (1997-01-01), Satake et al.
Vithayathil John J.
Vithayathil Joseph
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