Title of Invention

AN ELECTRICAL GENERATOR CAPABLE OF RUNNING IN BOTH SYNCHRONOUS AND ASYNCHRONOUS MODES

Abstract

Conventional way of power generation uses synchronous generators, with few exceptions driven by water turbines in remote areas. Power generation by synchronous generator with brushless excitation system suffers from the problem of 'synchronization', particularly when a number of such generators are used in parallel operation. Another problem is 'hunting' , the stability margin in which case is much below the 'thermal limit' owing to loss of synchronism. The conventional system also suffers from "regulation of frequency". The present invention aims at overcoming the above drawbacks and provides an electrical generator (1) and prime mover (4) attached thereto, characterized in that the said generator is equipped with an exciter (3) mounted on the shaft (2) connecting the said generator and prime mover, which in turn carries a stator current control device (5).

Full Text

The present invention relates to an electrical generator capable of running in both synchronous and asynchronous modes. More particularly the subject invention is concerned with power generation by an effective, efficient and economic system which is simple and at the same time substantially free from operational hazards. This invention also concerns a method of power generation by employing an asynchronous generator, using brushless excitation system, although brush and commutator system for its excitation may also be employed. A few prior patents exist in related field but none of them deals with or relates to an electrical generator capable of running in either asynchronous or synchronous mode. Some of these patents are, inter alia, as follows. US Patent No. 5920162 relates to servo mechanism operating on feed back signal from variable exciter, but does not disclose a generator capable of running both in synchronous and asynchronous modes. US Patent No. 4168459 deals with maintenance of uninterrupted power supply to loads and consumers, but does not disclose features of the present invention. UK Patent No, 509155 deals with method for improving power supply and control system for electric motors, but does not concern electrical generators operating on synchronous and/or asynchronous modes. UK Patent No. 1369844 relates to design and operation of an asynchronous or induction generator driven at variable speed aircraft turbines, differing substantially from the electrical generator through a brushless system. This present invention is based on a process of converging mechanical into electrical energy by adopting a totally novel concept which has not been anticipated or disclosed in the prior art In conventional mode of generating power, now-a-days bulk power generation is carried out by synchronous generator. Very small exceptions of using induction generators driven by water turbines are observed in remote areas. Power generation by synchronous generator with brushless excitation system suffers from the problem of "synchronization". particularly when a number of synchronous generators are used in parallel operation. Another problem confronting smooth production is "hunting". The stability margin in this case is also much below the "thermal limit" due mainly to the possibility of loss of synchronism. The conventional system also suffers from "regulation of frequency" by maintaining constant rotor speed with the help of governor mechanism, apart from the aforementioned problems of synchronization and hunting. In our proposed design, the main generator can be excited from an auxiliary generator which is called exciter. If the main generator is to run in the asynchronous mode in which its rotor is driven by the prime mover at a speed different from the synchronous speed at which the magnetic field rotates, the rotor windings of the main generator and the exciter mounted on the common shaft are to be electrically connected through their slip rings, which enables the rotor winding of the main generator to receive slip frequency currents required for excitation. In order that the generator may run in the synchronous mode, it is necessary to supply its rotor winding with (zero frequency) direct current through slip rings. For this purpose, current from the rotor winding of the exciter is to be rectified through a rectifier mounted on the common shaft. The principal object of this invention is to provide an electrical generator capable of running in both synchronous and asynchronous modes. A further object of this invention is to provide a system for using asynchronous generators by overcoming the problem of generation of rotating magnetic field at the starting and generating lagging reactive power. A still further object of this invention is to provide a system wherein slip frequency power is injected to the rotors of the asynchronous generators by means of exciters, which are auxiliary machines. Yet another object of this invention is to provide an electrical generator with an auxiliary machine or a system acting as an exciter capable of producing slip frequency current as well as direct current for injection into the rotor winding of the main generator. The foregoing objects are fulfilled by the present invention, which relates to an electrical generator capable of running in both synchronous and asynchronous modes, comprising a main generator and a prime mover attached thereto, characterized in that the said generator is equipped with an exciter for producing slip ring current for injection into the rotor windings of the main generator, said exciter being mounted on shaft connecting the said generator and prime mover, and wherein the prime mover carries a stator current control device. As indicated above, asynchronous generators have the drawbacks like the problem of producing rotating magnetic field at the starting, and generating lagging reactive power to supply the load. These drawbacks have been overcome by adopting a new technique of injecting slip frequency power to the rotors of the asynchronous generators, just as in the case of speed and power factor control of induction motors by auxiliary machines or exciters providing excitation to the desired level and extent. Slip ring induction motors are widely used in the industry for their high starting torque, better speed control and their rugged construction. They have additional advantages of low initial cost and a minimal maintenance cost. In the present invention, these machines are used as asynchronous generators for generating power. The cage rotor induction machine can also be used as asynchronous generator for power generation provided capacitive devices are connected across the stator windings as exciters. If the rotor of such a machine is run above synchronous speed by a prime mover, the machine generates electricity at negative slip (usually between 3% and 5%), which can be used for transmission and distribution through network. Requisite excitation may be provided with some auxiliary machine, such as, for instance, Leblanc, Walker or Scherbiu's machine. Some object of providing excitation may be achieved by electronic amplifiers using BJT, FET, IGBT or vacuum triode tubes with appropriate values of resistance, capacitance or inductance in the biasing circuit, as used in FM modulators. The prime mover at starting rotates the motor of asynchronous generator and its exciter below synchronous speed if it is connected to the power grid, or above synchronous speed if the system is an isolated one and not connected to the power grid. In case of power grid connection, the system first runs as an induction motor and the requisite magnetizing power is then provided by the power grid, while the core, windage and frictional losses are provided by the prime mover. But when the prime mover enhances the speed of the machine above synchronous speed in the same direction as the rotating magnetic field, the machine runs as generator, delivering electrical power to the connected busbar and drawing its excitation from the auxiliary machine acting as the exciter. For a further embodiment of the present invention, i.e. for an isolated system the machine starts generation due to remnant or residual magnetism of its stator and rotor core with capacitors connected to the stator winding. In this instance also auxiliary machines can be used as rotating capacitors since these provide magnetizing reactive power to the main generator. The electronic reactive power compensator as disclosed in Indian Patent Application No. 793/KOL/2005 can effectively be used for providing excitation. For this purpose, the aforesaid compensator device can be directly across the stator winding of asynchronous generator and as a consequence thereof, no auxiliary exciter is required. As an alternative both the electronic reactive power compensator and the auxiliary machine as exciter may be used to make the system full proof with the flexibility of reactive power control. If a direct connection is not made, then another asynchronous generator with small air gap can be used as exciter along with the electronic phase compensator or thyristor switched capacitor or synchronous condenser across its stator windings. If an auxiliary machine, viz. Leblanc machine, Walker machine or Scherbin's machine with brush and commutator assembly to be used as an exciter, then in place of variable resistors triac-diac assemblies may be used with their stator windings. The exciters are mounted on the same shaft of the main generator. If it is desired to make the system brushless, a second asynchronous generator of smaller rating is used which receices its rotor current from the rotor of the main generator. The asynchronous generator of smaller rating having a small air gap is used as an exciter, where capacitive devices are connected to its stator windings. The two rotor windings of the main generator and exciter are mounted on the same shaft and rotor windings are connected directly through the slip rings without any brush. The auxiliary machine which acts as an exciter can be used to produce slip frequency current as well as direct current for injection into the rotor winding of the main generator. When the exciter runs as a slip frequency generator, its stator winding is provided with a combination of series or parallel connection of variable resistors and variable capacitors. On the other hand, when the exciter runs as a direct current generator, the combinations of variable resistors and variable capacitors are disconnected and a source of direct current with proper magnitude and polarity which can be adjusted is connected across the stator winding of the exciter. When the exciter runs as a D.C generator and the main generator runs as a synchronous generator, the alternating current produced by the exciter in its rotor winding is rectified by a shaft-mounted rectifier (uncontrolled), the output of which is fed into the rotor circuit of the main generator and the shaft is driven at the synchronous speed (n, r.p.s) of the main generator where in which 'f' is the supply frequency in Hz and p, is the number of pole pairs of main generator. When the main generator runs in asynchronous mode, the shaft may run between the speeds n and n1, where n is the synchronous speed of the combination of the main generator and the exciter and n1 is the synchronous speed of the main generator as stated earlier. The shaft can also be made to run above the speed n1. The relation between n and n1 may be expressed as- where p1 has been stated earlier and p2 is the number of pole pairs of the exciter. The invention will now be more particularly illustrated by means of accompanying drawings wherein, Fig. 1 shows the electrical generator assembly of the present invention. Fig.2a is the block representation and Fig.2b is the diagramatic representation of Leblanc machine. Fig.3 shows the block representation of Walker machine. Fig.4 is the block representation of Scherbin's machine. Fig.5 depicts a frequency converter and its control device used as exciter and Fig.6 shows a brushless system with a second asynchronous generator of smaller rating. Referring to Fig.1 of the drawings, (1) is the main generator, (2) is the shaft connecting the main generator (1) and prime mover (4). Exciter assembly (3) is mounted on the main shaft (2) and is equipped with stator current control device (5). The arrow heads indicate direction of current flow to supply. Fig.2a showing a block diagrammatic representation of Leblanc machine used as an exciter (3). which has carbon brushes (6) and commutator (7) attached to the rotor (8). The direction of the arrow indicates flow of current to main generator rotor. Fig.2B depicts a diagrammatic representation of Leblanc machine, in which (9) is the stator carrying a "diac-triac assembly" (10). (8) stands for the rotor along with carbon brush (6) and commutator (7). Fig.3 illustrates block representation of Walker machine used as an exciter, in which the rotor (8) is attached to/connected with carbon brush (6) and commutator (7) assembly, which in turn is connected to the rotor of the main generator, as indicated by the direction of the arrows, through inductors in series (11). Fig.4 shows a block representation of Scherbin's machine, which may also be used as an exciter. This differs from the Walker machine only in the deployment of the inductors. As may be seen from the drawings, Scherbin's machine uses parallel inductors (12), and the arrow heads signify direction of connection with the rotor of the main generator. Fig.5 shows a diagrammatic representation of another embodiment of this invention which is frequency converter and its control device used as exciter. Incoming current from supply is made to pass through a variable ratio transformer (13), followed by passage through rotor (8) carrying brush and commutator assembly. This assembly in turn leads to the main generator rotor. Fig.6 of the drawings pertains to yet another embodiment of this invention and illustrates a diagrammatic representation of a brushless system which is in effect a variant of the set-up shown in Fig. 1 of the drawings, wherein sliprings are mounted on the main shaft connecting the prime mover and the main generator. The location of these devices is usually between said exciter and main generation unit as indicated in the drawing by (14). The system of the present invention is simple, yet cost effective. As the electronic components at different ratings are easily available at a reasonable cost in the open market, assembly/fabrication of the machine poses no problem. Apart from the conventional areas of power generation, the system of the present invention can be put to good use in non-conventional areas of power generation such as, for instance,- (i) wind, (ii) tidal waves, (iii) oceanic current, etc., the last named having been developed by Chinese Engineers using the principle of diffraction grating. Additional modifications and improvements of the present invention may also be apparent to those skilled in the art. Thus, the particular combination of parts described and illustrated herein is intended to represent only one embodiment of the present invention, and is not intended to serve as limitations of alternative devices within the spirit and scope of the invention. We claim: 1. An electrical generator capable of running in both synchronous and asynchronous modes, comprising a main generator (1) and a prime mover (4) attached thereto, characterized in that the said generator is equipped with an exciter (3) for producing slip ring current for injection into the rotor windings of the main generator, said exciter being mounted on shaft (2) connecting the said generator and prime mover, and wherein the prime mover carries a stator current control device (5). 2. An electrical generator as claimed in Claim 1, wherein excitation is provided by an auxiliary machine selected from the group of Leblanc, Walker or Scherbiu's machine, or by electronic amplifiers using BJT, FET, IGBT or vacuum triode tubes with appropriate values of resistance, capacitance or inductance in the biasing circuit, as used in FM modulators. 3. An electrical generator as claimed in Claims 1 and 2, wherein a frequency converter and its control device are used as the exciter. 4. An electrical generator as claimed in Claims 1 and 3, wherein the frequency converter includes commutator (7) and carbon brush (6) assembly, and a variable ratio transformer (13) serves as the control device, the entire assembly being used as the exciter. 5. An electrical generator as claimed in Claim 1, wherein the system is made brushless by introducing slip rings (14) mounted on the main shaft (2) between the main generator (1) and exciter (3), the said shaft being rotated by the prime mover (4). 6. An electrical generator as claimed in Claims 1 and 5, wherein a second asynchronous generator of smaller rating having small air gap which receives its rotor current from the rotor of the main generator is used as exciter, wherein slip rings (14) are mounted in its stator windings, and two rotor windings of main generator and exciter are mounted on the same shaft and the rotor windings are connected directly through the springs without any brush. 7. An electrical generator as claimed in Claim 1, wherein the exciter to act as a slip frequency generator, the stator winding thereof is provided with a combination of series or parallel connection of variable resistors or variable capacitors. 8. An electrical generator as claimed in Claim 1, wherein the exciter to act as a direct current generator, the combination of variable resistors and variable capacitors are disconnected and a source of direct current with adjustable magnitude and polarity is connected across the stator winding of the exciter. 9. An electrical generator capable of running in both synchronous and asynchronous modes, substantially as hereinfore described with particular reference to the accompanying drawings. Conventional way of power generation uses synchronous generators, with few exceptions driven by water turbines in remote areas. Power generation by synchronous generator with brushless excitation system suffers from the problem of 'synchronization', particularly when a number of such generators are used in parallel operation. Another problem is 'hunting' , the stability margin in which case is much below the 'thermal limit' owing to loss of synchronism. The conventional system also suffers from "regulation of frequency". The present invention aims at overcoming the above drawbacks and provides an electrical generator (1) and prime mover (4) attached thereto, characterized in that the said generator is equipped with an exciter (3) mounted on the shaft (2) connecting the said generator and prime mover, which in turn carries a stator current control device (5).

Documents:

00682-kol-2006-abstract.pdf

00682-kol-2006-claims.pdf

00682-kol-2006-correspondence others.pdf

00682-kol-2006-description(complete).pdf

00682-kol-2006-drawings.pdf

00682-kol-2006-form-1.pdf

00682-kol-2006-form-2.pdf

00682-kol-2006-form-3.pdf

00682-kol-2006-form-9.pdf

00682-kol-2006-power of attorney.pdf

682-KOL-2006-(06-09-2011)-CORRESPONDENCE.pdf

682-KOL-2006-(29-09-2011)-CORRESPONDENCE.pdf

682-KOL-2006-(29-09-2011)-OTHERS.pdf

682-KOL-2006-ABSTRACT.pdf

682-KOL-2006-AMANDED PAGES OF SPECIFICATION.pdf

682-KOL-2006-CANCELLED PAGES.pdf

682-KOL-2006-CLAIMS.pdf

682-KOL-2006-CORRESPONDENCE 1.1.pdf

682-KOL-2006-CORRESPONDENCE OTHERS 1.1.pdf

682-KOL-2006-CORRESPONDENCE-(12-12-2011).pdf

682-KOL-2006-DESCRIPTION (COMPLETE).pdf

682-KOL-2006-DRAWINGS.pdf

682-kol-2006-examination report.pdf

682-KOL-2006-FORM 1.pdf

682-kol-2006-form 18.pdf

682-KOL-2006-FORM 2.pdf

682-kol-2006-form 3.1.pdf

682-KOL-2006-FORM 3.pdf

682-kol-2006-form 9.pdf

682-KOL-2006-FORM-1-(12-12-2011).pdf

682-KOL-2006-FORM-13-(12-12-2011).pdf

682-kol-2006-granted-abstract.pdf

682-KOL-2006-GRANTED-CLAIMS.pdf

682-kol-2006-granted-description (complete).pdf

682-kol-2006-granted-drawings.pdf

682-kol-2006-granted-form 1.pdf

682-kol-2006-granted-form 2.pdf

682-KOL-2006-GRANTED-SPECIFICATION.pdf

682-kol-2006-pa.pdf

682-KOL-2006-REPLY TO EXAMINATION REPORT.pdf

682-kol-2006-reply to examination report1.1.pdf

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