AU2013209357A1 - System and method for generating an alternating current output signal - Google Patents

Abstract

Abstract A system and device for providing AC signal. The system includes: an AC generator that outputs an AC output signal and includes an AC rotor that communicates with a shaft that is rotated at a rotation speed; a speed sensor for 5 sensing the rotation speed; and a controller for controlling a magnetic field of the AC generator in response to the rotation speed; wherein the controller comprises a direct current (DC) generator that generates a current that is provided to the AC generator so as to control the magnetic field of the AC generator.

Description

Regulation 3.2 AUSTRALIA PATENTS ACT, 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT ORIGINAL Name of Applicant: VARIABLE WIND SOLUTIONS LTD. Actual Inventors: NADAM, Dror; BEN-DAVID, Rahamim Address for service AJ PARK, Level 11, 60 Marcus Clarke Street, Canberra ACT in Australia: 2601, Australia Invention Title: System And Method For Generating An Alternating Current Output Signal The following statement is a full description of this invention, including the best method of performing it known to us. 1 System and method for generating an alternating current output signal RELATED APPLICATIONS This application claims priority from US Provisional application 60/907,248, 5 filing date March 27 2007, hereby incorporated by reference as if fully set forth herein. FIELD OF THE PRESENT INVENTION The present invention is related to a system and device for generating electricity according to a constant output yet with potentially variable speed, and in ) particular, to such a system and device which feature a voltage regulation system. BACKGROUND OF THE PRESENT INVENTION The generation of electricity through the use of electrical generators is important for modern life. These generators require some source of external energy 5 to operate, which may for example be some type of fossil fuel and/or renewable energy. The generator then consumes the energy source in order to generate electricity. However, it is important that the power output remain constant in order for the generated electricity to be usable. The power output can remain constant if the shaft speed of the generator 0 remains constant. However, the shaft speed of the generator cannot always be held to a constant rate. Therefore, some generators have relied on maintaining at least a minimum speed, such that the power output provided is determined according to the 1a WO 2008/117290 PCIAL2008/000426 minimum speed of the shaft. If the shaft speed increases beyond the minimum, the excess power produced is discarded and hence is wasted. Various solutions have been attempted but none has completely solved the problem for alternating current (AC) generators. For example, US Patent No. 5 5541483 provides a method for controlling a direct current (DC) motor or generator, particularly those of the permanent magnet type. Due to functional differences between AC and DC generators, the described solution would not be operative for an AC generator. US Patent Application No. 2004/0257050 describes a method and device for 10 constant current generation, which attempts to overcome drawbacks associated with potentially variable shaft speed through controlling the current that is output by the generator, thereby achieving a constant level of output current. Therefore, the described invention relates to current stabilization which is relatively complex. Various attempted solutions for achieving stabilized voltage output have 15 included using inertia or friction as a mean of moderating the shaft speed fluctuations; using torque control in order to control the shaft speed; or converting the fluctuating AC electricity to DC electricity, and then converting it again into the standard grid AC power. Clearly all of these methods are very wasteful of energy. In an effort to use sources of renewable energy, systems and devices have 20 been introduced which use "natural" energy such as wind, through the use of wind turbines; sun, through the use of solar panels; water power, such as wave or tidal power; and the like. Wind energy may be particularly variable, given that wind tends to increase and decrease in power, and/or change direction, quite regularly. 2 WO 2008/117290 PCT/IL2008/000426 However, all of the "natural" energy sources may be expected to suffer from instability of power levels. Thus, for renewable energy, the ability to convert non stable mechanical power to stable electrical power has major demand in a variety of applications. 5 Various solutions have been proposed in this area to overcome the instability of renewable energy sources, particularly with regard to wind generation. For example, US Patent No. 7068015 provides a solution for wind power by adjusting the magnetic field according to the rotation speed of the wind turbine according to feedback determined by measuring output voltage or current. US Patent Application 10 No. 2004/0119292 provides a method for controlling the shaft speed of the generator by controlling its torque, a solution which is disadvantageous as noted above. The taught method further requires a diode rectifier for operation, which is another disadvantage. US Patent No. 5083039 controls the power output by controlling the magnetic 15 field of the generator, by controlling the stator current. However, changes to the stator current cause changes to the generator torque. In order to compensate for changes to the torque, the shaft speed is controlled by changing the pitch of the "wings" or blades of the turbine, which may be disadvantageous due to wind conditions, and which is disadvantageous in any case because it requires an 20 additional expenditure of energy. US Patent No. 6137187 is similarly disadvantageous as it requires a pitch control system. SUMMARY OF THE INVENTION 3 WO 2008/117290 PCT/IL2008/000426 The term "constant electrical output" means an alternating current output signal that has a constant peak voltage. The present invention provides a system and method for sensing a rotation speed of a shaft that rotates at a variable rotation speed and controlling the 5 magnetic field of the AC generator in response to the sensed speed. The controlling includes utilizing a DC generator that supplies a current to the AC generator. This current determines the magnetic field of the AC generator. The magnetic field can be controlled in a manner that maintains a peak voltage of an output signal of the AC generator substantially constant despite changes in the rotation speed of the shaft. 10 The electro magnetic force induced by an AC generator is governed by Faraday's law: EMF=6a x L x B x sin() In which: 15 EMF (electromagnetic field) is the output voltage. 6> is the tangential speed of the winding. L is the length of the winding that crosses the magnetic flux. B is the magnetic field intensity. sin(O) is the sin of the angle between the winding & the magnetic flux. 20 The above equation indicates that the peak voltage of an AC output signal of the AC generator is dependent on the shaft rotation speed, which in turn depends upon the mechanical power used to rotate the shaft. If the level of mechanical power is variable then the shaft rotation speed is in turn variable. However, even if 4.

WO 2008/117290 PCT/IL2008/000426 the shaft rotation speed is variable, the peak voltage should be maintained substantially constant. Therefore, the present invention does not require the shaft rotation speed to be constant, which is useful for a wide variety of applications, including but not 5 limited to power generation by renewable energy or "natural" energy sources or any other energy source having variable output. Instead, according to embodiments of the present invention, the measurement of the shaft rotation speed is used to control the magnetic field intensity (B) through a feedback or control mechanism (that includes a DC generator) according to the speed of rotation of the shaft, thereby 10 providing a constant voltage output and hence stable power generation. According to embodiments of the present invention, control of the magnetic field intensity of the AC generator is preferably provided with a DC generator, featuring a rotor winding connected to the rotor winding of the AC generator. The rotation speed of the shaft of the AC generator is measured; according to this 15 measurement, the operation of the DC generator is then used to increase or decrease the magnetic field intensity, thereby maintaining a constant voltage output even if the rotation speed of the shaft varies. The DC generator optionally and preferably has a separate power source. Preferably the amount of power required for management and control of the voltage output is relatively low as compared to 20 the output of the generator itself; for example, tests described below indicated that the control power required was less than 30W for a 5KW generator. Optionally and preferably, the rotor winding of the DC generator is connected to the rotor winding of the AC generator, for controlling the magnetic field of the AC generator. 5 WO 2008/117290 PCT/IL2008/000426 A system for providing an alternating current (AC) output signal, the system includes: an AC generator that outputs an AC output signal and comprises an AC rotor that communicates with a shaft that rotates at a rotation speed; a speed sensor for sensing the rotation speed; and a controller for controlling a magnetic field of the 5 AC generator in response to the rotation speed; wherein the controller comprises a direct current (DC) generator that generates a current that is provided to the AC generator so as to control the magnetic field of the AC generator. Conveniently, the DC generator includes a DC rotor that communicates with the shaft. 10 Conveniently, the DC rotor is connected to the AC rotor. Conveniently, the DC rotor is connected to the AC rotor by rigid wiring. Conveniently, the DC generator comprises a DC stator that if fed by an excitation voltage that has an amplitude that is responsive to the rotation speed. Conveniently, the controller includes a voltage regulation system that receives 15 rotation speed information from the speed sensor and determines an amplitude of an excitation voltage to be provided to the DC generator. Conveniently, the voltage regulation system determines the amplitude of the excitation voltage in response to a relationship between the rotation speed and a peak voltage of the AC output voltage. 20 Conveniently, the voltage regulation system includes: an analog to digital converter that converts analog rotation speed information to digital rotation speed information; a low pass filter for filtering the digital rotation speed information to provide filtered rotation speed information; a processor, for determining the 6 WO 2008/117290 PCT/IL2008/000426 excitation voltage in response to the filtered digital rotation speed information and to a relationship between the rotation speed and a peak voltage of the AC output voltage and a digital to analog converter, for converting a digital control signal outputted from the processor to an analog control signal that controls the amplitude 5 of the excitation voltage. Conveniently, the system includes a speed sensor that generated digital rotation speed information. Conveniently, the controller includes a voltage regulation system that receives rotation speed information from the speed sensor and a requested peak voltage of 10 the AC output signal and determines an amplitude of an excitation voltage to be provided to the DC generator. Conveniently, the shaft is rotated by a mechanical input element that is powered by a renewable energy source. Conveniently, the renewable energy is selected from a group consisting of 15 wind, water, solar and geothermal. Conveniently, the system further includes a mechanical input element that rotates the rotor. Conveniently, the system further includes a cooling fan that communicates with the shaft. 20 Conveniently, the controller controls the magnetic field of the AC generator so as to maintain a peak voltage of the AC output substantially constant despite changes in the rotation speed. 7 WO 2008/117290 PCT/IL2008/000426 Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The materials, methods, and examples provided herein are illustrative only and not intended to be limiting. BRIEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of 10 illustrative discussion of the embodiments of the present invention only, and are presented in order to provide what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the 15 description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the drawings: Figure 1 is of an exemplary, illustrative AC generator system according to the present invention; 20 Figure 2 is of an exemplary, illustrative method according to the present invention for control of the operation of the generator of Figure 1; Figure 3 is a more detailed but still schematic diagram of a voltage regulation system according to the present invention; 8 WO 2008/117290 PCT/IL2008/000426 Figure 4 is a schematic block diagram of an illustrative system according to the present invention for generation of a constant level of voltage by a generator that is at least partially powered by a renewable energy source or "natural" energy; Figure 5 shows results of a test of an exemplary system according to the 5 present invention Figure 6 illustrates in greater details a voltage regulation system and a rotation speed sensor, according to an embodiment of the invention; Figure 7 illustrates in greater details a voltage regulation system and a rotation speed sensor, according to an embodiment of the invention; 10 Figure 8 illustrates in greater details a voltage regulation system and a rotation speed sensor, according to an embodiment of the invention; Figures 9 and 10 illustrates the relationships between control current and rotation speed of the shaft for different values of the peak voltage of the AC output signal; and 15 Figure 11 illustrates a method according to an embodiment of the invention. DESCRIPTION OF THE VARIOUS EMBODIMENTS The present invention is of a system and device for providing constant voltage power (meaning - an alternating current output signal that has a substantially 20 constant peak voltage) by an AC generator, through control of the magnetic field intensity of the AC generator, wherein the control utilizes a DC generator. The magnetic field intensity is controlled by a current supplied by the DC generator and according to the rotation speed of the shaft of the AC generator, such that variations 9 WO 2008/117290 PCT/IL2008/000426 of the rotation speed of the shaft are compensated by changes to the magnetic field intensity. According to embodiments of the present invention, the DC generator, featuring a rotor winding electrically connected to the rotor winding of the AC 5 generator. The rotation speed of the shaft of the AC generator is preferably measured; according to this measurement, the operation of the DC generator is then used to increase or decrease the magnetic field intensity, thereby maintaining a constant voltage output even if the rotation speed of the shaft varies. Optionally and preferably, the rotor winding of the DC generator is connected 10 to the rotor winding of the AC generator, for controlling the magnetic field of the AC generator. The present invention may optionally be used with any type of mechanical power source, but is useful with regard to any mechanical power input source which is characterized by variability. Examples of such mechanical power input sources 15 include but are not limited to any type of renewable or "natural" energy source, including but not limited to wind, water, solar or geothermal. According to optional embodiments of the present invention, rather than providing a constant voltage output, optionally the magnetic field of the AC generator is controlled according to some criterion, such as a set point for example. The set 20 point may optionally be a minimal required set point or a maximal permitted set point for the voltage output, and/or a within a range of permitted set point values. The principles and operation of the present invention may be better understood with reference to the drawings and the accompanying description. 10 WO 2008/117290 PCT/IL2008/000426 Referring now to the drawings, Figure 1 is of an exemplary, illustrative AC generator system 100 for generating a constant level of output voltage. Generator system 100 features a connection to a mechanical power from a power source (not shown), which causes a shaft 106 of an AC generator 108 to rotate. Shaft 106 also 5 optionally features a cooling fan 104 for cooling the operations of generator system 100. AC generator 108 is preferably a three phase, double winding generator, and features a rotor 110 and a stator 112. Rotor 110 preferably features rotor winding 114 while stator 112 preferably features statorwinding 116. An auxiliary DC generator 118 is optionally and preferably installed onto shaft 10 106 as shown, for controlling the intensity of the magnetic field of AC generator 108. DC generator 118 features a DC rotor 120 with DC rotor winding 122, and a DC stator 124 with DC stator winding 126. DC rotor winding 122 is preferably connected to rotor winding 114 of AC generator 108 through a connector 130, which preferably comprises some type of wiring, which may optionally be rigid wiring as DC rotor 15 winding 122 rotates at the same speed as rotor winding 114. DC stator winding 126 is connected to a DC power source 128 through a suitable connector 132 as is known in the art. A rotation speed sensor (also referred to as speed sensor) 134 is preferably connected to shaft 106, for sensing the speed of rotation of shaft 106. 20 Rotation speed sensor 134 may optionally feature any suitable speed sensing device, including but not limited to a shaft encoder, resolver, tachometer, a Hall effect sensor or any type of proximity sensor that reads the motion of a mark point 11 WO 2008/117290 PCT/IL2008/000426 on the perimeter of shaft 106. The mark point may be any type of marking, including but not limited to, a notch, screw or hole. A voltage regulation system 136 is electrically connected to rotation speed sensor 134 and to DC power source 128. Voltage regulation system 136 is 5 preferably PLC (programmable logic controller) based, although any type of programmable or computational device, or digital circuit, or any device featuring software, firmware or hardware, or a combination thereof, could also optionally be used. AC generator system 100 preferably operates as follows. Mechanical power 10 (not shown) is supplied to shaft 106 of AC generator 108, causing shaft 106 to rotate. An initial excitation voltage is induced on DC stator 124 by DC power source 128, causing DC rotor 120 to be subjected to an external magnetic field. DC power source 128 also causes DC rotor 120 to rotate. The combination creates an EMF in DC generator 118, thus current (also referred to as control current) flows from DC 15 rotor 120 to rotor 110 of AC generator 108, thus creating a rotating magnetic field in rotor 110. As a result, an EMF is created in the output of AC generator 108 (ie AC generator 108 generates electricity). Rotation speed sensor 134 senses the speed of rotation of shaft 106. This information is fed to voltage regulation system 136, which controls DC power source 20 128 in order to change the excitation voltage in DC stator 124, thereby changing the EMF output by DC generator 118. In turn, this controls the magnetic field intensity of AC generator 108, by changing the current in rotor winding 114. 12 WO 2008/117290 PCT/IL2008/000426 The excitation voltage is optionally and preferably determined by voltage regulation system 136 according to a method shown in Figure 2. Figure 2 is of an exemplary, illustrative method according to the present invention for control of the operation of the generator of Figure 1. As shown, in stage 1, the actual rotation 5 speed (also referred to as shaft speed) of the AC generator shaft is measured by the rotation speed sensor. In stage 2, the voltage regulation system changes the value of the DC power source control input, which preferably causes the excitation voltage of the DC stator to be changed. Stage 1 is then repeated at least once, although optionally and preferably both stages are repeated as needed. 10 Preferably, initially the output voltage is measured in relation to the rotation speed in order to define the control voltage curve for a particular generator. Once this curve has been established for the generator, it is used for performing the above method. The method may optionally also be used for changing the "set point" of the 15 output signal of the AC generator. For example, such a method could optionally be used to increase or decrease the peak voltage of the output signal to be provided by the AC generator. Optionally, increasing or decreasing the peak voltage of the output signal could be useful under a variety of circumstances, for example if the level of input mechanical power to the AC generator were to change. As described 20 in greater detail below, such changes of input mechanical power may occur with regard to any type of energy source, but may be particularly prevalent with regard to renewable energy sources such as wind power or other types of "natural" energy sources. Preferably, such increasing or decreasing the level of output voltage is 13 WO 2008/117290 PCT/IL2008/000426 performed for any mechanical power input source which is characterized by variability. Figure 3 is a more detailed but still schematic diagram of a voltage regulation system according to the present invention. In stage 1, as previously described, the 5 control voltage curve is determined for each generator system, preferably during system integration. This stage is not necessarily repeated once the system is operational. Stages 2-5 are preferably repeated at least once and are more preferably performed continuously as necessary, as a loop. In stage 2, the AC generator is 10 operational and the AC generator's shaft is rotating. In stage 3, rotation speed of the shaft is measured by the rotation speed sensor as described in Figure 1. In stage 4, using the rotation speed of the shaft and the control voltage curve, the amount of control voltage to be output is determined. In stage 5, the control voltage is output by the control voltage output generator, which as described in Figure 1, is preferably 15 an associated DC generator. Figure 4 is a schematic block diagram of an illustrative system according to the present invention for generation of a constant level of voltage by a generator that is at least partially powered by a renewable energy source or "natural" energy. As shown, a system 400 features a mechanical power source 402, which is preferably a 20 device powered by some type of renewable energy source or "natural" energy, optionally and preferably selected from the group consisting of wind, solar, water and geothermal. System 400 also preferably features an AC generator system 404, 14 WO 2008/117290 PCT/IL2008/000426 which may for example be implemented as described with regard to Figures 1 and/or 2 and/or 3. Mechanical power source 402 is mechanically connected to a shaft 406 of an AC generator 408 which is part of AC generator system 404, thereby causing shaft 5 406 to rotate. Mechanical power source 402 is optionally and preferably a variable power source, in the sense that the output level of mechanical power may optionally be variable. Such variation causes variation in the speed of rotation of shaft 406. However, a voltage regulation system 410 of AC generator system 404 controls the magnetic field in AC generator 408, such that AC generator system 404 provides an 10 AC output signal that has a peak voltage that is maintained in a constant level, regardless of any variation of the speed of rotation of shaft 406. Among the many advantages of the exemplary embodiment of the present invention shown in Figure 4 is that the provision of a constant peak voltage of the AC output signal may be made without altering or affecting the structure or function of 15 the wind, water (hydroelectric), solar, geothermal or other type of turbine. Therefore, the function and design of the turbine itself may be selected for most effective capture of energy from the renewable energy source. No background art reference teaches or suggests such a system or device for generating a constant level of output voltage from a renewable energy source. 20 EXAMPLE 1 TESTING OF AN ILLUSTRATIVE SYSTEM This Example describes a test performed on an exemplary, illustrative non limiting system according to the present invention. The system featured a hybrid, 15 WO 2008/117290 PCT/IL2008/000426 dual winding three phase generator as is known in the art, which includes both a DC generator and an AC generator, product number ECO3-2S4 (Mecc Alte S.p.A., Italy); and a voltage regulation system based on a CQM-45 (Omron Inc., USA) Programmable Logic Controller (PLC). The set point voltage of the system was 285 5 Vac. The AC generator was powered by an electric motor connected to a variable speed motor driver. The speed of rotation of the AC generator shaft was then altered according to the speed of the motor. For each rotation speed, the excitation voltage of the auxiliary DC generator was changed, until the peak voltage of the AC output reached the set point value (285Vac). 10 Figure 5 shows the auxiliary excitation voltage values of the DC generator according to the speed of the shaft of the AC generator, as required to maintain a constant voltage output by the AC generator. Figure 6 illustrates in greater detail voltage regulation system 136 and rotation speed sensor 134, according to an embodiment of the invention. 15 Rotation speed sensor 134 provides an analog rotation speed information to analog to digital converter 161 that converts the analog rotation speed information to digital rotation speed information. The digital rotation speed information is provided to low pass filter 162 that filters the digital rotation speed information to provide filtered rotation speed information. Low pass filter 162 can apply a Fast Fourier 20 Transformation filtering process but this is not necessarily so. Processor, such as FPGA 164 determines the excitation voltage in response to the filtered digital rotation speed information and to a relationship between the rotation speed and a peak voltage of the AC output voltage. Information representing 16 WO 2008/117290 PCT/IL2008/000426 this relationship can be stored in the FPGA, wherein instructions that are executed by FPGA can be stored in a memory unit such as EPROM 165. Processor 164 outputs a digital control signal that represents the determination to digital to analog converter 166. Digital to analog converter 166 5 converts the digital control signal to an analog control signal that controls the amplitude of the excitation voltage. The analog control signal is fed to current driver 168 that provides a current signal that is provided to DC power source 128 and determines the excitation voltage outputted by power source 128. The signal outputted by current driver 168 or the analog control signal can be measured by 10 voltage gauge 171 and displayed on a display of the voltage gauge. A multiplexer 169 or other sampling circuit can be provided between current driver 168 and the output port of voltage regulation system 136 in order to enable such a measurement as well as the provision of the current signal to DC power source 128. Conveniently, digital to analog converter 166 includes a potentiometer that is 15 used for determining the output range of the digital to analog converter 166, especially for allowing it to output a zero voltage analog control signal. Protocol converter 163 or another port or interface can be used for providing to voltage regulation system 136 the required peak voltage. A port or an interface (or an additional port or interface) can be used for 20 providing web based management, as illustrated by figure 8. A web based management entity 175 of voltage regulation system 136 can permit data regarding rotation speed measurements and/or measurements of the analog signal outputted to DC power source 128 to be output to one or more external devices, and/or to 17 WO 2008/117290 PCT/IL2008/000426 permit one or more interactions of a user with the voltage regulation system 136. Web based management entity 175 can be software, firmware, hardware or a combination thereof. Referring back to figure 6, voltage regulation system 136 can be mounted on 5 a printed circuit board. Additionally or alternatively it can communicate with a web server (not shown) which can provide one or more web pages. These web pages act as an interface for the voltage regulation system 136 for the user, for example for simple configuration options, as is known in the art. Voltage regulation system 136 can output information to a display such as 10 rotation speed display 172 as well as to a display of voltage gauge 171 or a combination thereof. The information can be provided via a port or an interface. According to another embodiment of the invention the rotation speed sensor provides digital rotation speed information and not an analog rotation speed information. Figure 7 illustrates rotation speed sensor as including incremental 15 encoder 134'. Incremental encoder 134' generates digital rotation speed information thus various components (such as analog to digital converter 161 and digital low pass filter 162) of voltage regulation system 136 are not required. Figures 9 and 10 illustrate the relationships between control current and rotation speed of the shaft for different values of the peak voltage of the AC output 20 signal, wherein the curves in figure 10 correspond to the values included in the table of figure 9.. Multiple curves represent the relationship between control current and rotation speed of the shaft for different peak voltages of the AC output signal. 18 WO 2008/117290 PCT/IL2008/000426 Figure 11 illustrates method 300 according to an embodiment of the invention. Method 300 starts by stage 310 or by stage 320. Stage 310 includes determining a requested peak voltage of an AC output 5 signal to be outputted by a AC generator. Stage 320 includes receiving information representative of a requested peak voltage of an AC output signal to be outputted by the AC generator. Stages 310 and 320 are followed by stage 330 of sensing a rotation speed of a shaft. I0 Method 300 can include rotating the shaft by a mechanical input element. This stage is not shown for simplicity of explanation. The shaft can be rotated by a mechanical input element that is powered by a renewable energy source. The renewable energy can be wind, water, solar or a geothermal energy source. Stage 330 is followed by stage 340 of controlling a magnetic field of an AC 15 generator in response to the rotation speed. According to various embodiments of the invention, stage 340 can include at least one of the following stages or a combination thereof: (i) Stage 342 of determining an amplitude of the excitation voltage to be provided to a DC stator of a DC generator in response to a relationship 20 between the rotation speed and a peak voltage of the AC output voltage. The DC generator can have a DC rotor that communicates with the shaft. The DC rotor can be connected to the AC rotor, for example by rigid wiring. The amplitude can be determined so as to 19 WO 2008/117290 PCT/IL2008/000426 maintain a peak voltage of the AC output signal substantially constant despite changes in the rotation speed. (ii) Stage 344 of feeding the DC stator of the DC generator by the excitation voltage, wherein the excitation voltage has an amplitude that 5 is responsive to the rotation speed. (i) Stage 346 of providing a control current by the direct current (DC) generator to the AC generator; wherein the control current is responsive to the amplitude of the excitation voltage. This control current controls the magnetic field of the AC generator. 10 Stage 340 is followed by stage 350 of outputting, by the AC generator, an AC output voltage. The peak voltage of the AC output signal is responsive to the magnetic field of the AC generator. The AC generator includes an AC rotor that communicates with the shaft. Stage 342 of the determining can include at least one of the following stages 15 or a combination thereof: (i) stage 342(1) of receiving rotation speed information from the speed sensor; (ii) stage 342(2) of receiving a requested peak voltage of the AC output voltage; and (iii) stage 342(3) of determining the amplitude of the excitation voltage in response to a relationship between control current and rotation speed of the shaft; (iv) stage 342(4) of determining the amplitude of the excitation 20 voltage in response to a relationship between control current values, rotation speed of the shaft and a requested peak voltage of the AC output voltage. It is noted that by changing the requested peak voltage of the AC output signal, the "set point" of the system is changed. When the peak voltage is changed 20 the determination should select the relevant curve (or formula). Accordingly, stage 344 can involve selecting between curves such as those illustrated in figure 10. According to an embodiment of the invention the determination is executed in the digital domain. Thus, stage 344 can include: (i) converting analog rotation speed 5 information to digital rotation speed information; (ii) low pass filtering the digital rotation speed information to provide filtered rotation speed information; (iii) determining the excitation voltage in response to the filtered digital rotation speed information and to a relationship between the rotation speed and a peak voltage of the AC output voltage; (iv) generating a digital control signal that is responsive to the ) determining; and (v) converting the digital control signal to an analog signal that controls the amplitude of the excitation voltage. Method 300 can also include stage 370 of activating a cooling fan that communicates with the shaft. The cooling fan is activated by the shaft. Method 300 can also include stage 380 of displaying information. 5 It is noted that the mentioned above system and method alter the magnetic field of the AC generator without measuring the current or voltage generated by the AC generator. There is no need to estimate the power outputted by the AC generator or to compare the estimated power to a reference power. This simplifies the control scheme. 0 While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made. 21 The term 'comprising' as used in this specification and claims means 'consisting at least in part of'. When interpreting statements in this specification and claims which include the term 'comprising', other features besides the features prefaced by this term in each statement can also be present. Related terms such as 5 'comprise' and 'comprised' are to be interpreted in similar manner. 21a

Claims (40)

1. A system for providing an alternating current (AC) voltage, the system comprises: an AC generator that outputs an AC output signal and comprises an AC rotor that communicates with a shaft rotates at a rotation speed; a speed sensor for sensing the rotation speed; and a controller for controlling a magnetic field of the AC generator in response to the rotation speed; wherein the controller comprises a direct current (DC) generator that generates a current that is provided to the AC generator so as to control the magnetic field of the AC generator.

2. The system according to claim 1 wherein the DC generator comprises a DC rotor that communicates with the shaft.

3. The system according to claim 2 wherein the DC rotor is coupled to the AC rotor.

4. The system according to claim 2 wherein the DC rotor is coupled to the AC rotor by rigid wiring.

5. The system according to claim 1 wherein the DC generator comprises a DC stator that if fed by an excitation voltage that has an amplitude that is responsive to the rotation speed.

6. The system according to claim 1 wherein the controller comprises a voltage regulation system that receives rotation speed information from the speed sensor 22 WO 2008/117290 PCT/IL2008/000426 and determines an amplitude of an excitation voltage to be provided to the DC generator.

7. The system according to claim 6 wherein the voltage regulation system determines that amplitude of the excitation voltage in response to a relationship between the rotation speed and a peak voltage of the AC output voltage.

8. The system according to claim 6 wherein the voltage regulation system comprises: an analog to digital converter that converts analog rotation speed information to digital rotation speed information; a low pass filter for filtering the digital rotation speed information to provide filtered rotation speed information; a processor, for determining the excitation voltage in response to the filtered digital rotation speed information and to a relationship between the rotation speed and a peak voltage of the AC output voltage; and a digital to analog converter, for converting a digital control signal outputted from the processor to an analog signal that controls the amplitude of the excitation voltage.

9. The system according to claim comprising a rotation speed sensor that generated digital rotation speed information.

10. The system according to claim 1 wherein the controller comprises a voltage regulation system that receives rotation speed information from the speed sensor and a requested peak voltage of the AC output signaland determines an amplitude of an excitation voltage to be provided to the DC generator. 23 WO 2008/117290 PCT/IL2008/000426

11. The system according to claim 1 wherein the shaft is rotated by a mechanical input element that is powered by a renewable energy source.

12. The system according to claim 1 wherein the renewable energy is selected from a group consisting of wind, water, solar and geothermal.

13. The system according to claim I further comprising a mechanical input element that rotates the rotor.

14. The system according to claim 1 further comprising a cooling fan that communicates with the shaft.

15. The system according to claim 1 wherein the controller controls the magnetic field of the AC generator so as to maintain a peak voltage of the AC output signalsubstantially constant despite changes in the rotation speed.

16. The system according to claim 15 wherein the DC generator comprises a DC rotor that communicates with the shaft.

17. The system according to claim 16 wherein the DC rotor is coupled to the AC rotor.

18. The system according to claim 15 wherein the DC generator comprises a DC stator that if fed by an excitation voltage that has an amplitude that is responsive to the rotation speed.

19. The system according to claim 15 wherein the controller comprises a voltage regulation system that receives rotation speed information from the speed sensor and determines an amplitude of an excitation voltage to be provided to the DC generator. 24 WO 2008/117290 PCT/IL2008/000426

20. The system according to claim 15 wherein the shaft is rotated by a mechanical input element that is powered by a renewable energy source.

21. A method for providing an alternating current (AC) voltage, the method comprises: sensing a rotation speed of a shaft; controlling a magnetic field of an AC generator in response to the rotation speed; wherein the controlling comprises providing a current by a direct current (DC) generator to the AC generator; and outputting, by the AC generator, an AC output voltage; wherein a peak voltage of the AC output signalis responsive to the magnetic field of the AC generator; wherein the AC generator comprises an AC rotor that communicates with the shaft.

22. The method according to claim 21 wherein the controlling comprises providing the current by the DC generator; wherein the DC generator has a DC rotor that communicates with the shaft.

23. The method according to claim 22 wherein the DC rotor is coupled to the AC rotor.

24. The method according to claim 21 wherein the DC rotor of the DC generator is coupled to the AC rotor by rigid wiring.

25. The method according to claim 21 comprising feeding a DC stator of the DC generator by an excitation voltage that has an amplitude that is responsive to the rotation speed. 25 WO 2008/117290 PCT/IL2008/000426

26. The method according to claim 21 comprising receiving rotation speed information from the speed sensor and determining an amplitude of an excitation voltage to be provided to the DC generator.

27. The method according to claim 26 comprising determining the amplitude of the excitation voltage in response to a relationship between the rotation speed and a peak voltage of the AC output voltage.

28. The method according to claim 26 comprising: converting analog rotation speed information to digital rotation speed information; low pass filtering the digital rotation speed information to provide filtered rotation speed information; determining the excitation voltage in response to the filtered digital rotation speed information and to a relationship between the rotation speed and a peak voltage of the AC output voltage; generating a digital control signal that is responsive to the determining; and converting the digital control signal to an analog signal that controls the amplitude of the excitation voltage.

29. The method according to claim 28 comprising providing a digital rotation speed information by a rotation speed sensor.

30. The method according to claim 28 comprising receiving rotation speed information from the speed sensor and a requested peak voltage of the AC output signal and determining an amplitude of an excitation voltage to be provided to the DC generator.

31. The method according to claim 21 wherein the shaft is rotated by a mechanical input element that is powered by a renewable energy source. 26 WO 2008/117290 PCT/IL2008/000426

32. The method according to claim 21 wherein the renewable energy is selected from a group consisting of wind, water, solar and geothermal.

33. The method according to claim 21 comprising rotating the shaft by a mechanical input element.

34. The method according to claim 21 comprising activating a cooling fan that communicates with the shaft.

35. The method according to claim 21 comprising controlling the magnetic field of the AC generator so as to maintain a peak voltage of the AC output signal substantially constant despite changes in the rotation speed.

36. - The method according to claim 34 wherein the DC generator comprises a DC rotor that communicates with the shaft.

37. The method according to claim 35 wherein the DC rotor is coupled to the AC rotor.

38. The method according to claim 34 wherein the shaft is rotated by a mechanical input element that is powered by a renewable energy source.

39. The method according to claim 34 comprising feeding a DC stator of the DC generator by an excitation voltage that has an amplitude that is responsive to the rotation speed.

40. The method according to claim 34 comprising receiving rotation speed information from the speed sensor and determining an amplitude of an excitation voltage to be provided to the DC generator. 27