SYNCHRONOUS ELECTRIC GENERATOR

DESCRIPTION

TECHNICAL FIELD

The present invention relates in general to a method and an apparatus, of converting mechanical torque in constant frequency electricity regardless the rotating speed. It more particular relates to such a method and apparatus useful in providing constant frequency electricity to a utility grid. This methods and apparatus can be used in any electrical power production field such as wind generators, gas and hydraulic turbines, Stirling engines, etc. BACKGROUND ART Various different techniques have been employed in converting mechanical torque to constant frequency electricity for supplying it to a utility grid.

The most popular technique is the use of synchronous generator, this type of electric generator is made by one rotor holding a field winding which is magnetised by a DC electric current, called excitation current, and one stator holding a three-phase winding where the individual phase windings are distributed 120° apart in space.

When the rotor is rotated (e.g. by a turbine) the rotating magnetic field induces a voltage on the Stator windings. The stator coils induced voltages are sinusoidal with a magnitude that depends on the rotor DC current, differ by 120° in time and have a frequency determined by the rotating speed of the rotor and its number of poles (e.g. for a rotor with a pair of poles and rotating at 3000 rounds per minute the induced Stator voltage frequency is 50 Hz). For that reason the synchronous generators need to rotate at constant speed in order to produce a AC Voltage at constant frequency. The synchronous generator includes a device to control the rotor DC current for the AC electricity produced by the stator to be in phase with the utility grid voltage without any substantial phase shift and maintains the power factor above a mkdmum limit.

DISCLOSURE OF INVENTION

A method and apparatus are disclosed for converting mechanical torque generated electricity to constant frequency electricity for supplying it to a utility grid regardless the rotating speed of the apparatus. The disclosed method and apparatus use two stators and two rotors and a control unit providing two distinct excitation currents for the two rotors. These currents are not necessarily constant in time but they may oscillate with the same frequency and magnitude, but different phases. Assuming the two rotors poles have the same orientation and same winding shape, then the coils of the two stators are wound with a different angle with respect each others. This looks like two synchronous generators coupled (the two rotors have the same rotating speed and possibly they are on the same shaft). For simplicity in the following descriptions and Figures, we consider only one coil per each stator and the rotors having only a pair of poles. The functioning principle is very close to the synchronous generator described above, where the total produced voltage is the sum of the two stators coils induced voltages and its frequency is equal to the sum of the rotors spin and the excitation currents frequency (in example: with the rotors spinning at 46 Hz and the excitation currents frequency at 4 Hz, the total induced voltage frequency is equal to 50 Hz). Due to this particular lay-out, each of the stator coils produces a voltage without any specific frequency, but the sum of the stators' voltages is a perfect wave, its frequency is equal to the sum of the rotor coils rotating speed and excitation currents frequency. The control unit collects data from the sensors like the tachometer for rotor spin and position, the total voltage meter and 1he total current meter produced by the generator.

The current meter and the voltage meter monitors the current, the power produced by the generator and its power factor. The desired power factor of the AC electricity from the stators is achieved by rotors' currents control. A calculation of desired rotors' currents magnitude is then performed by the control unit. In case the power factors has decreased the above mentioned device increases excitation currents magnitude in order to keep the power factors above a minimum limit.

The tachometer monitors the shaft speed and position of the rotors and a calculation of desired rotors' currents frequency is then performed by the control unit in order to maintain the generated power at the desired frequency (e.g. 50 Hz for EU country , 60 for US) within a narrow tolerance. At the same time the calculation for currents' phases difference is performed keeping the phases difference at a predicted value. In case the rotors decrease their spin the above mentioned control unit increases the excitation currents frequency. These calculations may also include using closed loop controllers such as proportional , integral and derivative controllers.

The control unit generates two low power voltage analog outputs proportional to the two rotors currents. These are then amplified by operational amplifiers in order to obtain the proper currents magnitudes for the two rotors. The control unit can also takes care of disturbances like residual magnetism of the rotors or the mutual inductance differences. These effects may be included in the rotors currents calculation in the sense that the rotors currents can have bias to nullify tibe residual magnetism of the rotors or the excitation current can have different magnitudes in case the two generators have slightly different mutual inductance.

BRIEF DESCRIPTION OF DRAWINGS

The features of this invention and the manner of attaining them will become apparent and the invention itself will be best understood by reference to the description of certain embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

- Figure 1 a shows Generator "A", composed by rotor "A" and stator "A" (respectively ROT A and STAT-A), where α is the time dependant position of the rotor "A" with respect to the Stator "A", schematically represented by a single loop. - Figure Ib shows the trend of the rotor "A" excitation current (I_a)

- Figure Ic shows the trend of the induced voltage in the coil of the stator "A" (V_a).

- Figure 2a shows generator "B" , composed by rotor "B" and stator "B" (respectively ROTJB, and STATJB), where β is the time dependant position of the rotor "B" with respect to the stator "B", schematically represented by a single loop.

- Figure 2b shows the trend for the rotor "B" excitation Current 0Jb).

- Figure 2c shows the trend of the induced voltage in the coil of the Stator "B" (V_b ). - Figure 3 shows how the two stators coils are connected each, other and the Trend for total induced voltage V_ab (sum of the two voltage : V_a +V_b ).

- Figure 4 shows a symbolic block diagram for the generator and its control Unit (CJLJ) and how it is connected to Utility grid. Ih the figure are shown also the two rotors' currents operational amplifiers, (respectively AMP_A and AMPJB), the rotors' position and spin meter (m_w_2), the stators coils current meter (m_I_ab) and the stators coils Voltage meter (m__V_ab).

BESTMODE FOR CARRYING OUT THE INVENTION With first reference to figure 3, the above described electrical generator can be made by coupling two twin synchronous electric generators and including a control unit controlling the two excitation rotors currents. The two generators are twin in the sense their rotors have the same number of poles and same numbers of coils and impedance and their stators have the same number of winding with the same impedance.

These two generators rotate at the same speed, possibly with the two rotors on the same shaft, while the stators windings are connected each other,, in the sense that the coils of Stator "A" are connected electrically in series to the coils of Stator "B". Assuming the two rotors have the same orientation then the windings of the two stators are wound with a different angle -with respect each others, this angle is equal to 90 degree divided the number of pair of poles of the rotor. With reference to figure 4, the control unit collects signals from meters such as the Rotors spin meter (m_w_2), the current meter (m_I_ab) of the stators* coils the stators' coils voltage meter (m_V_ab).

The two rotors excitation currents are tuned by the control unit controlling their frequencies, magnitudes and their phases. The currents' phases differ by a quarter of current period.

The performance of the whole system is provided by the control unit (C_U) in order to match the requirements for any synchronous generator connected to utility grid such as power factor, fixed voltage frequency, voltage peak to peak value and produced Voltage to be in phase with utility grid voltage. With reference now to figure Ia ₅ Ib ₅ Ic, 2a 2b,2c,3and 4, the working principle is described assuming the two rotors ( Rot_A ₉ and RotJB) perform like magnetic dipoles rotating clockwise inside the two stators' coils respectively Stat_A and StatJB, represented schematically by a single loop.

Electromagnetic induction is the working principle of the synchronous electric generator. One fixed coil on the stator form a closed path placed in a region in which a magnetic field varies with time. An electromagnetic force (V) on this stator coil is generated and it depends on the rate of change of the magnetic flux. The time dependent magnetic flux can be provided either by a rotating permanent magnet or by a rotor fed by a constant current (synchronous generator ) rotating inside the stator coil . The induced voltage (V) is :

V- - dφ / dt where φ is the magnetic flux generated by the rotor. In case two devices (A and B), as described above, are properly coupled with two rotors' currents not stationary but oscillating and opportunely toned, it's possible to generate a total electromagnetic force with perfect wave having a frequency equal to the sum of the rotating speed and the currents frequency. Here below the description of functionmg principle assuming one coil per each stator and the rotors having only a couple of poles, see the attached figures for more details: OTfc^/magnetie flux generated by rotor "A" in stator coil "A" sin(β) ;magnetic flux generated by rotor "B" in stator coil "B" With reference in particular to figure Ib and figure 2b, the two rotors currents (respectively I_a and I_b) are not constant but are oscillating at the same frequency (w_l) but with different phase :

Where : TJ. ~ the rotors currents period (sec), w_J — the rotors currents frequency = 2 * π/T_l (rad/sec) Ijnax = the rotors current magnitude (Amp) .

With reference to figure 3, the two rotors are rotating with the same spin and the stator coil "B" has a position shifted by 90° with respect to stator coil "A" , where : a = the time dependant position of the rotor "A" respect the stator "A" β the time dependant position of the rotor "B" respect the stator "B".

The electromagnetic forces (V_a and V_b) induced on the fixed coils are function of the first time derivate of the magnetic fluxes :

The sum of the two induced electromagnetic forces is :

The total electromagnetic force (V_ab) of a couple of synchronous geaerators with rotors fed by two different currents appropriately controlled is a perfect wave and its frequency is the sum of the rotors currents frequency (w_l) and the rotor spin ( w_2).

Details about the electrical connections with the two rotors are not considered relevant to the disclosure of the present invention, in case the electrical connections to the two rotors are be provided by brushes, then Control Unit (C_U) and its operational amplifiers are external to the synchronous generators. In case the two generators are hrushless, then the operational amplifiers can be installed on the rotors shaft and they can be powered by a permanent magnet generator with diode bridge, in this case the control unit may communicate and drive these amplifiers with, an optical interface. Details about the utility grid breaker controls and actuation are not considered relevant to the disclosure of the present invention, these controls are normally made by standard unit in order to prevent spurious AC generated electricity flowing to utility grid. Details about the generator alignment, before utility grid breaker is closed, for the current regulation of the AC electricity produced to be in phase with the utility grid voltage without any substantial phase shift are not considered relevant to the disclosure of the present invention.

Details about the rotors' currents phase shift and stators coils relative position in case rotors have a numbers of poles greater the two and stators have a number of coils greater than one can be easily evaluated for any specific case. While particular details of the of the present invention have been disclosed, it is understood that various different modifications are passing and one contemplated within the true spirit and scope of the appended claims. For example while the description refers to a clockwise rotation of the rotors the same principle of the present invention could be applied to different rotating direction of each rotor or winding configuration for rotors or stater coils, For example it will become apparent to those skilled in the art that the control unit functions may be implemented in whole or in part either by software or by discrete circuits. There is no intention, therefore, of limitation to the exact abstract or disclosure herein.

INDUSTRIAL APPLICABILITY

The application fields for this invention are very wide. Consider all the fields where mechanical rotating energy is converted first into electric energy by DC generator then converted to AC voltage by a solid state device called "DC/AC Inverter ", this device is very costly and source of energy dispersion. Consider, also, all the fields where mechanical rotating energy is converted into electric energy by AC generator, in these cases the whole systems have a spin control devices in order to produce a fixed frequency voltage. For example a particular field of application for this new device could be that of the wind turbines. Thanks to its operability over a wide range of rotating speed and the ability to produce a fixed frequency AC voltage at any spin this new type of generator is a promising machine to convert energy from wind turbines. In fact, wind turbines coupled to synchronous generator need wind speed above a minimum limit to allow the generator to rotate at constant speed in order to be connected to the electrical grid network, in case of high wind speed regime, the wind generator need to rotate always at the same spin, reducing the overall efficiency. Other wind turbines, of smaller size, are coupled to permanent magnet generators, they operate at different spin based on the wind load but the produced voltage is first converted to a DC voltage, then an other device, called DC / AC INVERTER to convert the DC voltage into AC voltage for local usage or for the connection to external electrical grid.

The electric generator described herein coupled to a wind turbine can overcome all these aspects. It can operate at different wind speeds producing directly an AC voltage with a constant frequency (even at lower wind speeds ) and be connected to external power grid without any further conversion of its power (DC/AC Inverter ).