The present disclosure generally relates to wind turbines, and more particularly, relates to tachometers for determining the rotational speed of wind turbine generators.
Tachometers are commonly used in power generation system, such as wind turbines, to measure the rotational speed of generators for monitoring and/or control purposes. One configuration for a tachometer involves a relatively small, single-phase alternating current (AC) synchronous generator that is driven by the primary generator. More specifically, the single-phase generator is driven by a pinion that engages a gear on the shaft of the primary generator or some other arrangement. The single-phase generator is typically configured to output a signal frequency that is a multiple of the shaft speed such that standard frequency calculations or logic will yield the rotational speed of the primary generator shaft. While the single-phase generator provides adequate resolutions for high speed applications, such tachometer configurations may provide insufficient resolutions when used with low-speed applications. In particular, a single-phase generator driven by a primary generator shaft rotating at relatively low speeds does not offer a resolution that is capable of quickly and accurately detecting and differentiating between subtle changes in speed, often resulting in inaccurate readings.
Another configuration of generator tachometry is the use of optical shaft encoders. These devices attach directly to the shaft under measurement and generate good resolution with a pulse output exceeding 4096 pulse per revolution. Although adequate for most measurements they need to be attached directly to the generator shaft. In the case of many modern wind turbines this is difficult to achieve as some turbine have multiply generator with no external shaft and other turbines, as discussed below, are direct driven and have not output shaft for that connection.
One low-speed application of generators involves direct drive wind turbines. Direct drive wind turbines drive a large diameter, low-speed generator directly from the rotor of the wind turbine and do not use a speed-increasing gearbox. Many designs for direct drive generators for a wind turbine do not provide a central or main shaft upon which a gear for driving a tachometer may be conveniently mounted. Furthermore, the speed of the main shaft is so low that the resolution of such standard tachometer configurations would not adequately detect changes in the rotational speed of the primary generator. One alternative may be to mount the tachometer near an outer circumference of the primary generator where the large diameter of the generator provides a detectable surface speed that is much greater than that of the main shaft. However, the outer circumference of a wind turbine generator is typically not suited for fitment with a gear set for driving a tachometer, and adding a gear to the generator design would come at an unjustifiable cost.
Accordingly, it would be beneficial to provide a tachometer for low-speed generators, such as for direct drive wind turbines, which offer greater resolution and easier implementation at minimal cost. Moreover, there is a need for a tachometer that is capable of accurately detecting subtle changes in the rotational speed of generators while requiring minimal changes to the design of the generator and its setting.
In accordance with one aspect of the present disclosure, a tachometer for a generator is disclosed. The tachometer may include a plurality of filters configured to receive a plurality of generator phase signals, a plurality of zero-cross detectors and a logic circuit. The filters may be configured to convert each phase signal into a corresponding filtered signal. The zero-cross detectors may be configured to generate pulse signals responsive to zero-crossings detected in each filtered signal. The logic circuit may be in communication with each zero-cross detector and configured to receive the pulse signals. The logic circuit may logically combine the pulse signals into a combined signal, and generate a tachometer signal based on the combined signal, wherein the tachometer signal corresponds to a rotational speed of the generator.
In accordance with another aspect of the present disclosure, a generator system is disclosed. The generator system may include a multi-phase stator, a rotor rotatably disposed within the stator, a plurality of zero-cross detectors, and a logic circuit. The rotor may have a plurality of poles configured to electromagnetically interact with the stator and induce a phase signal in each phase while rotating relative to the stator. The zero-cross detectors may be in communication with the phase signals and configured to generate pulse signals responsive to zero-crossings detected in each filtered signal. The logic circuit may be in communication with each zero-cross detector and configured to receive the pulse signals. The logic circuit may logically combine the pulse signals into a combined signal, and generate a tachometer signal based on the combined signal, wherein the tachometer signal corresponds to a rotational speed of the generator.
In accordance with yet another aspect of the present disclosure, a method of determining a rotational speed of a generator is disclosed. The method may receive a phase signal from each phase of the generator, generate a pulse signal based on zero-crossings detected in each phase signal, logically combine the pulse signal from each phase into a combined signal, generate a tachometer signal based on the combined signal, and calculate the rotational speed of the generator based on the tachometer signal, the number of phases, and the number of poles of the generator.
Other advantages and features will be apparent from the following detailed description when read in conjunction with the attached drawings.
For a more complete understanding of the disclosed methods and apparatuses, reference should be made to the embodiments illustrated in greater detail on the accompanying drawings, wherein:
While the following detailed description has been given and will be provided with respect to certain specific embodiments, it is to be understood that the scope of the disclosure should not be limited to such embodiments, but that the same are provided simply for enablement and best mode purposes. The breadth and spirit of the present disclosure is broader than the embodiments specifically disclosed and encompassed within the claims eventually appended hereto.
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The TCU 32 may conduct further calculations in determining the actual rotational speed of the generator 34. For example, based on the frequency of the tachometer signal provided by the combinational logic circuit 52, and further based on the number of phases and poles of the generator 34, the TCU 32 may be able to calculate the rotational speed of the generator 34 using the following relationships
=2 (1)
=— (2)
where fo is the frequency of the tachometer signal, fφ is the frequency of the input phase signal, Nφ is the number of phases of the generator 34, Np is the number of poles of the generator 34, and ω is the rotational speed of the generator 34 in revolutions per minute. By combining equations (1) and (2), the rotational speed of the generator 34 may be determined using
=—. (3)
In alternative modifications, the combinational logic circuit 52 may be configured to calculate the rotational speed of the generator 34 using the relationships identified above. The resulting generator speed may then in turn be communicated to the TCU 32 and/or other appropriate controllers of the wind turbine 10, for instance, in terms of revolutions per minute rather than logic pulse signals, for additional analyses.
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In general, the present disclosure sets forth a tachometer for a low-speed generator, such as a direct drive wind turbine generator, which offers high resolution feedback for accurately determining the generator speed at minimal cost. More specifically, the disclosed tachometer observes each of a plurality of phase signals that is output by a multi-phase generator, and generates square wave or pulse signals corresponding to the phase signals using a series of zero-cross detectors. The tachometer then derives the frequency in each phase by tracking rise and fall transitions in the respective pulse signals. A combinational logic circuit of the tachometer combines the frequency information retrieved from each phase to result in a combined logic pulse signal or tachometer signal which exhibits a frequency that is a scalar multiple of the frequency in each observed phase signal. Based on the frequency of the resulting tachometer signal, the number of poles in the generator, and the number of phases in the generator, the logic circuit and the turbine control unit (TCU) is then able to compute the rotational speed of the generator.
As the disclosed tachometer derives generator speed based on a plurality of phase signals, received directly from the primary generator rather than indirectly through the single-phase of a secondary generator, the present disclosure offers higher resolution feedback and more accurate calculations, especially for low-speed applications such as direct drive wind turbines. Also, by eliminating the need for installation of an additional, single-phase generator and any gear sets associated therewith, the disclosed tachometer can be easily implemented in wind turbine settings having limited access to a main shaft and other related gearing assemblies. The simplicity of the present disclosure also minimizes costs, facilitates implementation with new installations, and enables retrofitment onto existing applications.
While only certain embodiments have been set forth, alternatives and modifications will be apparent from the above description to those skilled in the art. These and other alternatives are considered equivalents and within the spirit and scope of this disclosure and the appended claims.