Rotor Electrical Angle Assembly for Low-Speed Torque Control
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Solution Overview
Problem
Direct-drive wind turbine generators with outer rotors face challenges in accurately determining the electrical angle for precise torque control, especially at low speeds, due to the limitations of sensorless methods and the difficulty in installing shaft-mounted encoders.
Innovation Solution
An assembly comprising an encoder mounted on the stator to detect relative rotor rotation, combined with an electrical angle observer providing absolute angles, and a processing device that utilizes a gear ratio correction factor to determine the electrical angle with high precision, using high-frequency injection methods and closed-loop control algorithms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If sensorless (back-emf based) methods are employed for position estimation under power production mode, then the system avoids additional sensors and reduces cost, but these methods fail at very low speeds
Solution Approach 1:
The system switches between different position estimation methods based on operating conditions. At low speeds, high-frequency signal injection is used to generate position-dependent signatures, while at higher speeds, sensorless back-emf methods are employed. This parameter-based switching resolves the contradiction by adapting the estimation method to the specific speed regime.
Solution Approach 2:
High-frequency signal injection acts as an intermediary mechanism that enables position estimation at low speeds where direct sensorless methods fail. The injected signals create measurable responses that provide position information without requiring physical sensors, bridging the gap between sensorless operation and low-speed accuracy.
2Measurement precision
If shaft-mounted incremental encoders are used for high performance motion control, then position measurement precision is improved, but absolute rotor position cannot be determined and installation is difficult due to lack of rotor shaft
Solution Approach 1:
Instead of mounting the encoder on the rotating rotor shaft (traditional approach), the encoder is mounted on the stationary stator and contacts the rotor surface. This inversion of the mounting approach eliminates the need for a rotor shaft while maintaining measurement precision through the contactless or minimal-contact encoder design.
Solution Approach 2:
The encoder measures rotor position indirectly by detecting the position of rotor markings or features through the encoder wheel contact, rather than directly measuring shaft position. This copying approach enables accurate position measurement without requiring direct shaft access or mounting.
3Adaptability or versatility
If surface mounted PM and fractional slot concentrated winding machines are used, then machine design flexibility is improved, but position dependent signature for HFI sensorless methods is insufficient
Solution Approach 1:
High-frequency current signals are injected into the machine windings to excite position-dependent magnetic signatures. The interaction between the injected signals and the machine's magnetic circuit creates measurable vibrations or variations in the current that reveal rotor position information, enabling sensorless control even in machines with surface-mounted PMs.
Solution Approach 2:
The system utilizes changes in magnetic parameters and impedance characteristics at different rotor positions to generate detectable signatures. By monitoring how these parameters vary with rotor position during high-frequency signal injection, accurate position estimation is achieved despite the machine topology limitations.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enables accurate and reliable determination of the electrical angle, improving torque control and structural integrity by combining relative and absolute angle information, and dynamically correcting the gear ratio for precise rotor position feedback.
Implementation Method 1
an encoder wheel configured to contact a surface of the rotor to obtain relative rotor rotation information based on rotation of the encoder wheel
Implementation Method 2
Some machine technologies (e.g., interior permanent magnet (PM) machines) are well suited for alternative sensorless methods, like high-frequency signal injection (HFI) based observers
Implementation Method 3
a processing device coupled to communicate with the encoder and the electrical angle observer and configured to determine the electrical angle of the rotor based on the relative rotor rotation information and the absolute electrical angle
Data Source
AI summary
An assembly for determining the electrical angle of a rotor in an electrical machine is provided, such as a wind turbine generator. The assembly includes: (a) an encoder having an encoder wheel configured to contact a surface of the rotor to obtain relative rotor rotation information based on rotation of the encoder wheel, (b) an electrical angle observer configured to provide an absolute electrical angle, and (c) a processing device coupled to communicate with the encoder and the electrical angle observer and configured to determine the electrical angle of the rotor based on the relative rotor rotation information and the absolute electrical angle. Furthermore, a wind turbine generator including such an assembly, and a method of determining the electrical angle of a rotor in an electrical machine, such as a wind turbine generator, are provided.