Indirect Rotor Resistance Estimation via Stator Voltage Decay
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Solution Overview
Problem
Indirect field oriented control (FOC) of AC induction motors faces challenges in accurately estimating rotor resistance due to temperature variations, leading to misalignment of the synchronous frame with the rotor flux, which degrades motor performance, especially at low speeds and during temperature swings.
Innovation Solution
An improved indirect rotor resistance estimation system that uses a synchronous frame current regulator to set stator currents to zero, measuring the decay time or voltage to determine rotor resistance without additional sensors or hardware, allowing for real-time estimation during speed changes and temperature variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If rotor resistance is used to estimate slip frequency in indirect FOC, then slip frequency can be calculated, but rotor resistance varies with temperature causing misalignment of synchronous frame with rotor flux
Solution Approach 1:
The patent changes the measurement parameter from direct rotor resistance (which varies with temperature) to the decay characteristics of stator current/voltage after interruption. By measuring how the stator current decays after being set to zero, the system can infer rotor resistance at the current temperature without being affected by temperature drift, thus maintaining accurate rotor flux angle estimation across temperature variations.
Solution Approach 2:
The patent implements a feedback mechanism where the measured decay characteristics of stator current are continuously used to update the rotor resistance estimation. This closed-loop approach allows the system to adapt to temperature changes in real-time, ensuring the synchronous frame remains aligned with the rotor flux despite temperature variations.
2Measurement precision
If stator current is set to zero to measure decay characteristics, then rotor resistance can be estimated, but this requires additional control steps
Solution Approach 1:
The patent makes the existing synchronous frame current regulator perform an additional function: measuring rotor resistance. By using the same current regulator to both control motor operation and execute measurement sequences (setting current to zero and measuring decay), the system avoids adding separate measurement hardware or complex dedicated measurement circuits, thus limiting the increase in device complexity.
Solution Approach 2:
The system uses its own existing components (current regulator, voltage sensors) to perform the measurement function. The current regulator that normally controls motor operation is also used to create the measurement condition (setting current to zero) and measure the decay characteristics, making the system self-sufficient without requiring external measurement equipment.
3Speed
If indirect FOC is used for low speed control, then dynamic performance is improved, but current ripple and acoustic noise increase
Solution Approach 1:
The patent replaces mechanical measurement methods (such as torque sensors or direct position sensors) with an electrical measurement approach. By measuring the electrical decay characteristics of stator current, the system can accurately estimate rotor parameters and maintain precise control at low speeds without the mechanical complexity and associated noise of physical sensors, thus reducing current ripple and acoustic noise while maintaining low speed performance.
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 method enhances the accuracy of rotor flux angle calculation and motor performance by eliminating the need for torque or voltage sensors, maintaining alignment with the rotor flux even under varying conditions, and is compatible with standard inverter hardware and software frameworks.
Implementation Method 1
measuring the decay time or voltage to determine rotor resistance without additional sensors or hardware, allowing for real-time estimation during speed changes and temperature variations
Data Source
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AI summary
Indirect rotor resistance estimation for an AC induction motor is achieved by successively stepping the quadrature command to zero and the direct current command to a predetermined value causing the quadrature stator voltage to decay as a representation of rotor current decay; defining, in response to the decaying stator voltage reaching two spaced thresholds, a voltage/time difference, and retrieving from a storage device the rotor resistance associated with the voltage/time difference.