Rotor Position Detection Using Mutual Inductance in Brushless DC Motors
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Solution Overview
Problem
Existing methods for controlling brushless DC motors at standstill and low speeds require external sensors, which are costly, reduce reliability, and occupy valuable space, while existing sensorless techniques like back-EMF control are ineffective below 5-10% of nominal speed due to low or zero back-EMF voltage.
Innovation Solution
A device and method using mutual inductance between stator coils to determine rotor position by applying voltages to phases and sampling the third phase voltage, filtering out noise to accurately calculate mutual inductance and rotor position without external sensors, enabling effective control at zero and low speeds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If external electrically powered speed transducers and sensors are used to detect rotor position, then measurement precision is improved, but device complexity increases and reliability decreases
Solution Approach 1:
The motor windings themselves are used to generate the measurement signal through mutual inductance effects. The control unit applies voltages to two phases and measures the induced voltage in the third phase, allowing the motor structure to serve its own measurement function without external sensors.
Solution Approach 2:
The stator windings perform dual functions: they generate the magnetic field for motor operation and simultaneously serve as the measurement system for rotor position detection through mutual inductance. This eliminates the need for separate sensing components.
2Measurement precision
If external electrically powered speed transducers and sensors are used, then measurement precision is improved, but manufacturing cost increases
Solution Approach 1:
The stator windings perform dual functions: they generate the magnetic field for motor operation and simultaneously serve as the measurement system for rotor position detection through mutual inductance. This eliminates the need for separate sensing components.
Solution Approach 2:
The motor windings themselves are used to generate the measurement signal through mutual inductance effects. The control unit applies voltages to two phases and measures the induced voltage in the third phase, allowing the motor structure to serve its own measurement function without external sensors.
3Measurement precision
If external electrically powered speed transducers and sensors are used, then measurement precision is improved, but the space occupied in motor housing increases
Solution Approach 1:
The stator windings perform dual functions: they generate the magnetic field for motor operation and simultaneously serve as the measurement system for rotor position detection through mutual inductance. This eliminates the need for separate sensing components.
Solution Approach 2:
The motor windings themselves are used to generate the measurement signal through mutual inductance effects. The control unit applies voltages to two phases and measures the induced voltage in the third phase, allowing the motor structure to serve its own measurement function without external sensors.
4Device complexity
If back-EMF control technique is used for sensorless rotor position detection, then device complexity is reduced, but measurement precision deteriorates at low speeds
Solution Approach 1:
The invention changes the measurement parameter from back-EMF voltage (which is proportional to speed) to mutual inductance-induced voltage in the floating phase. This allows accurate rotor position detection at standstill and low speeds where back-EMF is negligible, while maintaining the simplicity of sensorless control.
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
Enables accurate and reliable rotor position detection and speed measurement at standstill and low speeds, eliminating the need for external sensors and improving cost efficiency and motor design by utilizing mutual inductance variations with rotor position.
Implementation Method 1
the voltage of the floating phase is a function of the rotor position due to the mutual inductance components
Data Source
Figure 1
Figure 2~3
Figure 4~6
AI summary
A device (1) is for determining a rotor position in a polyphase electric motor having a first phase, a second phase and a third phase. A power control unit (3) applies a first voltage on the first phase, and a second voltage on the second phase, the first voltage and the second voltage being periodic signals of opposite polarity, alternating between a first part and a second part of the alternating period, such as square waves. A sample unit (4) samples a third voltage on the third phase for acquiring a first sample at a first instant in the first part and a second sample at a second instant in the second part, and a difference value between the first sample and the second sample. The difference value represents a mutual inductance between the stator coils due to the rotor position. Finally, a determination unit (5) determines the rotor position based on the difference value.