Sensorless Brushless DC Motor Startup Using BEMF Alignment
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Solution Overview
Problem
Brushless DC motors face challenges in accurately detecting rotor position and rotation rate, especially when using sensorless methods, as the Back ElectroMotive Force (BEMF) signal becomes small at low speeds, leading to inefficiencies in start-up processes.
Innovation Solution
A twelve-phase technique is employed for starting a brushless sensorless DC motor, involving alignment of the rotor to the stator using a current mode operation where two of three phases are alternately energized, allowing one inductor terminal to float, and capturing the peak BEMF signal after a delay, with commutation to the next phase based on polarity changes or time-outs, enabling efficient start-up and torque maximization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If sensorless BEMF detection is used to reduce cost and complexity, then device complexity is reduced, but measurement precision deteriorates at low speeds
Solution Approach 1:
The patent applies preliminary action by implementing an alignment phase before normal operation, where current mode control is used to position the rotor at a predefined location. This preliminary positioning ensures that when BEMF detection begins, the rotor is in an optimal position where the BEMF signal is sufficiently strong for accurate measurement, thus resolving the low-speed measurement precision problem while maintaining sensorless operation
Solution Approach 2:
The patent changes operational parameters by switching between current mode control (during alignment) and voltage mode control with BEMF detection (during normal operation). It also adjusts the operating phase sequence to optimize BEMF signal strength, thereby improving measurement precision without adding physical sensors
2Device complexity
If rotor position is detected using BEMF signal in sensorless mode, then device complexity is reduced, but reliability deteriorates during start-up
Solution Approach 1:
The alignment phase serves as a preliminary action that reliably positions the rotor before BEMF-based commutation begins. This ensures that the motor starts with known rotor position information, making the subsequent sensorless operation more reliable during the critical start-up period
Solution Approach 2:
The patent creates equipotential conditions by using current mode control during the alignment phase to establish a stable magnetic field configuration. This stable starting condition ensures reliable transition to the BEMF detection phase, preventing unreliable operation during start-up
3Measurement precision
If Hall sensors are used to detect rotor position, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent uses the BEMF signal as an intermediary to infer rotor position without direct physical sensing. Instead of using Hall sensors to directly measure position, the system measures the BEMF voltage that naturally occurs in the windings and uses this signal to determine rotor position, thereby avoiding the complexity of sensor installation while maintaining functional capability
Solution Approach 2:
The patent replaces the mechanical sensor-based position detection system with an electrical field-based detection method. By substituting physical Hall effect sensors with electronic BEMF signal analysis, the system eliminates mechanical wear and sensor-related complexity while achieving the necessary measurement precision
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 effectively aligns the rotor with the stator and provides maximum acceleration and torque during start-up, improving the accuracy and efficiency of sensorless brushless DC motor operation, even at low speeds, while being cost-efficient.
Implementation Method 1
the position of the rotor is detected using a Back ElectroMotive Force (BEMF) signal
Implementation Method 2
a stationary portion or stator that produces a rotating magnetic field and a non-stationary portion or rotor in which torque is created by the rotating magnetic field
Data Source
AI summary
A method for starting a brushless DC motor. A rotor is aligned with a stator in accordance with a predetermined phase. After alignment, the rotor is positioned in accordance with another phase, two phases are skipped, a timer is set to a first count time, and the rotor is aligned with the stator in accordance with a third phase. Then the timer is restarted and the rotor is aligned with the stator in accordance with a fourth phase. After a first delay, first back electromotive force value is stored. The timer is stopped when the first back electromotive force value substantially equals a peak amplitude of opposite polarity. The timer is updated to a second count time that is substantially equal to a time at which the second timer was stopped. The process is repeated until the rotor has a position and a velocity that are suitable for normal operation.