BLDC Motor Phase Winding Waveform for Back-EMF Sensing
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Solution Overview
Problem
Existing methods for sensing the rotor position in multiphase brushless DC motors, especially in tri-polar mode, rely on complex systems and external timing signals to interrupt the driving current, increasing system complexity and reducing efficiency.
Innovation Solution
A method where each phase winding is driven with a stored waveform that is zero at the start, middle, and end of each driving phase, allowing for back EMF monitoring to determine the zero crossing point without external signals, using a stored waveform profile and digital processing to calculate the rotor position accurately.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If all three phase windings are driven continuously in tri-polar mode to minimize torque ripple, then torque smoothness is improved, but the ability to sense back EMF for rotor position detection is lost
Solution Approach 1:
The patent applies periodic action by introducing brief intervals where one phase winding is deactivated (set to zero) at regular commutation points. This periodic deactivation creates windows of opportunity to sense back EMF without continuously driving all three phases, thus maintaining torque smoothness while enabling position detection.
Solution Approach 2:
The patent uses preliminary action by pre-determining the commutation points based on electrical angles (e.g., 30°, 90°, 150°) and proactively deactivating phases at these predetermined points before back EMF sensing is needed. This allows the system to prepare sensing opportunities in advance without disrupting the overall continuous driving pattern.
2Measurement precision
If external timing signals are used to interrupt driving current for back EMF monitoring, then rotor position detection is enabled, but system complexity increases
Solution Approach 1:
The patent applies self-service by having the motor controller itself generate the phase deactivation signals based on its own commutation timing logic. The system uses its internal knowledge of electrical angles and commutation sequences to automatically create sensing windows, eliminating the need for external timing signals or additional complex monitoring hardware.
Solution Approach 2:
The patent makes the phase windings multi-functional by using them both for torque production during normal operation and for back EMF sensing during deactivation intervals. The same hardware components (phase windings, current paths) serve dual purposes, reducing the need for separate sensing windings or additional external signal generation circuits.
3Measurement precision
If phase windings are interrupted for back EMF monitoring, then rotor position can be determined, but power delivery is reduced during interruption intervals
Solution Approach 1:
The patent applies partial action by deactivating only one phase winding at a time for sensing purposes, while the other two phases continue to be driven and produce torque. This partial deactivation minimizes the impact on overall power delivery while still providing sufficient opportunity for accurate back EMF sensing and rotor position determination.
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 efficient rotor position sensing during normal multiphase operation without additional external signals, reducing complexity and torque ripple, and allowing for quick and accurate digital calculation of the zero crossing point.
Implementation Method 1
The magnetic field used to turn a permanent magnet rotor is generated using three (or more) interconnected phase windings in the stator of the motor
Implementation Method 2
the third undriven phase winding is used to monitor the back EMF voltage. By monitoring the back EMF voltage, the position of the rotor can be determined
Data Source
Figure 1
Figure 2~3
AI summary
The driving system for a tri-polar electric motor (100) comprises three phase windings (101u-1O1w). The winding drivers (102u-102w) drive each winding 101u-101w with a driving waveform (200) of the type shown in figure 2. The driving waveform (200) has a non-zero driving phase and intervals wherein the input is equal to zero at the start, middle and end of each driving phase. Using a driving waveform (200) of this type enables monitoring of the back EMF in the winding during each interval when the input is equal to zero. This enables regular monitoring of the zero crossing point of each winding (101u-101w) and hence of the position of the rotor. This enables the motor to operate efficiently without generating a torque ripple.