Brushless Motor Control for Stall Voltage Protection
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Solution Overview
Problem
Existing control systems for three-phase brushless motors, used as starter motors and AC power generators, face challenges in controlling motor coils without position sensors, especially during engine stall conditions, leading to potential excessive voltage application to external loads.
Innovation Solution
A brushless motor control apparatus that includes a three-phase bridge circuit with switching elements and diodes, a phase voltage detector, zero-cross point detector, and a regulator unit to manage engine rotations and battery voltage, allowing for phase control or interphase short circuiting to prevent excessive voltage application.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If sensorless drive control circuit is implemented to detect rotor position without sensors, then device complexity and cost are reduced, but measurement precision of rotor position detection deteriorates
Solution Approach 1:
The patent introduces an intermediary mechanism (sub-coil Su and zero-cross point detection) to indirectly detect rotor position without requiring direct sensors on the rotor. The sub-coil detects voltage changes caused by rotor movement, and the zero-cross point detector translates this into position information, serving as a mediator between the rotor's physical position and the control system's digital processing.
Solution Approach 2:
The patent replaces the mechanical sensor-based detection system (hole devices mounted on rotor) with an electrical field-based detection system using sub-coils and voltage detection. This substitution eliminates mechanical contact and physical mounting requirements while achieving rotor position detection through electromagnetic principles.
2Productivity
If phase control is performed based on zero-cross points of AC output voltage, then battery charging control is improved, but reliability deteriorates when engine rotations are low and zero-cross points cannot be detected
Solution Approach 1:
The patent implements dynamic switching between two control modes based on operating conditions: at normal engine speeds, zero-cross point-based phase control is used for efficient battery charging; at low engine speeds where zero-cross points are undetectable, the system dynamically switches to interphase short-circuit control to prevent excessive voltage. This dynamic adaptation ensures reliability across the full operating range.
Solution Approach 2:
The patent applies beforehand cushioning by preparing an alternative control mode (interphase short-circuit control) in advance for low-speed conditions. The control unit monitors engine rotation and proactively switches to the pre-prepared backup control strategy before voltage anomalies occur, preventing excessive voltage application to external loads rather than reacting after the problem arises.
3Reliability
If interphase short circuit control is performed at low engine rotations, then excessive voltage application to external loads is prevented, but productivity for battery charging is reduced
Solution Approach 1:
The patent applies partial action by implementing interphase short-circuit control selectively only when necessary (at low engine rotations where zero-cross points are undetectable), rather than continuously. This partial application of the safety control maintains reliability when needed while avoiding unnecessary productivity loss during normal operating conditions where full battery charging capability can be utilized.
Data Source
Figure 1
Figure 2A~2B
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AI summary
Even in a situation other than engine normal operation such as engine stall, the output voltage of the three-phase brushless motor is prevented from causing the excessive voltage to be applied to an external load. A brushless motor control apparatus (10) of the present invention calculates the number of rotations of an engine based on a cycle of zero-cross points of the phase voltage Vsu induced to a sub-coil Su of a three-phase brushless motor (1). Then, when the number of rotation of the engine is smaller than the first number of rotations N1, before it becomes impossible to detect the zero-cross points, the control state of the three-phase brushless motor 1 is changed from the state of phase control by a phase control regulator unit (24) with respect to the AC output voltages Vu, Vv, and Vw to the state of interphase short circuit of motor coils by a short regulator unit (25). Then, when the number of rotations of the engine exceeds the first number of rotations No and the phase voltage Vsu becomes the voltage level such that the zero-cross points can be detected, the control state is changed again to the state of phase control by the phase control regulator unit (24).