Variable Switching Threshold for PWM Motor Control Stability
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Solution Overview
Problem
The existing PWM control methods for alternating-current motors experience instability due to frequent switching between sinusoidal wave and overmodulation modes, leading to chattering, which affects the stability of voltage control.
Innovation Solution
A control device and method that includes a pulse width modulation control unit, a mode-switching determining unit, and a determination value setting unit, which variably sets a switching determination value based on the power conversion operation state to prevent frequent switching between sinusoidal wave and overmodulation modes, ensuring stable control by adjusting the carrier frequency and switching rate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If fixed threshold voltage is used for mode switching, then control simplicity is maintained, but chattering occurs due to frequent mode switching
Solution Approach 1:
The patent applies dynamics by making the threshold voltage variable rather than fixed. The threshold voltage is dynamically adjusted based on the operating state (such as motor speed or load conditions), allowing the switching criterion to adapt to changing operational requirements. This dynamic adjustment prevents the system from频繁ly switching modes around a fixed threshold, thereby eliminating chattering while maintaining control simplicity.
Solution Approach 2:
The patent implements parameter changes by modifying the threshold voltage parameter according to operating conditions. Instead of using a constant threshold value, the system changes the threshold parameter dynamically based on factors like motor speed, load torque, or other operational parameters. This parameter adaptation allows smooth transitions between control modes and prevents oscillatory behavior.
2Reliability
If dead time is provided for switching elements, then short-circuit current is prevented, but voltage output changes significantly during mode transitions
Solution Approach 1:
The patent applies preliminary action by anticipating the voltage output changes that occur during mode transitions. Before switching between overmodulation PWM control and sinusoidal wave PWM control, the system pre-adjusts the threshold voltage to account for the expected impact of dead time on voltage output. This preliminary adjustment ensures smooth transitions without significant voltage fluctuations.
Solution Approach 2:
The patent implements feedback by monitoring the actual voltage output and using this information to dynamically adjust the threshold voltage. The system continuously compares the expected voltage output with the actual output (considering dead time effects) and modifies the switching threshold accordingly. This closed-loop approach maintains voltage output stability during mode transitions while preserving the protective dead time.
3Power
If overmodulation PWM control is used, then voltage amplitude exceeds carrier wave, but switching rate reduction causes control instability
Solution Approach 1:
The patent applies dynamics by making the carrier frequency variable in synchronous PWM control during overmodulation. Instead of using a fixed high-frequency carrier, the system dynamically adjusts the carrier frequency based on motor speed and load conditions. This dynamic carrier frequency adjustment optimizes the switching rate for each operating point, preventing control instability while maintaining the high voltage amplitude characteristics of overmodulation control.
Data Source
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AI summary
When performing PWI control in accordance with an overmodu!ation mode, an ECRU ;amiably sets a switching determination value, which is used for determination of switching control modes between the overmodulation mode and a sinusoidal wave modulation mode, based on a switching state of` an inverter at present (S200). Then, the ECU compares the degree of modulation, which is calculated from voltage command values, with the switching determination value (S210) so as to determine whether to switch to the sinusoidal wave modulation mode (S220) or to maintain the overmodulation mode (S230). In particular, when an influence of a dead time is likely to cause generation of a voltage command that requires switching to the overmodLClation mode just after switching to the sinusoidal wave modulation mode, the switching determination value is variably set to prevent the transition from the overmodulation mode to the sinusoidal wave modulation mode. This prevents chattering in which the control modes are frequently switched therebetween.