Motor System PWM Signal Inversion for Star Point Potential Control
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Solution Overview
Problem
The existing technique for reducing power loss in a combination of an inverter and a three-phase AC motor using sub-reactors and parallel connections of switching elements fails to maintain the loss reduction effect when the potential of the star connection point changes, leading to inefficiencies in the motor system.
Innovation Solution
A motor system comprising three switching circuits connected in parallel, each with four switching elements and two sub-reactors, and a controller that adjusts PWM signals to invert the switching operations based on current values and potential conditions, ensuring that motor-side potentials of sub-reactors remain lower than switching element-side potentials to achieve loss reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If sub-reactors are connected between midpoints of parallel switching circuits and the motor coil to reduce reverse recovery currents, then power loss is reduced, but the loss reduction effect is lost when the star connection point potential equals the DC input terminal potential
Solution Approach 1:
A signal adjusting module is introduced as an intermediary between the PWM signal generation and the switching circuits. This module monitors the star connection point potential and adjusts the PWM signals accordingly to maintain the voltage condition across sub-reactors, ensuring the loss reduction effect remains effective under all operating conditions
Solution Approach 2:
The system implements feedback by monitoring the star connection point potential and using this information to dynamically adjust the PWM signals. When the star connection point potential approaches the DC input terminal potential, the signal adjusting module modifies the PWM duty cycles or timing to maintain the necessary voltage differential across sub-reactors for loss reduction
2Reliability
If switching elements are connected in parallel to reduce load on each switching element, then reliability is improved, but power loss increases due to reverse recovery currents between parallel circuits
Solution Approach 1:
Sub-reactors are extracted and connected between the midpoints of parallel switching circuits and the motor coil to specifically address and remove the harmful reverse recovery currents that cause power loss, while preserving the reliability benefits of parallel switching element configuration
Solution Approach 2:
The sub-reactors convert the harmful reverse recovery currents into beneficial controlled current paths, using the inductance of sub-reactors to limit and control the reverse recovery current flow, thereby transforming a loss mechanism into a controllable parameter that maintains system reliability
Data Source
AI summary
A motor system may include three switching circuits, each of which includes two upper switching elements and two lower switching elements. A controller includes a signal output module, a signal distribution module, and a signal adjusting module. The signal output module outputs upper and lower PWM signals. The signal distribution module distributes each of the upper (lower) PWM signals alternately to the first upper (lower) switching element and the second upper (lower) switching element of corresponding one of the switching circuits. The signal adjusting module inverts all the PWM signals output by the signal output module when (1) each of currents flowing through two of the three coils has a negative value and all the three upper PWM signals are at HIGH level, or (2) each of currents flowing through two of the three coils has a positive value and all the three lower PWM signals are at LOW level.