Arithmetic Device Motor Drive Current Calculation
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Solution Overview
Problem
Existing motor drive devices face challenges in accurately calculating phase currents when driving three-phase motors with square waves, leading to reduced calculation accuracy compared to sine wave drives.
Innovation Solution
The proposed solution involves an arithmetic device and motor drive unit that utilize a shunt current to calculate the effective current value by detecting the shunt current at specific timing points and using a correction coefficient to improve accuracy, even during square wave motor drives.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If bus current is detected at zero-cross timing to calculate phase currents, then calculation process is simplified, but calculation accuracy is reduced when driving with square wave
Solution Approach 1:
The patent applies preliminary action by detecting the bus current at a specific timing (when the switching element transitions from ON to OFF state) before the actual motor operation requires the current data. This timing selection ensures that the current value reflects the actual motor phase current more accurately, especially during square wave operation, while still maintaining a simplified single-point detection approach rather than requiring continuous multi-phase measurements.
2Measurement precision
If multiple detection points are used to improve current calculation accuracy, then measurement precision improves, but processing load increases
Solution Approach 1:
The patent extracts only the essential and most informative current measurement point from the complete current waveform cycle. By selecting specifically the timing when the switching element transitions from ON to OFF state, the system extracts the most relevant current data needed for accurate motor control calculations, discarding the need to process multiple less informative measurement points, thus reducing computational load while maintaining accuracy.
3Ease of operation
If bus current detection timing is fixed at zero-cross, then detection process is simplified, but accuracy deteriorates under square wave drive conditions
Solution Approach 1:
The patent applies dynamics by making the detection timing adaptive to the switching state rather than fixed at zero-cross. The detection timing dynamically follows the switching element's transition from ON to OFF state, which occurs at different points in the waveform depending on the drive mode (sine wave or square wave). This dynamic timing adjustment maintains simplicity in the detection process while significantly improving accuracy under square wave drive conditions.
Data Source
AI summary
An arithmetic device and a motor drive device including the arithmetic device capable of performing high accuracy motor current calculation, include a control unit performing an arithmetic processing including: acquire a duty ratio; calculate a waiting time according to the duty ratio and a waiting coefficient; detect a shunt current value at a detection timing after lapse of the waiting time; acquire, as a zero-cross current value, the shunt current value; calculate an average value using the shunt current values; and calculate an effective current value by correcting the average value using a correction coefficient.