Three Phase Inverter Current Detection Using Shunt Resistors
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Solution Overview
Problem
Conventional methods for detecting phase current in three-phase PWM inverters face challenges such as complexity in control software, current distortion, and limited flexibility in layout and vibration resistance, particularly in compact and vibration-sensitive applications like vehicle-mounted air-conditioner compressors.
Innovation Solution
The solution involves providing a current detector between the lower arm switching elements and the minus side of the direct-current power source for two phases, deducing an identical ON period from the upper arm switching elements' ON period across all phases, allowing detection of current for all three phases using only two current detectors, which simplifies control software and maintains unchanged electric supply periods, thereby avoiding current distortion and enabling carrier cycle shortening effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If three current detectors are provided for three phases, then current detection precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent extracts the current detection function from all three phases and implements it only in two phases. By placing current detectors in only two phases and using Kirchhoff's current law to calculate the third phase current, the system reduces the number of detectors from three to two while maintaining detection capability.
Solution Approach 2:
The patent creates a virtual copy of the current detection by calculating the third phase current through mathematical computation based on the detected two phase currents. This computational copy replaces the physical third detector, achieving the same functional result with fewer hardware components.
2Measurement precision
If ON period of upper arm switching elements is increased for current detection, then current detection capability is improved, but current distortion occurs
Solution Approach 1:
The patent applies partial action by deducting only a portion (identical ON period) from the upper arm switching elements' ON period, rather than completely extending or modifying it. This partial modification enables current detection in two phases while avoiding the excessive action that would cause current distortion in all three phases.
3Reliability
If hall devices are used for current detection, then vibration resistance is improved, but device size increases
Solution Approach 1:
The patent extracts the current detection function from all three phases and implements it only in two phases using hall devices. This reduction in the number of hall devices from three to two decreases the overall inverter size while maintaining vibration resistance through the robust hall effect sensors that are used.
Data Source
AI summary
A small and highly vibration-proof inverter capable of detecting a phase current without requiring the development of complicated control software and without causing any current distortion is provided. Shunt resistors (15, 16) serving as a kind of current detector are provided, respectively, between lower arm switching elements X, Y, Z) in only two phases—and the minus side of a DC power supply. An identical ON period is reduced from the ON periods of upper arm switching elements (U, V, W) in the carrier cycle entirely for three phases. Consequently, phase current is detected for the two phases provided with the shunt resistors (15, 16). The inverter can detect the phase current without complicating the control software and without causing any current distortion, by controlling inverter circuit (10) with control software incorporated in control circuit (14) through connection lines (18).