Electric Motor Cable Disconnection Detection via Current Difference
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Solution Overview
Problem
Existing methods for detecting disconnections in power connection lines between electric motors and power conversion devices in hybrid and electric vehicles face challenges, such as difficulty in detecting small currents and potential erroneous determinations due to zero current phases, especially in AC electric motors.
Innovation Solution
A control device for electric motors that includes a controller and current sensors to detect phase currents, calculate command current values, and determine disconnections based on the difference between detected and command current values, using integration and threshold ratios to accurately detect cable disconnections.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If phase current is compared with a determination value to detect disconnection, then disconnection detection can be performed, but detection becomes difficult for small currents depending on setting of the determination value
Solution Approach 1:
The patent changes the detection parameter from direct current magnitude comparison to comparison of current change rates (derivatives). By detecting the rate of change of phase current rather than the absolute current value, the system can reliably detect disconnections even when currents are small, as the abrupt change in current flow pattern remains detectable regardless of magnitude.
Solution Approach 2:
Instead of detecting disconnection by checking whether current flows (presence detection), the patent inverts the approach by detecting disconnection through the absence of current change patterns (change detection). When a disconnection occurs, the current change rate exhibits a specific pattern that differs from normal operation, allowing detection without relying on current magnitude thresholds.
2Reliability
If disconnection is detected based on phase current and change speed of phase current, then disconnection can be identified, but erroneous determination may occur when electric motor stops at specific rotation angle where current does not flow
Solution Approach 1:
The patent incorporates feedback by continuously monitoring the relationship between command current (desired current based on torque requirements) and actual phase current. By comparing what the current should be versus what it actually is, and analyzing the rate of change of this difference, the system can distinguish between zero current due to motor position and zero current due to disconnection, eliminating erroneous determinations.
Solution Approach 2:
The patent performs preliminary calculation of command current values based on torque commands and motor state before actual current measurement. This allows the system to establish expected current patterns in advance, so when actual current deviates from expected patterns in a way that indicates disconnection rather than normal zero-current phases, the detection can proceed accurately even at specific rotation angles.
Data Source
AI summary
A control device for an electric motor includes: a controller configured to apply switching control to an inverter configured to execute power conversion in accordance with a torque command, to thereby control AC power to be supplied to the electric motor; and a current sensor configured to detect a phase current, which flows through an AC cable configured to connect the electric motor and the inverter to each other, wherein the controller includes a disconnection detection unit configured to acquire the phase current detected by the current sensor as a phase current detection value, calculate based on the torque command a phase current command value directed to the electric motor, and determine presence or absence of a disconnection of the AC cable in each phase from a transition result of a difference value between the phase current command value and the phase current detection value in each phase.