Open Phase Fault Detection in Motor Circuits
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Solution Overview
Problem
Existing methods for diagnosing open phase faults in multi-phase electric motor circuits are prone to false results, especially at low speeds and low torque conditions, where PWM waveforms have a low frequency and frequent zero crossings, leading to incorrect identification of open phases.
Innovation Solution
A method that determines the presence of an open phase fault by monitoring the q-axis motor current, motor speed, and demanded phase voltage, satisfying specific threshold conditions, including the q-axis current demand amplitude, motor current fraction, and saturated phase voltage, to output a signal indicative of an open phase fault, with additional checks for low and zero speeds to ensure robust detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple current samples are taken to avoid false fault detection, then reliability of fault detection is improved, but at low speeds and low torque conditions, frequent zero crossings cause false positive fault indications
Solution Approach 1:
The patent changes the diagnostic parameters from simple current magnitude thresholding to a comprehensive evaluation involving q-axis current demand amplitude, actual q-axis current magnitude, demanded phase voltage, and motor speed. This multi-parameter approach allows the system to distinguish between genuine open-phase faults and normal zero-crossing conditions, resolving the contradiction between reliability and measurement precision.
Solution Approach 2:
The patent implements feedback by continuously monitoring multiple parameters (current demand, actual current, voltage demand, speed) and using this information to dynamically adjust fault detection decisions. The controller compares demanded versus actual values and uses this feedback loop to avoid false positives while maintaining sensitivity to real faults.
2Device complexity
If simple current sampling is used for fault detection, then device complexity is reduced, but false detection occurs at low speeds due to PWM waveform characteristics
Solution Approach 1:
The patent makes the existing motor control parameters serve multiple functions: they continue to control motor operation while simultaneously providing fault detection information. The q-axis current, phase voltage, and speed parameters used for normal control are also used for diagnostic purposes, eliminating the need for separate detection hardware and maintaining reliability without increasing device complexity.
3Loss of time
If fault detection is performed continuously, then fault detection speed is improved, but false positives increase at low speeds with low frequency PWM waveforms
Solution Approach 1:
The patent applies dynamic thresholds and conditional logic that adapt to operating conditions. The fault detection algorithm dynamically evaluates whether observed current characteristics represent a fault or normal operation based on real-time speed and torque conditions, allowing continuous monitoring without generating false positives at low speeds.
Data Source
AI summary
A method of diagnosing an open phase fault condition comprising the steps of: determining the q axis motor current in the motor, the motor speed, the q axis motor current demand, and the demanded phase voltage from the controller and outputting a signal indicative of that an open phase fault condition of motor is present when the following conditions are met: (1) the q axis current demand amplitude is above a threshold; (2) the q axis motor current is below a first predefined fraction of the demanded q axis motor current; (3) the demanded phase voltage is greater than a second predefined fraction of the saturated phase voltage value of the motor circuit; and (4) the motor speed is below a threshold speed.