DC Bus Current Sensor for Ground Isolation Fault Detection
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Solution Overview
Problem
Existing electric motor control circuits face challenges in detecting ground isolation faults due to measurement errors, bandwidth limitations, and resonance issues, which can lead to undetected faults and capacitor damage.
Innovation Solution
An electric motor control circuit that continuously monitors AC current flow through a capacitive shunt circuit connected to a chassis ground, using an AC current transducer to detect faults and control the inverter to interrupt current flow when exceeding a predetermined threshold, thereby preventing damage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If phase current sensors are used to measure electric current through each phase cable and arithmetically sum them to detect ground isolation faults, then fault detection capability is provided, but measurement errors are cumulative and can cause error in the overall current signal
Solution Approach 1:
The patent extracts the fault detection function from multiple phase current sensors and concentrates it into a single DC bus current sensor. Instead of summing measurements from multiple sensors (which accumulates errors), the invention directly measures the total current through the DC bus, eliminating cumulative measurement errors while maintaining fault detection capability.
Solution Approach 2:
The DC bus current sensor serves multiple functions: it monitors overall current flow, detects ground isolation faults, and provides protection against capacitor damage. This single sensor replaces the need for multiple phase current sensors, reducing system complexity and measurement error accumulation while maintaining comprehensive monitoring capability.
2Reliability
If phase current sensors with magnetic and electrical response characteristics are used for fault detection, then current measurement is provided, but bandwidth/step response limitations and sensor saturation occur causing missed fault detection
Solution Approach 1:
The patent replaces magnetic-based phase current sensors with a DC bus current sensing system that uses resistive voltage measurement. This substitution eliminates magnetic saturation and bandwidth limitations associated with inductive sensors, providing faster response time and higher reliability for detecting ground isolation faults.
3Reliability
If DC voltage sensors are used to measure voltage between positive/negative DC bus and chassis ground to detect ground isolation faults, then fault detection is provided, but measurement errors are cumulative and bandwidth/response time limitations cause missed faults
Solution Approach 1:
The patent extracts the voltage measurement function from multiple DC voltage sensors (positive bus to ground, negative bus to ground) and consolidates it into a single measurement across the entire DC bus. This eliminates cumulative measurement errors from multiple sensors while maintaining the ability to detect ground isolation faults through differential voltage measurement.
4Measurement precision
If data sampling is performed at or near zero crossing for fault detection, then measurement is provided, but resonance elements in fault impedance cause fault current to be aliased out
Solution Approach 1:
The patent implements preliminary protective action by continuously monitoring DC bus current and detecting ground isolation faults in real-time. The system is configured to detect faults at any point in the AC cycle, not waiting for zero crossing, and immediately interrupts current flow through the inverter switches to prevent capacitor damage. This preliminary action eliminates the aliasing problem by ensuring fault detection occurs before resonance effects can mask the fault current.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Effectively detects ground isolation faults and prevents capacitor damage by accurately measuring AC current flow and interrupting current flow when faults occur, ensuring reliable operation of the electric motor control circuit.
Implementation Method 1
Capacitors can be electrically connected in parallel with the high impedance resistors to provide low impedance shunt paths for high frequency electric noise currents
Implementation Method 2
continuously monitoring AC electric current flow through a capacitive shunt circuit electrically connected between the positive DC electric power bus and the negative DC electric power bus
Data Source
AI summary
An electric motor control circuit includes an electric energy storage device electrically connected via DC power buses to an inverter circuit that connects via an alternating current circuit to an electric machine. A capacitive shunt circuit connects between the power buses. Current flow through the capacitive shunt circuit to the chassis ground is monitored. A fault is identified when the current flow through the capacitive shunt circuit to the chassis ground exceeds a threshold.