Capacitive Discharge Shield for EV Motor Bearing Voltage
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Solution Overview
Problem
Common mode voltage discharge through bearings in electric traction motors causes severe stress and physical damage, such as pitting and degradation of bearing grease, leading to early failures, and existing solutions like bearing protection rings and ceramic bearings either add cost or transfer the issue to other components.
Innovation Solution
Implementing a common mode voltage discharge shield capacitively coupled to stator windings and grounded through a strap, providing a low impedance path for voltage discharge, thereby reducing or eliminating discharge through bearings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If bearing protection rings with brushes are used to provide a lower resistance path, then bearing discharge current is addressed, but device complexity and cost increase
Solution Approach 1:
The patent extracts the harmful common mode voltage from the bearing discharge path by introducing a separate capacitive coupling path. The discharge shield capacitively couples to the stator windings and provides an alternative discharge route through the grounding strap, removing the voltage stress from the bearings without adding mechanical protection rings or brushes to the bearing assembly.
Solution Approach 2:
The patent introduces a discharge shield as an intermediary element between the stator windings and ground. This shield capacitively couples to the stator end turns and provides a mediated path for common mode voltage discharge through the grounding strap, preventing direct discharge through bearings while avoiding complex mechanical protection structures.
2Reliability
If ceramic bearings are used to electrically isolate bearing paths, then bearing discharge is prevented, but cost increases and availability decreases
Solution Approach 1:
The patent replaces the mechanical electrical isolation approach (ceramic bearings) with an electromagnetic field-based solution. Instead of using non-conductive ceramic materials in the bearing races, the patent uses capacitive coupling through a discharge shield to provide electrical isolation and alternative discharge path, allowing standard steel bearings to be used.
3Reliability
If brushes are used to create discharge path, then bearing discharge is reduced, but weight increases
Solution Approach 1:
The discharge shield acts as an intermediary that provides electrical discharge path capability without requiring physical contact elements like brushes. The capacitive coupling mechanism allows voltage discharge through the grounding strap without the need for sliding contact components that would add weight to the rotating assembly.
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
Prevents discharge through bearings, reducing electrical discharge machining (EDM) and circulating current, while being cost-effective and not transferring the issue to other components.
Implementation Method 1
The common mode voltage discharge shields are capacitively coupled to the end turns to a ground strap, providing a low impedance path for common mode voltages
Data Source
AI summary
Common mode voltage discharge shields are disposed over end turns of stator windings for a three phase electric traction motor within an electric vehicle. The common mode voltage discharge shields are capacitively coupled to the end turns to a ground strap, providing a low impedance path for common mode voltages accumulating at a neutral for electric traction motor as a result of high frequency switching of the motor drive voltage to produce desired torque. An insulating layer separates the common mode voltage discharge shields from the end turns. Accumulation of the common mode voltage in excess of a bearing breakdown voltage, and the associated potential for discharge through bearings between the stator and rotor and electrical discharge machining, is reduced or eliminated.