Capacitive Shield Slot-Wedge for Stray Current Mitigation
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Solution Overview
Problem
High voltage gradients in electric machines cause stray currents that can damage bearings and mechanical devices, existing solutions such as insulated bearings and capacitive shielding face challenges like mechanical strength and cost, and require case-specific inductors or filters for standardization.
Innovation Solution
A slot-wedge with a layered structure of electrically insulating sheet portions and electric conductor strips between them, forming a capacitive shield to reduce capacitively coupled currents between windings and the core structure, which can be made from fiber reinforced resin or magnetically amplifying materials.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If insulated bearings are used to reduce bearing currents, then bearing protection is improved, but cost increases
Solution Approach 1:
The patent introduces an intermediary capacitive shield structure between the winding and the core/bearing path. This shield acts as a mediator that blocks high-frequency stray currents from reaching the bearings through capacitive coupling, thereby protecting bearings without requiring expensive insulated bearings. The shield is electrically connected to ground and positioned in the slot to intercept stray currents before they can couple capacitively to the bearing structure.
2Reliability
If thick insulator layers are used between bearing and frame, then impedance against high frequency stray currents is improved, but mechanical strength deteriorates
Solution Approach 1:
The patent places a capacitive shield as an intermediary element between the winding and the bearing support structure. This shield provides the necessary electrical isolation and impedance against high-frequency stray currents without requiring thick insulator layers on the bearing itself. The shield is positioned in the slot and connected to ground, creating an electrical barrier that protects the bearing path while allowing the use of mechanically strong, thin insulator layers or no insulator layer on the bearing.
3Reliability
If frequency converter output inductors or filters are used to reduce voltage gradients, then stray current reduction is improved, but device complexity and standardization difficulty increase
Solution Approach 1:
The patent extracts the stray current mitigation function from the frequency converter system and relocates it to the motor itself through the capacitive shield structure. Instead of requiring external inductors or filters in the frequency converter that would need to be customized for different cable lengths and applications, the shield is integrated into the motor slot structure. This extraction of the protective function to the motor level simplifies the overall system and enables standardization.
Solution Approach 2:
The capacitive shield structure serves multiple functions: it provides capacitive coupling to ground for stray current protection, acts as an additional insulation barrier, and can be integrated with the slot wedge structure. This multi-functionality allows a single standardized component design to serve various protection needs without requiring case-specific inductors or filters in the frequency converter.
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
The capacitive shield effectively reduces stray currents between coil sides and the rotor/stator core, enhancing mechanical strength and reducing the need for expensive insulated bearings or case-specific filters, while allowing for standardization.
Implementation Method 1
one or more electric conductor strips between the sheet portions without being inside material of any of the sheet portions so as to constitute a capacitive shield when the slot-wedge is between adjacent teeth of the electric machine
Data Source
Figure 1
Figure 2a~2b
AI summary
A slot-wedge (101) for an electric machine comprises sheet portions (102, 103) which are one on the other so that the sheet portions constitute a layered structure. Furthermore, the slot-wedge comprises one or more electric conductor strips (105) between the sheet portions so as to constitute a capacitive shield when the slot-wedge is between adjacent teeth of the electric machine and when the electric conductor strips of the slot-wedge are connected to a constant electric potential.