Coaxial Servo Motor Cable Reducing Parasitic Capacitance
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Solution Overview
Problem
Conventional three-phase servo motor power cables exhibit high parasitic conductor to shield/ground capacitance, leading to electrical interference, ground fault detector saturation, and safety hazards, which are not adequately addressed by existing solutions, particularly in stringent military applications.
Innovation Solution
A coaxial three-phase servo motor power cable design featuring a shielded twisted triple cable with a flexible nonconductive conduit filled with air, where the innermost shield is not electrically connected to the back shells, and a braid shield is electrically terminated at both ends, significantly reducing parasitic capacitance and using polytetrafluoroethylene (TEFLON) for the conduit, which has a low relative permittivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional cable designs with conductors in close proximity to shield/ground are used, then electrical insulation and mechanical stability are improved, but parasitic conductor to shield/ground capacitance increases
Solution Approach 1:
The patent transitions from a conventional parallel conductor-shield arrangement to a coaxial geometry where conductors are positioned at the center along the longitudinal axis, surrounded by concentric shielding layers. This dimensional reconfiguration reduces the effective capacitance coupling between conductors and shield/ground while maintaining electrical insulation integrity.
Solution Approach 2:
The patent introduces an intermediary coaxial structure with multiple shielding layers (inner shield, outer shield) and dielectric materials positioned between conductors and shields. This intermediary configuration reduces direct capacitive coupling while maintaining electrical insulation, thereby reducing parasitic capacitance without sacrificing reliability.
2Object-generated harmful factors
If common mode inductors are inserted to reduce parasitic capacitance effects, then RF tuned circuit effectiveness is improved, but device size, weight, and power dissipation increase
Solution Approach 1:
The patent extracts the parasitic capacitance problem from the electrical circuit domain and addresses it at the physical cable structure level through coaxial geometry and shielding arrangements. This eliminates the need for external common mode inductors, thereby reducing system weight while still achieving the goal of reducing parasitic capacitance effects.
Solution Approach 2:
The patent replaces the mechanical/electrical solution of common mode inductors with a structural/coaxial cable design that inherently reduces parasitic capacitance through its geometry and material arrangement. This substitution eliminates heavy inductive components while achieving the same functional objective.
3Reliability
If dielectric material with high relative permittivity is used for electrical insulation, then insulation performance is improved, but parasitic conductor to shield/ground capacitance increases
Solution Approach 1:
The patent changes the geometric parameters of the cable structure (coaxial arrangement, conductor positioning, shielding layer distances) to reduce capacitance coupling. By optimizing the physical configuration rather than relying solely on dielectric material properties, the patent achieves reduced parasitic capacitance while maintaining insulation performance.
Solution Approach 2:
The patent employs composite cable construction with multiple materials including dielectric insulators, conducting shields, and jacketing materials in a coaxial configuration. This composite structure allows optimization of both insulation performance and parasitic capacitance reduction through material selection and geometric arrangement.
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
This design reduces parasitic conductor to shield/ground capacitance, minimizing electrical interference and power dissipation, enabling smaller, lighter, and more efficient servo electronic equipment that meets stringent EMI and safety criteria.
Implementation Method 1
the electrically nonconductive conduit defining a void space surrounding the electrically insulating sheath inside of the electrically nonconductive conduit
Implementation Method 2
The value of parasitic conductor to shield/ground capacitance in a particular cable design is dependent on the dielectric material that is used in the cable
Data Source
AI summary
A coaxial three-phase servo motor power cable is comprised of a shielded twisted triple cable which transmits three phase electrical power from a servo amplifier to a servo motor, with the shield being electrically insulated and mechanically floating within an air void that exists inside a conduit having a diameter much greater than the shielded twisted triple cable. The conduit is covered by a braid shield, which is covered by shrink tubing which may be overlaid by one or more additional signal wires, with this entire assembly being further covered by an over braid shield, which is covered by an outer insulating sheath. Both the conduit braid shield and over braid shield are electrically terminated to back shells at each end of a cable assembly.