A variable frequency drive (VFD) controls motor speed by adjusting the frequency and voltage supplied to the motor. While this provides precise control and energy-saving opportunities, the electrical connection between the drive and motor requires careful cable selection. High-frequency switching, voltage pulses, and common-mode currents can create electromagnetic interference (EMI) and place additional demands on the grounding and shielding arrangement.
Symmetrical shielded cable for VFD motor and industrial drive applications
Symmetrical shielded cable is designed for applications where the arrangement of phase conductors, protective grounding conductors, and shielding needs to be considered as one coordinated electrical system. In a commonly used symmetrical configuration, three phase conductors are arranged with three smaller protective grounding conductors distributed symmetrically around them. A metallic screen provides an additional conductive path for electromagnetic interference when correctly designed and terminated.
Unlike a conventional power cable selected mainly by voltage rating and current-carrying capacity, symmetrical shielded cable must also be evaluated according to conductor geometry, grounding continuity, screen construction, and installation requirements.
- Symmetrical distribution of protective grounding conductors
- Metallic screen construction and electrical continuity
- Coordination between grounding, shielding, and EMC installation
- Suitable insulation and sheath for the operating environment
1. What Is Symmetrical Shielded Cable?
Symmetrical shielded cable is a power cable construction in which the protective grounding conductors are distributed in a balanced arrangement around the main phase conductors, together with a metallic shielding system. It is commonly considered for motor connections supplied by variable frequency drives, where the electrical characteristics of the cable can influence electromagnetic compatibility and grounding performance.
In a typical three-phase symmetrical design, the cable contains:
- Three phase conductors: Carry electrical power from the VFD output to the motor.
- Symmetrically arranged grounding conductors: Provide distributed protective grounding paths around the phase conductors.
- Metallic screen or shield: Helps control electromagnetic emissions and provides a conductive path for interference currents.
- Insulation and outer sheath: Electrically isolate the conductors and protect the cable against environmental and mechanical influences.
Typical symmetrical conductor arrangement - actual construction must follow the product drawing.
The symmetrical arrangement is an important distinction. Instead of relying only on one separate protective earth conductor positioned beside the phase conductors, the grounding conductors are distributed around the cable core. This can help create a more balanced geometry and a more consistent high-frequency return path.
However, the term does not define one universal construction. The number, cross-sectional area, position, and material of the grounding conductors, as well as the type of metallic screen, must be confirmed from the manufacturer's construction drawing.
2. Why Symmetrical Conductor Arrangement Matters in VFD Systems
A VFD does not supply the motor with a simple sinusoidal voltage waveform. Its switching devices generate fast voltage transitions, which can produce high-frequency currents through the motor cable, protective earth system, motor frame, and surrounding conductive structures.
2.1 A More Balanced Grounding Geometry
When protective grounding conductors are distributed symmetrically around the phase conductors, the cable can provide a more balanced physical arrangement than a design with a single grounding conductor located on one side. This geometry is relevant to the distribution of high-frequency return currents and the electromagnetic behavior of the cable.
2.2 Support for High-Frequency Return Current Paths
High-frequency common-mode currents may travel through parasitic capacitances between the motor winding, motor frame, cable screen, and protective grounding system. Symmetrically positioned grounding conductors can provide distributed grounding paths along the cable. Together with a properly designed metallic screen and suitable bonding at both ends, this arrangement can help engineers manage high-frequency current paths.
2.3 Coordination Between Cable and EMC Installation
Shielding performance is not determined by the cable alone. The connection between the cable screen and the drive cabinet, motor terminal box, and protective bonding system is equally important. A cable with a suitable symmetrical construction can support an EMC-oriented installation, but poor termination or discontinuous bonding may undermine the intended result.
3. Typical Construction of Symmetrical Shielded Cable
The construction may vary according to voltage rating, motor power, installation environment, and EMC requirements. The following table describes common functional layers rather than a fixed product specification.
| Component | Typical Function | Selection Considerations |
|---|---|---|
| Phase conductors | Transmit three-phase power from the VFD to the motor. | Conductor material, cross-sectional area, current rating, and conductor class. |
| Conductor insulation | Provides electrical insulation between conductors and surrounding layers. | Insulation material, rated voltage, temperature, and electrical stress. |
| Symmetrical grounding conductors | Provide distributed protective grounding paths around the phase conductors. | Number, cross-sectional area, position, and earthing requirements. |
| Metallic screen | Helps control electromagnetic emissions and provides a conductive path for interference currents. | Screen material, coverage, continuity, and termination method. |
| Inner covering | Provides separation and support between cable elements where required. | Material compatibility and construction stability. |
| Outer sheath | Protects the cable against environmental and mechanical influences. | Temperature, oil, chemical, moisture, UV, and abrasion requirements. |
3.1 Phase Conductor Design
The phase conductors carry the motor's operating current. Copper is commonly used where electrical conductivity and compact conductor dimensions are important. The required conductor cross-sectional area should be determined from motor current, installation conditions, cable length, ambient temperature, grouping, and applicable electrical standards.
3.2 Symmetrical Protective Grounding Conductors
The grounding conductors are a defining element of the symmetrical construction. In a common three-phase design, multiple grounding conductors are positioned at balanced intervals around the phase conductor group.
This distributed arrangement can support a more uniform grounding geometry and provide multiple paths for protective and high-frequency currents. The grounding conductor size must still comply with applicable protective earthing requirements. Symmetrical positioning does not automatically mean that the grounding conductors can be undersized.
3.3 Metallic Shielding System
The metallic screen helps manage electromagnetic interference generated by the VFD switching process. Depending on the cable design, the screen may use copper wires, copper tape, a combination of metallic elements, or another specified conductive construction.
Different screen constructions have different electrical and mechanical characteristics. Screen coverage, continuity, electrical resistance, and the connection method at both ends can all influence the effectiveness of the installed shielding system.
4. Symmetrical Shielded Cable vs. Conventional Shielded VFD Cable
Both constructions are intended for motor-drive installations where electromagnetic interference needs to be considered. Their key difference is not simply whether a metallic screen is present. The arrangement of the protective grounding conductors is also an important part of the design.
| Comparison Point | Symmetrical Shielded Cable | Conventional Shielded VFD Cable |
|---|---|---|
| Phase conductors | Typically designed for three-phase motor power. | Typically designed for three-phase motor power. |
| Grounding arrangement | Grounding conductors are distributed symmetrically around the phase conductors in the relevant construction. | May use a separate grounding conductor or another specified arrangement. |
| Metallic screen | Includes a metallic screen as specified by the design. | Includes a metallic screen as specified by the design. |
| Design emphasis | Coordinates conductor geometry, grounding paths, and shielding. | Depends on the specified power conductor, grounding, and shielding construction. |
| Selection priority | Useful where a symmetrical grounding arrangement is required by the drive-system design. | Suitable when its specified grounding and shielding arrangement meets system requirements. |
Neither construction is automatically suitable for every VFD installation. The right choice depends on the drive manufacturer's recommendations, motor requirements, cable length, EMC design, installation method, and applicable standards.
For a broader overview of shielded motor cable options, see our VFD Shielded Motor Cable page. For applications where the main requirement is shielding in a general control circuit, refer to our Shielded Control Cable page.
5. Applications of Symmetrical Shielded Cable
Symmetrical shielded cable is relevant to industrial motor-drive systems where grounding geometry and electromagnetic compatibility are important design considerations.
Typical VFD-to-motor cable installation in an industrial facility
5.1 Industrial Pumps and Fans
Variable speed pumps and fans are widely used in water treatment, HVAC, process plants, and industrial utilities. Their motors may operate at different speeds according to system demand. A suitable symmetrical shielded motor cable can support the connection between the VFD and motor while helping engineers coordinate grounding and shielding requirements.
5.2 Conveyors and Material Handling Systems
Conveyor lines often contain multiple motors and drive units operating near sensors, control wiring, communication cables, and automation equipment. Electromagnetic interference management is therefore an important part of the electrical installation. Cable selection should consider both motor power requirements and the routing relationship between power and signal circuits.
5.3 Manufacturing and Process Equipment
Industrial production lines may use VFD-controlled motors for mixers, extruders, machine tools, compressors, and other equipment. Cable construction should be coordinated with motor power, operating temperature, cable routing, and plant EMC practices.
5.4 Water and Wastewater Facilities
Pumping stations and treatment facilities commonly use variable speed drives to control flow and pressure. The cable's outer sheath must be selected for the installation environment, while grounding and shielding should follow the drive manufacturer's recommendations and facility electrical requirements.
6. How to Select the Right Symmetrical Shielded Cable
6.1 Confirm VFD and Motor Requirements
Check the drive output voltage, rated current, switching characteristics where specified, motor connection requirements, and cable types approved or recommended by the drive manufacturer. Also confirm whether the system specifically requires a symmetrical grounding conductor arrangement.
6.2 Check Conductor Size and Current-Carrying Capacity
The phase conductor size should be selected according to actual operating current and installation conditions. Relevant factors include motor rated current, ambient temperature, cable grouping, installation method, cable length, voltage drop, and applicable ampacity correction factors.
6.3 Verify the Grounding Arrangement
Ask the manufacturer for a cable cross-section drawing. Confirm the number and position of grounding conductors, their cross-sectional area, and their intended relationship with the metallic screen.
6.4 Specify the Shield Type
Determine whether the application requires a copper wire screen, copper tape screen, combined screen, or another specified construction. The choice should be based on EMC requirements, mechanical conditions, cable dimensions, and termination method.
6.5 Select the Appropriate Sheath Material
The outer sheath should match the operating environment. Consider temperature, oil and coolant exposure, moisture, UV exposure, abrasion, mechanical impact, and whether the installation is fixed or moving.
6.6 Confirm Voltage Rating and Applicable Standards
The voltage rating must be confirmed from the product datasheet. The term "symmetrical shielded cable" describes a construction concept; it does not establish the rated voltage, insulation system, or compliance with a particular standard.
7. Installation Considerations: Grounding and Shield Termination
7.1 Maintain Shield Continuity
The metallic screen should be connected using a termination method compatible with the cable design and the drive manufacturer's EMC instructions. Avoid unnecessary interruptions, long unshielded sections, or termination arrangements that compromise screen continuity.
7.2 Use Suitable EMC Cable Glands
Where required, use EMC cable glands or screen termination accessories that provide reliable electrical contact with the metallic screen. The termination must be compatible with the screen construction and cable dimensions.
7.3 Bond the Drive and Motor Correctly
Grounding and bonding should follow the drive manufacturer's instructions and applicable electrical codes. The symmetrical grounding conductors and metallic screen must be connected according to the cable design and system grounding scheme.
7.4 Separate Motor Power Cables from Sensitive Signal Cables
Where practical, route VFD motor cables separately from low-level sensor, encoder, and communication cables. If they must cross, follow equipment and project installation guidelines.
7.5 Consider Cable Length and Output Filtering
Long motor cable runs can increase the effects of reflected voltage waves and cable capacitance. Depending on the drive, motor, and cable arrangement, an output reactor, dv/dt filter, or sine-wave filter may be required. These measures should be selected by the system designer.
8. Technical Information to Confirm Before Ordering
| Specification Item | Information to Provide or Confirm |
|---|---|
| Product construction | Symmetrical grounding arrangement and cable cross-section drawing. |
| Rated voltage | Project voltage designation and required insulation system. |
| Phase conductor | Material, cross-sectional area, and conductor class. |
| Grounding conductors | Number, position, material, and cross-sectional area. |
| Insulation | Required insulation material and temperature rating. |
| Metallic screen | Screen type, material, coverage, and electrical requirements. |
| Outer sheath | Material and environmental resistance requirements. |
| Operating conditions | Ambient temperature, installation method, cable length, and fixed or moving use. |
| Standards and approvals | Applicable project standards, testing, and certification needs. |
| Delivery requirements | Cable length, packaging, marking, and documentation. |
9. Frequently Asked Questions
What makes symmetrical shielded cable different from ordinary shielded cable?
The main distinction is the arrangement of the protective grounding conductors. In a common symmetrical design, multiple grounding conductors are distributed around the phase conductors rather than being positioned as a single separate earth conductor. The cable also includes a metallic shielding system.
Is symmetrical shielded cable suitable for VFD motor connections?
It can be used for VFD-to-motor connections when its voltage rating, conductor size, insulation, grounding arrangement, screen construction, and sheath properties meet the drive and motor requirements.
Does symmetrical shielded cable eliminate electromagnetic interference?
No. A suitable symmetrical conductor arrangement and metallic screen can help manage interference, but overall EMC performance also depends on screen termination, grounding, bonding, cable routing, motor construction, and drive configuration.
Are the grounding conductors and metallic screen the same thing?
No. Protective grounding conductors and the metallic screen have different design functions. Their electrical bonding and termination must follow the cable design and applicable installation requirements.
Can symmetrical shielded cable be used for long-distance VFD installations?
It may be suitable if the cable and drive system are designed for the required distance. Longer cable runs can introduce reflected-wave voltage stress and cable capacitance, so the drive manufacturer should confirm whether output filtering or other measures are required.
Can this cable be used in continuously moving equipment?
Not automatically. A cable designed for fixed installation may not have the conductor stranding, sheath, and mechanical construction needed for continuous flexing. Confirm the movement type, bending radius, cycle requirements, and installation arrangement.
10. Specify Symmetrical Shielded Cable for Your Drive System
Selecting symmetrical shielded cable requires coordination between motor power conductors, protective grounding arrangement, metallic screen, and installation method. A clear construction drawing and complete technical specification help ensure that the supplied cable matches the electrical and EMC requirements of the project.
When evaluating a cable, focus on the actual conductor layout, grounding conductor dimensions, screen construction, rated voltage, insulation system, and sheath properties rather than relying on the product name alone.
For related product information, you can also review our VFD Shielded Motor Cable and Armored VFD Cable pages. Each construction should be selected according to its own electrical, mechanical, and installation requirements.
Hot Tags: symmetrical shielded VFD cable、symmetrical grounding conductor cable、VFD motor cable、EMC shielded cable、variable frequency drive cable、symmetrical earth conductor cable, Armored VFD Cable, variable frequency drive cable, vfd cable specification, vfd kabel, vfd power wiring, vfd shielded cable grounding
