By TEBAOFLEX Jyen. Originally published on July 19, 2026.
Symmetrical motor cables can improve the high-frequency return path between a variable-frequency drive and its motor. A 3+3 construction distributes three protective-earth conductors around three phase conductors. A four-core cable places one PE conductor alongside three phases. Neither label alone determines the complete drive system’s EMC performance or bearing life.
Why bearing currents require a system diagnosis
PWM drives produce common-mode voltage. Motor capacitances, grounding paths, cable layout and the driven machine influence the resulting currents. Bearing damage may involve electrical discharge, circulating currents or shaft-grounding currents; lubrication, mechanical alignment and loading must also be investigated. A fluted bearing is not evidence that a particular four-core cable alone caused the failure.
ABB describes several bearing-current mechanisms and mitigation measures. Symmetrical cabling and appropriate grounding are part of the solution; motor insulation, insulated bearings, shaft grounding and filtering may also be needed. Use the recommendations for the actual motor-drive combination, not a universal power threshold.
Compare complete cable constructions
- Record phase-conductor area, total PE arrangement, shield construction, conductor resistance and voltage rating.
- Check the drive manufacturer’s permitted cable type and length, output-filter requirements and termination arrangement.
- Compare measured capacitance and shield properties under stated conditions; optical braid coverage alone is not a complete EMC rating.
- For moving machinery, require separate qualification for bending or torsional duty. A symmetrical electrical layout does not prove dynamic endurance.
Protective earth and EMC shielding have different jobs
The PE system must satisfy the applicable protective-conductor and fault-clearing requirements. A metallic shield must not be counted as protective-earth capacity merely because it is conductive. Its cross-section, continuity, connections and permitted protective function must be established by the responsible designer.
Illustrative geometry only: three 16 mm² PE conductors have a combined nominal area of 48 mm². This sum does not establish fault withstand or approve the cable for a given phase size. Check the applicable sizing method, conductor material, fault duration and termination capability.
Termination and commissioning
- Connect every specified PE conductor using an approved terminal arrangement; do not leave split-PE cores floating.
- Use the drive and motor manufacturers’ prescribed shield bonds, generally aiming for a short, broad high-frequency connection where specified.
- Do not assume a long shield pigtail has the same high-frequency performance as a circumferential connection.
- Verify protective bonding and equipment safety before energization. Shaft-voltage measurements require appropriate probes, bandwidth and competent personnel.
What evidence should a purchase order request?
Ask for an exact construction drawing, declared ratings, compatible glands and terminals, and any claimed EMC or endurance reports. If bearing-current mitigation is a project objective, define the motor, drive, switching conditions, cable route and diagnostic method. Do not accept an isolated peak-to-peak voltage as a universal bearing-safety pass/fail criterion.
A 3+3 cable can be a sound choice when recommended for the installation, but it is not a promise of zero bearing current, zero interference or a particular service life.
Sources and scope
ABB: bearing currents and mitigation
These references explain the relevant methods or manufacturer guidance. They do not certify a TEBAOFLEX product or establish results for a supplied batch.
Discuss your specification
For a project-specific cable specification, send the duty, installation drawing and required acceptance criteria to TEBAOFLEX. WhatsApp: +86 185 9591 3361. Request documented product ratings and test evidence before approval.