Motor cable for variable frequency drives — symmetrical construction against bearing currents

The guide on cable shield earthing (single-ended/double-ended) covers how a motor cable's shield is earthed to limit EMC radiation. This article covers a related but separate choice that must already have been made before that earthing question arises: which physical construction the cable itself must have to prevent the motor itself from suffering damage via its bearings.

Why an ordinary 4-core cable falls short here

A variable frequency drive switches its output transistors with steep, high-frequency edges. These switching edges cause a common-mode voltage between the motor winding and the motor housing/shaft, which can couple onto the rotor via the parasitic capacitance of the winding. With an ordinary supply cable that has three phase conductors and a single PE conductor (positioned asymmetrically relative to the phases), the magnetic field around the cable is not fully symmetrical, and the cable provides no low-impedance, symmetrical return path for this common-mode current. The result: the common-mode current seeks an alternative path back to the source — often straight through the motor bearings, which over time can cause pitting in the bearing races (EDM damage) and premature bearing wear.

The symmetrical construction of a VFD motor cable

A cable specifically designed for variable-frequency-drive motor applications ("VFD cable" or "VSD cable") solves this in two ways:

the earth conductor(s) are not laid as a single core alongside the three phases, but distributed symmetrically around the phase conductors (for example as several smaller earth cores, positioned evenly around the circumference) — this makes the magnetic field around the cable nearly symmetrical and provides a low-impedance return path for the common-mode current that lies close to the phase conductors;
a fully enclosing shield (foil and/or braid) around the whole, earthed low-impedance at both ends (drive and motor) via a 360°-encompassing gland connection — see the guide on cable shield earthing for why this specifically must be double-ended for variable-frequency-drive motor cables, unlike a low-frequency instrumentation cable.

Together, the symmetrical earth conductors and the double-earthed shield give the common-mode current a low-impedance path back to the drive, instead of through the motor bearings and the building's earthing installation.

IEC TS 60034-25 and insulated bearings

IEC TS 60034-25 (guidance for the use of induction motors with converter supplies) describes, besides the cable requirements, an additional measure for larger motors: fitting an insulated bearing on the non-drive end (NDE), generally recommended from around frame size 280 and up, to break a circulating bearing-current path via the motor shaft that can still occur on larger machines even with a correctly executed symmetrical motor cable. This is an additional measure, not a replacement for the symmetrical cable construction and the double-ended shield earthing.

Practical relevance

When replacing an ordinary supply cable with a variable-frequency-drive fed version (for example when fitting a VFD onto an existing motor), it must be checked that the motor cable is specifically designed for VFD use (symmetrical earth conductors, full shield) — a standard installation cable with a single PE core, even if correctly sized for current-carrying capacity otherwise, lacks the symmetrical construction needed to limit bearing currents.

Common mistakes

Applying a standard 4-core installation cable (3 phase + 1 PE) to a variable-frequency-drive motor circuit without checking whether a VFD-specific cable with symmetrical earth conductors is required.
Earthing the motor cable's shield single-ended (as with a low-frequency signal cable) instead of double-ended with a 360° gland connection — see the guide on cable shield earthing.
Relying on the cable alone for larger machines without considering the additional insulated-NDE-bearing measure that IEC TS 60034-25 recommends for larger frame sizes.
Attributing bearing damage to lubrication or load without investigating the possibility of common-mode bearing currents in a motor fed via a variable frequency drive.