Installing variable-speed drives — EMC grounding and bearing currents

The STO guide covers the functional safety of a variable-speed drive (Safe Torque Off). This article covers an entirely different aspect of the same installation: how the EMC grounding of the motor cable and wiring itself must be carried out, laid down in the product standard IEC 61800-3 — and why incorrectly executed grounding can eventually lead to bearing damage, independent of any safety function.

Why an ordinary shielded cable is not enough

A variable-speed drive switches the voltage to the motor at high frequency (PWM — pulse width modulation). Those fast switching edges create high-frequency disturbance currents that can radiate from the motor cable or flow back to the drive. A shielded motor cable is needed to give that high-frequency current a controlled path — but the way the shield is grounded determines whether that actually works.

360° grounding versus a shield "pigtail"

A common installation mistake: the cable shield is connected to the earth rail using a single wire (a "pigtail"). That works for low-frequency currents, but the high-frequency disturbance components of a PWM drive see too high an impedance in that single wire to be effective.

The correct execution in IEC 61800-3 practice:

• The shield is grounded 360° around its circumference, directly at the cable entry into the terminal box of both the drive and the motor — using a dedicated EMC cable gland that lets the full circumference of the shield make contact, not a single wire.
• If a full 360° gland is not possible, a flattened, twisted shield (with a width of at least one-fifth of its length) is an acceptable alternative — still better than a thin pigtail.
• The shield is grounded at both ends (drive side and motor side), not only one.

Bearing currents: a consequence of the same switching voltage

Besides radiation, the PWM switching voltage also creates a common-mode voltage across the motor shaft, which can discharge to ground through the bearings. An ordinary grounding conductor in the motor cable does not offer a low-impedance return path for these high-frequency currents — designing a drive system therefore requires multiple grounding conductors placed symmetrically around the phase conductors, rather than a single PE core, to effectively return the low-frequency noise.

Left untreated, these bearing currents can over time cause fluting and pitting on the bearing surface — creeping wear that only leads to bearing failure and a noticeable noise after months, without any electrical fault ever occurring in the classic sense. Common countermeasures:

• Insulated bearings (ceramic or with an insulating coating) on the non-drive end of the motor.
• Shaft grounding brushes/rings that actively discharge the buildup of charge on the shaft to the motor housing.
• Common-mode chokes or sine-wave filters for long cable runs, which dampen the high-frequency common-mode component already at the drive.

Note: which combination of measures is needed (insulated bearings, grounding ring, filter) depends on the motor power, cable length and switching-frequency setting of the drive — this article gives the principle, not a universally applicable recipe for every installation.

Practical relevance

When installing or replacing a variable-speed drive system (for example a pump or fan in an industrial building or a horticultural business), it is important to connect the motor cable with a 360°-grounded shield instead of a pigtail, and for long cable runs or nearby sensitive equipment (measurement and control systems) to consider in advance whether a common-mode choke or sine-wave filter is needed — fixing a bearing problem afterwards is considerably more expensive than getting the installation right the first time.

Common mistakes

1. Connecting the cable shield with a pigtail instead of a 360° EMC gland — a pigtail is barely effective against high-frequency disturbance currents.
2. Grounding the shield at only one end (for example only at the drive) — this disrupts the intended return path for the high-frequency current.
3. Attributing bearing damage to lubrication or load without considering the possibility of bearing currents — especially for a motor driven by a variable-speed drive and installed without insulated bearings or a grounding ring.

Related

• Safe Torque Off (STO) for variable-speed drives
• Replacing a motor
• EMC separation of power and data cables