{"id":2524,"date":"2026-11-08T06:00:00","date_gmt":"2026-11-08T06:00:00","guid":{"rendered":"https:\/\/xnhang.com\/?p=2524"},"modified":"2026-11-06T22:02:32","modified_gmt":"2026-11-06T22:02:32","slug":"vfd-bearing-currents","status":"publish","type":"post","link":"https:\/\/xnhang.com\/pt\/blog\/vfd-bearing-currents\/","title":{"rendered":"Correntes de rolamento VFD: Evid\u00eancias e Revis\u00e3o de Mitiga\u00e7\u00e3o"},"content":{"rendered":"

VFD bearing currents are unwanted electrical currents that can pass through motor bearings in a variable frequency drive system. They can contribute to bearing damage, but noise, vibration or an early bearing failure alone does not prove the cause. A useful investigation combines the motor failure evidence with the drive, cable, bonding and driven equipment arrangement. Select mitigation only after identifying the likely current path. This guide explains what buyers and maintenance teams should record, which questions to ask and how to verify a proposed remedy without treating one accessory as a universal solution.<\/p>\n

Understand the system boundary<\/h2>\n

A motor supplied by a PWM drive experiences a different electrical environment from a motor connected directly to a sinusoidal supply. Switching produces rapid voltage changes. The drive, cable, motor frame, rotor and surrounding installation form a system with several possible paths for high-frequency current. A review limited to the drive’s power rating misses much of that system.<\/p>\n

The practical starting point is a single installation record: drive model, motor model, cable construction and length, termination arrangement, protective bonding and the mechanical connection to the driven machine. Add any filters or shaft protection already installed. Use the actual as-built arrangement, because a cable or motor replacement can change the conditions even when the drive settings are unchanged.<\/p>\n

The ASHRAE handbook chapter on motors and drives<\/a> discusses VFD-related shaft voltage, bearing current detection and mitigation. Its central engineering distinction is useful here: measures that interrupt one path do not necessarily address every way that voltage or current can reach a bearing.<\/p>\n

Distinguish the likely mechanisms<\/h2>\n

Capacitive coupling can place voltage on the rotor and shaft. Under some conditions, charge can discharge through the bearing lubricant film. Another mechanism involves circulating current through a loop that includes the shaft, bearings and frame. Other shaft-connected equipment may also form part of the available return path.<\/p>\n

These descriptions explain why the location and type of protection matter. A bearing insulation proposal, a shaft grounding proposal and an output filter proposal do different jobs. Their suitability depends on the complete motor and installation. Do not select a design solely from a horsepower cutoff quoted without its original assumptions.<\/p>\n

The EASA overview of shaft and bearing currents<\/a> explicitly distinguishes causes and the need for different remedies. Its public description is an overview, not a substitute for access to the full training recording or an equipment-specific engineering assessment.<\/p>\n

\"Front
Keypad illustration from the website reference. Blank small labels and display carry no settings or diagnostic readings.<\/figcaption><\/figure>\n

Preserve failure evidence before replacing parts<\/h2>\n

Ask the repair provider to retain and identify the removed bearings. Record the drive end and non-drive end, the installation orientation, service history and observed condition. Obtain clear photographs and the repair provider’s explanation of the damage. Keep the distinction between an observed pattern and a confirmed root cause in the report.<\/p>\n

Lubrication, contamination, alignment, mechanical loading and installation practices still require review. A bearing that failed on a VFD-driven machine has not automatically failed because of the VFD. Conversely, replacing the bearing without reviewing a suspected electrical mechanism can leave the new bearing exposed to the same conditions.<\/p>\n

Build a timeline. Note whether failures began after a drive conversion, a change of motor, longer cable routing, a filter removal or a change to the driven machine. Record normal speed range and operating duty. A precise timeline helps an engineer select useful checks and avoids unrelated changes made simply because they are easy to try.<\/p>\n

Match the proposed action to the evidence<\/h2>\n

This decision table organizes a technical discussion; it does not prescribe a retrofit. The motor and drive manufacturers should review the final configuration, including any effect on warranty, protection, cooling, maintenance or the connected machine.<\/p>\n

\n\n\n\n\n\n\n\n\n\n\n
Review area<\/th>\nQuestion to resolve<\/th>\nEvidence needed before approval<\/th>\n<\/tr>\n<\/thead>\n
Cable and termination<\/td>\nDoes the installed arrangement provide the intended high-frequency return path?<\/td>\nCable details, termination photographs and the matching installation instructions<\/td>\n<\/tr>\n
Frame bonding<\/td>\nDoes the installation match the required bonding arrangement?<\/td>\nAs-built bonding record and checks selected by a competent engineer<\/td>\n<\/tr>\n
Shaft grounding<\/td>\nWhich current path is the device intended to divert?<\/td>\nMotor-specific proposal, mounting details and maintenance requirements<\/td>\n<\/tr>\n
Bearing insulation<\/td>\nWhich loop is interrupted, and where can current go afterward?<\/td>\nMotor bearing arrangement and review of coupled equipment<\/td>\n<\/tr>\n
Output treatment<\/td>\nDoes the proposed filter address the identified stress or current mechanism?<\/td>\nCompatibility statement for drive, motor, cable and operating range<\/td>\n<\/tr>\n
Verification<\/td>\nWhat observation will demonstrate that the targeted condition changed?<\/td>\nAgreed before\/after method, operating conditions and follow-up record<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

A low-resistance connection checked for one purpose does not, by itself, establish the behavior of an entire installation at switching frequencies. Likewise, a component advertised for output voltage stress should not automatically be described as a solution to all common-mode current paths. Require the supplier to state the function being addressed.<\/p>\n

Review cable and bonding details before adding accessories<\/h2>\n

Obtain the drive manufacturer’s cable and grounding instructions for the exact model. Compare the installed cable type, routing and terminations with those instructions. Record deviations and ask for a written disposition. If a shield has been modified during maintenance, capture that detail rather than assuming the original design is still present.<\/p>\n

The existing VFD cable guide<\/a> explains the procurement information needed for the output cable. For a bearing current review, add the actual bonding and termination evidence. A cable described as suitable for VFD service still needs an appropriate installation at both ends.<\/p>\n

Keep electrical safety bonding intact. Never disconnect a protective conductor as an experiment to see whether noise or shaft voltage changes. Measurement and corrective work require appropriate procedures and competent personnel. A shortcut that removes a required safety path is not a valid diagnostic method.<\/p>\n

Assess shaft grounding and insulated bearings together<\/h2>\n

A shaft grounding device is intended to provide a route that bypasses a bearing for the current it is designed to handle. Its effectiveness depends on the application and the condition of the contact or connection. Ask how access, contamination, wear and maintenance will be managed over the service life.<\/p>\n

Insulating a bearing can interrupt a particular loop, but a proposal must also consider capacitive coupling and alternative routes through other bearings or the driven machine. Request a drawing that identifies which bearing is insulated and why. Avoid approving a parts list that leaves the current path unexplained.<\/p>\n

For a replacement motor, include the drive application in the inquiry from the beginning. Give the motor supplier the drive and cable information, rather than asking for a bearing modification after delivery. The motor supplier and drive supplier should agree on the combined arrangement and identify any unresolved site assumptions.<\/p>\n

\"Unconnected
Illustration of drive system documentation review; no wiring, operating measurement or protection accessory is shown.<\/figcaption><\/figure>\n

Do not confuse carrier settings with a complete remedy<\/h2>\n

Carrier frequency is part of the switching environment, but changing it can also affect noise, losses and permitted drive loading. It should not be adjusted randomly after a bearing complaint. Preserve the original settings and request a technically justified change with a defined verification method.<\/p>\n

The carrier frequency article<\/a> covers the broader setting tradeoffs. This investigation has a narrower purpose: establish whether the proposed setting change addresses the suspected bearing current mechanism and remains within the equipment’s limits. A quieter motor after adjustment does not prove that an electrical bearing problem has been removed.<\/p>\n

The Jim Pytel lecture below explains rectification and PWM inversion. It provides background for understanding the switched waveform; it is not a bearing current measurement tutorial. The article’s diagnostic and procurement checks should remain usable without relying on a video demonstration.<\/p>\n