The Complete Guide to Online Truing
On generators with brushed excitation systems, the collector rings are where the work quietly happens. They carry field current to the rotor through a set of carbon brushes, and the quality of that brush-to-ring connection has an outsized effect on reliability. Over time, mechanical and electrical wear leaves rings grooved, out-of-round, or blemished. The result is higher brush vibration, arcing, and, in the worst cases, a collector ring fire and a forced outage.
The traditional fix is to take the unit offline and grind the rings on turning gear. Online truing takes a different approach: it restores the ring surface while the generator keeps running at full speed and temperature. This guide explains what online truing is, why collector rings need it, how it compares to offline grinding, and what to expect from the process.
Table of Contents
- What Is Online Truing?
- Why Collector Rings and Commutators Need Truing
- Online Truing vs. Offline Grinding
- What to Expect During the Online Truing Process
- The Benefits of Online Truing
- Online Truing as Part of Cutsforth's Generator Services
What Is Online Truing?
Online truing is a specialized machining process used to restore and resurface collector rings and commutators while the generator remains online and operating at full speed and temperature. You may also see it referred to as collector ring grinding, slip ring grinding or polishing, collector ring machining, slip ring maintenance, or commutator surfacing. Whatever the name, the goal is the same: return the ring surface to a true, round, smooth condition so the brushes ride cleanly and conduct current reliably.
The defining feature is that no outage is required. There is no need to take the unit offline, and no need to remove brush rigging or enclosures. Field service technicians perform the work on the running machine, which means the process reflects the generator's real operating condition rather than an approximation of it.
Why Collector Rings and Commutators Need Truing
Collector rings and commutators are grooved and worn by the constant mechanical and electrical action of the brushes riding on them. Unfiltered air and oil contamination accelerate that wear. Several distinct problems develop over time, and each one degrades the brush-to-ring connection:
Out-of-roundness and flat spots. Electrical erosion creates low or flat spots on the ring surface. These are often invisible to the naked eye, but they drive up brush vibration and chatter, which in turn causes brush holder problems.
Arcing. A compromised brush-to-ring connection produces visible arcing. Carbon deposits, poor spring pressure, and out-of-round rings are common culprits. Left unaddressed, arcing can escalate into a catastrophic ring fire and a forced outage.
Spiral groove wear. Many collector rings have a machined spiral (or helical) groove that helps maintain the surface film and brush contact. As the groove wears down, selective-action problems appear. When any part of the groove wears below roughly 0.03 inches or goes missing, it should be restored rather than left to fail.
Online truing directly addresses out-of-roundness, flat spots, and surface blemishes. When the groove itself needs to be re-cut, that is a related but separate service, spiral groove restoration, typically performed at a scheduled outage.
Online Truing vs. Offline Grinding
The biggest advantage of online truing is accuracy, and it comes from a simple physical fact: a round ring on turning gear does not always equal a round ring online. When the machine is running, the rotor sits at its true operational center and the rings are at thermal equilibrium. Grinding under those conditions references the operational center, so material is removed exactly where it needs to be, and no more. Offline grinding, performed cold and at low speed, cannot replicate that.
| Attribute | Offline (Traditional) Grinding | Online Truing |
|---|---|---|
| Unit status | Requires taking the generator offline | Performed while the generator runs at full load |
| Downtime and lost revenue | Outage time and lost generation | No downtime, no lost revenue |
| Brush rigging and enclosures | Often must be removed | No removal needed |
| Reference point for machining | Cold ring on turning gear | True operational center at temperature and speed |
| Accuracy of the round surface | Good, but does not reflect operating conditions | Often more accurate; optimizes brush-to-ring contact |
| Effect on brush vibration | May not fully resolve running vibration | Reduces brush vibration and chatter |
Because the work is done at operating speed and temperature, online truing is often better than offline grinding at reducing brush vibrations and optimizing the brush-to-ring connection. This is why even brand-new rings on large rotors sometimes need truing after start-up: a shift in operational center can produce brush vibrations greater than three mils once the unit is running.
What to Expect During the Online Truing Process
Online truing is a controlled, measurable process. Technicians assess ring condition by measuring the movement of the brush with a vibration analyzer, reading run-out in mils of displacement. As a rule of thumb, a round ring registers a low reading, generally under two mils, while readings above six mils exceed the OEM recommended limit and indicate a ring that needs attention.
A few practical points on how the service runs:
Time required. The process typically takes anywhere from two to ten hours, depending on the condition of the collector rings or commutator.
Load. The work can be performed safely at full load. In most cases there is no need to reduce load on the generator. Only in rare situations of very high arcing or high brush vibration might a temporary load reduction be needed for truing to take place.
Who performs it. Experienced field service technicians carry out the work on site, following established safety practices for working on energized collector systems.
The Benefits of Online Truing
The value of online truing shows up across cost, uptime, and safety:
No lost generation. Because the unit stays online, there is no outage to schedule and no revenue lost to downtime. For many plants, this alone makes online truing more cost-effective than offline machining.
Greater accuracy. Machining at operating speed and temperature, referenced to the true operational center, produces a surface optimized for real running conditions.
Lower brush vibration. A truer ring means smoother brush ride, which reduces the vibration and chatter that lead to brush holder problems.
Reduced risk of failure. By restoring the surface and improving the brush-to-ring connection, online truing helps prevent the arcing and ring damage that can escalate into a ring fire and forced outage.
Online Truing as Part of Cutsforth's Generator Services
Cutsforth has performed collector ring and commutator work across gas and steam turbines, hydroelectric units, and nuclear, coal, and wind generation, as well as chemical plants, paper mills, and oil and gas refineries. Online truing is one part of a broader generator services slate that also includes spiral groove restoration, which returns a worn or missing helical groove to optimal condition in a fraction of the time traditional methods require, brush rigging inspections, and emergency services when time-critical support is needed.
That service capability is backed by decades of expertise rooted in generator maintenance. Cutsforth is also known for its EASYchange removable brush holders, which allow the brush and brush box to be safely removed as a unit while the generator is running, its shaft grounding systems, and collector ring manufacturing. On the monitoring side, Cutsforth pairs this hardware with multiphysics condition monitoring, the InsightCM software platform, and reliability services, so plants can not only restore ring surfaces but also track brush and ring health continuously and catch problems earlier.
The authoritative body behind much of the industry guidance on brush, collector ring, and excitation maintenance is the Electric Power Research Institute (EPRI), an independent nonprofit whose research informs best practices across power generation. To learn more about the service itself, see the Cutsforth Online Truing page, or explore the full range of generator services and monitoring solutions at Cutsforth.com.
About the Author
John Pasquarette is an Austin-based product and marketing leader with a background spanning National Instruments and Monolith. He writes about industrial reliability, condition monitoring, and predictive maintenance for reliability engineers and plant teams. Connect with John on LinkedIn.