What Happens If the Commutator in a DC Motor Is Damaged or Malfunctioning?
If the commutator is not healthy, the motor stops behaving like a stable torque machine and starts acting like a noisy, resistive, partially random arc generator. Current shifts to the wrong places, some bars run hot, brushes erode quickly, insulation ages fast, and given enough run time the motor either trips protection, burns itself, or damages whatever is feeding it. That is the simple story.
Table of Contents
1. What “damage” to a commutator really does
You already know the textbook role of the commutator, so skip the basics and look at what changes when the surface is no longer uniform.
A worn, pitted, or rough commutator breaks the clean sliding contact that brushes rely on. Instead of a nearly constant area of contact, you get a contact patch that is shrinking and expanding as the rotor turns. That means current density pulses. Pulsed current density means local heating, small arcs at the trailing edge of the brush, and more copper and carbon being thrown into the air.
Grooves, scoring, or heavy discoloration are early, visible versions of this. They show that some segments are carrying more current, or carrying it in a harsher way, than others. You might still get the rated torque for now, but you are buying it with extra heat and more brush wear per hour.
If the commutator is mechanically out of round, things move from untidy to unstable. Brush contact pressure alternates as the high and low spots go by, so the brush “bounces” and arcs more on each revolution. That bounce chips edges, breaks springs, and sometimes breaks the brush itself.
At the extreme, damage opens a path for flashover: the arc no longer stays inside a small zone under the brush but runs across multiple bars or even across the full circumference, briefly shorting large parts of the winding. That is when commutator failure becomes an event, not just a maintenance note.
2. Electrical behaviour once the commutator is compromised
From the circuit side, a bad commutator mainly shows up as noise, heat, and the wrong current at the wrong time.
The first thing that usually shifts is operating current. Poor contact and higher resistance in certain segments force the motor to draw more current to develop the same torque. That extra current is not “useful torque” current; much of it is just I²R loss in bars, brushes, and leads. Efficiency drops, sometimes quite sharply, even though the nameplate load has not changed.
Next comes arcing and electrical noise. When brushes ride over pitted, dirty, or uneven bars, the gap opens and closes fast, creating frequent small arcs. Those arcs produce EMI, radio noise, and visible sparking. They also spit copper and carbon dust into the air gap, which settles between bars and across insulation, slowly building conductive paths that were never intended to exist.
In heavier damage, adjacent commutator segments begin to share current through contamination or carbon tracking. That partially bypasses the commutation pattern the motor was designed for, shifting current into coils that should be off or nearly off at that rotor angle. Now torque ripple increases and some coils see higher RMS current than they were designed for, even with the same supply.
If the issue progresses to a flashover, the electrical picture is simple and unpleasant: a near short between brushes, a sharp current spike, and usually a protective trip or fuse operation. Insulation can be punched through in a single event.
3. Mechanical and performance symptoms you can’t ignore
On the shaft side, a damaged commutator mainly shows up as inconsistency.
Torque, which should be relatively smooth for a multi-pole machine, starts to “beat.” You notice a speed wobble on lightly loaded drives or a larger than usual ripple in line current on fixed-speed applications. On closed-loop systems, speed or position controllers may start working harder, with more aggressive corrections for no obvious external cause.
Vibration and noise often go up together with commutator damage. Some of this is simply brush chatter on an out-of-round or badly finished surface. Some of it is electromagnetic: uneven current and torque create a repeating disturbance once per revolution or once per commutator pattern. In small motors this may barely register; in large machines, it can be obvious even at a distance.
Heat is the quiet symptom. You can have a motor that “sounds fine” yet runs noticeably hotter because of extra resistive losses at the interface and in overloaded coils. Unless someone is looking at temperature trends, this can carry on for a long time and quietly shorten insulation life.
At the very end, the performance story is blunt: the motor becomes unreliable to start, slower to accelerate, unwilling to carry its rated load without tripping, and eventually unable to run at all without severe sparking.
4. From small scars to failure: a practical progression
In the field, commutator issues rarely jump straight from “looks OK” to “catastrophic.” You usually move through a sequence that roughly looks like this, even though individual machines have their own quirks.
First you see cosmetic change: a slightly uneven film, faint marks where brushes ride, a bit more dust than usual. That is not failure, but it is the machine telling you its commutation conditions have shifted.
Then come definite mechanical features: grooves along the direction of rotation, ridges, localized dark bands, or spots that remain after cleaning. At this stage, torque may still be acceptable but wear rate is accelerating. Brush life shortens, and contact conditions are heading toward instability.
If this is ignored, geometry problems appear. The commutator runs out of round or develops high bars, either from uneven wear, contamination, or thermal effects. Now contact pressure varies by angle, and the brush assembly experiences alternating forces that it was not designed for. Broken springs, broken brushes, and noisy arcing follow.
Beyond that, structural damage begins: lifted segments, cracked risers, or burned spots where arcs have stayed too long in one place. Once you reach this stage, the motor is at real risk of flashover and serious armature damage during operation.
5. Quick mapping: visible damage vs what really happens
Here is a compact way to think about observable commutator conditions and what they actually mean for the machine.
| Visible condition on commutator | Electrical effect inside the motor | What the motor tends to do | Risk if you keep running |
|---|---|---|---|
| Light discoloration and a uniform, smooth film | Contact resistance is slightly higher but evenly distributed, current sharing between segments is still acceptable | Runs close to normal, maybe a bit warmer; brush wear is slightly higher than ideal, but behaviour is stable | Low short-term risk; long-term, you are eating into brush and commutator life faster than necessary |
| Narrow grooves or scoring following the direction of rotation | Current density concentrates on raised edges, encouraging localized arcing and more dust production | Slightly rougher sound, modest increase in sparking, less predictable brush life between sets | Medium risk; waveform quality and efficiency degrade and the machine is moving toward instability if not corrected |
| Out-of-round surface or high bars | Contact force varies with angle; brushes bounce and arcs occur each time a high spot passes under the brush | Audible chatter, visible sparks, frequent brush chipping or spring failures, more vibration | High risk; one mechanical incident or overload can trigger a flashover or sudden failure of brushes |
| Burned or heavily darkened segments, sometimes in patches | Repeated arcing has raised resistance and damaged insulation around specific bars; current commutates poorly in those coils | Noticeable torque ripple, hot smell near the machine, more frequent trips under load | High and growing; the motor is already in a failure mode and can damage windings or power electronics if kept in service |
| Loose or lifted commutator segments | Connection between winding and bar is compromised; some coils carry intermittent or no current, others carry too much | Hard starts, severe sparking, occasional refusal to start or sudden stalls | Very high; this is a stop-running-now situation if you want to avoid major armature work or replacement |
| Carbon and copper dust packed between bars | Conductive paths bypass intended insulation, creating partial shorts between segments | Random, erratic sparking, heating at low load, and sometimes unexplained trips or nuisance fuse operations | Very high; the machine is prone to flashover and can fail dramatically under a routine disturbance |
This is deliberately compressed. In practice, you can see combinations of these states on the same rotor.
6. System-level fallout around the motor
A damaged commutator rarely hurts only itself.
On the supply side, increased current draw and frequent transients make protection devices work harder. Contactors, breakers, and fuses see more inrush-like events, more often. Cables and terminals run hotter than they should for the nominal load, especially in older installations where margins were already thin.
Drive electronics suffer too. Solid-state DC drives or rectifier front ends experience higher ripple current, sharper dv/dt and di/dt edges, and more back-EMF disturbances from irregular commutation. Depending on the design, this can show up as nuisance trips, derating requirements, or premature failure of semiconductors and filter components. On the plant-wide level, EMI from heavy sparking can disturb nearby instrumentation, especially analog sensors, radio systems, and poorly shielded communication lines. You see it as odd spikes in trends or as random behaviour in equipment that shares the same supply or tray.
So a damaged commutator is not just a component problem. It becomes a power-quality and reliability issue for the surrounding hardware.
7. What usually fails first when you push your luck
If you keep running with a compromised commutator, something has to give. Often it is not the commutator itself that fails first, at least not visibly.
Brushes are typically the first clear casualty. They wear rapidly, chip, or glaze. As they deteriorate, arcing becomes more intense, feeding back into even faster commutator wear. At this point, someone may think the motor has a “brush problem” when the root cause is geometry or contamination on the commutator surface.
Next is insulation around the commutator and in the first turns of the armature winding. Repeated local heating, high dv/dt from arcs, and contamination weaken varnish and slot insulation. Eventually a turn-to-turn fault or bar-to-bar fault appears. When you inspect the machine later, you often find burnt areas exactly where commutation had been poor for a long time.
Mechanical damage to the commutator body itself tends to be the endgame: loose segments, cracked support, lifted risers. Once those appear, the motor either fails rapidly or is taken out of service quickly because the symptoms are now too obvious to ignore.
8. How experienced technicians tend to react
In real workshops and plants, nobody has infinite budget or time, so decisions about a damaged commutator are usually made in layers.
If inspection shows only film issues and light marking, the usual move is to clean, dress the brushes, maybe polish the commutator lightly, and then monitor. The aim is to reset conditions closer to what the motor expects without invasive work.
If grooves, scoring, or modest out-of-round are present, the standard path is more mechanical: turning the commutator to restore roundness, undercutting the mica if needed, and fitting new brushes matched to the new surface. This is also the point where people start asking hard questions about loading, alignment, and the environment that created the wear pattern in the first place.
Once there are lifted segments, burned bars, or evidence of repeated flashover, the motor moves into the “major repair or replace” zone. Armature rewinding, commutator replacement, or complete motor replacement become realistic options, and the decision comes down to frame size, age, and how critical the application is.
Through all of this, the unspoken rule is simple: a DC motor is only as reliable as its commutation. If the commutator is compromised, the rest of the design can be perfect and the motor will still behave like a temporary solution.
9. A short takeaway to keep in mind
If you want a single sentence version for your own notes, it could be this: a damaged commutator turns clean DC torque into a mix of extra heat, unstable current, and growing risk of flashover, and it usually does so long before the motor actually stops.
Everything else you measure or repair is just that one fact showing itself in different ways.
