Brush sparking is one of the most visible signs of trouble in a commutator-type electric machine. It can appear in DC motors, generators, universal motors, and in machines with slip rings — wound rotors and brush-excited synchronous machines.
A small, even level of sparking is sometimes allowed by the design and duty of a particular machine. Strong, unstable or localized sparking, however, is never normal — it burns the commutator or slip rings, wears brushes faster, overheats the brush holders, damages windings, and can trigger a flashover.
Sparking does not always mean the brushes simply need replacing. The cause can be a misplaced brush rocker, a commutator defect, disrupted commutation, overload, an armature interturn short, faulty interpoles, or mechanical vibration — so diagnostics need to cover the whole brush-commutator assembly, the armature, the magnetic system, the bearings, the load and the supply.
A brush provides the electrical contact between the stationary part of the machine and the rotating commutator or slip ring. Brushes are made from electrographite, graphite, carbon-graphite or metal-graphite materials chosen for the machine’s voltage, current and speed.
A spring presses the brush against the commutator, and as it rotates a sliding electrical contact forms between them that must simultaneously carry current steadily, spark minimally, run at an acceptable temperature, wear evenly, and build up the right working film.
A commutator is made of individual copper bars insulated from one another; each is connected to a matching section of the armature winding. As the commutator turns, the brush moves from one bar to the next and briefly short-circuits the winding section in between.
In that section, the current has to, in sequence:
This process is called commutation. If the current fails to change in time before the bar leaves the brush, an electric arc forms between the brush edge and the bar. The bigger the mismatch between the actual commutation process and the required current change, the stronger the sparking.
Not always. Light, even, spot sparking that does not spread around the commutator can be acceptable for a particular design — during brief overload, a sudden load change, reversing, or the starting of a traction motor.
Sparking should be considered dangerous when it is:
| Sparking pattern | Likely cause | What to check |
|---|---|---|
| Light, even, at every brush | Bedding-in, duty cycle | Contact area, current, documentation |
| Strong under every brush | Overload, neutral shifted | Current, rocker, interpoles |
| Only under one brush | Spring force, sticking, lead | Brush holder, spring, flexible lead |
| Repeats once per revolution | A bar or coil defect | Commutator, solder joints, armature winding |
| Only under load | Armature reaction, overload | Interpoles, compensation, current |
| After a brush change | Wrong grade or bedding-in | Grade, contact area, spring force |
| After commutator turning | Run-out, roughness, high mica | Geometry, undercutting |
| With heavy vibration | Imbalance, bearings, shaft | Vibration, run-out, bearings |
| On slip rings | Contamination, force, current asymmetry | Rings, brushes, rotor phases |
| Bright arcs between brush zones | Flashover | Immediate shutdown and inspection |
A brush is chosen not just by size but by resistivity, allowable current density, hardness, friction coefficient and its ability to support commutation. A grade that fits physically but does not match the machine’s electrical characteristics causes heavy sparking, faster wear and commutator damage.
Brushes of different grades have different resistance, hardness and film-forming ability, so current splits unevenly across parallel brushes — some carry too much current and spark, others carry almost none.
Contact over only a small area raises current density there, causing local heating, sparking and charring of the brush edge. After correct bedding-in, the running face should follow the commutator’s curvature as closely as possible.
A light spring lets the brush lose stable contact, bounce and spark; excessive force raises friction, heat and wear. Force should be about equal across all brushes of a given polarity, and checked with a gauge rather than by hand.
Contamination, a deformed holder, or the wrong brush size stop it from moving freely and following commutator run-out. Once a brush hangs up, the spring can no longer maintain contact, and an arc and local burning follow.
An incorrect mounting angle, or too great a distance to the commutator, produce an unstable contact, brush vibration and worse commutation.
Partially broken strands, a poor connection to the brush, or a loose terminal raise the lead’s resistance, causing local heating and an uneven current split between brushes.
Brush position relative to the magnetic neutral is critical for commutation. If the rocker has shifted — from an assembly mistake, a repair error, or a rotation-direction change made without correction — the commutated coil develops an extra EMF, and one edge of the brush starts sparking.
Carbon or copper dust, oil, moisture or process contamination change the contact resistance and can create conductive paths between bars. A correct, thin working film on the commutator surface, by contrast, supports stable contact and even wear — problems appear when the film is missing, patchy, too thick, or charred.
If the distance from the axis of rotation to the commutator surface varies, the brush cannot keep following the surface, especially at high speed, and periodically lifts off, creating an arc. Causes include incorrect turning, a bent shaft, a loose commutator, or worn bearings.
If the inter-bar insulation is not recessed below the copper after turning, the brush partly rides on mica — contact with the copper is interrupted, causing vibration and sparking. Copper burrs, sharp edges, or metallic dust left in the slots after undercutting produce the same effect.
Armature coils are joined to the commutator bars by soldering or welding. A poor joint causes local heating, a changed current in that coil, and sparking that repeats at the same spot on the commutator every revolution.
When part of a coil shorts, that section has altered electrical parameters, causing current asymmetry, local overheating and disturbed commutation. With an open coil, current is interrupted, and as the matching bars pass under the brush an arc forms, repeating in the same position every revolution.
Interpoles create a field that compensates the reactive EMF of commutation. Swapped polarity after a repair, an open interpole winding, or an interturn short in it worsen commutation instead of improving it — sparking can then be very strong even with a healthy commutator.
Under overload, armature current rises, current density under the brushes increases, and commutation becomes harder — sparking grows with the load. In drives with shock or pulsating loads, armature current changes quickly, and if the commutation system cannot compensate fast enough, sparking appears at every torque spike.
Machines fed from thyristor or other converters can see significant current ripple, which degrades commutation, brush heating and the state of the working film. A fault in the converter itself — current asymmetry, missed pulses — is not fixed by brush maintenance either.
In a DC motor, a reduced field current raises speed, shifts the neutral, worsens commutation and increases sparking. Causes include an open field circuit, a poor contact, a faulty regulator, or an interturn short in the field winding.
Slip-ring machines have no commutation in the DC-motor sense, but their brushes can still spark from unstable contact. Common causes: contaminated or out-of-round rings, corrosion, weak spring force or a stuck brush, an overloaded rotor circuit, a broken flexible lead, shaft vibration, and poor connections in the rotor circuit.
If brushes spark on slip rings, check the current in every rotor phase separately and compare the condition of all the brushes, rather than replacing only the one under the visible spark.
A flashover is a powerful arc that spreads across the commutator surface between brush zones of opposite polarity. It is one of the most dangerous conditions: possible causes include heavy sparking, contamination with carbon or copper dust, overvoltage, a sudden overload, field weakening, or excessive speed.
At the first sign of a flashover, shut the machine down immediately — the consequences can include melted bars, destroyed brushes and brush holders, insulation breakdown and total destruction of the commutator.
A slight, even level of sparking can appear during initial bedding-in. Strong arcs, local flare-ups, overheating or rapid charring are not normal.
Possible causes: the wrong grade, insufficient bedding-in, wrong spring force, sticking, fitting new brushes onto a damaged commutator, or mixing brush types.
Only if insufficient force is genuinely the cause. Excessive force will cause overheating and faster wear instead.
A defect is likely in that specific brush holder, spring, flexible lead, or contact surface.
Check the interpoles, the compensating winding, the neutral position, overload, and the shape of the current.
The main causes are contamination, out-of-round rings, wrong spring force, vibration, poor contacts, and current asymmetry in the rotor.
An electric arc runs at high temperature, damaging the brushes, the commutator copper and the insulation, and can escalate into a flashover.
EPR (Elektropromremont) diagnoses, repairs and tests commutator-type electric machines, generators and slip-ring machines.
The scope of work includes:
This material is for informational purposes. The values, diagnostic methods, scope of work and recommendations given here are general and do not replace the manufacturer’s technical documentation. The final decision for a specific machine is made from its own diagnostics and inspection, taking into account its type, power, design, duty, operating history and applicable standards.
We inspect the brush-commutator assembly, the armature and the magnetic system, pin down the cause of the sparking, and carry out the repair needed.