A multiple brush commutator is a DC machine commutator fitted with two or more brushes per brush arm — and often multiple brush arms around the circumference — so that armature current splits across several parallel carbon contacts instead of one. Unlike a single-brush setup that can only carry maybe 200-400 A before the contact patch overheats, multiple brushes share the load and keep current density inside the safe 8-12 A/cm² window. We use this in traction motors, rolling-mill drives, and large lap-wound DC machines where a single armature handles 2,000 A or more without flashover.
Multiple Brush Commutator Interactive Calculator
Vary armature current, brush contact area, paths, and density limit to see required brushes per arm and operating current density.
Equation Used
The calculator divides total armature current by the number of parallel paths, allowable brush current density, and one brush contact area. The theoretical result is rounded up to give the brush count per arm. With 25 mm x 40 mm brushes, the worked example contact area is 10 cm2.
- Current shares equally between parallel brushes on the same arm.
- For a lap-wound DC armature, parallel paths equal pole count.
- Brush contact face area is entered in cm2.
- Final brush count is rounded up to the next whole brush.
How the Multiple Brush Commutator Works
The job of any commutator is to reverse current direction in each armature coil at the instant the coil crosses the magnetic neutral zone. Stack enough current onto a single brush and two things go wrong fast — the carbon overheats and the trailing edge of the brush starts arcing as the segment leaves contact. A multiple brush commutator solves both problems by splitting the total armature current across parallel brush paths, with each brush carrying a fraction of the load. If you notice blue arcing only on one brush in a stack of four, that brush is taking more than its share — usually because its spring pressure is high or the pigtail resistance is lower than its neighbours.
For a lap-wound armature, the number of parallel paths equals the number of poles. A 6-pole lap-wound mill motor has 6 parallel paths, which means you need 6 brush arms — and each arm typically carries 2, 3, or 4 brushes side-by-side along the commutator axis. Each individual brush sees current density around 8-12 A/cm² of contact face. Push past 15 A/cm² and the brush film breaks down, copper transfers to the carbon, and you get the streaky black-and-copper pattern that signals impending flashover. The brushes are also staggered axially across the commutator face so the wear track spreads evenly — if all brushes ride the same band, you grind a groove into the copper inside 2,000 hours.
Brush spring pressure matters more than people expect. Spec is typically 17-25 kPa (about 2.5-3.6 psi) on the contact face. Drop below 14 kPa and the brush bounces, arcs, and pits the segments. Push above 30 kPa and friction losses climb, the brush wears in 600 hours instead of 4,000, and the commutator runs hot. The pigtail (the flexible copper braid carrying current from the brush to the brush holder) must have its own resistance matched within ±5% across all brushes on a single arm — otherwise current sharing goes off and you cook one brush while the others loaf.
Key Components
- Carbon Brushes: Sliding contacts made from electrographitic or metal-graphite carbon. Each brush typically measures 25 × 32 × 50 mm for a 1,000 A class machine, with hardness graded to match commutator copper hardness (HV 70-110 for the copper, brush tuned softer).
- Brush Holders: Box-shaped guides that constrain each brush to slide radially against the commutator. Clearance between holder wall and brush is held to 0.1-0.2 mm — tighter and the brush sticks, looser and it cocks under load.
- Brush Arms (Brush Yokes): Insulated arms that mount the holders around the commutator periphery. Number of arms equals number of poles for lap windings. Arm-to-arm angular position must be set within ±0.5° of the geometric neutral plane or commutation suffers.
- Brush Springs: Constant-force or coil springs that push each brush onto the commutator at 17-25 kPa. Pressure drift over brush wear life must stay within ±10% of nominal — otherwise current sharing degrades as the brush shortens.
- Pigtail Shunts: Flexible copper braids that carry current from the brush body to the brush holder terminal. Resistance matched within ±5% across parallel brushes on the same arm so each brush carries its fair share of current.
- Commutator Segments: Hard-drawn copper bars insulated with mica strips 0.6-0.8 mm thick. Mica is undercut 1.0-1.5 mm below the copper surface so the brush rides on copper only — if mica protrudes, brushes chatter and arc.
- Risers: Slotted copper tabs at the rear of each segment where the armature coil ends are soldered or TIG-welded. A bad riser joint shows up as a single hot segment under thermal imaging.
Who Uses the Multiple Brush Commutator
Multiple brush commutators show up wherever a DC machine has to handle currents beyond what a single brush set can manage — historically that means traction, steel mills, and large industrial drives. You will not find them on a small fractional-horsepower motor, but on anything from 50 kW upward they become the only practical option for getting current in and out of the armature without the brush gear melting.
- Rail Traction: GE 752 traction motor on EMD SD40-2 locomotives — 6 brush arms each with 4 brushes per arm, handling 1,050 A continuous at 600 V DC.
- Steel Rolling Mills: ABB DMI 400 reversing mill motor at ArcelorMittal hot strip lines — 8 brush arms with 3 brushes each on the main commutator, sized for 2,400 A armature current.
- Mine Hoists: Siemens 4-quadrant DC hoist drive at Kiruna iron ore mine — multiple brush sets allow regenerative braking with full reversal of current direction.
- Marine Propulsion: Legacy turboelectric drives on the SS United States and earlier Liberty ship auxiliaries used multi-brush DC propulsion motors rated 20,000 SHP per shaft.
- Heavy DC Welding: Lincoln Idealarc R3R-500 motor-generator welder uses a multi-brush commutator on the DC generator armature to deliver 500 A at 40 V DC for stick and submerged-arc welding.
- Paper Mill Sectional Drives: Voith DC mill motors on Kraft paper machine drying sections — multiple brush sets manage the 1,500-3,000 A armature currents typical of 750 kW machines.
The Formula Behind the Multiple Brush Commutator
The core sizing question is: how many brushes do you need per arm so that no single brush exceeds its current density limit? Push the count too low and you bake the brushes inside 500 hours. Push the count too high and you waste commutator length, friction climbs, and the brush gear becomes a maintenance headache. The sweet spot is where each brush sits comfortably at 60-75% of its rated current density — that gives you margin for the inevitable mismatch between brushes without forcing constant babysitting.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| nb | Number of brushes required per brush arm | count | count |
| Iarm | Total armature current | A | A |
| P | Number of parallel paths (equals pole count for lap winding, 2 for wave winding) | count | count |
| Jmax | Allowable current density at brush contact face | A/cm² | A/in² |
| Abrush | Contact face area of a single brush | cm² | in² |
Worked Example: Multiple Brush Commutator in a 1.2 MW cement-plant kiln drive DC motor
A cement plant in Alberta is rebuilding the brush gear on a 1.2 MW GE Type CD-808 DC kiln-drive motor. The armature is 8-pole lap-wound, rated 1,800 A continuous at 700 V DC. Each carbon brush measures 25 mm × 40 mm on the contact face, electrographitic grade EG-389 with a rated current density of 11 A/cm². The maintenance team needs to confirm the existing 3-brushes-per-arm configuration is sized correctly, and check what happens at the 60% partial-load condition the kiln runs at during start-up and at the 110% short-term overload during clinker upset.
Given
- Iarm = 1800 A
- P = 8 parallel paths
- Abrush = 10 cm² (25 × 40 mm)
- Jmax = 11 A/cm²
Solution
Step 1 — at nominal 1,800 A armature current, find current per parallel path:
Step 2 — divide by the safe current density and brush face area to get brushes needed per arm at nominal load:
Round up to 3 brushes per arm. That gives each brush an actual current density of 225 / (3 × 10) = 7.5 A/cm² — sitting at 68% of the rated 11 A/cm² ceiling. That is the sweet spot — comfortable margin without wasting brush gear.
Step 3 — at the 60% kiln start-up load (1,080 A total):
At 4.5 A/cm² the brush is loafing — current density is below the threshold needed to maintain a stable patina film on the copper. You will see the commutator surface go from chocolate brown to streaky pink within 200 hours of light-load running, and brush wear drops to almost nothing. Not a failure mode, but the brushes will need re-bedding once the kiln returns to full load.
Step 4 — at the 110% overload condition (1,980 A total):
Still inside the 11 A/cm² limit, with 25% headroom. The brush set will handle a 110% upset for the few minutes a clinker jam takes to clear without flashover. Push to 130% sustained and you cross the threshold — at that point you would need a 4-brush-per-arm rebuild.
Result
The nominal answer is 3 brushes per arm operating at 7. 5 A/cm² — exactly what GE specified for the original CD-808 brush gear. At 60% kiln start-up load each brush sees 4.5 A/cm² and the commutator film starves; at 110% upset each brush sees 8.25 A/cm² with comfortable margin to the 11 A/cm² ceiling. If you measure higher current density on one brush than predicted — say one brush running 9 A/cm² while its two neighbours run 6.5 — the cause is almost always pigtail resistance mismatch (one braid corroded or undersized), uneven spring pressure across the three brushes on that arm, or a cocked brush sitting only on its leading edge because the holder clearance has worn past 0.3 mm. Confirm with a clamp meter on each pigtail and a spring gauge on each brush before you blame the brush grade.
Multiple Brush Commutator vs Alternatives
The decision is rarely between multi-brush commutator and nothing — it is between multi-brush DC, brushless DC with electronic commutation, or AC induction with VFD. Each has a current-handling, maintenance, and cost profile that lands differently depending on duty cycle and environment.
| Property | Multiple Brush Commutator (DC) | Brushless DC (BLDC) | AC Induction with VFD |
|---|---|---|---|
| Continuous current handling per machine | Up to 6,000 A on large mill motors | Practically limited to ~500 A by inverter switches | Up to 4,000 A on large drives |
| Brush maintenance interval | 2,000-4,000 hours brush life, monthly inspections | None — no brushes | None — no brushes |
| Speed range with full torque | 10:1 to 100:1 below base speed | 1000:1 with encoder feedback | 20:1 to 100:1 with vector VFD |
| Capital cost (per kW at 500 kW class) | Lowest for retrofits, highest for new builds | High — inverter dominates cost | Lowest for new installations |
| Reliability in dusty/abrasive environments | Brush dust accumulates, requires sealed enclosure | Sealed, excellent | Sealed, excellent |
| Overload capability (short-term) | 200-300% for 30 seconds | 150-200% limited by inverter | 150-200% limited by VFD |
| Typical service lifespan (commutator/armature) | 20-40 years with brush/commutator rebuilds | 30+ years, electronics may need replacement | 30+ years |
Frequently Asked Questions About Multiple Brush Commutator
One brush is hogging the current. Either its pigtail braid is shorter or has lower resistance than the other three, or its spring pressure is significantly higher, forcing better contact at the expense of the neighbours. Pull the brush, measure pigtail resistance with a 4-wire milliohm meter, and check spring force with a gauge — you want all four matched within ±5% on resistance and ±10% on pressure.
If both check out, look at brush face condition. A glazed mirror finish on three brushes and a matte finish on the fourth tells you the matte one bedded in faster (often because it was a different production batch) and is now carrying disproportionate load. Replace the whole set from one batch lot, never mix-and-match.
More brushes per arm spreads current and gives redundancy — if one brush fails, the others carry the load briefly while you shut down. A wider brush gives you the same contact area in a single piece, but if it cracks (and wide brushes do crack from thermal stress) you lose all the current at once.
Rule of thumb: stay below 50 mm individual brush width on machines above 1,000 A. Above that width, the brush face cannot maintain uniform pressure across its length and you get edge-loading. For high-current applications, multiple narrower brushes per arm always wins over fewer wide brushes.
Brush friction loss is the missing term. At 17-25 kPa contact pressure and surface speeds of 25-40 m/s typical for medium-frame DC motors, friction can contribute 30-50% as much heat as the electrical I²R loss. The combined loss raises commutator temperature substantially above what an electrical-only calculation predicts.
If your measured temperature is even higher — say 120°C — check brush spring pressure. A pressure drift up to 35 kPa from a fatigued spring doubles friction heating. Also check that the mica undercut is still 1.0-1.5 mm deep; if it has filled with carbon dust the brush rides partly on insulator and the local I²R spikes.
Lap winding gives you P parallel paths where P is the pole count — 6 poles means 6 paths and 6 brush arms. That is the right choice for high-current, low-voltage machines because each brush arm only carries Iarm/P. Wave winding gives just 2 parallel paths regardless of pole count, so each brush arm carries half the total armature current — fine for high-voltage low-current designs but brutal on brush gear at high currents.
For an 800 kW machine at 600 V DC (~1,335 A), lap winding with 6 poles gives 222 A per arm — easily handled with 3 brushes per arm. Wave winding would force 667 A per arm and require 8+ brushes per arm or much larger brush faces. Lap wins for this size class.
That is normal bedding-in. New brushes have flat faces that contact the curved commutator on a thin line until they wear to match the radius. During this period contact area is a fraction of nominal, current density spikes locally, and you get visible sparking. It typically resolves in 1-4 hours of light-load running.
To shortcut the process, use a brush-seating stone (a soft abrasive block held against the running commutator) that throws abrasive dust under the brushes and forces them to bed in within minutes. If sparking persists beyond 8 hours of run-in, you have a different problem — likely a commutator out of round (TIR above 0.05 mm) or brush neutral position misaligned by more than 1°.
Briefly, yes — at reduced load. On a 6-arm lap-wound machine, removing one arm forces the remaining 5 to share the current. Each remaining brush sees 20% more current density than designed, which is survivable for hours but not days. Limit armature current to 75% of nameplate to bring brush current density back inside the rated Jmax.
The bigger risk is magnetic asymmetry. With one arm out, current distribution around the armature is no longer symmetric, and you get a sideways magnetic pull that loads the bearings unevenly. Fine for a 4-hour limp-home to finish a heat or a shift, but not a continuous operating mode.
References & Further Reading
- Wikipedia contributors. Commutator (electric). Wikipedia
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