How to Select a Carbon Brush Grade: Current Density, Peripheral Speed and Contact Drop

Quick Answer

Selecting a carbon brush grade means checking the machine's operating point against the grade's limits, in a fixed order. Current density is the armature or ring current divided by the total contact area (t × a) of the brushes of one polarity; compare it with the grade's nominal value — 12 A/cm² (about 77 A/in²) for an electrographite grade such as EG4 — at full load and at the lowest sustained load. Peripheral speed is v = π × D × n / 60, with D in metres and n in rpm; it must stay below the grade's maximum — 60 m/s for EG4 — with margin for runout. Contact voltage drop governs commutation and electrical loss: higher-drop grades damp commutation sparking but waste more power; EG4 runs at 0.9–1.9 V per pair of brushes. Friction — 0.25 maximum for EG4 — sets mechanical loss and heating, which at high speed can exceed the electrical loss. Spring pressure must suit the grade and the vibration level; EG4's nominal value is 20 kPa. Finally, check the environment — humidity, contaminants, temperature — and the duty: brushes that run for long periods far below their nominal current density fail to maintain a stable film and wear the commutator, a problem known as light load.

The usual way a brush grade is chosen is that nobody chooses it. The machine came with a grade, the stores reorder it, and it works — until the machine's duty changes, a different supplier's "equivalent" arrives, or the plant installs a dust extraction fan that dries the air inside the motor enclosure. Then the commutator darkens, the brushes start to spark, and the question arrives: is it the brushes?

Often it is the match between the brushes and the machine. This guide sets out how that match is checked — with numbers, not appearance — so a grade can be confirmed, changed or rejected on evidence.

What You Need Before Starting

• Machine type — DC motor, DC generator, exciter, universal motor, slip-ring induction motor, synchronous machine
• Voltage and current — rated, and the actual load profile: how much time at full load, part load, overload
• Speed — rated and maximum, including field-weakening range for DC motors
• Commutator or slip-ring diameter
• Brush arrangement — number of brush arms or rings, brushes per arm, brush size t × a × r
• Duty and environment — continuous or intermittent, reversing, shock loads, vibration, ambient temperature, humidity, dust, oil, chemicals
• The present grade and how it behaves — wear rate, film colour, sparking

Step 1: Current Density

Current density is the single most important number in brush selection.

J = I / (N × t × a)

where I is the current through all brushes of one polarity (the armature current for a DC machine), N is the number of brushes of that polarity, and t × a is the contact area of one brush.

Worked example: a DC mill motor

A 400 kW, 440 V DC motor at about 93% efficiency draws roughly 980 A of armature current. It has four brush arms — two positive, two negative — each with five brushes of 25 × 32 mm.

• Contact area per brush: 2.5 × 3.2 = 8.0 cm²
• Brushes per polarity: 2 arms × 5 = 10
• Total contact area per polarity: 80 cm²
• Current density at full load: 980 / 80 ≈ 12.2 A/cm²

That sits at the nominal current density of an electrographite grade such as EG4 (12 A/cm²). Short overloads above nominal are normal for DC drives; what matters is that the sustained operating point is near the grade's design value.

Check part load as well

The same motor at 25% load runs at about 3 A/cm². If it spends most of its life there, the brushes are lightly loaded — and light load causes its own problems.

The light-load problem

At low current density the brushes cannot maintain a stable commutator film. Friction rises, the brush faces glaze, the brushes become noisy and start to chatter, and over weeks the commutator threads and grooves as copper is picked up on the brush faces — on a machine that is barely working. It is common on oversized motors, standby machines that run unloaded, and drives whose process changed after installation. A commutator that runs cold makes it worse: below about 40°C a proper film does not form at all.

There is no single threshold; it depends on the grade. Some electrographite grades are uncomfortable for long periods below about 7–8 A/cm², while grades designed for light load run for long periods at 3–4 A/cm². Ask the brush supplier to confirm the lower limit for the actual duty.

The remedies, in order:

1. Remove brushes to raise current density. Take the same number from every arm, and choose positions so that each commutator track still runs under equal numbers of positive and negative brushes; the machine or brush supplier can confirm which positions to leave empty. First check that the remaining brushes stay within the grade's rating at peak load.
2. Change to a grade suited to light load, selected for film stability at the actual current density. Resin- or wax-treated grades are generally less sensitive.
3. Control the environment. Humidity and contaminants have more effect on a lightly loaded film.

For the mill motor above, removing two brushes from each arm would lift the 25%-load figure from about 3 to about 5 A/cm². At full load, though, the remaining 48 cm² per polarity would carry about 20 A/cm², acceptable only as a short peak. If the motor still sees sustained full load, a grade change is the better remedy.

Step 2: Peripheral Speed

v = π × D × n / 60

with D the commutator or ring diameter in metres and n the speed in rpm.

EG4's maximum permissible peripheral speed is 60 m/s. The mill motor is comfortably inside it at both speeds. The exciter at 55 m/s is within the limit but close to it, and close to the limit, commutator runout, vibration and brush holder condition matter much more. The 400 mm ring at 3,000 rpm is above it, and would need a grade rated for higher speed or a smaller ring diameter.

High speed also raises friction heat and makes brushes more sensitive to bounce. If the machine runs near the grade's speed limit, check spring pressure and holder condition more often.

Step 3: Contact Voltage Drop and Commutation

Every brush has a voltage drop across its sliding contact — mostly in the film, not the brush body. It is quoted per pair of brushes (one positive, one negative in series) or per brush; check which.

Why it matters for commutation. In a DC machine, as each commutator bar passes under the brush, the coil connected to it is short-circuited by the brush and its current must reverse. The brush contact resistance helps limit the circulating current in that short-circuited coil. A higher contact drop damps commutation sparking on machines that commutate with difficulty; a lower contact drop reduces electrical losses on machines that commutate easily.

Why it matters for losses. Electrical brush loss is roughly the current times the contact drop. For the 980 A mill motor with a drop of around 1.4 V per pair, that is about 1.4 kW.

Brush makers draw these bands differently, so compare the published voltage figures, measured per pair of brushes, rather than class names such as "low" or "medium".

Step 4: Friction and Mechanical Loss

Friction between brush and commutator produces heat, noise and mechanical loss.

Friction loss ≈ μ × p × A × v

For the mill motor: 20 brushes of 8 cm² at 20 kPa give a total normal force of about 320 N. With a friction coefficient of 0.25 — EG4's maximum — the friction force is about 80 N, and at 39.3 m/s the friction loss is about 3.1 kW, more than twice the electrical contact loss. At base speed it is about 1.6 kW.

That is why low-friction grades matter on high-speed machines, and why spring pressure should be correct rather than generous.

Step 5: Spring Pressure

Spring pressure keeps the brush in steady contact. Too little and the brush bounces, arcs and burns; too much and friction, heating and mechanical wear rise. The grade's datasheet gives a nominal value — 20 kPa for EG4 — and the machine's vibration level decides whether to work above it.

Typical ranges published by brush makers:

Datasheets quote spring pressure in several units: 20 kPa ≈ 2 N/cm² ≈ 204 g/cm² ≈ 2.9 psi.

Check spring pressure with a spring balance at the brush's working position, and check that all springs on a machine are equal; brush makers recommend keeping the spread within about 15%. Uneven pressure makes brushes share current unevenly, and the hard-pressed brushes overheat.

Step 6: Environment

The brush and its film are sensitive to the atmosphere inside the machine.

• Humidity. The commutator film needs a small amount of water vapour to form and stay stable. Brush makers put the optimum at about 8–15 g of water per cubic metre of air. Below about 2 g/m³ the film breaks down and the brushes wear rapidly into fine powder, a condition known as dusting; above about 25 g/m³ commutator deterioration becomes common. Very dry air is found at high altitude, in cold dry winters and in enclosed machines with heated or desiccated cooling air. Outside air at −10°C holds only about 2 g/m³ even when saturated, so a machine ventilated with winter air can run close to the dusting threshold. For comparison, air at 20°C and 50% relative humidity holds about 8.7 g/m³.
• Silicone vapour. Silicone from varnishes, sealants, greases or cable insulation decomposes under sparking at the sliding contact into abrasive silica. In totally enclosed machines, brush wear can reach 10–15 times normal. Keep silicone products away from brush machines, or ventilate generously with fresh air.
• Oil and oil mist. Oil contaminates the film, causes erratic contact and glazing, and binds brush dust into conductive deposits.
• Corrosive gases — hydrogen sulfide, chlorine, ammonia, acid vapours — attack the copper and the film.
• Abrasive dust — cement, coal, mineral dust — grooves the commutator and accelerates wear. Filtered ventilation pays for itself.
• Temperature. High brush and commutator temperatures accelerate oxidation and thicken the film. A commutator that runs too cold (below about 40°C, typically on an over-ventilated or lightly loaded machine) does not form a proper film at all.

Step 7: Duty and Mechanical Conditions

• Reversing duty — the brush holder must suit both directions; radial holders are usual, and the brush grade must commutate well in both directions
• Shock and vibration — rolling mills, crushers, marine propulsion and traction: higher spring pressure, rubber top cushions, split brushes
• Frequent starts and overloads — the grade must tolerate short-time current densities well above nominal
• Slip rings — no commutation, so contact drop matters less and current density, speed and film stability matter more. See our guide to slip ring and exciter brushes

Matching Grade Families to Applications

Our guide to carbon brush types and grades covers each family in detail.

Changing Grade: How to Do It Properly

1. State the reason — wear rate, sparking, film condition, a change in duty
2. Check the new grade against current density, peripheral speed, contact drop, friction and spring pressure for this machine
3. Prepare the commutator or rings — clean off the old film, and correct any runout, high bars or high mica first; a new grade will not fix a mechanical fault
4. Replace the complete set — never mix grades on one machine
5. Bed the brushes in before full load
6. Monitor film colour, sparking and wear over the following weeks, and record the wear rate

Common Selection Mistakes

1. Choosing by appearance or by "equivalent" tables alone. Two brushes that look identical can have very different contact drop and friction.

   Prevention: Select against the machine's current density, speed, commutation and environment, and treat cross-reference tables as a starting point only.

2. Checking current density only at full load. The motor spends its life at a third of rating and the brushes run lightly loaded.

   Prevention: Calculate current density at the typical sustained load, and remove brushes symmetrically or change grade if it is low.

3. Ignoring peripheral speed after a speed increase. Field weakening, a new drive or a gearbox change pushes the commutator past the grade's speed limit.

   Prevention: Recalculate peripheral speed whenever the maximum speed changes.

4. Generous spring pressure "to be safe". Friction loss and wear rise sharply.

   Prevention: Set spring pressure to the grade's nominal value and adjust only for proven vibration.

5. Mixing grades on one machine. Uneven current sharing, uneven film and sparking.

   Prevention: Change complete sets.

6. Blaming the grade for a mechanical fault. High mica, runout or a worn holder causes sparking that no grade will cure.

   Prevention: Inspect and correct the commutator and holders before changing grade. See our commutator troubleshooting guide.

7. Silicone products used near the machine. Rapid, unexplained brush wear.

   Prevention: Ban silicone sealants, greases and sprays in and around brush machines.

8. No record of wear rate. Problems are noticed only when the brushes run out.

   Prevention: Record brush length at each inspection and track wear in millimetres per 1,000 hours.

Supply from Kasko Makine

Kasko Mak branded EG4 electrographite carbon brushes

Applications: turbogenerator exciters and excitation slip rings on units below 200 MW, rolling mill drive motors, marine propulsion motors, DC generators and converters, single-armature converters, low-power DC motors and general-purpose machines, synchronous motors and slip-ring induction motors.

Construction: 25 × 32 × 40 mm standard, other sizes to order, to drawing or from samples; single or twin tamped copper shunts; forked, ring, flat-blade or bare terminals; top rubber cushion, contact shoe and radial or bevelled contact face options.

Engineering support: Send the machine data — current, voltage, speed, commutator or ring diameter, brush arrangement and size, duty and environment — with the present grade and its behaviour. We will calculate current density and peripheral speed, check EG4 against the operating point, and quote.

Send your machine data for a grade check and quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading: Carbon Brush Series

• Carbon Brushes: Types, Grades and How They Work
• Replacement Carbon Brushes: Dimensions, Shunts and Terminals
• Commutator and Carbon Brush Problems: Diagnosis Guide
• Slip Ring and Exciter Brushes
• Industrial Electric Motors: IE Classes and Enclosures

Frequently Asked Questions

Q: How do you calculate carbon brush current density?
A: Divide the current flowing through all brushes of one polarity by their total contact area. For a DC machine that is the armature current divided by the number of positive brushes multiplied by each brush's tangential thickness times axial width. A motor drawing 980 A through ten 25 × 32 mm brushes per polarity runs at 980 ÷ 80 cm², about 12.2 A/cm².

Q: How is peripheral speed of a commutator calculated?
A: Peripheral speed in metres per second equals π multiplied by the commutator or ring diameter in metres multiplied by the speed in rpm, divided by 60. A 500 mm commutator at 1,500 rpm runs at about 39 m/s; a 350 mm exciter commutator at 3,000 rpm at about 55 m/s. The result must stay below the brush grade's maximum permissible peripheral speed with margin.

Q: What is the light-load problem with carbon brushes?
A: When brushes run for long periods far below their nominal current density, they cannot maintain a stable commutator film. Friction rises, the brush faces glaze, and over weeks the commutator threads and grooves as copper transfers to the brushes. The threshold depends on the grade: some electrographites are uncomfortable below about 7–8 A/cm², while light-load grades run at 3–4 A/cm². Remedies are removing brushes symmetrically or changing to a grade suited to the actual load.

Q: What does contact voltage drop mean for a carbon brush?
A: It is the voltage lost across the sliding contact, mostly in the surface film, quoted per brush or per pair of brushes in series. A higher contact drop helps damp commutation sparking on machines that commutate with difficulty; a lower drop reduces electrical loss. EG4 electrographite brushes have a contact drop of 0.9 to 1.9 V per pair.

Q: What spring pressure should carbon brushes run at?
A: Start from the grade's nominal value, which is 20 kPa for EG4 electrographite brushes. Stationary industrial machines typically run at about 18–25 kPa, and traction, crane, marine and rolling mill drives with heavy vibration at about 25–50 kPa. Too little pressure causes bouncing, arcing and burning; too much raises friction, heating and wear. Check every spring with a spring balance and keep them within about 15% of each other.

Q: Why do carbon brushes wear quickly in dry air?
A: The commutator film that lets brushes slide with low wear needs a small amount of water vapour to form and remain stable. The optimum is about 8–15 g of water per cubic metre of air; below about 2 g/m³ the film breaks down and brushes wear rapidly into dust, a condition called dusting. It occurs at high altitude, in cold dry winters and inside enclosed machines with heated or dried air. Treated grades and humidity control are the usual remedies.

Q: Can I change the brush grade on a motor?
A: Yes, if there is a reason such as high wear, sparking or a changed duty, and the new grade is checked against the machine's current density, peripheral speed, contact drop, friction and spring pressure. Correct any mechanical faults on the commutator first, replace the complete set rather than mixing grades, bed the new brushes in, and monitor film, sparking and wear afterwards.