Precious metal versus carbon brushes in real duty cycles
Which brush system belongs in a given application, and why does the answer depend more on the duty cycle and the supply voltage than on the torque?
The brush and commutator are the only sliding electrical contact in a brushed DC motor, and they are the part that decides its life. Two families are used in compact motors: precious metal contacts, usually a silver or palladium alloy running on a segmented precious metal commutator, and carbon or graphite brushes running on copper. They are not interchangeable, and the choice is made by the electrical duty rather than by the mechanical load.
Contact drop, and why supply voltage decides the choice
Every brush contact drops voltage. That voltage never reaches the winding and never becomes torque. It becomes heat at the contact. A precious metal system typically drops on the order of a tenth of a volt per contact; a carbon system drops on the order of three quarters of a volt to a volt per contact. With two contacts in the circuit, the totals are roughly 0.2 V and 1.6 V.
Pbrush = Vd · I
- Pbrush
- power dissipated in the brush contacts, W
- Vd
- total contact drop across both brushes, V
- I
- motor current, A
This single relationship explains most of the application boundary. Battery-powered mechanisms at 3 V and 6 V are the natural home of precious metal contacts, because a fixed 1.6 V penalty is unaffordable there. Line-derived 24 V systems can spend it without noticing.
What each system tolerates
| Property | Precious metal | Carbon on copper |
|---|---|---|
| Contact drop | Low, roughly 0.1 V per contact, and stable at low current | High, roughly 0.8 V per contact, and less stable at very low current |
| Current capability | Limited. Sized for low currents; heavy current erodes the contact quickly | Higher. Carbon carries larger currents and tolerates the heat |
| Tolerance of arcing | Poor. Arcing transfers and erodes precious metal rapidly | Good. Carbon erodes gradually and its film re-forms |
| Repeated starts and stalls | The limiting duty. Each inrush is a high-current event through a low-current contact | More tolerant, though still the dominant wear event |
| Wear mode | Erosion and material transfer. Failure often looks like rising contact resistance | Mechanical wear of the brush, plus a surface film on the commutator |
| Electrical noise | Lower at low current, with less arcing energy | Higher. Arcing is a broadband noise source |
| Sensitivity to contamination | High. Films from outgassing or silicones raise contact resistance | High, and in a different way: a poisoned film accelerates wear sharply |
| Dry or vacuum environments | Workable, subject to material and lubricant selection | Poor without a special grade. Graphite film formation depends on adsorbed moisture |
The duty cycle is the life variable
Brush life is not a number that belongs to a motor. It belongs to a motor operated in a stated way. The events that consume brush material are not distributed evenly across running time.
- Starts. At the instant of a start there is no back-EMF, so current is limited only by winding resistance. Every start passes something close to stall current through the contact. A mechanism that starts 900 times an hour is a fundamentally different duty from one that runs continuously for the same total minutes.
- Reversals. A reversal is a start preceded by a braking event, and it often carries more current than a start from rest.
- Stalls. A designed-in stall, such as driving a valve against a hard seat, holds stall current through the contacts for the duration. See the article on locked-rotor events.
- Speed. Sliding distance accumulates with revolutions, so a high-speed motor covers its brush life in fewer hours at the same current.
- Current level. Wear rate rises with current density, and above the contact's rating it rises steeply.
This is why a life figure quoted in hours, with no duty cycle attached, cannot be transferred to a new application. Two applications with identical hour counts can differ by an order of magnitude in brush wear, and the difference is entirely in the number of starts.
Contamination, the failure that surprises people
Brush systems are sensitive to what is in the air around them. Volatile silicones are the classic case: silicone from a sealant, a gasket, a lubricant or a cleaning product can be drawn into the motor and decompose at the contact, forming an insulating deposit. The result is rapidly rising contact resistance and a motor that fails early for reasons entirely unrelated to its rated life. The same mechanism applies to some plasticizers and to solvent vapors in an enclosed assembly.
Choosing, and what to test
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Start from the supply voltage
Below roughly 6 V, the contact drop of a carbon system is a large fraction of the available voltage, and precious metal is usually the only workable choice.
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Then check the current
Compare the continuous and peak current to the contact's capability, not to the winding's. A precious metal system in a duty that repeatedly draws stall current is being asked to do the wrong job.
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Count the starts, not the hours
Express the duty as starts and reversals per hour alongside total running time. This is the number that predicts brush life ordering between candidates.
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Consider a brushless option honestly
Where the duty is high-start-count, long-life, or sealed and inaccessible, a brushless motor removes the wear item entirely at the cost of a drive. That is a system decision, not a motor decision, and it belongs in the review early rather than after a brush life test fails.
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Test at the duty cycle, not at a convenient one
Accelerating a brush life test by running continuously removes the very events that consume the brush. A life test that does not reproduce the start count does not measure the application.
No published brush life figure substitutes for that test. The wear rate depends on current, speed, spring force, commutator surface, temperature and the local atmosphere, and those interact. What a review can do is get the family and the sizing right so that the life test is a confirmation rather than a discovery.
Commutation and the electrical environment
Each time the commutator breaks contact with a coil, the current in that coil has to stop. Winding inductance resists that, so the collapsing field produces a voltage spike across the opening contact, and if it is large enough an arc bridges the gap. That arc is what erodes the contact and what radiates electrical noise.
- Winding inductance sets the severity. A higher inductance winding, or a higher current, produces a more energetic break.
- Suppression helps and is not free. Capacitors across the brushes or across the terminals reduce arcing and noise; they also load the drive and can interact badly with a PWM controller.
- PWM interacts with the contact. Chopping the supply means the contact carries a rapidly switched current, and the effective current at the contact is not the average the meter shows.
- Current sensing in the drive sees the brush ripple. That ripple is useful, since it disappears when the rotor stops, but it also has to be filtered before it is used as a control signal.
This matters to the brush choice because the same suppression that protects a precious metal contact costs voltage, board area and cost. Where the electrical environment is hostile, a brushless machine removes the contact rather than protecting it, and that comparison belongs in the review before a brush family is chosen.