What Are Motor Brushes?
What Are Motor Brushes?
A motor brush is a sliding electrical contact. A rotating shaft cannot be hard-wired to a stationary circuit — any rigid wire would twist and snap the moment the shaft turns. The brush solves this with a controlled sliding interface: a block held in a brush holder presses against a rotating conductive surface, maintaining continuous electrical contact through friction rather than a fixed joint.
In a brushed DC or universal motor that rotating surface is a 換向器 — a cylinder of copper segments separated by insulating mica. In AC wound-rotor machines and generators the surface is a 滑環 (a continuous ring, no switching). The brush’s job is identical in both: move current across the moving boundary without breaking the circuit.
Why Carbon, Not Metal?
The choice of carbon/graphite is a deliberate engineering compromise between conflicting needs:
| 要求 | Why carbon/graphite wins |
|---|---|
| Electrical conductivity | Carbon composites conduct enough current while keeping contact resistance high enough to limit arcing |
| Sacrificial wear | Soft graphite wears preferentially, protecting the hard, expensive copper commutator |
| Self-lubrication | Graphite’s layered crystal structure slides with very low friction and deposits a protective film |
| 熱穩定性 | Carbon stays stable where copper would soften, deform, or weld to the contact |
| Film formation | A “patina” (copper oxide + graphite) builds on the commutator, lowering friction and noise over time |
How Motor Brushes Work: The Commutation Cycle
Follow the current and you can see exactly what the brush does in each rotation:
- DC is applied to the brush terminals. Current enters through the positive brush.
- Current crosses the sliding contact into the commutator segment touching that brush, then into the connected armature coil.
- The energized coil becomes an electromagnet and the Lorentz force (F=B·I·L) pushes it against the stator field, 產生扭矩.
- The rotor turns, carrying the commutator with it. A few degrees before the coil would reach the “dead” aligned position (zero torque), the brush crosses to the next segment.
- Current reverses in that coil and feeds the next optimally-placed coil — so torque always acts in the same rotational direction.
- The cycle repeats dozens to thousands of times per second, giving smooth, continuous rotation.
With many coils (a typical motor has 9–24, large machines over 100 換向片), torque ripple stays small. Manufacturers such as maxon even use an odd number of commutator bars so only one brush commutates at a time — this reduces torque ripple and the energy switched per step, cutting brush fire and EMI.
Three Actions Happen at Once
| 行動 | How | Failure if absent |
|---|---|---|
| Spring pressure | 150–400 g/cm² (≈15–30 kPa) keeps the brush on the commutator | Bouncing → arcing, intermittent power |
| Current conduction | Carbon carries current across the sliding gap with low loss | Open circuit → motor stops |
| Controlled wear | Soft graphite conforms to the surface and wears slowly | Commutator scoring, rapid failure |
For the failure modes this wear produces, 請參閱我們的指南 why brushed motors spark, and for the brushless alternative read BLDC 馬達的缺點.
Brush Material Types Compared
刷子 “年級” is the carbon-graphite matrix tuned with additives (銅, silver, resins) for the motor’s voltage, 當前的, 速度, and environment. Using the wrong grade causes premature wear or commutator damage.
| Brush type | 作品 | 接觸電阻 | Best application |
|---|---|---|---|
| Pure carbon | Carbon | 高的 | Low current density, small DC motors |
| Graphite | Natural graphite | 中等的 | Lubrication-critical, general purpose |
| Carbon-graphite | Carbon + graphite | Medium–high | 均衡; 電動工具, 工業設備 |
| Electrographitic | Heat-treated carbon-graphite | 中等的 | 電動工具, 牽引力, high-speed motors |
| Metal-graphite (銅) | Graphite + 銅 | 很低 | 大電流, 低壓, heavy industrial |
| Metal-graphite (silver) | Graphite + silver | 很低 | 精確, high current density |
| Precious metal | Bronze body + silver-plated tip | Extremely low (〜50毫歐) | Small motors, low current, 電池 (maxon EB) |
Graphite vs Precious-Metal Brushes (maxon)
On small DC motors, OEMs choose between two fundamentally different commutation systems. maxon’s technical notes summarize the trade:
| Property | Graphite brushes (國標) | Precious-metal brushes (EB) |
|---|---|---|
| Contact body | ~50% 石墨 + 50% 銅 | Spring-bronze with silver-plated tip |
| 換向器 | Copper alloy | Silver alloy |
| 接觸電阻 | 更高 | Extremely low (〜50毫歐) |
| 空載電流 / 摩擦 | 更高 (more drag) | 很低 |
| Current capability | 高的; tolerates start/stop & current peaks | 低的; damaged by high current / brush fire |
| 電磁干擾 | Commutation spikes | 低的 (uniform pattern); CLL further suppresses |
| 典型用途 | 更大的電機, reversing, 脈寬調製, 伺服 | Small motors, 連續的, 電池, tachometers |
Precious-metal brushes gain life through CLL (capacitor long-life) 技術: an RC filter across adjacent segments damps the inductive arc at commutation, reducing electro-erosion and EMI.
工程數據: Brush Pressure, Drop, Wear & 生活
These are the numbers a maintenance or design engineer actually uses. Ranges reflect typical small-to-industrial brushed motors.
| 範圍 | Typical value | 筆記 / source |
|---|---|---|
| Brush contact pressure | 15–30 kPa (≈150–400 g/cm²) | Too low → bounce/arc; too high → rapid wear |
| Contact voltage drop | 0.5–2 V per brush set | Loss that never reaches the armature |
| Current density (碳) | ~10 A/cm² typical | Metal-graphite runs higher |
| Wear rate | 0.01–0.1 mm per hour | Allows planned replacement |
| Brush service life | 500–5,000 hours | High current = short; light load = long |
| Precious-metal contact R | 〜50毫歐 | maxon EB commutator |
| Commutator bars | Odd number | Reduces torque ripple & brush fire (maxon) |
Sparking Acceptance Limits (國際電工委員會 60034-1)
國際電工委員會 60034-1 定義四個換向火花等級. A service tech should never accept Class 3; 班級 2 僅允許在銘牌上規定的過載條件下使用.
| 班級 | 描述 | 可以接受? |
|---|---|---|
| 1 | 無火花; 換向器和電刷不變 | 適用於所有負載 |
| 1½ | 僅刷子邊緣有微弱火花 | 允許連續 |
| 2 | Permissible under stated overload conditions | 允許, 但調查 |
| 3 | 危險的火花, 火災/侵蝕風險 | 永遠不能接受 |
What the Research Says About Brush Wear
- Electrical vs mechanical wear: A City University of Hong Kong study on sliding contacts (brush load 50–800 gf, speed 1,000–35,000 rpm, current 0–20 A) found brush wear is 電力 (arc erosion) at low load and 機械的 at high load — so both under- and over-springing accelerate failure (Louie, CityU HK thesis, 2007).
- Sparking mechanism: Current reversal during commutation dissipates stored winding energy at the brush edge, raising local current density and igniting arcs that drive electro-erosion (Tribology International, commutator wear, 2002).
- Arc vs sliding wear: In automotive fuel-pump DC motors, short commutation arcs produced mostly mechanical sliding wear, but longer arcs caused arc erosion — a wear model separates the two regimes (IEICE Trans. 電子產品, 2010).
- Tribolayer failure: One of two identical motors failed at 1,200 小時 (the other ran 1,500 小時) because a thick oxide/carbon layer built up on the commutator and brush — confirming that a healthy commutator film is essential to life (Surface & Coatings Technology / Tribology, 2015).
Where Motor Brushes Are Used
Brushed motors remain the default wherever low cost, 簡單的控制, and high starting torque matter more than maintenance-free life.
| 應用 | Why brushes fit | Brush concern |
|---|---|---|
| 電動工具 (演習, 研磨機) | 高啟動扭力, 便宜的, repairable | High current → frequent brush changes |
| 家用電器 | 低成本, simple drive | 安靜的, low-spark grade |
| 汽車 (開胃菜, 交流發電機, 座位) | Robust, high inrush tolerance | 振動, 溫度 |
| 牽引力 & 工業的 | 高電流密度 (metal-graphite) | Commutator maintenance |
| 發電機 & 風力發電機 | Slip-ring current transfer | Continuous duty wear |
| 航太 / specialized | Electrographitic at altitude | Extreme environment grade |
Robotic and precision actuators often skip brushes entirely for gear-reduced BLDC instead; our piece on why robotic arms need speed reducers covers that trade-off.
選用指南: How to Choose the Right Brush
- Match the current and voltage: 高電流 / low voltage → metal-graphite (銅); low current / battery → precious-metal; general purpose → carbon-graphite or electrographitic.
- Match the duty: start/stop, reversing, or PWM drives need graphite brushes; continuous low-current running suits precious-metal (with CLL).
- Set spring pressure to spec: target 15–30 kPa. Never guess — pressure is calibrated to brush grade and speed.
- Verify commutator condition: a grooved or out-of-round commutator destroys even the correct brush. Re-surface before fitting new brushes.
- Confirm dimensions & grade code: brush size, lead style, and the manufacturer’s grade marking must match the original.
- Replace as a set with springs: always fit new pressure springs together with the brushes so pressure stays correct.
Common Engineering Mistakes With Motor Brushes
| 錯誤 | Why it hurts |
|---|---|
| 電刷等級錯誤 | 過早磨損, commutator scoring, or excessive sparking |
| Wrong spring pressure | Too low → bounce/arc; too high → mechanical wear & 熱 |
| Using emery cloth on the commutator | Copper grit embeds between bars, 火花惡化－使用換向石 |
| 忽略雲母底切 | Carbon builds on the mica, bridges segments, 驅動火花 |
| Replacing brushes but not springs | Old springs lose tension → low pressure → arcing |
| Mixing brush types in one motor | Uneven wear and current sharing → hot spots |
| Skipping commutator resurfacing | New brushes wear to a bad profile in hours |
Motor Brush Troubleshooting Table (問題 → 原因 → 解決方案)
| 問題 | 可能的原因 | 解決方案 |
|---|---|---|
| 火花過多 | 刷子磨損, wrong grade, grooved commutator, low spring pressure | 更換符合規格的電刷; 重新表面換向器; set pressure |
| 電刷磨損快 | Too much pressure, abrasive dust, wrong grade, 高電流 | Reset pressure; clean environment; fit correct grade |
| Motor weak / 低速 | Brushes worn short, 接觸不良, oxidized commutator | 更換電刷; clean/seat commutator; check film |
| Overheating at brushes | High pressure, overduty current, arcing | 減少負載; correct pressure; verify grade |
| 磨削 / rough running | Commutator out of round, embedded grit | Re-machine or replace commutator; resurface |
| Intermittent power | Bouncing brush, loose lead, worn spring | Seat brush; tighten lead; replace spring |
| 馬達無法啟動 | Brushes gone, open lead, 被卡住的軸承 | Fit new brush set; check circuit; free shaft |
For the full diagnostic sequence (including winding and insulation tests), use our DC motor troubleshooting guide.
常見問題解答
What are motor brushes made of?
Most are a carbon-graphite composite. Grades range from pure carbon and natural graphite through carbon-graphite and electrographitic, to metal-graphite (copper or silver) for high current, and precious-metal (silver-plated bronze) for small low-current motors. The mix is tuned to the motor’s voltage, 當前的, 速度, and environment.
What is the difference between graphite and precious-metal brushes?
Graphite brushes (~50% 石墨 + 50% 銅) handle high current and start/stop peaks but add friction and commutation spikes. Precious-metal brushes have extremely low contact resistance (〜50毫歐), very low friction, and low EMI, but are limited to small, low-current, continuous-duty motors — and are protected from arc damage by CLL capacitor technology.
How long do motor brushes last?
通常 500 到 5,000 營業時間. High-current power-tool motors sit at the short end; lightly loaded small motors at the long end. Wear runs about 0.01–0.1 mm per hour, which is why brushes are designed as planned-replacement wear parts.
What brush pressure should be used?
Usually 15–30 kPa (about 150–400 g/cm²), calibrated to the brush grade and motor speed. Too little pressure lets the brush bounce and arc; too much causes rapid mechanical wear and overheating. Always replace the pressure springs with the brushes.
Is brush sparking normal?
A faint spark at the brush edges (國際電工委員會 60034-1 班級 1 或 1½) 是可以接受的. 危險的火花 (班級 3) is never acceptable and signals worn brushes, a grooved commutator, wrong grade, or incorrect spring pressure that must be corrected before continued operation.
Can a motor run without brushes?
Only if it is brushless. 有刷直流, universal, AC wound-rotor, and generator designs all need brushes or slip rings to transfer current across the rotating boundary. 無刷直流 (無刷直流電機) replaces the mechanical contact with electronic commutation — see our BLDC overview for the trade-offs.
為什麼選擇 Greensky 拉絲 & Brushless Motion
When a brushed motor reaches end-of-brush-life — or when your design wants to skip brushes altogether — Greensky supplies both paths from one source, built to 國際電工委員會 60034 和 NEMA MG 1 尺寸，以便它們可以放入現有的安裝座中:
- Brushed PMDC & universal motors with documented commutator specs, brush-grade options, and replacement-brush programs for maintenance teams.
- 無刷 (無刷直流電機) motors and gear motors with matched Hall sensors and drives — eliminating brush wear, 火花四射, and commutation maintenance entirely.
- Integrated gear motors that multiply torque at the output shaft, reducing the current (and brush stress) the motor core must handle — see our gearbox vs gear motor guide.
- 法蘭相容性: IEC B5/B14 and NEMA C-face with customized pilot diameters — see our 馬達法蘭導軌.
- 低訂購量 OEM/ODM: 小批量用於備件計劃和定制軸/編碼器配置.
相關閱讀
- 什麼是直流電機? 類型, 原則 & 公式
- 有刷馬達為什麼會產生火花? 原因 & 修復
- How to Troubleshoot a DC Motor: Step-by-Step Guide
- 工程師應了解的 BLDC 馬達缺點
- How to Short a DC Motor (Short-Circuit Braking)
- 什麼是馬達法蘭? IEC 與 NEMA 安裝
- 交流電機與直流電機: 選擇哪個
- 齒輪箱與齒輪電機: 差異 & 選擇
- 同步電機與感應電機: 主要差異
- 為什麼機械手臂需要減速器
參考
- 國際電工委員會 60034-1 — 旋轉電機: 評級和表現 (激發課堂, 溫度限制). webstore.iec.ch/publication/67467
- 國際電工委員會 60034-30-1 — 旋轉馬達的效率等級 (IE1-IE5). webstore.iec.ch/publication/67784
- 一氧化氮鎂 1 — 電動機和發電機 (安全, 熱的, 安裝尺寸). nema.org/standards/view/mg-1-motors-and-generators
- IEEE 43-2013 — 旋轉機械絕緣電阻測試的建議實施規程. standards.ieee.org/ieee/43/4385
- IEEE 112 — Standard Test Procedure for Polyphase Induction & 直流電機 (損失 & 反電動勢法). standards.ieee.org/ieee/112/4213
- 我們. DOE：電動機效率測定 & Repair Guidance. energy.gov/eere/amo/articles/definition-electric-motors
- SKF — 電動馬達的軸承維護與潤滑. skf.com/us/products/maintenance-products/bearing-maintenance
- maxon — Brushed DC motor commutation: graphite vs precious-metal brushes, CLL. maxongroup.com/medias/sys_master/8798093410334.pdf
- FAULHABER — DC-Motors Technical Information (precious-metal commutation). faulhaber.com/en/technical-information
- Louie, Y.T. (CityU HK, 2007) — Tribological characteristics of brush/commutator sliding contact (PV factors, wear regimes). scholars.cityu.edu.hk/en/theses/theses(e7add903-b2e5-4635-9695-c59f031b68d6).html
