How to Short a DC Motor: Short-Circuit (Dynamic) Braking Explained
What Does “Shorting a DC Motor” Mean?
The phrase is ambiguous, and the distinction decides whether the procedure is safe or destructive. There are two unrelated meanings:
- Controlled short-circuit braking (dynamic braking): with the supply removed, the armature terminals are tied together so the coasting rotor’s back-EMF circulates a braking current. This is a designed, repeatable stopping method.
- Accidental internal short: a fault inside the motor — inter-turn short, 地絡, or commutator segment short — that bypasses normal current paths and overheats or arcs. This is a failure to diagnose; 私たちのを参照してください DC motor troubleshooting guide.
Two Meanings, Two Outcomes
| 意味 | When It Occurs | Safe? | 結果 |
|---|---|---|---|
| Controlled short-circuit braking | Supply off, terminals tied | はい, by design | 速い, repeatable stop |
| Accidental internal short | Winding / commutator fault | いいえ | Overheat, arc, failure |
| Powered-terminal short | Supply on, terminals shorted | Never | Welded contacts, 焼けた巻線 |
How Short-Circuit Braking Works (ステップバイステップ)
Short-circuit braking exploits the motor’s own generator action. The sequence is:
- Supply disconnected; the rotor keeps spinning from inertia.
- Back-EMF appears: the rotating armature in the field generates Eb = kえ・おお (Faraday induction).
- Terminals shorted: an H-bridge turns both low-side switches on, or a contactor ties the armature leads.
- Braking current flows: with supply voltage = 0, 私ブレーキ =Eb / Rある circulates through the windings.
- Opposing torque is produced: Tブレーキ = kT・私ブレーキ acts against the direction of rotation.
- Energy becomes heat: the stored kinetic energy ½Jω² is dissipated in Rある.
- Decay to zero: as speed falls, えb falls, 私ブレーキ falls, torque falls — and it is exactly zero when the shaft stops.
Why It Must Be Power Off
If the supply is still connected and you short the terminals, you place the full supply voltage across Rある: I = V供給 / Rある — the same magnitude as locked-rotor inrush (6–10× rated), but now sustained. Contacts weld and windings burn. Braking current is self-limiting because it is driven by Eb, which collapses with speed; fault current is limited only by V供給 and Rある. That single fact is the whole safety difference.
Short-Circuit Braking vs Other Braking Methods
| Method | Circuit State | Braking Torque | Energy Fate | 一般的な使用方法 |
|---|---|---|---|---|
| Coast (開ける) | Terminals open | ~0 (friction only) | None recovered | ファン, low-precision stops |
| Dynamic / short-circuit | Terminals shorted | High at speed, decays to 0 | Dissipated as heat in motor | E-stop, コンベア, locks |
| Regenerative | Back to supply via converter | Medium–high | Recovered 60–80% | EV, 無人搬送車, mobile robots |
| Plugging (reverse V) | Reverse supply applied | 非常に高い | Wasted as heat | Fastest stop (high stress) |
エンジニアリングデータ: Braking Formulas & 作業例
Short-circuit braking is governed by the same relations as motoring, with supply voltage set to zero. These let you size the event and prove it stays within limits.
| 量 | 式 | Diagnostic / Design Use |
|---|---|---|
| 逆起電力 | えb = kえ・おお | Source of braking current |
| Braking current | 私ブレーキ =Eb / Rある (V供給 = 0) | Peak current at the moment of shorting |
| Braking torque | Tブレーキ = kT・私ブレーキ (kT=kえ はい) | Stopping force, decays with speed |
| Instantaneous power | Pブレーキ = 私ブレーキ²・Rある =Eb²/Rある | Heat dumped into windings |
| Kinetic energy to dissipate | えkin = ½·J·ω² | Total energy that becomes heat |
作業例: 24 V PMDC Braking Event
Take a 24 V PMDC with Rある ≈ 0.5 Ω and kえ such that back-EMF is ≈22 V at 3,000 回転数 (314 ラド/秒) under rated load. Suppose it is disconnected and short-braked from 2,000 回転数 (209 ラド/秒):
- Back-EMF at that speed: えb ≈ 22 × (209 / 314) ≈ 14.7 V.
- Peak braking current: 私ブレーキ = 14.7 / 0.5 = 29.4 あ — about 7.8× the 3.75 A rated current.
- Instantaneous braking power: Pブレーキ = 29.4² × 0.5 ≈ 432 W, versus only ~7 W copper loss in normal running.
- Stored kinetic energy (rotor J ≈ 5×10⁻⁴ kg·m²): えkin = ½·5×10⁻⁴·209² ≈ 11 J.
The peak current is brief (it collapses as the shaft slows), but 432 W into a 7 W-rated winding is why the event must be short and current-limited — either by duration, by adding a braking resistor, or by PWM. Continuous or repeated shorting without limits exceeds the IEC 60034-1 熱クラス.
Energy Recovery Efficiency by Braking Type
| Braking Type | Energy Recovered | 注意事項 |
|---|---|---|
| Coast | 0% | All energy lost to friction |
| Dynamic short-circuit | 0% (100% → heat) | Simplest; heats the motor |
| Regenerative | 60–80% | Needs DC-DC converter / smart driver |
| Plugging | Negative (extra from supply) | Fastest, most wasteful |
絶縁温度制限 (IEC 60034-1)
| 絶縁クラス | 最大巻線温度 | 使用 |
|---|---|---|
| クラスB | 130 ℃ | 一般産業用 |
| クラスF | 155 ℃ | 要求の厳しい / 同封された |
| クラスH | 180 ℃ | ひどい / トラクション |
Best Applications for Short-Circuit Braking
| 応用 | Why Short Braking Fits | ウォッチアイテム |
|---|---|---|
| クレーン, ホイスト, エレベーター | Prevents load drift on power loss | Add mechanical backup; limit current |
| 無人搬送車 / mobile robots | 正確な, fast positioning stop | H-bridge low-side short is built in |
| 電動工具 | Quick spindle stop | Limit repeat cycles (熱) |
| Smart locks / バルブ | Deterministic stop on de-energize | Confirm torque at low speed |
| コンベヤー | Emergency stop without regen hardware | Audit duty cycle |
Which Braking Method to Choose
| If You Need… | Recommended Method |
|---|---|
| Fastest emergency stop | Dynamic short-circuit (or plugging if stress allowed) |
| Energy savings on frequent start/stop | Regenerative |
| Simplest, lowest-cost circuit | Dynamic short-circuit via H-bridge |
| Lowest mechanical stress | Coast |
セレクションガイド: How to Design a Controlled Short-Circuit Brake
For procurement and design teams specifying a DC motor with braking, follow six steps so the stop is fast but never destructive:
- Guarantee power-off plus short: the driver must disconnect the supply and short the armature — an H-bridge with both low-side switches on, or a braking contactor.
- Calculate peak braking current: 私ブレーキ =Eb,最大 / Rある; confirm it is within the switch and winding ratings (expect several× rated).
- Add a braking resistor if needed: if Iブレーキ exceeds limits, insert Rext so I = Eb / (Rある + Rext); size the resistor for the transient energy, not just wattage.
- Check the thermal class: single and repeated braking energy must stay under the IEC 60034-1 insulation limit (Class B/F/H); derate for high duty cycles.
- Rate the driver: MOSFET / contactors must handle the peak and repetitive current and the back-EMF voltage.
- Add protection: 過電流, 温度, and — for hoists or elevators — a mechanical backup, since electrical braking can fail.
よくあるエンジニアリングの間違い
| 間違い | なぜ痛いのか |
|---|---|
| Shorting a powered motor | Welded contacts, 焼けた巻線 (sustained inrush) |
| No current limiting | Exceeds switch and winding rating |
| Ignoring the thermal class | Insulation degradation on repeated braking |
| Confusing fault short with braking | Misreads a failure as “normal braking” |
| Using plugging unknowingly | Highest stress; supply energy dumped as heat |
| No mechanical backup on hoists | Load drop if the electrical brake fails |
DC Motor Short-Fault Troubleshooting Table (問題→原因→解決策)
| 問題 | 考えられる原因 | 解決 |
|---|---|---|
| Winding runs hot, low resistance | Inter-turn short | Surge / メガボディ; 巻き戻し |
| Breaker trips, メガ <1 MΩ | Ground fault (winding to frame) | IEEE 43 test; dry out or rewind |
| スパーキング, uneven bars | Commutator segment short (copper/carbon) | クリーン, アンダーカットマイカ, resurface |
| Overheats after braking | Repeated shorting exceeds thermal class | Add resistor / derate duty cycle |
| Excessive braking current | No limit, low Rある | Add Rext, PWM limit |
| Motor will not release (locked) | メカニカルバインド, not electrical | Check load and bearings |
| Weak braking near standstill | Back-EMF falls to zero at stop | Inherent; add mechanical brake |
| Unexpected jerk / ストップ | Wrong braking method selected | Re-evaluate the method table |
よくある質問
Is it safe to short a DC motor?
Only as a controlled, power-off procedure — short-circuit (dynamic) 制動. Never short the terminals of a motor that is still connected to its supply; that sustains locked-rotor inrush current and can weld contacts or burn windings.
What current flows when you short a DC motor?
With the supply off, the braking current is I = Eb / Rある, driven by the rotor’s back-EMF. It peaks at the moment of shorting (highest speed) and falls to zero as the shaft stops — unlike a powered short, which is limited only by supply voltage and Rある.
What is the difference between short-circuit braking and a shorted winding?
Short-circuit braking is an intentional, external connection of the armature terminals after power-off, used to stop the motor. A shorted winding is an internal fault (inter-turn, 地面, or commutator) that overheats and arcs and must be diagnosed and repaired.
Can short-circuit braking damage the motor?
はい, if unmanaged. The instantaneous braking power can be tens of times the rated copper loss, so repeated or sustained shorting exceeds the IEC 60034-1 熱クラス. Limit current with a resistor or PWM and respect the insulation temperature limit.
Is dynamic braking energy-efficient?
No — dynamic (short-circuit) braking recovers 0% of the energy; it dissipates all of it as heat in the motor. Regenerative braking recovers 60–80% but needs a converter and smart driver.
How do I limit the braking current?
Insert a braking resistor Rext so I = Eb / (Rある + Rext), use PWM chopping, or rely on the H-bridge’s current-control loop. Size the resistor for the transient braking energy, not just continuous wattage.
Why Choose Greensky for DC Motors with Braking?
When your application needs a controlled stop, Greensky は完全な DC ポートフォリオを提供します — PMDC, 起毛した, そしてブラシレス (BLDC) モーターと統合ギアモーター — with braking options engineered to IEC 60034 およびNEMA MG 1 熱限界:
- Built-in braking paths: H-bridge-ready windings and optional braking resistors so dynamic braking is current-limited by design.
- Thermal headroom: Class F/H insulation for high-duty-cycle braking without derating.
- フランジの互換性: IEC B5/B14 and NEMA C-face — see our モーターフランジガイド.
- 低MOQ OEM/ODM: custom shaft, エンコーダ, and brake-resistor configurations for spares and new designs.
関連書籍
- How to Troubleshoot a DC Motor (Fault Diagnosis)
- DCモーターとは? 種類, 原理 & 数式
- エンジニアが知っておくべき BLDC モーターの欠点
- モーターフランジとは? IEC と NEMA の取り付け
- AC モーターと DC モーター: どれを選ぶか
- ギアボックスとギアモーター: 違い & 選択
- 同期モーターと誘導モーター: 主な違い
- ロボットアームに減速機が必要な理由
参照
- IEC 60034-1 — 回転電機: 評価と性能 (サーマルクラス, braking limits). webstore.iec.ch/publication/67467
- IEC 60034-30-1 — 回転電機の効率クラス. webstore.iec.ch/publication/67784
- MGはありません 1 — モーターと発電機 (安全性, 熱, 取り付け). nema.org/standards/view/mg-1-motors-and-generators
- IEEE 43-2013 — 回転機械の絶縁抵抗試験の推奨方法. standards.ieee.org/ieee/43/4385
- IEEE 112 — 多相誘導電動機の標準試験手順 (損失 & 逆起電力方式). standards.ieee.org/ieee/112/4213
- 私たち. DOE — 電気モーターの効率の決定 & 修理. energy.gov/eere/amo/articles/determination-electric-motors
- ROHM — Brushed DC Motor: Short Braking (principle & 応用). techweb.rohm.com/product/motor/brushed-motor/brushed-motor-basic/206
- maxon — DC motor braking and application notes. maxon.com/en-us/technologies/tech-papers
- SKF — Bearing maintenance under braking inertia loads. skf.com/us/products/maintenance-products/bearing-maintenance
- Siemens — Drive braking functions (DC braking / regenerative). siemens.com/global/en/products/drives.html
