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 (dynamisk bromsning): 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, ground fault, or commutator segment short — that bypasses normal current paths and overheats or arcs. This is a failure to diagnose; see our DC motor troubleshooting guide.
Two Meanings, Two Outcomes
| Meaning | When It Occurs | Safe? | Resultat |
|---|---|---|---|
| Controlled short-circuit braking | Supply off, terminals tied | Ja, by design | Fast, repeatable stop |
| Accidental internal short | Winding / commutator fault | Nej | Overheat, arc, failure |
| Powered-terminal short | Supply on, terminals shorted | Never | Welded contacts, burnt windings |
How Short-Circuit Braking Works (Step by Step)
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 = kE·ω (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, jagbroms = Eb / Ra circulates through the windings.
- Opposing torque is produced: Tbroms = kT·Ibroms acts against the direction of rotation.
- Energy becomes heat: the stored kinetic energy ½Jω² is dissipated in Ra.
- Decay to zero: as speed falls, Eb falls, jagbroms 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 Ra: I = Vförse / Ra — 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 Vförse and Ra. That single fact is the whole safety difference.
Short-Circuit Braking vs Other Braking Methods
| Method | Circuit State | Braking Torque | Energy Fate | Typisk användning |
|---|---|---|---|---|
| Coast (open) | Terminals open | ~0 (friction only) | None recovered | Fans, low-precision stops |
| Dynamic / short-circuit | Terminals shorted | High at speed, decays to 0 | Dissipated as heat in motor | E-stop, transportörer, locks |
| Regenerative | Back to supply via converter | Medium–high | Recovered 60–80% | EV, AGV, mobile robots |
| Plugging (reverse V) | Reverse supply applied | Very high | Wasted as heat | Fastest stop (high stress) |
Engineering Data: Braking Formulas & Worked Example
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.
| Quantity | Formula | Diagnostic / Design Use |
|---|---|---|
| Back-EMF | Eb = kE·ω | Source of braking current |
| Braking current | jagbroms = Eb / Ra (Vförse = 0) | Peak current at the moment of shorting |
| Braking torque | Tbroms = kT·Ibroms (kT=kE in SI) | Stopping force, decays with speed |
| Instantaneous power | Pbroms = Ibroms²·Ra = Eb²/Ra | Heat dumped into windings |
| Kinetic energy to dissipate | Ekin = ½·J·ω² | Total energy that becomes heat |
Worked Example: 24 V PMDC Braking Event
Take a 24 V PMDC with Ra ≈ 0.5 Ω and kE such that back-EMF is ≈22 V at 3,000 RPM (314 rad/s) under rated load. Suppose it is disconnected and short-braked from 2,000 RPM (209 rad/s):
- Back-EMF at that speed: Eb ≈ 22 × (209 / 314) ≈ 14.7 V.
- Peak braking current: jagbroms = 14.7 / 0.5 = 29.4 A — about 7.8× the 3.75 A rated current.
- Instantaneous braking power: Pbroms = 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): Ekin = 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 thermal class.
Energy Recovery Efficiency by Braking Type
| Braking Type | Energy Recovered | Anteckningar |
|---|---|---|
| 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 |
Insulation Temperature Limits (IEC 60034-1)
| Isoleringsklass | Max Winding Temp | Use |
|---|---|---|
| Klass B | 130 °C | General industrial |
| Klass F | 155 °C | Demanding / enclosed |
| Class H | 180 °C | Harsh / traction |
Best Applications for Short-Circuit Braking
| Ansökan | Why Short Braking Fits | Watch Item |
|---|---|---|
| Kranar, hissar, hissar | Prevents load drift on power loss | Add mechanical backup; limit current |
| AGV / mobile robots | Exakt, fast positioning stop | H-bridge low-side short is built in |
| Power tools | Quick spindle stop | Limit repeat cycles (heat) |
| Smart locks / valves | Deterministic stop on de-energize | Confirm torque at low speed |
| Transportör | 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 |
Urvalsguide: 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: jagbroms = Eb,max / Ra; confirm it is within the switch and winding ratings (expect several× rated).
- Add a braking resistor if needed: if Ibroms exceeds limits, insert Rext so I = Eb / (Ra + 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: MOSFETs / contactors must handle the peak and repetitive current and the back-EMF voltage.
- Add protection: over-current, temperatur, and — for hoists or elevators — a mechanical backup, since electrical braking can fail.
Common Engineering Mistakes
| Mistake | Why It Hurts |
|---|---|
| Shorting a powered motor | Welded contacts, burnt windings (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 (Problem → Cause → Solution)
| Problem | Likely Cause | Lösning |
|---|---|---|
| Winding runs hot, low resistance | Inter-turn short | Surge / megger kropp; rewind |
| Breaker trips, megger <1 MΩ | Ground fault (winding to frame) | IEEE 43 test; dry out or rewind |
| Sparking, uneven bars | Commutator segment short (copper/carbon) | Clean, undercut mica, resurface |
| Overheats after braking | Repeated shorting exceeds thermal class | Add resistor / derate duty cycle |
| Excessive braking current | No limit, low Ra | Add Rext, PWM limit |
| Motor will not release (locked) | Mechanical bind, not electrical | Check load and bearings |
| Weak braking near standstill | Back-EMF falls to zero at stop | Inherent; add mechanical brake |
| Unexpected jerk / sluta | Wrong braking method selected | Re-evaluate the method table |
Vanliga frågor
Is it safe to short a DC motor?
Only as a controlled, power-off procedure — short-circuit (dynamic) bromsning. 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 / Ra, 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 Ra.
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, ground, or commutator) that overheats and arcs and must be diagnosed and repaired.
Can short-circuit braking damage the motor?
Ja, 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 thermal class. 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 / (Ra + 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 supplies a full DC portfolio — PMDC, borstat, and brushless (BLDC) motors plus integrated gear motors — with braking options engineered to IEC 60034 and NEMA MG 1 thermal limits:
- 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.
- Flange compatibility: IEC B5/B14 and NEMA C-face — see our motor flange guide.
- Low-MOQ OEM/ODM: custom shaft, kodare, and brake-resistor configurations for spares and new designs.
Related Reading
- How to Troubleshoot a DC Motor (Fault Diagnosis)
- What Is a DC Motor? Typer, Principle & Formler
- BLDC Motor Disadvantages Engineers Should Know
- What Is a Motor Flange? IEC vs NEMA Mounting
- AC vs DC Motor: Vilket man ska välja
- Gearbox vs Gear Motor: Skillnader & Selection
- Synchronous vs Induction Motor: Viktiga skillnader
- Why Robotic Arms Need Speed Reducers
Referenser
- IEC 60034-1 — Rotating Electrical Machines: Rating and Performance (thermal classes, braking limits). webstore.iec.ch/publication/67467
- IEC 60034-30-1 — Efficiency Classes for Rotating Electrical Machines. webstore.iec.ch/publication/67784
- NEMA MG 1 — Motors and Generators (säkerhet, thermal, mounting). nema.org/standards/view/mg-1-motors-and-generators
- IEEE 43-2013 — Recommended Practice for Insulation Resistance Testing of Rotating Machinery. standards.ieee.org/ieee/43/4385
- IEEE 112 — Standard Test Procedure for Polyphase Induction Motors (loss & back-EMF methods). standards.ieee.org/ieee/112/4213
- U.S. DOE — Electric Motor Efficiency Determination & Reparera. energy.gov/eere/amo/articles/determination-electric-motors
- ROHM — Brushed DC Motor: Short Braking (principle & ansökan). 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
