DC Motor Stall Current Calculator
Estimate brushed DC motor stall current, total series resistance, voltage drops, I²R loss, current-limit behavior, stall torque, and cold/hot winding resistance. The calculator is intended for locked-rotor stress checks and motor-driver margin review.
MOT-003 does not simulate motor inductance, PWM ripple, battery electrochemistry, thermal rise, driver switching loss, or motor acceleration. It provides steady-state stall references that must be checked against real motor and driver datasheets.
Engineering tool
DC Motor Stall Current Calculator
Estimate brushed DC motor locked-rotor current, series voltage drops, I²R loss, current-limit behavior, stall torque, and hot winding resistance.
Calculation mode
Parameter panel
Result console
- Ideal Stall Current
- 12A
- Winding Power Dissipation
- 144W
- Winding Power Dissipation
- 0.144kW
- Stall Mechanical Power
- 0W
- Winding Voltage
- 12V
This is a winding-limited reference using only winding resistance, not a guaranteed real system stall current. High stall resistive loss requires careful safe-duration, thermal, and protection review.
DC Motor Stall Current Formula Audit
| Adopted Motor Type | Brushed DC motor locked-rotor / stall reference model. |
|---|---|
| Stall Definition | Shaft speed is zero. |
| Back EMF at Stall | At ω=0, ideal back EMF approaches zero. |
| Basic Stall Current Formula | Istall = V / R. |
| Total Series Resistance Definition | Rtotal = Rwinding + Rbattery + Rdriver + Rwire + Rother. |
| Resistance Components | Every resistance input is treated as a series resistance in ohms. |
| Voltage Drop Formula | Vi = I x Ri. |
| Winding Copper Loss Formula | Pwinding = I²Rwinding. |
| Total Resistive Loss Formula | Ptotal = I²Rtotal. |
| Current Limit Model | Ieffective = min(Iresistance, Ilimit). |
| Effective Current Definition | Ideal reference current after applying a controller-current limit. |
| Kt Stall Torque Formula | τstall ≈ Kt x Ieffective. |
| Mechanical Stall Power Definition | Pmech = τω = 0 because ω=0 at stall. |
| Cold / Hot Resistance Model | R2 = R1[1 + α(T2 - T1)]. |
| Copper Temperature Coefficient Convention | Default α=0.00393/°C is a common room-temperature copper reference, not a universal exact constant. |
| Inductance / Transient Boundary | Initial current rise depends on L di/dt and is not equal to instant V/R. |
| Battery Resistance Boundary | Battery internal resistance is simplified and varies with chemistry, SOC, temperature and pulse duration. |
| Driver Resistance Boundary | Driver resistance should be total effective conduction-path resistance, not an assumed topology. |
| Double-Counting Boundary | Do not add resistance already included in measured motor terminal resistance. |
| Safe Stall Duration Boundary | Safe stall time must come from motor and driver datasheets. |
| MOT-002 Scope Boundary | Back-EMF analysis remains separate. |
| MOT-006 Scope Boundary | Full motor copper-loss analysis remains separate. |
Formula
Formula reference
DC motor stall-current formulas
The model is a steady-state locked-rotor reference and does not describe the initial inductive current transient.
ω = 0, so E ≈ 0Rtotal = Rwinding + Rbattery + Rdriver + Rwire + RotherIstall = V / RtotalVi = I × RiPloss,i = I²RiIeffective = min(Iresistance, Ilimit)τstall ≈ Kt × IeffectivePmech,stall = τ × 0 = 0Rhot = Rcold[1 + α(Thot - Tcold)]Variable definitions
- V
- applied motor voltage
- E
- back EMF, approximately zero at stall
- Rtotal
- total series resistance
- Istall
- resistance-limited stall current
- Ieffective
- ideal current-limit reference
- Kt
- torque constant in N·m/A
- α
- resistance temperature coefficient
DC Motor Stall Current Formula Audit
| Adopted Motor Type | Brushed DC motor locked-rotor / stall reference model. |
|---|---|
| Stall Definition | Shaft speed is zero. |
| Back EMF at Stall | At ω=0, ideal back EMF approaches zero. |
| Basic Stall Current Formula | Istall = V / R. |
| Total Series Resistance Definition | Rtotal = Rwinding + Rbattery + Rdriver + Rwire + Rother. |
| Resistance Components | All resistance entries are series resistances in ohms. |
| Voltage Drop Formula | Vi = I x Ri. |
| Winding Copper Loss Formula | Pwinding = I²Rwinding. |
| Total Resistive Loss Formula | Ptotal = I²Rtotal. |
| Current Limit Model | Ieffective = min(Iresistance, Ilimit). |
| Kt Stall Torque Formula | τstall ≈ Kt x Ieffective. |
| Mechanical Stall Power Definition | Pmech = τω = 0 because ω=0 at stall. |
| Cold / Hot Resistance Model | R2 = R1[1 + α(T2 - T1)]. |
| Copper Temperature Coefficient Convention | Default α=0.00393/°C is a common room-temperature copper reference. |
| Inductance / Transient Boundary | Initial current rise depends on L di/dt and is not instant V/R. |
| Battery Resistance Boundary | Battery resistance is a simplified first-order term, not a chemistry model. |
| Driver Resistance Boundary | Driver resistance should be total effective conduction-path resistance. |
| Double-Counting Boundary | Do not add resistance already included in measured motor terminal resistance. |
| Safe Stall Duration Boundary | Safe stall duration must come from actual motor and driver specifications. |
| MOT-002 Scope Boundary | Back-EMF analysis remains separate. |
| MOT-006 Scope Boundary | Full motor copper-loss analysis remains separate. |
Worked Examples
| Example | Calculation | Result |
|---|---|---|
| 12 V, 1 Ω winding | Istall = 12 / 1 | Istall = 12 A |
| Winding loss for 12 A, 1 Ω | P = I²R = 12² × 1 | 144 W |
| Stall mechanical power | Pmech = τ × 0 | 0 W |
| 12 V with 1 Ω + 0.1 Ω + 0.05 Ω + 0.05 Ω | Rtotal = 1.2 Ω | I ≈ 10 A |
| Same voltage budget | Vwinding=10 V, battery=1 V, driver=0.5 V, wire=0.5 V | Sum = 12 V |
| Same loss budget | I²R per element | 100 W + 10 W + 5 W + 5 W = 120 W |
| 12 A ideal, 8 A current limit | Ieffective = min(12, 8) | 8 A |
| 12 A ideal, 20 A current limit | Ieffective = min(12, 20) | 12 A |
| Kt = 0.1 N·m/A, 12 A | τ = 0.1 × 12 | 1.2 N·m |
| Kt = 0.1 N·m/A, current-limited to 8 A | τ = 0.1 × 8 | 0.8 N·m |
| 24 V, 2 Ω | Istall = 24 / 2 | 12 A |
| 0 V, positive resistance | Istall = 0 / R | 0 A |
| R = 0 | V/R is undefined | Rejected |
| Negative resistance | Invalid physical magnitude | Rejected |
| 1 Ω at 20°C to 100°C | Rhot = 1[1 + 0.00393×80] | ≈1.3144 Ω |
| 12 V hot winding current | Ihot = 12 / 1.3144 | ≈9.1296 A |
| Cold vs hot | Rhot > Rcold | Hot stall current is lower |
| Voltage budget round-trip | Sum I×Ri | Returns source voltage |
| Current/drop round-trip | Drop / resistance | Returns current |
| 1 mΩ | Unit conversion | 0.001 Ω |
| 1000 mA | Unit conversion | 1 A |
| Limit equal boundary | Ilimit = Iresistance | Current limiting not active |
Engineering Notes
DC Motor Stall
At stall, shaft speed is zero and ideal back EMF approaches zero.
Locked Rotor
Locked rotor is a severe electrical and thermal stress condition, not a normal operating state.
Stall Current
The simplest reference is V/R, but real systems include more resistance and current limiting.
Back EMF
Back EMF is handled in MOT-002; MOT-003 assumes stall where E≈0.
Winding Resistance
Measured motor terminal resistance may already include brush/contact effects.
Battery Resistance
Battery internal resistance can reduce available current and create voltage sag.
Driver Resistance
For an H-bridge, enter the total effective conduction path resistance.
Wire Resistance
Wire and connector resistance matter strongly in low-voltage high-current motors.
Current Limit
Driver current limiting is implementation-dependent and may be peak, average, foldback or thermal.
Stall Torque
Torque can be estimated from Kt and current, but saturation and heating can reduce accuracy.
Copper Loss
I²R loss heats windings and series elements during stall.
Cold Resistance
Cold copper resistance is lower, so initial stall-like current can be higher.
Hot Resistance
Hot winding resistance increases and usually reduces current.
Motor Inductance
Inductance controls startup current rise and is outside this steady-state calculator.
Common Mistakes
- Adding nonzero back EMF at stall.
- Calling V/Rwinding a guaranteed real stall current.
- Ignoring battery internal resistance.
- Ignoring driver on-resistance.
- Ignoring wire and connector resistance.
- Double-counting series resistance already included in measured winding resistance.
- Writing stall mechanical power as VI.
- Assuming zero mechanical power means no heating.
- Writing I²R loss as IR.
- Forcing the current limit even when it is above the resistance-limited current.
- Using Kt with the wrong current units.
- Using cold winding resistance as if it were hot steady-state resistance.
- Ignoring inductive startup transient behavior.
- Assuming a driver current limit is perfectly constant.
- Assuming a motor can remain stalled indefinitely.
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Open CalculatorFAQ
Support reference
FAQ
How do I calculate DC motor stall current?
For a simplified steady-state locked-rotor estimate, use Istall = V / Rtotal, where Rtotal includes winding and other explicit series resistances.
Why is back EMF zero at stall?
Back EMF is proportional to speed. At stall the shaft speed is zero, so ideal back EMF approaches zero.
Why is real stall current lower than V divided by winding resistance?
Battery internal resistance, driver resistance, wire resistance, connector resistance, brush/contact resistance, current limiting, and supply sag can reduce real stall current.
How does battery internal resistance affect stall current?
Battery internal resistance adds to total series resistance and creates voltage sag under high current, reducing the current available at the motor.
How does motor-driver resistance affect stall current?
Driver on-resistance adds series resistance and dissipates I²R heat in the switching devices or H-bridge path.
How do wire and connector resistance affect motor stall current?
Wire and connector resistance drop voltage at high current and convert power into heat, especially with low-voltage motors.
How do I calculate winding copper loss at stall?
Use Pwinding = I²Rwinding, where I is the stall-current reference and Rwinding is motor winding resistance.
Why is mechanical power zero at stall?
Mechanical shaft power is torque times angular speed. At stall the speed is zero, so mechanical output power is zero.
Can a motor still overheat when mechanical power is zero?
Yes. Electrical input power and I²R winding loss can be very high at stall even though shaft output power is zero.
How do I estimate stall torque from current?
If Kt is known, use τstall ≈ Kt × I. This is a first-order reference and can deviate with saturation and temperature.
How does a driver current limit affect stall current?
The ideal current reference becomes min(resistance-limited current, controller current limit), but real behavior depends on the current-control implementation.
Why does motor winding resistance increase when hot?
Copper resistance rises with temperature, commonly estimated near room temperature with R2 = R1[1 + α(T2 - T1)].
Is cold stall current higher than hot stall current?
Usually yes. Cold winding resistance is lower, so the initial stall-like current can be higher than the hot winding current.
How does motor inductance affect startup current?
Inductance prevents current from jumping instantly to V/R. The V/R value is a steady-state locked-rotor reference, not the initial transient current.
Is V/R the instantaneous startup current?
No. At the instant voltage is applied, winding inductance and driver behavior affect current rise.
How long can a motor remain stalled?
Safe stall duration depends on motor winding thermal limits, driver limits, protection, airflow, and datasheet ratings.
What is the difference between stall current and rated current?
Rated current is a normal operating or continuous current. Stall current is a locked-rotor stress condition and is often much higher.
Engineering Disclaimer
This calculator gives simplified locked-rotor estimates. Real stall current, safe duration, protection settings, and thermal margin depend on the motor, battery, wiring, driver, controller, duty cycle, cooling, and manufacturer ratings.