Stall Torque Calculator shows how motor voltage, winding resistance, torque constants, and the gear ratios determine stall current, output torque, and heat at zero rotor speed.
Calculate Motor Stall Torque, Current, and Overheating Risk with the Stall Torque Calculator
This Stall Torque Calculator is built for EV conversion builders, e-bike and e-scooter tuners, and anyone selecting a motor and gear ratio for a drivetrain. It shows what a motor and its supply voltage will do the instant the rotor is locked, which is the highest-stress moment an electric drivetrain sees.
How the Stall Torque Calculator Reads Your Motor Specs
Enter your supply voltage, the motor’s terminal phase resistance in ohms, its torque constant Kt (in lb-ft/A or Nm/A depending on the unit system you pick), and the total drivetrain gear ratio. The calculator assumes a locked rotor, so there’s no back-EMF working against the applied voltage.
The Formula the Stall Torque Calculator Runs On
Stall current follows straight from Ohm’s law: since a locked rotor generates no back-EMF, the full supply voltage drives current through the winding resistance alone. That’s $I_{stall} = V / R$.
Motor stall torque then comes from the motor’s torque constant: $T = K_t \times I_{stall}$. This is the standard torque-current relationship used across DC brush, BLDC, and PMSM motor datasheets, where Kt is the slope of a motor’s torque-versus-current curve.
One mistake happens right here: some multimeters read line-to-line resistance across two phase leads, which is twice the true per-phase value on a wye-wound motor. Halving that reading before you enter it avoids under-stating your stall current.
Output torque after the drivetrain is $T_{out} = T \times gear$. The calculator treats this as a pure mechanical multiplier and doesn’t subtract gear-mesh losses, so real output torque will run a little lower than the number shown.
Electrical power at stall is $P = V \times I_{stall}$. Because a locked rotor does no mechanical work, all of that power turns into heat in the windings. This is worth stating plainly: heating comes from current, not voltage. Raising supply voltage while current stays the same barely changes how hot the motor gets, since resistive heating scales with current squared, not voltage.
The heating-rate figures assume a normalized 1 kg mass of copper winding, since the calculator has no way to know your motor’s actual winding mass. Treat it as a way to compare stall severity across setups, not a literal prediction for your specific motor.
The same logic applies to the battery-side numbers. C-rate and runtime are calculated against a fixed 100Ah reference pack, not your actual battery, so scale them to your own pack size before relying on them.
The “time to 200°C” figure is a reference point, not a universal failure line. It represents Class N insulation, one tier among several defined in NEMA MG-1 and IEC 60085. Most production motors use Class F (155°C) or Class H (180°C) insulation, so a real motor could reach its actual limit sooner than this calculator’s 200°C reference suggests. Running any motor at stall for more than a few seconds risks winding damage regardless of which insulation class it carries.
Valid Input Range for the Stall Torque Calculator
Terminal resistance must be greater than zero. At exactly zero, the stall current formula divides by zero and returns a nonsensical infinite result, which is why the calculator enforces a small positive minimum.
Gear ratio has a floor of 1:1. Values below that would represent an overdrive gearset rather than a reduction, which this calculator isn’t built to model.
Motor Winding Insulation Temperature Limits
| Insulation Class | Maximum Winding Temperature |
| Class A | 105°C |
| Class B | 130°C |
| Class F | 155°C |
| Class H | 180°C |
| Class N | 200°C |
| Class R | 220°C |
Values per NEMA MG-1 and IEC 60085 insulation class ratings. Check your specific motor’s nameplate or datasheet for its actual class.
Common Stall Torque Calculator Input Mistakes
Entering line-to-line resistance instead of per-phase resistance, which doubles the true value and understates stall current.
Pasting a Kt value from a datasheet in the wrong unit system, since lb-ft/A and Nm/A aren’t interchangeable numbers even though they describe the same physical constant.
Assuming a controller’s rated battery current is the same as stall phase current. Many simple controllers only regulate battery-side current, so actual phase current at stall can run far higher than the battery rating suggests.
Stall Torque and Motor Overheating Questions
What does “stall” actually mean for an electric motor?
Stall means the rotor is fully locked while voltage is still applied, so rotational speed is zero. Since there’s no back-EMF at zero speed, the motor draws its highest possible current at that moment.
Does higher voltage or higher current cause a motor to overheat?
Current. Resistive heating in the windings scales with current squared, not voltage. A motor run at higher voltage but the same current runs only slightly hotter, from small increases in friction and iron losses.
What happens if I run a motor at a higher voltage than it’s rated for?
Current draw and heat both climb, and components in the controller (FETs, capacitors) may exceed their own voltage ratings. Winding insulation can also break down if voltage stress exceeds what it was designed to hold.
How do you find a motor’s torque constant if the datasheet doesn’t list it?
Divide measured stall torque by measured stall current: $K_t = T / I_{stall}$. This is the same relationship the Stall Torque Calculator uses in reverse to solve for torque.