Stall Torque Calculator finds motor stall torque from voltage, resistance, and Kt using I=V/R and torque=I×Kt, then shows geared output torque, C-rate, battery drain, and heat risk.
Stall Torque Calculator: The Core Relationship
A stall torque calculator derives the twisting force an electric motor produces when its rotor is locked stationary. This value represents the theoretical maximum torque the motor can generate under any condition, limited only by supply voltage and internal winding resistance. At zero RPM, back‑EMF is absent, and Ohm’s law dictates the current that will flow through the armature — a current far higher than any normal operating point.
Understanding stall torque is central to electric vehicle drivetrain design. It determines the absolute stress on motor shafts, gear teeth, and axle components during a locked‑rotor event. Engineers use this figure to size contactors, fuses, and battery cables, and to calibrate motor controller current limits that prevent the motor from ever reaching that destructive state.
What Stall Torque Actually Means
In a permanent‑magnet DC or brushless motor, torque output is proportional to phase current. The proportionality constant is the torque constant, Kt, typically given in Newton‑metres per amp or pound‑feet per amp. At any shaft speed, the effective voltage driving current through the windings equals the supply voltage minus the back‑EMF generated by the spinning rotor.
When the rotor is held at standstill, back‑EMF drops to zero. The full supply voltage now pushes current through the motor’s internal resistance. That current reaches a peak dictated solely by Ohm’s law, and the resulting torque is the stall torque.
This locked‑rotor condition is not a normal operating mode. It occurs briefly during startup before the rotor begins moving, or during a mechanical bind that prevents rotation. Even a few seconds at stall can overheat windings past their insulation rating, making this a critical safety boundary, not a performance target.
The Stall Torque Equation
The calculation moves through two simple physical relationships. First, locked‑rotor current follows Ohm’s law for a resistive circuit. Then, motor torque scales linearly with that current through the torque constant.
Stall Current = Supply Voltage / Terminal Phase Resistance
Motor Stall Torque = Stall Current × Torque Constant (Kt)
Where:
Supply Voltage is the DC bus voltage applied to the motor controller and motor, in volts.
Terminal Phase Resistance is the phase‑to‑phase winding resistance measured at the motor terminals, in ohms.
Torque Constant (Kt) is the manufacturer‑specified ratio of torque produced per amp of phase current, in Nm/A or lb‑ft/A.
Worked Imperial Example
A 48‑volt system powers a brushless motor with 0.12 ohms terminal resistance and a Kt of 0.037 lb‑ft/A.
Stall current: 48 V ÷ 0.12 Ω = 400 A
Motor stall torque: 400 A × 0.037 lb‑ft/A = 14.80 lb‑ft
If the motor drives a 10.5:1 gear reduction, the torque at the output shaft becomes 14.80 lb‑ft × 10.5 = 155.40 lb‑ft. This is the static twist the drivetrain must withstand before any motion occurs.
Metric Variant
When Kt is given in Nm/A, the formula remains identical but yields Newton‑metres. For the same 48 V, 0.12 Ω motor, a typical metric Kt is 0.050 Nm/A. The stall current is still 400 A, giving motor stall torque of 20.00 Nm. With the same 10.5:1 gearing, output torque reaches 210.00 Nm.
Both unit systems produce equivalent physical values; the choice depends on the manufacturer’s data sheet conventions. The conversion between lb‑ft and Nm is 1 lb‑ft ≈ 1.3558 Nm.
From Stall Torque to Drivetrain Load
Gear reduction multiplies torque while reducing shaft speed. At stall, no speed exists, so the torque multiplication through the gearbox is fully realized as static torsion on every downstream component.
A 10:1 ratio turns a modest motor stall torque into an axle‑breaking load, which is why driveline engineers must design shafts, universal joints, and differentials to survive this theoretical peak, even if the controller will normally clamp current before reaching it.
After the gearbox, the torque at the wheel hubs or output flanges is the figure used to evaluate half‑shaft strength, bolt shear margins, and gear tooth bending stress. This output torque is also expressed in alternative units like inch‑pounds or kilogram‑centimetres when working with legacy component specifications or international suppliers.
The Thermal Reality of Stall
With the rotor locked, electrical input power converts entirely into heat inside the windings. No mechanical work is done, so the heat load equals the product of supply voltage and stall current. For the 48 V, 400 A example, that is 19.2 kW of pure resistive heating dumped into a few kilograms of copper and steel.
Copper windings have a specific heat capacity of roughly 385 J/(kg·°C). At 19,200 W, a 1 kg winding mass heats at nearly 50 °C per second under adiabatic conditions — meaning no cooling and no heat transfer to the stator core. Starting from a 25 °C ambient, winding temperature reaches the 200 °C insulation failure threshold in about 3.5 seconds.
Real motors have some thermal mass beyond just the copper, but the order of magnitude remains stark: stall conditions destroy windings in seconds unless the controller intervenes. This is why every EV motor controller implements instantaneous over‑current protection and why stall torque is never a sustained operating point.
Battery Demands During a Locked Rotor
A stall event draws the highest current the battery will ever see. That current translates directly into a C‑rate requirement for the battery pack. A 100 Ah pack delivering 400 A operates at 4C — four times its one‑hour capacity rating per hour. Battery cells have maximum continuous and pulse discharge ratings; stall current can easily exceed both if left unchecked.
At 400 A, the battery depletes at 6.67 Ah per minute, or 0.32 kWh per minute in a 48 V system. An ideal 100 Ah pack would drain in 15 minutes, but in practice, voltage sag under such a heavy load triggers the battery management system’s undervoltage cutoff long before that. Usable capacity shrinks dramatically at high discharge rates, and cell heating compounds the problem.
These figures inform contactor ratings, fuse selection, and wiring gauge. Every connection in the high‑current path must survive the stall current for at least the time it takes the controller to detect and interrupt the fault.
Practical Limits and Controller Intervention
Production EV motor controllers never allow current to reach the theoretical stall value. Current sensors on each phase feed a fast control loop that clamps phase current to a predetermined limit, often well below the stall current. This limit balances acceleration performance with thermal safety and driveline durability.
Even during a true mechanical bind, the controller senses the absence of back‑EMF and the rapidly rising current. Within milliseconds, it either folds back the current or disables the inverter entirely. The stall torque therefore acts as an upper bound for sizing, not an operating regime.
Some applications, such as electric power steering or brake actuators, intentionally operate near stall when holding a position against load. In those cases, the motor is designed with sufficient thermal mass, potting, and heat sinking to survive the continuous locked‑rotor current without exceeding insulation temperature limits. These motors are specified differently, with a continuous stall torque rating that accounts for cooling.
Why Stall Torque Matters in System Design
Every component in the torque path — from the motor shaft keyway to the tire contact patch — experiences its highest static load during a locked‑rotor event. Engineers apply a safety factor to the calculated stall torque at each stage. Motor mounts, gearbox housings, and chassis pickup points all see reaction forces proportional to this peak, even if it is rarely or never reached in service.
Stall torque also sets the baseline for vehicle launch capability. A vehicle’s initial acceleration from rest depends on the motor’s low‑speed torque, which is fundamentally capped by the controller current limit — itself a fraction of stall current. Knowing the true stall torque helps calibrate that limit and predict driveline shock under aggressive throttle application.
In motorsport and off‑road EV applications, where drivers may intentionally load the drivetrain against the brakes to pre‑tension the suspension, understanding the stall threshold prevents gearbox damage from torque spikes that exceed the traction limit.
Stall Torque vs. Rated Torque
Rated torque is the continuous output the motor can sustain without exceeding its thermal class at a specified speed and cooling condition. Stall torque is typically three to ten times higher than rated torque, depending on winding resistance and voltage. The large gap between these numbers underscores why locked‑rotor protection is mandatory in any EV system.
Motors with very low internal resistance produce proportionally higher stall currents. A high‑performance motor designed for 800 V and 0.01 Ω phase resistance would theoretically draw 80,000 A at stall, generating torque in the thousands of Newton‑metres. In reality, inductance, controller switching speed, and current sensor limits prevent this, but the calculation still defines the worst‑case scenario for component ratings.
Extending the Idea to Other Motor Types
AC induction motors and synchronous reluctance machines have more complex stall behaviors because the torque‑producing current component depends on the controller’s ability to establish a rotating magnetic field at zero speed. Sensorless field‑oriented control can produce stall torque comparable to a PM motor if a starting algorithm injects a known current vector.
Induction motors, however, suffer from reduced torque at stall due to slip‑dependent rotor flux, making their locked‑rotor current‑to‑torque ratio less favorable than that of permanent‑magnet machines.
Regardless of motor type, the fundamental principle holds: stall torque is the product of available voltage, winding resistance, and the machine’s torque‑per‑amp characteristic. A stall torque calculator that captures these variables gives the engineer a rapid sanity check long before finite‑element simulation or dynamometer testing begins.