Stall current is the maximum amount of electrical current a motor draws when its shaft is completely prevented from spinning. At zero speed, the motor pulls as much current as the circuit will allow, limited only by the resistance of its internal windings and the supply voltage. This is the highest current the motor will ever draw during normal operation, and understanding it matters for sizing power supplies, protecting circuits, and avoiding motor damage.
Why Current Spikes When a Motor Stops
To understand stall current, you need to know one thing about how electric motors work: a spinning motor generates its own opposing voltage. As the motor’s rotor turns inside a magnetic field, it produces a voltage that pushes back against the supply voltage. This is called back-EMF (electromotive force), and it acts like a natural current limiter. The faster the motor spins, the more back-EMF it produces, and the less current flows through the windings.
A motor spinning freely with no load produces almost as much back-EMF as the supply voltage itself. The difference between those two voltages is small, so very little current flows. That’s why an unloaded motor barely draws any power.
Now imagine that motor hitting a wall. The rotor stops. Back-EMF drops to zero because nothing is spinning. With no opposing voltage, the full supply voltage pushes current through the motor’s windings, which have very low resistance. The result is a massive surge of current: the stall current.
How to Calculate Stall Current
The math is straightforward Ohm’s law. With no back-EMF to oppose the supply voltage, the only thing limiting current is the winding resistance:
Stall current = Supply voltage รท Winding resistance
For example, a small 12V DC motor with 2 ohms of winding resistance would have a stall current of 6 amps. That same motor running freely might draw only 0.2 amps. The difference between those two numbers is entirely due to back-EMF doing its job at speed and disappearing at stall.
This is why stall current can be five, ten, or even fifteen times higher than normal running current. The ratio depends on the motor’s design, but the jump is always dramatic. If your power supply or wiring isn’t rated for stall current, you’ll blow fuses, trip breakers, or damage components the moment the motor gets stuck.
Stall Current vs. Locked Rotor Amps
You’ll sometimes see the term “locked rotor amps” (LRA) used in HVAC, appliance, and industrial motor specs. This is essentially the same concept as stall current, applied to AC motors. When an AC motor first starts up, the rotor is stationary, so there’s no back-EMF yet. The motor briefly draws a surge of current identical to what it would draw if the rotor were physically locked in place.
The key difference is timing. Starting current is a brief spike that lasts only until the motor gets up to speed, typically a fraction of a second to a few seconds. Locked rotor current (true stall current) can persist indefinitely if something mechanically prevents the rotor from turning. That sustained high current is where the real danger lies.
Stall Torque and Its Link to Current
Stall current and stall torque are directly related. Torque in an electric motor is proportional to current: more current flowing through the windings creates a stronger magnetic force trying to turn the rotor. At stall, the motor is drawing maximum current and therefore producing maximum torque. If you know a motor’s torque constant (a value listed on its datasheet), you can multiply it by the stall current to get the stall torque.
This relationship is useful when selecting motors for robotics or mechanical projects. If you need a motor to push through a certain load, comparing stall torque values tells you the upper limit of force each motor can deliver. Just keep in mind that operating a motor continuously at or near stall torque will destroy it.
What Happens If a Motor Stays Stalled
All that current flowing through thin copper windings generates enormous heat. A motor designed to run at 1 amp might be pulling 10 amps at stall, and all of that energy converts directly to heat since no mechanical work is being done. The consequences escalate quickly.
First, the winding insulation breaks down. The thin enamel coating on the copper wire softens and melts, allowing adjacent wires to short-circuit. This creates even more current flow and more heat in a destructive feedback loop. Second, if the temperature climbs high enough, the permanent magnets inside the motor begin to lose their magnetism. This partial demagnetization is irreversible and permanently reduces the motor’s power output, even after it cools down. The higher winding resistance from heat, combined with weakened magnets, means the motor will never perform to its original specifications again.
In AC motors, prolonged locked rotor conditions can damage both the stator and rotor windings. This is why motors in compressors, pumps, and industrial equipment use protection devices like thermal overload relays and circuit breakers that cut power if current stays elevated too long.
Why Stall Current Matters in Practice
If you’re building a robot, selecting a motor driver, or sizing a power supply, stall current is one of the most important specs to check. Your motor driver needs to handle the stall current without burning out, even if the motor only runs at a fraction of that current during normal operation. A jammed wheel, an unexpected obstacle, or a mechanical binding can stall a motor without warning.
For the same reason, fuses and wiring in motor circuits should be rated above stall current if you want the motor to push through temporary overloads, or rated below it if you want the fuse to blow and protect the motor before damage occurs. The choice depends on whether you’d rather lose the fuse or risk the motor.
Most motor datasheets list stall current alongside free-running current and rated current. If it’s not listed, you can measure the winding resistance with a multimeter (with the motor disconnected from power) and divide your supply voltage by that resistance. The result gives you a reliable estimate of stall current without ever needing to actually stall the motor and risk damage.