DC Motor Stall Current

DC motor stall current

The most dangerous situation for a DC motor is having the shaft locked (stalled) while it stays energised. Why? Because the current it draws at that moment is the highest possible, given by a plain application of Ohm's law: I = V ÷ R, where V is the applied voltage and R the armature resistance (the resistance of the windings, usually low, from a fraction of an ohm to a few ohms). The reason is the back electromotive force: when the motor spins it generates a back-EMF that opposes the applied voltage and limits the current; but with the shaft at rest (zero speed) there is no back-EMF at all, and nothing holds the current back except the small resistance of the copper — which yields an enormous current, often 5 to 10 times (or more) the normal running current. This stall current brings serious consequences: the heat generated (proportional to I²·R) shoots up and can burn out the windings in seconds, destroy the driver or H-bridge (whose transistors cannot withstand the current), and overload the power supply. That is why motor systems include stall protection: stall detection (through high current or absence of movement), current limiting in the driver, and fuses and breakers sized for that current. The stall current is likewise the point of maximum torque (stall torque = K_t·I_stall) on the torque-speed curve — the motor delivers its greatest torque precisely when it is locked, which helps in applications that demand high starting torque but turns harmful if sustained. Interestingly, there are scenarios in which stalling is momentary and tolerable: starting any motor briefly draws a current close to the stall value, before it accelerates and builds back-EMF. A prolonged stall, however, is destructive. Working out the stall current is essential to size fuses, drivers, supplies and the thermal protection of any DC motor drive. Enter the applied voltage and the armature resistance.