Stall Current (Locked-Rotor Current)
The current a motor draws while the rotor is stationary. Because the motor is not turning, no back-EMF is produced and only the winding resistance limits the current; the value reaches typically 5–8 times the rated current. In driver selection this peak value is as decisive as the average current.
Roughly, the stall current is the supply voltage divided by the armature resistance. At a 24 V supply, a winding resistance of 0.4 ohm means about 60 A. This current also appears briefly at start-up and, depending on the load, can last 0.5–3 seconds.
In practice it has three consequences:
- Driver selection: The peak current window of the module must cover this pulse. The values 45 A (5 s) for the KS250 and 40 A (15 s) for the PT500 are stated as such in the product data.
- Fuse selection: A fast-blow fuse blows needlessly on this pulse; a slow-blow characteristic is preferred.
- Ramp: A soft start lowers the pulse to 2–4 times the rated value and relieves both the driver and the battery.
To measure the stall current you can lock the motor mechanically and take a brief current reading; the measurement should not last more than a few seconds. For a correct current measurement the clamp meter must be of the Hall type.
Where this term is used: Locked rotor and thermal protection