COGGING torque is defined as the attraction/interaction of the magnetic poles (typically on the rotor) to the steel teeth (typically on the stator lamination) within an un-energized motor.
STALL torque is the torque produced by a mechanical device whose output rotational speed is zero. It may also mean the torque load that causes the output rotational speed of a device to become zero, i.e., to cause stalling.
Basically - cogging means the motor cannot begin rotating, while stalling means the motor cannot maintain rotation.
Cogging is a "bad thing"; if the machine will not rotate, it is of no use for a process. Stalling is also bad (in most instances) because it is when the machine stops working. There are a few specific instances where the intended design is for "continuous stall". In metal rolling, the interstand looper is a tensioning device for the strip. It applies uses a lever arm to produce fixed tension; this is effectively a "stall" in terms of rotation.
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