Torque is the force applied to a lever, multiplied by its distance from the lever's fulcrum but it is often thought of as "rotational force." It is the basic measure of a motor, but most motors produce different amounts of torque depending on thier current rotational speed. The maximum force a motor can resist, keeping its shaft still against that force, is called the holding torque. Torque can also be defined as the the power output of an engine divided by its current rotational speed. The varying torque output over a range of speeds can be measured with a dynamometer, and shown as a torque curve. Internal-combustion engines produce useful torque only over a limited range of rotational speeds. Electric motors produce torque over a much broader range of speeds but most produce less torque at very low or very high speeds.
Stepper motors produce maximum torque when the shaft is not moving and the stators are aligned with the coils (holding torque) and torque decreases with increasing speed. The dynamic, "detent" or "pull out" torque required to continue moving from one step to the next, is normally much less than holding torque for stepper motors. See "measuring stepper torque" In a stepper motor, torque is directly proportional to Ampere-turns or current times the turns of wire through which it passes. Voltage influcences how quickly the current builds, and how far it builds, but it is the resulting current, and not the voltage that builds the inductive field in the stepper motor which holds position.
When purchasing motors for which the pull out torque is not known, it can be estimated at less than half, and often less than 1/10th the holding torque.
Torque is expressed as distance times Force. The SI standard unit is Newton - Meters (Nm) but more common units are grams per centemeter (g-cm) or ounce inches (oz-in)
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