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How Does Holding Torque Differ From Pullout Torque?

by Jeff Kordik

The first thing many new users ask about step motors is: "what's the difference between holding torque and pullout torque?". Which one matters to me (as Herb Tarlek might say)? If you apply a constant current to one winding of a step motor, torque is produced according to this formula:


The chart below plots the torque of a step motor winding as the shaft is turned. This is shown in electrical degrees. For a 1.8 degree step motor, the mechanical angle, or shaft angle, is 1/50 the electrical angle so this sinusoidal torque curve repeats every 7.2 degrees.


If we apply current to phase B instead, we'll see the same curve, but it will be offset by 90 degrees. Applying current to both phases provides the sum of the individual torque curves as seen below in the green trace. The green dot represents the point where the shaft could no longer resist the applied torque: this is the holding torque of the motor. That's how much torque can be applied before the motor totally loses position.


As a respected fellow engineer once told me, "if all you want is holding torque, buy a bolt." There is rarely a reason to pay for a step motor and driver just to prevent something from moving. The rest of us buy a motion control system so that we can make something move. Seriously, when people ask me what I do, I usually say "I make things move." It's actually a good conversation starter. Perhaps, then, we need to be more concerned about how much torque we can use to move an object. Applying a constant current to both phases of a two phase step motor can make it move, but only until the shaft moves far enough to the stable equilibrium, where the torque reaches zero. In this case, that's at 135 degrees.


If we want to keep the shaft turning, we'll need to switch the current to another combination of phases and polarities. This is also called "commutating" the motor. The next plot shows the torque resulting from the four combinations of phase currents.


If we commutate at exactly the right angle, we can sustain the following torque as we spin the motor:


The minimum torque on this dynamic curve is called the pullout torque. This is the amount of torque we can generate while the motor is moving without causing it to lose position.

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