Reading a Stepper Motor Torque-Speed Curve Before You Size a Drive
Holding torque says very little about what a stepper can do while turning. Here is how to read the curve, what pull-in and pull-out torque mean, and how much margin to leave.
A stepper motor's holding torque is the number most often quoted, and the one most often misread. It describes the motor at standstill and tells you almost nothing about what it can do once it starts turning.
What the curve actually shows
A torque-speed curve plots available torque against step rate for one specific combination of drive voltage, phase current, microstep setting and wiring. Change any one of those and the curve moves — which is why two suppliers can quote the same motor with noticeably different curves.
Why torque falls with speed
Every step is a current step through an inductive winding. As the step rate rises, the winding has less time to reach its rated current before the next step arrives, so the average torque produced per step drops. A higher bus voltage pushes current into the winding faster and extends the useful speed range — the main reason a 48 V drive holds torque further up the curve than a 24 V one.
Pull-in and pull-out torque
Most curves carry two lines. Pull-out torque is what is available at a given speed while the motor is already running. Pull-in torque is the maximum load the motor can start and stop without a controlled acceleration ramp. If your motion profile ramps smoothly, design against the pull-out curve, but keep margin for resonance and friction.
A practical sizing routine
- Calculate the torque the axis needs at the working speed, including friction, gravity on vertical axes and acceleration.
- Add at least 30–50% margin — stepper torque drops quickly near the top of the curve.
- Check the required speed sits well inside the curve at your actual supply voltage, not the datasheet's best case.
- Compare rotor inertia with reflected load inertia; a large mismatch makes the system prone to resonance and missed steps.
Where a closed-loop stepper helps
A closed-loop stepper with an encoder can recover from occasional position error and will report a stall instead of silently losing steps. It does not create torque the motor does not have — the curve still applies — but it makes the limit visible and safer to work near.
Send us the axis data — moving mass, speed, acceleration and guide friction — and we will check the curve against your profile and recommend a motor and drive combination with the margin already worked out.