cyclogramHolding torque is not what it gives
The holding torque on the catalogue's first line is what a stepper holds at standstill. Turning, it gives its pull-out torque, which starts near the holding torque and falls away with speed as the windings run out of time to fill with current. Without the motor's own curve the page takes it flat up to a corner speed and falling as 1/n beyond, and lets the move use it over a safety factor:
T_pull(n) = T_hold n ≤ n_c T_pull(n) = T_hold · n_c / n n_c < n ≤ n_max usable = T_pull / SF SF = 2 by default step rate = n / 60 · 360 / θ_step steps/sA worked example
A generic NEMA 23 stepper — 1.2 N·m holding torque, the torque holding to 300 rpm, a 1500 rpm maximum — on the default 10 mm ball screw, moving 100 mm in 1 s in thirds with a 1.5 s dwell:
- The peak speed is 0.15 m/s, so the motor turns at 900 rpm, 3000 steps/s at 1.8° a step.
- At 900 rpm the pull-out torque is 1.2 · 300 / 900 = 0.4 N·m; over the safety factor of 2 the move may use 0.2 N·m.
- The move needs 0.0665 N·m at its peak, a margin of 67 %.
- The motor moves for 40 % of the 2.5 s cycle, under the 50 % a stepper at full current tolerates.
- The load is 2.71 times the rotor's inertia.
Where steppers stop
The same motor on the default move — 200 mm in 0.5 s, 3600 rpm at the motor — fails twice. 3600 rpm is past its 1500 rpm maximum, and at 1500 rpm its usable torque is 0.12 N·m against the 0.289 N·m the move needs. Above about 1000 rpm a stepper has little torque left; a servo or a closed-loop stepper suits such a move better.
The duty cycle
A stepper draws its full current standing still, so it heats over the whole cycle, not just while it moves. The page warns — marked Hot — when the motor moves for more than half the cycle: lengthen the dwell, or let the driver reduce the standstill current.
Questions
What safety factor should a stepper have?
2 is the usual rule, 1.5 where the load and the friction are well known. Open loop forgives nothing: a stall is a lost position, found only at the next homing.
Does microstepping change the torque?
It smooths the motion and refines the resolution, but it does not add torque. Size on the pull-out torque your driver's curve gives.
Can I enter the pull-out curve?
Yes. A motor file can carry the pull-out curve as speed–torque pairs, and the page then uses it instead of the generic shape.
Why does the driver's supply voltage matter?
A higher supply fills the windings faster, so the pull-out torque holds to a higher speed. Type the corner speed for your driver and voltage, or use its curve.
What inertia ratio suits a stepper?
About 5 when it drives the load directly and 10 through a gearbox is the usual rule; beyond it the motor tends to miss steps on the ramp.
Sources
- Oriental Motor, Technical Reference, stepping motor selection — the safety factor on the pull-out torque and the duty limit of a standard stepper at full current.
Related guides
- Axis sizing calculatorThe whole guided flow: motion, mechanism, motor, regeneration, and the move as a bar.
- Servo motor sizingPeak torque at speed, RMS over the cycle, maximum speed and inertia ratio, each with its margin.
- Inertia ratioThe load's inertia at the motor through a screw, a belt or a gearbox, and the ratio that fixes it.
- Move time and motion profilePeak speed, acceleration and jerk from a distance and a move time; trapezoid, triangle and S-curve.
- Ball screw motor sizingSpeed, reflected inertia, torque per phase, RMS, critical speed and D·N for a ball screw axis.
Every term on this page, defined: the axis and motor sizing glossary.