cyclogramWhat a servo can give
A servo's catalogue gives a rated torque, which it holds for ever without overheating, and a peak torque, which it gives for a short while. Both hold up to the rated speed; above it the motor runs at constant power and the torque falls, to nothing at the maximum speed. Without the motor's own curves, the page draws that generic envelope:
T_cont(n) = T_rated n ≤ n_rated T_cont(n) = T_rated · n_rated / n n_rated < n ≤ n_max T_int(n) = the same with T_peakThe default move peaks at 3600 rpm on a 3000 rpm servo, so the 400 W generic motor's 3.82 N·m peak torque is derated to 3.82 · 3000 / 3600 = 3.18 N·m where it is needed.
The four checks
| Check | Against | Required | Limit | Margin |
|---|---|---|---|---|
| Peak torque | the intermittent torque at the speed of the peak | 0.284 N·m | 3.18 N·m | 91 % |
| RMS torque | the rated torque | 0.138 N·m | 1.27 N·m | 89 % |
| Max speed | the maximum speed | 3600 rpm | 6000 rpm | 40 % |
| Inertia ratio | the limit, 10 by default | 2.92 | 10 | 71 % |
The margin is the share of the limit still unused. Below zero the row fails; below 10 % it reads Close — you can set that threshold, and the inertia-ratio limit, under Limits in step 4.
When the inertia ratio fails
The smallest generic servo, 50 W, has the torque for the default move — 42 % margin on the peak — but its rotor is so light that the load is 30.4 times its inertia. The page suggests the gearbox that would bring the ratio to 10, i = √(30.4 / 10), about 1.8 : 1. The inertia ratio guide works through it.
Steppers, BLDC and AC induction motors
- Stepper — sized on its pull-out torque at speed over a safety factor, with the duty cycle and the step rate checked too. See the stepper guide.
- BLDC motor — a rated torque and a limited-duty torque, both constant over the speed range; the load inertia is checked against the maker's permissible figure when you give it.
- AC induction motor on a drive — the rated torque up to the base speed, constant power beyond, and a breakdown torque that falls with the square of the speed in field weakening. Its inertia ratio is shown as a caution only, as there is no servo loop to tune.
Questions
Why check the RMS torque?
Because heat decides whether a motor survives the cycle. The RMS torque over the whole cycle — the moves, the dwell and the rest — is the steady torque that would warm the motor as much, and it must stay under the rated torque.
What margin should I keep?
The page reads a margin under 10 % as Close. Many designers keep 20 to 30 % on the peak and the RMS for unknown friction, wear and a faster cycle later; set your own threshold under Limits in step 4.
What if the move runs above the rated speed?
Then the torque available is lower: the page derates the peak and rated torque at constant power above the rated speed, and checks the peak against the derated value at the speed where it happens.
Can I use a motor's real speed–torque curves?
Yes. A motor brought in from a file can carry its own curves as speed–torque pairs; the page interpolates them instead of the generic envelope.
Can I load a whole motor catalogue?
Yes. Drop JSON or CSV files — or a folder of them — onto the Motor files box in step 4. Common column names and units are understood, and the motors join My motors in this browser.
Related guides
- Axis sizing calculatorThe whole guided flow: motion, mechanism, motor, regeneration, and the move as a bar.
- Stepper motor sizingPull-out torque at speed over a safety factor, duty cycle, step rate and inertia ratio.
- Inertia ratioThe load's inertia at the motor through a screw, a belt or a gearbox, and the ratio that fixes it.
- Ball screw motor sizingSpeed, reflected inertia, torque per phase, RMS, critical speed and D·N for a ball screw axis.
- Brake resistor sizingThe energy an axis returns while braking, what the drive's bus takes, and the resistor for the rest.
Every term on this page, defined: the axis and motor sizing glossary.