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Servo Motor Sizing and Motion Control: A Practical Guide
Size a servo motor from the motion profile, not the load weight alone: calculate the reflected inertia, the peak torque needed to accelerate it, the RMS torque over the whole cycle, and the maximum speed. The motor must exceed all of them with margin, and the load-to-motor inertia ratio should stay low enough for stable tuning.
Published · Last reviewed by Geek Engineering senior engineering review.
Servo, stepper or VFD
| Technology | Best for | Limitation |
|---|---|---|
| Servo motor and drive | Fast, precise point-to-point moves, synchronized axes, electronic cams | Higher cost; needs tuning |
| Stepper motor | Low-cost positioning with light, consistent loads | Can lose steps under load unless closed-loop; torque falls at speed |
| Induction motor with VFD | Speed control of conveyors, pumps and fans | Not designed for precise positioning |
The sizing steps
- Define the motion profile: distance, move time, dwell, and how often it repeats.
- Calculate load inertia and reflect it through the mechanism, such as a gearbox, belt or ball screw, to the motor shaft.
- Calculate the peak torque to accelerate the total inertia, plus friction and any external force such as gravity on a vertical axis.
- Calculate RMS torque over the full cycle, which sets motor heating.
- Check maximum speed against the motor's speed and torque curve.
- Select a motor and drive that exceed peak torque, RMS torque and speed with margin, then check the inertia ratio.
Inertia ratio and tuning
The inertia ratio compares the reflected load inertia with the motor's rotor inertia. A high ratio makes the axis harder to tune and more prone to oscillation, especially with compliant couplings or belts. Gear reduction lowers the reflected inertia by the square of the ratio, which is why a gearbox is often the fix for an axis that will not tune.
Mechanical stiffness matters as much as the motor: a loose coupling or long belt limits how fast the axis can settle, whatever the drive settings.
Coordinated motion
Multi-axis machines synchronize axes through the controller, using electronic gearing, cams or robotics functions, over a deterministic motion network such as EtherCAT, SERCOS or EtherNet/IP with CIP Motion. Safety functions such as safe torque off and safe speed monitoring are built into many modern servo drives and can simplify the safety design.
Key takeaways
- Size from the motion profile: peak torque, RMS torque and speed.
- Keep the inertia ratio low; gearing reduces it by the ratio squared.
- Mechanical stiffness limits performance as much as the motor.
- Use servo-integrated safety functions where they simplify the design.
Related questions
What inertia ratio is acceptable for a servo?
There is no single rule, and modern drives tolerate higher ratios than older ones. Lower ratios tune more easily and settle faster. Check the drive manufacturer's guidance for the application, and reduce the ratio with gearing when tuning is difficult.
Why does my servo axis vibrate or hunt?
Common causes are a high inertia ratio, mechanical compliance in couplings or belts, resonance in the machine frame, and gains set too high. Use the drive's tuning and frequency analysis tools, and fix the mechanics before compensating in software.
Can a stepper replace a servo to save cost?
Sometimes, for light, consistent loads and moderate speeds. Where loads vary, speed is high or a missed position is costly, a servo or closed-loop stepper is the safer choice.
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