KEVOS® MAKEOS · Power Transmission
Servo or Motor Duty Cycle Check (Move Profile, RMS and Peak)
For this move profile, what torque does the motor need to accelerate, run and stop the load, and is it within the motor’s continuous and peak ratings?
Duty-cycle sizing of a motor for a repeated trapezoidal move: acceleration, running, deceleration and dwell segments from the inertia at the motor, the top speed and the segment times, segment torques including friction or load torque, the RMS torque over the cycle (as the RMS torque tool) against the continuous rating, the peak torque against the peak rating, the inertia ratio and the energy returned on each deceleration.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Load inertia at the motor shaft | kg·m² | Inertia of the load and transmission referred to the motor shaft (divide load inertia by the ratio squared). |
| Motor rotor inertia | kg·m² | Inertia of the motor rotor (and brake, if fitted). |
| Top speed | rpm | Top speed of the move at the motor. |
| Acceleration time | s | Time to accelerate from rest to top speed. |
| Running time at top speed | s | Time at top speed. |
| Deceleration time | s | Time to decelerate to rest. |
| Dwell time | s | Rest time before the next move. |
| Friction or load torque while moving | N·m | Torque for friction and load while the axis moves (positive, opposing motion). |
| Holding torque during dwell | N·m | Torque the motor holds at rest (for a vertical axis without a brake). |
| Motor continuous (rated) torque | N·m | Continuous torque rating of the motor (from the maker). |
| Motor peak torque | N·m | Peak torque rating of the motor and drive (from the maker). |
| Motor maximum speed | rpm | Maximum speed of the motor. |
| Largest load-to-rotor inertia ratio | Largest inertia ratio you accept for good servo tuning (your value). |
Method and formulas
- ω = 2π n ÷ 60; total inertia J_t = J + J_m.
- Acceleration torque = J_t ω ÷ t_a + T_f; running = T_f; deceleration = T_f − J_t ω ÷ t_d (negative = braking); dwell = holding torque.
- RMS = √(Σ T² t ÷ Σ t) (as the RMS torque tool); peak = the largest |T|.
- Inertia ratio = J ÷ J_m; energy returned per stop ≈ J_t ω² ÷ 2 − T_f ω t_d ÷ 2.
- Example: the move needs 15.566 N·m to accelerate; the RMS torque is 5.4835 N·m against 6 N·m continuous.
Limitations
- Trapezoidal profile with constant acceleration; S-curves lower the peak slightly.
- Motor heating also depends on speed; check the maker’s speed-torque curve.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a servo motor for an indexing or pick-and-place axis
- Check a motor for a faster cycle
- Estimate the braking resistor energy per stop
Who it is for
- Automation engineers
- Machine designers
- Controls engineers
What you get
- Segment torques
- RMS and peak torque against the ratings
- Inertia ratio and energy returned per stop
Interpreting the result
- Longer dwell lowers RMS torque; shorter acceleration raises the peak sharply.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2569–2580, 180–186. Topic reference; no source tables or recommended settings are reproduced.
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Open the Servo or Motor Duty Cycle Check (Move Profile, RMS and Peak) calculator
