Engineering purpose
Use the sequence below to establish a defensible starting point, then confirm the decision with model-specific performance, tolerances and test evidence.
Continuous Torque Is Thermally Defined
Continuous torque is the torque sustainable under declared ambient, mounting, cooling and speed conditions without exceeding permitted temperatures. Changing the installation or airflow can change the usable continuous rating.
Intermittent Ratings Need a Time Basis
S2, S3 and other duty declarations are meaningful only with their duration, cycle and test conditions. A 15-minute rating cannot automatically be repeated without adequate cooling time.
- Record on-time and rest time
- Check starts per hour
- Confirm whether the motor reaches thermal equilibrium
Peak Torque Has Several Limits
Peak torque may be limited by drive current, winding temperature, magnet demagnetisation, commutation, shaft strength, gearing or traction. The lowest applicable limit governs the system.
Use the Full Cycle for Thermal Review
RMS torque can assist preliminary comparison when torque is approximately proportional to current, but it does not replace a thermal model. Low-speed operation, drive losses and poor cooling can make two equal-RMS cycles thermally different.
Worked example
Preliminary RMS torque for a repeating cycle
Given
- 4 N·m for 10 s
- 1.5 N·m for 40 s
- 0 N·m for 10 s during an unpowered rest interval
Method
- Square each torque and multiply by its duration.
- ΣT²t = 4² × 10 + 1.5² × 40 + 0² × 10 = 250.
- Divide by 60 s and take the square root: Tᵣₘₛ = 2.04 N·m.
Engineering result: Use 2.04 N·m as an initial current-related heating comparison only. Confirm low-speed cooling, peak-current duration, non-torque-dependent losses and the selected motor’s thermal data.
Common mistakes
What to Avoid
- Treating stall torque as a usable continuous rating
- Quoting peak torque without duration
- Ignoring mounting and ambient conditions
- Applying RMS torque without checking speed-dependent cooling
Selection check
Confirm Before Selection
- Declared duty type and duration
- Continuous and peak current limits
- Cooling and mounting condition
- Motor, drive and gearbox peak limits
SDT engineering note
Apply General Knowledge with Product-Specific Evidence
This guide supports preliminary engineering. Final selection must be confirmed against the complete application, selected motor and drive data, declared operating conditions, applicable standards and agreed validation criteria.