Engineering purpose
Use the sequence below to establish a defensible starting point, then confirm the decision with model-specific performance, tolerances and test evidence.
Losses Create the Thermal Load
Copper, iron, magnet, bearing, windage, drive and gearbox losses vary with torque, speed, switching and temperature. Nameplate efficiency at one point cannot represent the full cycle.
The Installation Rejects Heat
Housing contact, mounting flange, airflow, enclosure, adjacent heat sources and altitude influence the thermal path. A motor tested on a substantial metal plate may run hotter in an insulated product.
Duty Determines Temperature Accumulation
Short events can be acceptable when followed by adequate cooling, but repeated peaks may accumulate heat. Record every state and the worst repeated cycle.
Protect the Limiting Component
Windings are not always the only limit. Magnets, bearings, lubricant, electronics, encoder and surrounding plastics may have lower acceptable temperatures.
Verify Where Calculation Is Uncertain
Use representative supply, load, mounting, airflow and ambient conditions. Instrument the relevant components, allow stabilisation and record the complete cycle.
Worked example
Estimate steady temperature rise from total loss
Given
- Estimated total dissipated loss: 35 W
- Thermal resistance from the defined reference point to ambient: 1.6 K/W
- Ambient temperature: 40°C
Method
- Estimated rise at the reference point = 35 × 1.6 = 56 K.
- Estimated stabilised reference-point temperature = 40 + 56 = 96°C.
- Compare winding, magnet, bearing, electronics and surface limits using the appropriate internal thermal model.
Engineering result: The simple estimate indicates thermal verification is essential; it does not establish winding temperature by itself.
Common mistakes
What to Avoid
- Using average mechanical power as heat loss
- Ignoring ambient and mounting differences
- Ending a test before thermal stabilisation
- Monitoring the housing while neglecting internal component limits
Selection check
Confirm Before Selection
- Losses at all operating points
- Ambient, altitude, airflow and mounting
- Cycle duration and starts per hour
- Temperature limits, sensors and acceptance criteria
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.