Actuator Thermal Derating Calculator estimates whether a linear actuator has enough thermal margin for the entered current, duty cycle, ambient temperature, and cooling assumptions. It is intended for early actuator selection when repeated motion, high load, or warm environments may reduce usable performance.
Calculator
Enter conservative values. If you do not know motor resistance or thermal resistance, use the result as a sensitivity study rather than a final design value.
What This Calculator Solves
Actuators are often selected from force, stroke, speed, and voltage. Those specs are necessary, but they do not tell the whole story when the actuator cycles repeatedly or works in a warm enclosure. Electric motors create heat. Gearboxes and screws also create losses. If the heat cannot escape quickly enough, the actuator may slow down, trip a controller, soften grease, age seals, damage windings, or lose useful life.
Thermal derating is the act of reducing usable load, current, duty cycle, or force rating when the temperature environment becomes more demanding. A short demonstration may work perfectly, while the same actuator in a production cycle overheats after repeated operation. This calculator gives a first-pass estimate of that risk.
The calculator is intentionally conservative. It focuses on current-related heat and a simple thermal resistance model. That is not a complete motor thermal simulation, but it is useful for finding obvious problems before committing to a product, bracket, controller, enclosure, or duty cycle.
Formula
The heat estimate uses duty-weighted copper loss:
P_heat = I^2 x R x duty
The estimated temperature rise is:
Delta T = P_heat x R_thermal
The estimated operating temperature is:
T_operating = T_ambient + Delta T
The thermal margin is:
Margin = T_limit - T_operating
The derated force estimate scales the entered actuator force by the square root of remaining thermal margin ratio. This is a screening approximation, not a manufacturer rating curve.
Worked Example
Assume a 24 V actuator runs at about 6 A under load. The winding resistance estimate is 1.2 ohm, duty cycle is 25%, thermal resistance is 3.5 deg C/W, ambient temperature is 25 deg C, and the chosen conservative limit is 80 deg C. The duty-weighted heat loss is lower than continuous heat loss because the actuator rests for part of the cycle, but the temperature still rises if the heat cannot leave the motor and housing fast enough.
| Input | Example value | Meaning |
|---|---|---|
| Operating current | 6 A | Higher current increases heat by the square of current. |
| Motor resistance | 1.2 ohm | Resistance turns current into copper loss. |
| Duty cycle | 25% | Only part of the cycle creates running heat. |
| Thermal resistance | 3.5 deg C/W | Higher value means poorer cooling. |
| Temperature limit | 80 deg C | Conservative upper screen for the model. |
How To Interpret The Results
Duty-weighted heat loss
This is the estimated average heat being created by the actuator motor during the repeated cycle. Because current is squared, doubling current creates four times the copper loss before duty cycle is applied.
Estimated operating temperature
This combines ambient temperature and calculated temperature rise. A higher ambient temperature directly reduces margin. An actuator that works in a cool open shop may not have the same margin inside a sealed cabinet, outdoor enclosure, vehicle compartment, or hot factory cell.
Thermal margin
Positive margin means the simplified model is below the selected limit. Small positive margin still deserves caution. Negative margin means the design should be revised before selecting parts.
Preliminary derated force
This output is a screening guide. It does not rewrite the product rating. It helps show how much usable force may need to be reduced when the thermal environment is worse than the reference condition.
Practical Selection Workflow
- Estimate real loaded current. Current under load is more useful than no-load current for thermal screening.
- Enter duty cycle honestly. Include repeated moves, short rest periods, testing, and likely user behavior.
- Use the warmest ambient condition. Do not size from room temperature if the actuator will run outdoors, inside machinery, or near heat sources.
- Check margin before buying. Low margin means you should reduce load, reduce duty cycle, improve cooling, or select a more suitable actuator.
- Then check adjacent limits. Verify force, stroke, speed under load, controller current, wiring voltage drop, bracket loads, and side loading.
Inputs Explained
Ambient temperature
Ambient temperature is the starting point for heat rise. Every degree of ambient temperature reduces available margin by the same amount. This is why outdoor, vehicle, marine, and enclosed installations deserve extra attention.
Operating current
Operating current is a strong heat indicator. Current rises with load, friction, poor geometry, low voltage, worn components, or actuator operation near its force limit.
Duty cycle
Duty cycle is the percentage of time the actuator is powered and creating heat. A high-force actuator that runs briefly may stay cool, while a moderate load repeated constantly may become the harder thermal case.
Thermal resistance
Thermal resistance approximates how easily heat leaves the motor and actuator body. Lower thermal resistance means better cooling. Higher thermal resistance can represent insulation, poor airflow, sealed covers, or a hot mounting environment.
Rated force and reference margin
Rated force is used for the simple derating estimate. Reference margin is the assumed thermal window under nominal conditions. If you have manufacturer temperature test data, use that data instead of generic assumptions.
Common Mistakes
- Using no-load current: no-load current can badly understate heat during real lifting, pushing, or sliding.
- Ignoring enclosure temperature: sealed housings, cabinets, and vehicle compartments can run much hotter than room air.
- Assuming force rating equals continuous force: many actuator applications are intermittent, and duty cycle matters.
- Not checking voltage drop: low actuator terminal voltage can increase current and worsen heating.
- Skipping physical testing: the model is useful, but temperature stickers, sensors, or controller telemetry give better real-world evidence.
When To Use A Different Calculator
References And Review Basis
The calculator uses standard electrical heat-loss and thermal-resistance relationships. For final actuator selection, compare the result with FIRGELLI product specifications, duty-cycle guidance, controller current limits, installation details, and measured test data.
- OpenStax University Physics, Volume 2: direct-current circuits and resistive power relationships.
- NIST SI unit guidance for unit-consistent engineering calculation display.
- FIRGELLI actuator product data for voltage, current, force, speed, stroke, and duty-cycle constraints.
Frequently Asked Questions
What is thermal derating for a linear actuator?
Thermal derating is the reduction in usable load, current, or duty cycle required when heat buildup or ambient temperature leaves less cooling margin.
Why does actuator heat matter?
Heat affects motor winding temperature, grease, seals, gearbox wear, electronics, and long-term reliability. A system can pass a short force test but still overheat during repeated cycling.
Is duty cycle the same as thermal derating?
No. Duty cycle describes run time versus rest time. Thermal derating estimates whether the heat created during that duty cycle can stay below a chosen temperature limit.
Should I use current or force for the estimate?
Current is often the better heat input because motor copper loss is related to current squared. Force is still important because higher force usually increases current.
Can this calculator approve a safety-critical actuator design?
No. It is a preliminary screen. Final designs need product data, test data, installation review, environmental checks, and appropriate safety factors.
What should I do if the calculator shows negative margin?
Reduce load, reduce duty cycle, lower current, improve cooling, choose a stronger actuator, change the mechanism geometry, or add rest time between cycles.
About The Author
This calculator is published by FIRGELLI Automations and reviewed under the engineering direction of Robbie Dickson, Chief Engineer and Founder of FIRGELLI Automations. It is intended to help engineers, builders, and product designers screen actuator heat and duty-cycle decisions before choosing components.
