Motor overheating — what the calculation says about temperature and limits

Hot motors are rarely bad luck. Load and cooling usually disagree: oversized prop, tight Kv, continuous use near peak, poor airflow — or you notice too late. eCalc estimates case temperature and compares peak power/current to manufacturer limits.

What the gauges mean

From the xcopterCalc help:

• est. Temperature: green ~0–70 °C, yellow 70–90 °C, red above (overheat risk)
• Practical rule in the help: case should not exceed ~80 °C / 180 °F; above 100 °C is very critical
• Electric Power / Current: maximum vs motor limit (short, help: up to about 15 s) — green below ~70 % of limit, yellow up to limit, red over

Blue/red messages are advisories, not syntax errors. eCalc does not monitor max physical RPM — respect manufacturer limits.

Cooling grades (Excellent → Very poor) change the temperature estimate. Open mounts help; sealed fuselages without vents hurt. Same idea in propCalc for airplanes — Airplane drive.

Mechanism

Motor losses become heat. Without enough heat rejection, case temperature rises. Hover is a continuous current problem; punch-outs may use the peak limit only briefly. Treating peak amps as continuous overheats by design — Setup too marginal.

Also: running far below the current of best efficiency can raise waste heat disproportionately (help note). Hover current should sit near or slightly below the optimum point.

Rules of thumb

1. Set cooling realistically in eCalc.
2. Read hover temperature and max temperature separately.
3. Try smaller prop / less pitch or Kv/cells before “stronger motor”.
4. ESC headroom: help suggests extra margin for poor cooling and partial load.
5. Calibrate loggers — built-in telemetry can deviate significantly per the help.

Worked example: 5” quad gets hot after 90 s

Assumptions:

• AUW 700 g, 4S 1300 mAh, props 5×4.8 (aggressive), motors ~2000 Kv
• Cooling Medium
• Hover throttle OK, but est. Temperature yellow/red and peak current near limit

Typical fixes to re-simulate:

1. Drop to 5×4.3 or 5×4 — watch peaks and temperature.
2. Improve airflow and update the cooling grade.
3. Lower AUW or improve prop efficiency (Sizing).
4. Pick a better-matched Kv/Rm from the database — not “anything stronger”.

Steps in xcopterCalc (airplanes: propCalc)

1. Enter the setup in xcopterCalc.
2. Set Motor Cooling deliberately.
3. Calculate → est. Temperature @ Hover and @ Maximum.
4. Read Power/Current vs limit (15 s peak context).
5. Change one variable (prop, pitch, Kv, cells, cooling), recalc.
6. On the bench: short runs, feel/IR the case — the calc supports measurement; it does not replace it.

FAQ

Is “briefly hot” OK?
Brief yellow spikes may be fine; sustained case temperatures above ~80 °C risk damage. Manufacturer data wins.

Wattmeter lower than eCalc?
Charge state, sag and logger error. For peaks use charge state full and a reference meter.

Only one motor hot?
Mechanics, uneven cooling or a bad ESC — compare sides.