What to Correct When Selecting a Geared Motor for High Temperature or High Altitude

Start with the answer: there is no universal derating percentage. Some standard systems do give simplified correction methods, but they do not replace the manufacturer's correction curve for that model.

Motors of different insulation classes, cooling methods and protection degrees do not share a correction curve, so "so much height, so much off" points at the right effect with an unusable number.

High temperature and high altitude need three separate corrections:

1. Motor output — P_out = P_rated · k_T · k_alt, each coefficient read from that model's manufacturer curve;
2. Gearbox thermal rating — both ambient temperature and altitude pull it down;
3. Lubrication and cooling method — high and low temperature adjust in opposite directions.

The order cannot be reversed: environmental data → motor correction curve → gearbox thermal rating → lubrication and cooling last.

1. Why there is no universal derating percentage

Because the correction coefficient is not a constant. It depends on properties of the unit itself:

Change any one of the four and the percentage changes. So the correct move is not to apply a number, but to read the correction curve for this unit.

That is the central stance of this article: the correction curve comes from the manufacturer; a general formula cannot supply it.

2. High temperature and high altitude are different physics

The two are routinely lumped together, yet the mechanisms differ:

The altitude row deserves its own section

Most people stop at "cooling is worse up high" and miss the second effect: thin air weakens dielectric strength at the same time, which shows up in creepage distance and clearance.

The two effects do not surface at the same speed:

• Weaker cooling → the unit runs hot straight away, and it is visible quite quickly;
• Lower dielectric strength → the problem may only appear after a long period of running.

Not every protection degree is affected: it depends on how the clearance and the creepage distance are designed. The insulation coordination for a high-altitude project should be reviewed by the electrical designer.

The second is quieter and more dangerous. A motor that behaved at low altitude can turn up with an insulation anomaly months after being moved up — by which time tracing it back to the selection stage is genuinely hard.

3. Motor side: read the two coefficients separately

Usable motor output is corrected multiplicatively:

P_out = P_rated · k_T · k_alt

P_rated is the nameplate rated power, k_T the ambient-temperature correction coefficient and k_alt the altitude correction coefficient.

Two points:

1. The two coefficients are looked up separately, not merged into a single "plateau heat factor", because temperature and altitude act on different physical processes.
2. The values must come from that motor model's manufacturer correction curve. They are not given here — the curve differs with insulation class, cooling method and protection degree.

Why multiply rather than add? Each coefficient trims the same rating by a proportion, and when the two act together those proportions compound. Adding them overestimates the result, because two proportional cuts do not stack into their sum.

4. Insulation life is very sensitive to temperature rise

The relation can be written as:

L / L_0 = 2^(−ΔT / A)

L_0 is the reference life, L the actual life, ΔT the temperature rise above the reference, and A the temperature-rise constant at which insulation life halves.

The relation says one thing plainly: temperature rise cuts life by multiples, not proportionally. The value of A depends on the insulation class and has to be confirmed against manufacturer data — it is not given here. The relation is there for qualitative understanding of how temperature rise affects life; it is not a basis for predicting life.

What deserves more attention is that the effect accumulates:

At the same excess temperature, sustained overtemperature removes far more life than occasional overtemperature. Occasional excess accelerates ageing only briefly; sustained excess pins the insulation at a worse operating point for the whole run.

That is why the judgement "a few degrees over during the hottest week of summer is fine" is dangerous — what matters is how long it stayed over.

5. Gearbox side: the thermal rating falls with the environment

The gearbox side has no insulation to worry about, only heat rejection. Its thermal rating relation is the one used for gearboxes generally:

P_th = k · A · ΔT / (1 − η)

Here k is the heat-transfer coefficient, A is the heat-dissipation area — a different quantity from the life-halving temperature-rise constant A in the insulation relation above, and the two must never be mixed — ΔT is the difference between oil temperature and ambient, and η is efficiency.

The temperature difference itself is:

ΔT = T_limit − T_ambient

So two paths pull the rating down: a higher ambient temperature reduces `ΔT`; a higher altitude reduces `k`.

The full derivation sits in the separate article on the difference between mechanical load capacity and thermal rating. This article does not repeat it and only uses its conclusion.

6. Lubrication: high and low temperature pull in opposite directions

Of the three corrections this is the one most easily skipped, because it never shows up in "is there enough power".

The "wide spread" row is the typical plateau case: daytime may be warm, night drops sharply, so the high-temperature and low-temperature constraints exist together. A simple "pick a high viscosity" or "pick a low viscosity" is wrong in both directions here.

Oil type, viscosity grade and change interval are set out in the lubrication guide; this article does not repeat them.

7. The order of correction cannot be reversed

1. Settle the environmental data (max/min temperature, altitude, ventilation)
   ↓
2. Read k_T and k_alt from the manufacturer curve and compute usable output
   ↓
3. Check the gearbox thermal rating (P_th falls with the environment)
   ↓
4. Adjust lubrication and cooling method

Why can it not be reversed? Two reasons:

1. Select first, check the environment afterwards, and you will rework. By the time the environmental data lands the selection is already made and the capacity has already been cut, so the unit has to be chosen again.
2. You tend to correct only one side. The instinct is to correct the motor, because the motor has nameplate data, and to forget that the gearbox thermal rating needs correcting too.

The most practical piece of process advice in this article: put the environmental data in as an input, not a late check.

8. Condition table

9. Site checklist

Data collection:

1. Confirm the maximum ambient temperature (peak value, not average).
2. Confirm the minimum ambient temperature (affects lubrication and starting).
3. Confirm the altitude.
4. Confirm the mounting position and ventilation (outdoor / indoor / sealed cabinet).
5. Confirm the protection degree requirement (dust, anti-condensation, pollution degree).

Motor-side correction:

1. Record the motor nameplate data, especially the insulation class.
2. Read k_T and k_alt from the manufacturer curve.
3. Compute the usable output P_out and compare it with the actual load.

Gearbox-side correction:

1. Check the thermal rating P_th against the actual ambient temperature and altitude.
2. Decide whether a fan, a larger frame or oil cooling is needed (the same options as in the thermal rating article).

Lubrication and commissioning:

1. Confirm oil type and viscosity grade against the upper and lower ambient limits.
2. For high-altitude or wide-spread sites, confirm whether preheating or a special starting method is required.
3. Record the actual temperature rise after commissioning, as the basis for later reviews.

10. Common mistakes and applicability boundaries

Common mistakes:

1. Applying a universal derating percentage. Curves differ with insulation class and cooling method, so a single number will be off.
2. Merging high temperature and high altitude into one coefficient. Different physics, so they are looked up separately.
3. Correcting the motor and forgetting the gearbox thermal rating. Both sides need correcting.
4. Using an average ambient temperature. Use the peak value.
5. Ignoring the effect of altitude on insulation. Thinking only about cooling and missing the insulation coordination.
6. Adding the correction coefficients. They multiply.
7. Selecting the model before checking the environment. Rework is certain.
8. Correcting cabinet-mounted equipment against outdoor temperature. Inside a sealed cabinet it can be clearly hotter.

Applicability boundaries:

• The three-correction split, the mechanism differences, the two relations and the correction order in this article apply to geared motor selection for high-temperature and high-altitude environments.
• This article gives no derating percentage of any kind, and in particular none of the "every so much height, take off so much" kind.
• This article gives no numerical values for `k_T`, `k_alt` or `A` — they must be read from that model's manufacturer correction curve and its insulation class.
• This article does not tally which failure modes dominate at altitude; it gives the correction process and the curves to consult.
• The specific selection must follow the product correction curves and the requirements of the individual unit; this article does not replace a verification calculation for a specific project.

11. Environmental data list

Items 3 (altitude) and 7 (insulation class) are the two most often missed — the first gets treated as a cooling factor only, the second is rarely volunteered. Yet these two decide whether a high-altitude project can run safely.

12. About this article and the publisher

This article was written by the technical team at Chengfa Reducer (Changzhou) Co., Ltd. (CHINFAR) and released after the company's technical review. CHINFAR products are manufactured and delivered through the joint manufacturing system shared with HUIGUANG.

Related pages:

• Modular geared motors
• Geared motor overheating: causes and a site-check order
• Gearbox lubrication guide: oil type, fill volume and change interval
• Contact the CHINFAR technical team

The overheating article covers "it is already hot, how do I investigate"; this one covers "how do I correct for the environment at the selection stage" — opposite directions, complementary.

13. Next step

Send the ten items above to sales@chinfar.com. For a high-altitude project, state item 3 (altitude) and item 7 (insulation class) explicitly — cooling problems show up as heat at once, insulation problems may only surface months later.

To be clear: this article gives no derating percentage and no numerical values for the correction coefficients; the coefficients must be read from that model's manufacturer curve, and the specific selection must follow the product data and the requirements of the individual unit.