Buying Motors for High Altitude and Hot Environments: The Derating Calculation

The same electric motor that runs trouble-free for years in a factory at sea level may start to struggle far sooner than expected at a mine site at 2000 metres or at a plant where summer temperatures reach 45 degrees C. The reason is not the motor's quality but the laws of physics: motors are designed for a specific ambient temperature and altitude, and outside these limits the cooling becomes insufficient. On the purchasing side, the name of this situation is derating, and when it is not managed properly it ends with the early failure of a newly bought motor. As a company that, since 1979, has manufactured and sold motors to every geography of Turkiye - from the high-altitude mine sites of Eastern Anatolia to the plants on the hot plains of the Southeast - HEM Motor knows very well what this calculation means in the field. In this article we explain how to take derating into account when buying a motor for altitudes above 1000 metres and ambient temperatures above 40 degrees C, when you need to step up to a higher power, and what information you should write into your order note.

What Is Derating, and Why Does It Affect Purchasing?

When an asynchronous motor runs, all of its losses turn into heat, and this heat must be expelled to the ambient air over the body. The nominal power on the motor nameplate is, by general convention, valid for an ambient temperature of 40 degrees C and altitudes up to 1000 metres above sea level. When one of these two conditions is exceeded, the motor's heat-dissipation capacity decreases; if the motor is forced to deliver its nameplate power, the winding temperature rises above the limit allowed by the insulation class. Overheating of the insulation does not stop the motor at that moment; it stealthily consumes the insulation life. That a sustained rise in winding temperature rapidly shortens insulation life is one of the most fundamental tenets of motor engineering.

The conclusion from the purchasing point of view is this: if you are buying a motor for a high-altitude or hot site, you should base the decision not on the power your mechanical load needs but on the power the motor can deliver under those ambient conditions. This often means either choosing a motor from a higher power rating or sizing the motor for those conditions together with the manufacturer. Derating is not a quality defect or a matter of negotiation; it is a fact arising from the physics of cooling, valid for every manufacturer in the world. The difference lies in whether the manufacturer does this calculation on your behalf.

Why Is a Motor Strained at High Altitude?

As altitude rises, the air thins; that is, the mass of the same volume of air, and therefore the heat it can carry, decreases. Even though the motor's fan turns at the same speed and blows the same volume of air, this air carries less heat than at sea level. The result: the winding temperature of a motor running at the same load increases as the altitude rises.

The general approach used in the field can be summarised as follows: no correction is needed up to 1000 metres; for every 100 metres above 1000 metres, a derating of roughly 1 percent in the motor's loadability is foreseen. For example, at a plant at 2000 metres the continuous loadability of the motor falls to a band of about 88-92 percent of the nameplate power; at 3000 metres this ratio approaches the level of the low 80s. These ratios are approximate values that vary according to the manufacturer, the motor series and the cooling design; the exact value should be confirmed with the manufacturer's engineering department before ordering. What matters is to see the principle: buying a 30 kW motor for a crusher plant at 2000 metres because "there's exactly a 30 kW load" means running the motor continuously in the overheating zone.

The overlooked second effect of high altitude is the magnitude of the day-night temperature difference: on motors that stop on cold nights and run during the day, the risk of condensation increases. That is why we recommend you also evaluate the anti-condensation heater option when ordering for high-altitude sites.

Derating at Ambient Temperatures Above 40 degrees C

On the ambient temperature side the logic is the same: the hotter the cooling air, the less heat the motor can expel. For environments above the standard design limit of 40 degrees C, the common engineering approach is roughly as follows: at a 45 degrees C ambient the motor should be loaded to about 95 percent of its nameplate power, at 50 degrees C to 90 percent, and in the 55-60 degrees C band to 80-85 percent. These values too are approximate ratios that vary from series to series; but the order of magnitude is enough to make a decision in the field.

The critical question here is this: what is the real ambient temperature of your motor? The shade temperature on the weather report and the temperature at the point where the motor sits can be very different. A fan motor mounted in a roof space, a pump motor inside a sun-exposed sheet-metal cabin, a conveyor motor next to a furnace or boiler room may be running in a micro-environment above 50 degrees C while the outside air is 35 degrees C. Before purchasing, you should take a measurement at the point where the motor will be mounted under summer conditions, or at least make a realistic estimate. At the other end of the temperature scale, we covered the requirements of motors that will run in cold environments in our article on cold storage fan and compressor motors.

What Is Seen in the Field if Derating Is Neglected?

The symptoms of a motor bought without taking derating into account reveal themselves over time and are misdiagnosed in most plants:

• Thermal tripping in the afternoon: A motor that runs trouble-free in the morning tripping the thermal in the hottest hours of the day is a classic symptom of derating. In the field it is usually said that "the relay is faulty" and the setting is raised; this means disabling the protection and sacrificing the winding.
• Excessive heat on the body: The motor body heating up too hot to touch is a sign that the winding temperature is also walking on the limit.
• Darkening of the paint colour: Long-term overheating leaves a trace as a colour change in the body paint and as hardened, run bearing grease.
• Recurring winding failures: A motor at the same point going for a winding renewal every few years should suggest a sizing error before motor quality.

If you have a motor in your plant showing these symptoms, ordering its replacement at exactly the same power serves no purpose other than carrying the problem to the new motor. A replacement purchase is the most correct opportunity to fix the derating calculation.

Both High and Hot: The Combined Effect

A feature of Turkiye's geography is that the two conditions are often seen together: it is common for a mine site at 1500 metres to measure 38-40 degrees C in the shade during the summer months, and the point where the motor sits may be hotter than that. In this case the altitude and temperature deratings are evaluated together; when the two effects superimpose, the motor's safe loadability can fall even below 80 percent of the nameplate power. The practical rule: calculating according to only one of the two conditions is not enough; report both the altitude of the site and the hottest-month ambient temperature, and make the selection with these two data.

The Decision to Step Up to a Higher Power: When and How?

The simplest and most commonly applied way of managing derating in the field is to choose a motor from a higher standard power rating. Let us make it concrete with an example: your mechanical load requires 22 kW and your plant is at 2200 metres. When a derating of about 10-12 percent is foreseen, the power that a 22 kW motor can safely deliver at that site falls to a band of 19-20 kW; that is, it cannot carry the load. When the next standard power, 30 kW, is chosen, the motor easily meets around 26 kW even after derating and runs in a comfortable zone below the 22 kW load.

Let us give a second example from the temperature side: imagine a 15 kW fan motor that will run inside a sheet-metal cabin at a plant in the Southeast. If the cabin interior temperature reaches 50 degrees C in summer, the motor's safe loadability falls to roughly 90 percent, that is around 13.5 kW, and it cannot carry the 15 kW load. Here the solution is either to choose a higher power of 18.5 kW or to improve the cabin ventilation and order the motor with a temperature-reserve winding. In both examples the common lesson is the same: the power on the nameplate is not the power on your site.

The points to watch when stepping up to a higher power are these:

• The frame may grow: The frame type and shaft diameter of the higher-power motor may change; check the base, coupling and pulley dimensions against the new frame.
• The protection is resized: The thermal relay and breaker must be selected according to the new motor's nameplate current.
• Avoid over-oversizing: Choosing a motor far above the need runs the motor continuously at low load and in an inefficient zone. The target is for the post-derating capacity to meet the load with a margin of 10-20 percent.
• Discuss the alternative with the manufacturer: In some cases, instead of stepping up to a higher power, it is possible to produce a solution in the same frame with a higher insulation class and a temperature-reserve special winding. The advantage of our being a manufacturer comes into play exactly here.

The efficiency class is also part of this decision: because it produces less heat at the same load, an IE3 motor and IE4 motor with lower losses offers a natural temperature reserve in harsh ambient conditions. On demanding sites, preferring a high-efficiency motor is not only an energy-saving but also a thermal-safety decision.

Site Measures to Consider Together With Motor Selection

A correctly sized motor is the most important half of the equation; the other half is the cooling conditions of the point where the motor will be mounted. A few measures, almost cost-free when planned together with the purchasing decision, lighten the derating burden: building a simple sunshade that prevents direct sun from hitting a motor in an open site, leaving the cabin's ventilation cross-section according to the motor's fan flow on motors mounted inside cabins, not positioning the motor's fan intake side too close to a wall or equipment, and writing the periodic cleaning of the cooling fins into the maintenance plan on dusty sites. These measures do not eliminate the need for derating; but they keep the motor on the safe side, within the calculated reserve. If you share the installation photo or layout sketch with us at the order stage, we make these recommendations specific to your site.

Order Notes for Hot-Region Plants and High-Altitude Mine Sites

The information you should add to your order note when requesting a quotation enables the supplier to do the correct sizing for you. A checklist you can copy and use:

1. The site's altitude: In metres, an approximate value is enough (e.g. "the site is at 1850 m altitude").
2. The hottest-month ambient temperature: The highest temperature expected at the point where the motor will be mounted (e.g. "48 degrees C is measured at the motor location in summer").
3. The mechanical power need: The power the load actually wants and the operating regime (continuous or intermittent, how many starts per hour).
4. The type of application: Crusher, belt conveyor, fan, pump, mill, etc.; the load character determines the starting difficulty.
5. The dust and the mounting location: Open site, closed building, inside a cabin; the dust density affects the fouling rate of the cooling fins and the protection-class need.
6. Supply information: The voltage level and, if any, the voltage fluctuation; on remote sites, voltage drop combined with derating worsens the picture.

On a request that arrives with this information, we recommend a clear configuration that includes, where needed, a higher power, where needed a temperature-reserve winding, a protection class suited to the dust, and a cast iron motor frame. Our mining sector electric motor series, the ready equivalent of these configurations for mining sites, is sized with high altitude and heavy dust conditions in mind. For the load-side details of motor selection at crushing-and-screening plants, you can also look at our article on crusher and stone-crushing plant motor selection.

The Difference of Buying From the Manufacturer: Let Us Do the Calculation

An intermediary selling motors from a catalogue is often content with merely noting down your altitude and temperature data. At HEM Motor, as an industrial electric motor manufacturer, however, these data go directly into the sizing: the winding design, insulation class, fan and cooling selection are determined according to your site; where necessary, a custom configuration is produced for you instead of a standard stock motor. Thanks to our production range spanning 0.55 kW to 355 kW and our stock strength across Turkiye, the higher power you will need after the derating calculation is often ready on the shelf. The early failure of an incorrectly sized motor creates a loss many times greater than the price difference from the right motor; let us do the calculation together, before you buy.

Frequently Asked Questions

If we are only slightly above 1000 metres or 40 degrees C, is derating still needed?

In conditions just above the limit (for example 1200 metres, 42 degrees C) the derating ratio is small, and if your load is significantly below the motor's nameplate power, a standard selection is often sufficient. However, if the motor will be loaded close to its nameplate power, even a small derating means operating on the limit. In that case, report the site's data at the quotation stage; the decision is clarified with a small calculation.

Instead of derating, can we just buy the motor as is and load it less?

Technically you can; derating already means "loading the motor less." The problem is that this cannot be guaranteed in the field: when a large stone enters the crusher or the fan blade gets fouled, the load does not ask about your plan. If load control is not in your hands, the safe way is to reserve the capacity on the motor side, that is with a higher power or a special winding.

For high altitude, should an aluminium frame or a cast iron frame be preferred?

The frame material does not change the derating calculation; the choice depends more on the mechanical conditions. In the impact, vibration and heavy-dust conditions of mine and crusher sites, a cast iron motor frame stands out in terms of endurance. If you state the site conditions in your order note, we offer the frame recommendation together with the sizing.

Get a Quote

Tell us your plant's altitude, hottest-month temperature and your load; let us do the derating calculation and offer you the right power, the right winding and the right frame in a single quotation. You can reach us by phone at +90 (532) 345 49 86 or create a request through our contact us page. HEM Motor - motors that have worked at every altitude and in every climate of Turkiye since 1979.