Temperature Rise of Dry-Type Transformer: Guide to Thermal Performance

The temperature rise of a dry-type transformer is one of those technical details that can look simple on a datasheet but actually tells you quite a lot about how the transformer will perform in real operation.

Whenever a transformer is energized, it produces heat. Some comes from the magnetic core, while more heat is usually generated in the windings as the load increases. That heat has to get out. If it stays trapped inside the transformer, temperatures rise, insulation ages faster, and reliability can suffer over time.

In basic terms:

Temperature rise = Transformer temperature − Ambient temperature

So, if the surrounding air is 40°C and the average winding temperature reaches 140°C, the winding temperature rise is 100 K.

Dry-type transformers are cooled by air rather than insulating oil. That makes airflow, ventilation, winding construction, insulation materials, and cooling ducts especially important. In practice, the transformer itself and the room where it is installed should be considered as one thermal system.

There are several sources of heat, but core loss and winding loss are the big ones.

Core loss occurs whenever the transformer is energized. It mainly comes from hysteresis and eddy currents in the magnetic core. Unlike winding loss, it does not disappear when the transformer is lightly loaded.

Winding loss is more closely tied to load current. The resistive component is commonly expressed as:

P = I²R

This is worth paying attention to because the current is squared. If the current increases, winding heating can increase much faster than you might expect. At 80% of rated current, for example, the I²R component of winding loss is roughly 64% of the value at rated current, assuming the other conditions stay similar.

There are also stray and additional losses. Leakage flux can induce currents in metallic structural parts, creating localized heating. Harmonic currents can make the situation more complicated, too. Equipment such as UPS systems, variable-frequency drives, solar inverters, EV chargers, and other power-electronic loads may introduce harmonics and increase transformer heating.

So, the final temperature is basically a balance: how much heat the transformer produces versus how much heat it can get rid of.

Ambient temperature matters as well. A transformer operating in a hot electrical room has less thermal margin than the same transformer installed in a cooler, well-ventilated space.

Non-resin dry-type transformers use insulation systems that do not depend on complete epoxy resin encapsulation. Depending on the design, the windings may use varnish, fiberglass, Nomex, air insulation, or other solid insulation materials.

Because there is no thick cast-resin layer around the windings, airflow can play a particularly important role. The design needs to provide enough space for heat to move away from the active parts without creating unnecessary thermal bottlenecks.

The heat dissipation surface of non-resin dry-type transformers is essentially the area available for transferring heat into the surrounding air.

Heat generated in the core and windings has to travel through insulation and structural components before it reaches the air. A good thermal design gives this heat a reasonably direct path out of the transformer.

Winding geometry matters here. Radial and axial air channels can help cooler air enter the lower sections of the transformer while warmer air rises and leaves through the upper sections. In naturally cooled transformers, this circulation happens mainly through convection.

It sounds simple, but installation can easily get in the way.

If a transformer is installed too close to a wall, ventilation openings are blocked, or the electrical room has poor air circulation, the actual operating temperature may be much higher than expected.

For larger units, forced-air cooling can be used to increase airflow and provide additional capacity. But simply putting a fan next to the transformer is not enough. The air needs to reach the intended cooling paths inside the transformer.

The heat load of non-resin dry-type transformers is mainly related to their total losses under actual operating conditions.

Core loss is present whenever the transformer is energized, while winding loss changes with the load. As current approaches the rated value, winding heating increases considerably because of the I²R relationship.

The connected load itself also deserves attention. A transformer supplying a straightforward linear load may have a fairly predictable thermal profile. A transformer feeding UPS equipment, variable-speed drives, rectifiers, or other nonlinear loads can be a different story.

Harmonic currents can increase additional losses and cause extra heating in the windings and metallic parts. This is particularly relevant in data centers, industrial plants, renewable energy projects, and EV charging installations.

For these applications, transformer selection should not be based only on the nameplate kVA. Harmonic content, load profile, expected operating hours, and future load growth may all need to be considered.

The temperature rise of non-resin dry-type transformers is determined by the balance between heat generation and heat dissipation.

When a transformer is first energized, its temperature gradually increases. As the temperature difference between the transformer and the surrounding air becomes larger, heat dissipation also increases. Eventually, the transformer reaches a relatively stable thermal condition.

But the temperature is not the same everywhere.

The core, windings, insulation, clamps, and structural components can all operate at different temperatures. Some sections of the winding can become considerably hotter than the average winding temperature. These areas are generally called hot spots.

That distinction matters because insulation aging is strongly influenced by the hottest areas. Measuring the outside of the transformer can provide useful information during maintenance, but it does not necessarily tell you what is happening inside the winding.

IEC 60076-11 provides requirements for temperature-rise testing of dry-type transformers. Common reference values include approximately 80 K for Class B insulation, 100 K for Class F, and 125 K for Class H, depending on the applicable standard and transformer design.

Resin dry-type transformers, especially cast-resin transformers, use epoxy resin as an important part of the winding insulation system.

The resin provides electrical insulation and mechanical support while helping protect the windings from moisture, dust, and other environmental contaminants. At the same time, it becomes part of the heat-transfer path.

That means the thermal properties of the resin matter. So do the winding arrangement, conductor size, resin thickness, cooling ducts, and total transformer losses.

Cast-resin transformers are commonly used in commercial buildings, industrial plants, renewable energy installations, transportation infrastructure, and other applications where environmental protection and reliable insulation are important.

The temperature rise of a resin dry-type transformer core mainly comes from core losses.

Hysteresis and eddy-current losses occur whenever the core is energized. Their level depends on the magnetic material, operating frequency, flux density, lamination thickness, and overall core design.

Using high-quality electrical steel and keeping the magnetic flux density within an appropriate range can help reduce core losses. But reducing losses is only part of the job. The heat that remains still needs a good path out of the transformer.

Excessive stray flux or poor magnetic design can also create localized heating in the core and nearby metallic components. This is why electromagnetic and thermal design need to be considered together rather than separately.

During factory temperature-rise testing, the transformer is operated until it reaches a stable thermal condition. The measured results help confirm that it can operate at its rated conditions without exceeding the specified limits.

The temperature rise of resin dry-type transformer windings is one of the most important thermal considerations in cast-resin transformer design.

Current flowing through the conductors produces heat. That heat then has to travel through the insulation and resin before it can reach the surrounding air.

As a result, conductor size, current density, winding arrangement, insulation thickness, resin thermal properties, cooling paths, and load losses all affect the final winding temperature.

And again, the temperature inside a winding is not perfectly uniform. An internal hot spot may be significantly hotter than the outer surface of the resin.

This is why an infrared thermometer is useful for checking external conditions during maintenance, but it cannot by itself determine the actual internal hot-spot temperature.

This becomes even more important with demanding loads. Industrial machinery, data centers, renewable energy equipment, and power-electronic systems can create rapidly changing or relatively high load levels. A transformer selected for these applications should be evaluated based on its actual operating profile, not just its nominal kVA rating.

The insulation thermal class is another important part of the temperature-rise picture.

Insulation Class	Maximum Insulation-System Temperature	Common Temperature-Rise Reference
Class B	130°C	80 K
Class F	155°C	100 K
Class H	180°C	125 K

For example, if the ambient temperature is 40°C and the specified temperature rise is 100 K, the average winding temperature would be approximately 140°C.

That does not mean every point in the winding is exactly 140°C. Internal hot spots can be higher.

A higher insulation class provides greater thermal capability, but it should not be interpreted as a reason to deliberately operate the transformer hotter. Keeping operating temperatures under control still helps reduce insulation aging and provides greater thermal margin.

Both resin and non-resin dry-type transformers can provide reliable thermal performance when properly designed.

Feature	Non-Resin Dry-Type Transformer	Resin Dry-Type Transformer
Winding insulation	Varnish, air, solid insulation, etc.	Typically epoxy resin
Main thermal consideration	Air circulation and cooling ducts	Resin thermal path and winding design
Environmental protection	Depends on insulation system	Generally strong
Mechanical protection	Depends on construction	Strong encapsulation
Cooling	AN or AN/AF	AN or AN/AF
Typical applications	Industrial and general distribution	Commercial, industrial, renewable energy and infrastructure

There is no simple rule saying that one type always has a lower temperature rise.

A well-designed non-resin transformer can dissipate heat effectively through open winding structures and good airflow. A cast-resin transformer, meanwhile, can combine strong mechanical and environmental protection with good thermal performance when its resin and winding design are properly engineered.

The better choice depends on the application, installation environment, required insulation system, loading conditions, and project specifications.

The basic idea is pretty straightforward: generate less heat and remove the heat that is generated more effectively.

Low-loss core materials, properly sized conductors, optimized winding structures, and effective cooling channels can all help reduce temperature rise during the design stage.

Correct transformer sizing is just as important. A transformer that operates continuously at or close to full load will naturally experience greater thermal stress than one with some spare capacity.

The installation environment matters too. There should be enough space around the transformer for air circulation, and ventilation openings should remain clear. If the transformer is installed inside an electrical room, the room's ventilation or air-conditioning system should account for the heat released by the transformer.

For transformers designed for AN/AF operation, forced-air cooling can provide extra capacity during periods of high demand. Fans and controls should be checked regularly so that the additional cooling is actually available when needed.

Routine maintenance can prevent many avoidable thermal problems. Dust buildup can restrict airflow, while loose electrical connections may create localized hot spots. Where nonlinear loads are present, checking harmonic currents can also help identify unexpected sources of transformer heating.

Temperature has a direct relationship with insulation aging. Continuous operation at excessive temperatures can accelerate insulation deterioration and eventually shorten transformer service life.

Still, temperature rise should not be considered on its own. Insulation class, ambient temperature, loading, cooling method, and transformer design all work together.

When selecting a dry-type transformer, it is therefore better to look beyond the basic voltage and kVA ratings. Important thermal specifications include temperature rise, insulation class, cooling method, no-load loss, load loss, ambient temperature, installation altitude, harmonic loading, and expected load profile.

The temperature rise of dry-type transformers is an important part of evaluating transformer performance and reliability. Core, winding, stray, and additional losses create heat, while the transformer's construction and installation environment determine how effectively that heat is removed.

For non-resin dry-type transformers, the heat dissipation surface, airflow, and winding cooling ducts play a particularly important role. For resin dry-type transformers, the thermal properties of the resin and the design of the encapsulated windings are equally significant.

The temperature-rise value should never be viewed as an isolated figure. It makes much more sense when considered alongside insulation class, ambient temperature, transformer loading, cooling method, losses, altitude, and harmonic content.

For industrial plants, commercial buildings, renewable energy projects, data centers, and other demanding applications, choosing a dry-type transformer with the right thermal design can help keep operating temperatures under control, slow insulation aging, and support reliable operation over the transformer's service life.

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