1. Introduction
Flip the switch on almost any modern building and there's a good chance a dry-type transformer is behind it. In my experience it's one of the most dependable pieces of gear we've got in power distribution, and unlike the old oil-filled kind it cools itself with air instead of liquid. That one difference is exactly why, in this line of work, temperature is the thing you can never afford to stop watching.
There are two products that handle that job, and honestly, on their own neither one does you much good:
The temperature controller - I think of it as the brain. It watches the winding temperature and decides when to cool, warn, or shut things down.
The cross-flow cooling fan - the lungs. It lays a wide, even sheet of air across the transformer to carry the heat away, and it does it quietly.
In this write-up I'll walk you through what these two actually are, how they do their job, where they came from, and where I think the industry is heading.
A dry-type transformer installed indoors, showing the flat, fin-like cooling surfaces on the windings.
2. Dry-Type Transformers: The Context
2.1 Definition
Let me start with the basics. A dry-type transformer is exactly what the name says: no oil, no liquid insulation at all. The windings and core sit out in open air and shed heat by circulation - either on their own (natural convection) or with a fan's help. For me the big selling point is what's missing: no oil means no fire hazard, no spill containment to worry about, and a smaller footprint. That's why I keep seeing them tucked into commercial buildings, factories, hospitals, metro stations, data centers, and renewable-energy sites.
Cutaway diagram of a cast-resin dry-type transformer, with the windings and internal air ducts clearly labeled.
2.2 Brief History
Dry-type units aren't new, but the cast-resin version most of us work with today really took off from the 1970s onward. Stricter fire codes pushed buildings away from oil-filled transformers, and that opened the door.
2.3 The Thermal Challenge
Here's the part that keeps me focused. With no oil to soak up and move heat around, a dry-type transformer's temperature really comes down to three things:
Load - the current through the windings makes heat (that's the I-squared-R loss you might remember from school).
Ambient temperature - we usually rate it at 40 °C.
Cooling method - either AN (air natural) or AF (air forced, i.e. fan-assisted).
The insulation wrapped around the windings is graded by the highest temperature it can take continuously, and we rate it against IEC 60076-11 and IEC 60085. In dry-type units the common classes sit in the F and H range.
If the windings run hotter than their class allows, the insulation ages fast and can fail outright - I've watched it happen. That's the whole reason we need constant temperature watching, and why, once things warm up, we add forced-air cooling. Which is precisely what the two products below are for.
A simple bar chart of insulation classes (F, H) with their continuous-temperature limits marked.
3. Dry-Type Transformer Temperature Controllers
3.1 Definition
So what is a dry-type transformer temperature controller? To me it's a small electronic instrument whose only real job is reliability. It reads the winding temperature - all three phases, sometimes the core too - shows it to you, and then automatically:
starts and stops the cooling fans when the temperature crosses the set points;
raises an alarm if things get too hot;
sends a trip signal to the breaker at the danger point;
flags faults - a broken sensor, say;
talks to SCADA or whatever remote monitoring you've got.
In plain terms: it stops the transformer from cooking itself. And that's the number-one cause of early insulation failure I get called about.
3.2 How It Works
Under the hood, the controller senses temperature with Pt100 RTD sensors - platinum thermometers that read 100 Ω at 0 °C - embedded in or strapped onto the windings. Here's the chain, start to finish:
Pt100 sensors (3 phases + core)
resistance changes with temperature
signal conditioning and A/D conversion
digitized temperature
microprocessor (compares against set points, runs the logic)
outputs
fan start/stop → over-temp alarm → trip signal → RS485 / Modbus
Those thresholds - say, fan on at 100 °C, alarm at 130 °C, trip at 155 °C for Class F - are adjustable, so you can match the controller to the transformer's insulation class and to how you actually want it run.
Video: Real product footage - a dry-type transformer equipped with PT100 temperature controller and cooling fan.
Watch on YouTube - Dry-type transformer with PT100 temperature controller & cooling fan
3.3 Core Functions at a Glance
Day to day, a controller is quietly doing five things: measuring, deciding, cooling, warning, and reporting. Most of the time you'll never notice it - and honestly, that's exactly the point. The best ones are the ones you forget are even there.
3.4 Types of Controllers
Mechanical (analog) controllers - bimetallic strips or capillary thermostats. They're simple and cheap, but not very accurate, and I rarely see them in new equipment anymore.
Digital (microprocessor) controllers - this is the standard today, and the one I'd spec. Accurate to about ±1 °C, multi-channel, fully programmable, and built to talk to other systems. In China this family is widely known as the BWDK series (BWDK-3207-type units are typical); elsewhere you'll see BWD-3K and similar.
3.5 Brief History
1970s–1980s: mechanical thermostats and the first electronic monitors show up alongside cast-resin dry-type transformers.
1980s–1990s: we develop microprocessor-based digital controllers - the BWDK series - in China, and they quickly become the default for dry-type protection: three-phase measurement, fan logic, and alarm/trip outputs all in one box.
2000s–present: digital displays, black-box recording, RS485/Modbus, and networking for substation automation.
3.6 Standards & Compliance
If you're exporting controllers to North America or other regulated markets - and we do - certification to something like UL 61010-1 isn't optional. It's the gate you have to pass, full stop.
4. Cross-Flow Cooling Fans
4.1 Definition
A cross-flow fan - also called a tangential or transverse-flow fan - is the odd one out in the fan world, and the one I reach for most on transformers. Air goes across the wheel: in through the blades on one side, through the hollow center, and out the other side. The wheel itself is a long cylinder lined with many small, forward-curved blades.
For transformer cooling, you mount these along the cooling ducts - under or beside the enclosure - and they lay a wide, even band of airflow right along the long, flat cooling surface of the windings. Which is exactly the shape of air a transformer wants.
A cross-section of a cross-flow fan showing air entering one side of the wheel and exiting the other.
4.2 Working Principle
Here's how it works. A motor spins a long cylindrical wheel full of curved blades.
Air enters across one side of the blade row.
Inside the wheel, an eccentric vortex forms and the air passes through the blades twice - in, then out.
It exits along the full length of the wheel as one uniform, straight band of air.
Because the airflow is generated across the whole length instead of from a single point, it stays even over a wide area. And that's the big win over a normal axial fan - the reason I'd pick cross-flow for a transformer every time.
Video: Real CFD simulation of cross-flow fan airflow.
Watch on YouTube - How a cross-flow fan works (Longwell)
4.3 Brief History
1892
French engineer Paul Mortier invents the cross-flow fan (patented 1893). That long-wheel, even-output idea was decades ahead of its time - I still find it clever.
1950s–1970s
Cross-flow fans become a staple in HVAC, most visibly in room air conditioners, where their slim shape and even flow were a perfect fit.
1980s–present
The design moves into industrial equipment cooling, including dry-type transformer forced-air cooling - the place where long, uniform airflow and low noise decide the purchase.
4.4 Why Cross-Flow for Transformer Cooling? (vs. Axial Fans)
When I talk to transformer makers, they reach for cross-flow fans when uniform coverage, low noise, and a slim mount matter most. That's usually the case in a building or a subway station - places where you can't have a loud fan screaming all day.
4.5 Construction & Materials
An industrial cross-flow cooling fan is typically built from:
a long cylindrical impeller (often aluminum or engineered-plastic blades on a metal shaft);
a shaded-pole or EC motor;
a scroll-shaped housing that shapes the airflow;
mounting brackets and guards.
When we build ours, I pay the most attention to the impeller balance and the motor - those are what decide whether the fan runs quiet and lives a long life.
A labeled exploded view of a cross-flow fan, with the impeller, motor, housing, and brackets called out.
5. The System: Controller + Fans Working Together
Here's the thing I want you to take away: these two products are really one closed-loop system, not two separate boxes.
Winding temp rises → Pt100 senses it → controller hits the set point
fan relay closes → cross-flow fans run (forced air, AF rating)
temp drops below the hysteresis point → fans stop on their own
if it keeps climbing: alarm → trip signal to the breaker
This "cool on demand" approach is what I'd call the whole point:
Extends transformer life - insulation stays inside its rated class.
Saves energy - fans spin only when they have to, not all day (I like that part).
Adds safety - over-temp protection heads off catastrophic failure.
Enables supervision - remote monitoring turns a dumb part into a managed asset.
A one-page system diagram: sensor → controller → fan → breaker, with the feedback loop drawn in.
5.1 Typical Applications
I've seen these systems spec'd into just about every setting you can name:
Indoor distribution substations and switchgear rooms
Commercial buildings, hospitals, airports, and metro systems
Industrial plants - steel, cement, chemical, mining
Renewable energy: wind nacelles, solar stations
EV charging infrastructure
Data centers and UPS systems
Video: Real product footage of cross-flow cooling fans applied to dry-type transformers.
Watch on YouTube - Resin-insulated dry-type transformer, fan automatic start/stop
5.2 Industry Trends
Where I see the industry going:
Smart monitoring: controllers with RS485/Modbus, IoT gateways, and cloud dashboards for predictive maintenance. We're shipping more of these every year.
Higher-efficiency fans: EC motors cut power use and allow variable-speed cooling.
Quieter operation: better impeller and aerodynamic design for noise-sensitive urban sites.
Higher compliance: UL, CE, IEC certification is increasingly required for export - and it's getting harder to skip.
Digital twin / AI analytics: temperature history (the black-box data) is being used to model aging and tune loading. I think that's the most interesting one of the lot.
If you're specifying temperature controllers or cooling fans for your own dry-type transformers, it's worth seeing how the pros build them. At Perigon Intelligent (Shanghai) Limited, we design and manufacture both - the controllers and the cross-flow fans - as one matched thermal-management package.
Have a look at our dry-type transformer cooling solutions :
6. Glossary
Quick reference - the terms I keep using above:
Pt100 - a platinum resistance temperature sensor (100 Ω at 0 °C); the standard way we measure winding temperature.
AN / AF - Air Natural (cooling by convection) vs. Air Forced (fan-assisted cooling).
Insulation class (e.g., F, H) - the maximum continuous temperature the winding insulation can handle.
BWDK - the common Chinese designation for digital dry-type transformer temperature controllers.
EC motor - an electronically commutated motor; efficient, variable-speed, common in modern fans.
Cross-flow fan - a tangential fan that moves air across its long cylindrical impeller for even, wide output.
A note before you go: this primer is an industry-knowledge overview for general education, not a spec sheet. For any application-specific decision, I'd always refer to the transformer manufacturer's documentation, the applicable standards (IEC 60076-11, UL 61010-1, and the like), and the controller/fan manufacturer's own technical specifications.