Most people rarely think about transformers unless their work depends on them. They stay in the background, unnoticed in daily life. Still, their impact is everywhere. Each time you switch on a light, charge a phone, or use any electrical device, a transformer is making that possible behind the scenes.
Its job is simple, but essential. A transformer adjusts electrical energy from one voltage level to another so it can be safely used.
At its core, a transformer has two main parts:
- A primary coil that receives incoming voltage
- A secondary coil that changes that voltage to a different level
If you want a deeper explanation of how these coils function, you can explore a full guide on electrical transformers.
The type most people rely on directly is the distribution transformer. These units reduce high voltage to levels suitable for everyday use in homes, factories, malls, and airports. Because power systems differ across locations, various transformer designs exist, each built to meet specific needs.
Differences Between Liquid-Filled vs. Dry-Type Transformers
Transformers generally fall into two main groups: liquid-filled and dry-type.
Liquid-Filled Transformers
Liquid-filled transformers use a dielectric fluid to both cool and insulate their internal windings. This cooling happens through natural convection, where the fluid circulates and carries heat away.
Most of these transformers have an insulation rating of about 120°C, though other options exist. A common choice includes designs that use enhanced Kraft paper (TUK).
Their sealed tank structure makes them well suited for outdoor use. Because the fluid transfers heat efficiently, these transformers tend to cool better than dry-type units. As a result, they are also more capable of handling overload conditions without damage.
Dry-Type Transformers
Dry-type transformers are mainly intended for indoor environments. They usually have a higher insulation rating, around 220°C.
Instead of liquid, they rely on surrounding air for cooling. Air flows naturally around the windings, which is why these units often have ventilated enclosures.
They typically run at higher internal temperatures compared to liquid-filled transformers, since they produce more heat during normal operation. In overload situations, they are generally less resilient unless specially designed.
Because they do not use any fluid, dry-type transformers do not require containment systems. This makes them a safer and more practical option for indoor installations.
Types of Liquid-Filled Transformers
Liquid-filled transformers come in several designs, each built for specific environments and uses.
Padmount Transformers
“Padmount” refers to a compartmentalized, tamper-resistant transformer installed on a concrete pad. Because of their secure design, they are commonly placed in public spaces such as airports, campuses, and commercial buildings.
These units are typically painted green so they blend into outdoor surroundings. At the front, a cabinet houses cable connections along with controls and monitoring components.
Inside, padmount transformers may include bayonet fuses and under-oil rotary load-break switches. Since both protection and switching are built in, they can operate without additional switchgear. This all-in-one setup often makes them a cost-effective choice.
Substation Transformers
Substation transformers feature a stronger, more durable tank structure. Their control and monitoring equipment is mounted outside the tank for easy access.
Many of these transformers use external cooling fans to support higher load capacity. They are usually installed in industrial locations that are not open to the public.
They rarely operate alone. Instead, they work alongside other electrical equipment as part of a larger system known as a substation, which is where they get their name.
Pole-Mount Transformers
Pole-mount transformers are installed on utility poles and are widely used in power distribution. Most are single-phase units, making them common in residential areas where electricity is reduced for home use.
For larger commercial or industrial needs, multiple units can be grouped together to supply three-phase power. However, in many areas, underground systems—such as small padmount transformers—are gradually replacing single-phase pole-mounted units.
Submersible / Network Transformers
Submersible, also called network or vault-type transformers, are designed for underground installation. Their construction accounts for limited space and higher surrounding temperatures.
They are often used in dense urban areas to supply power to apartments and office buildings. These transformers are built to withstand harsh conditions, including extended exposure to water during flooding.
Types of Dry-Type Transformers
Low Voltage Dry-Type Transformers
Low voltage dry-type transformers are used when both the primary and secondary voltages are 600 volts or less. They are commonly installed inside commercial or industrial buildings to raise or lower the building’s service voltage so a specific piece of equipment can run at its required level. Because of their floor-mounted enclosures and flexible use, they are often called general-purpose dry-type transformers.
Medium Voltage Dry-Type Transformers
Medium voltage dry-type transformers are usually built in a unit substation-style enclosure. They are often installed in a line with other distribution gear, typically with a medium-voltage switch on the high side and low-voltage switchgear on the other end. In some cases, a single transformer can be installed as a standalone unit, with power cables entering the primary and secondary through junction boxes or air terminal chambers.
Cast Coil Transformers
Cast coil transformers are a tougher variation of medium voltage dry-type units. Their windings are fully embedded in a solid epoxy resin, which seals and protects them. This design makes them a strong choice in damp or harsh environments where open windings on a standard dry-type could absorb moisture or be damaged.
Autotransformers
An autotransformer uses a single coil that serves as both the primary and the secondary winding. Because there is no separate secondary, it does not provide electrical isolation between input and output, so it is not suited for applications where isolation is required. The shared winding reduces copper and core material, which usually makes these transformers smaller and more economical for many everyday step-up or step-down needs. When the required voltage change is only about plus or minus 5–20 percent, a buck-boost style transformer can often handle the job effectively.
Drive-Isolation Transformers
Drive-isolation dry-type transformers are built for loads with motor drives, such as variable frequency drives. Like standard isolation transformers, they have separate primary and secondary windings, but they are also designed to withstand heavier, non-linear loading and higher harmonic levels. These transformers are K-rated, typically starting at a K-4 rating, and include a 200 percent rated neutral conductor plus an electrostatic shield between the windings. They are suitable when the system has more harmonic content than normal and extra robustness is needed.
Encapsulated Transformers
Encapsulated transformers are another type of cast-resin unit. Their windings, and often the core, are completely surrounded by a mix of sand and epoxy or similar resin that is cured into a solid shell. This sealed construction keeps out moisture, dust, and other contaminants, making them well suited for tough environments where ordinary ventilated dry-type transformers would be exposed to dirt and humidity.
If you are unsure which transformer type is right for your project, it usually helps to talk through your electrical and site requirements with a specialist who can match a unit to your application. Maddox keeps thousands of new and remanufactured transformers in stock and ready to ship, so they can typically provide a suitable option on short notice. You can contact their sales team directly to discuss your needs and request a quote for the transformer that best fits your project.
FAQ
What is the main difference between liquid-filled and dry-type transformers?
Liquid-filled transformers use dielectric fluid for cooling and insulation, while dry-type transformers rely on air or resin systems and are commonly used indoors.
Where are padmount transformers typically used?
Padmount transformers are usually installed outdoors in public or commercial areas, providing tamper-resistant, low-profile power distribution.
When should I choose a dry-type transformer?
Dry-type transformers are ideal for indoor installations, or where fluid containment is not desired, and are common in commercial and industrial buildings.
What are cast coil and encapsulated transformers used for?
Cast coil and encapsulated transformers are designed for harsh or high-moisture environments, protecting windings with resin or resin-and-sand encapsulation.
How do I know which transformer type is best for my project?
The best transformer depends on voltage levels, location (indoor or outdoor), environmental conditions, load profile, and whether you need isolation; consulting a transformer specialist is recommended.