A complete guide to transformer core material, windings,
insulation, transformer tank material, and more — by
You probably walk past a transformer every day without a
second thought — mounted on a pole, humming quietly outside a factory, or
tucked inside a substation. But have you ever wondered what a transformer is
actually made of, and why those specific materials were chosen?
The answer matters more than you think. The transformer
core material, the type of winding wire, the insulation quality, the transformer
tank material — every single choice directly affects efficiency, safety,
lifespan, and cost. At Makpower Trans-Systems, we've been building
high-performance transformers from our facility in Kolkata for decades, and
material selection is something our engineering team takes very seriously.
This guide breaks down the full transformer material list
— part by part — so you understand exactly what goes into a well-built
transformer and why it matters.
1. Transformer Core — The Magnetic Heart of the Machine
The core of transformer is made up of a material that
can carry magnetic flux efficiently. It forms the path through which magnetic
energy travels from the primary winding to the secondary winding — and the
quality of this path determines how much energy gets wasted as heat.
Which material is used in the core of a transformer?
The most widely used material in the core of a transformer is Cold
Rolled Grain Oriented (CRGO) Silicon Steel, also known as grain-oriented
silicon steel. It has high magnetic permeability, which means it allows
magnetic flux to pass through easily, and very low core losses (hysteresis and
eddy current losses), which keeps the transformer running efficiently.
Here's a quick overview of common transformer core materials:
•
CRGO Silicon Steel — Industry standard. Best
combination of low loss and high permeability. Used in most distribution and
power transformers.
•
Amorphous Steel (Metallic Glass) — Significantly
lower core losses than CRGO — up to 70% less. Ideal for energy-efficient
transformers.
•
Soft Iron (in older designs) — High permeability
but higher losses. Rarely used in modern industrial transformers.
The core is typically laminated — stacked in thin sheets
separated by insulating varnish. This lamination breaks the eddy current path,
dramatically reducing energy loss. Thinner laminations = less loss = higher
efficiency.
To understand how the core fits into the complete build, see
our page on the transformer manufacturing process.
2. Transformer Windings — The Electrical Conductors
The windings are coils of conducting wire wound around the
core. The primary winding receives electrical energy from the source; the
secondary winding delivers transformed voltage to the load. The quality and
material of the winding directly affects electrical efficiency, temperature
rise, and mechanical durability.
Transformer coil material: Copper vs. Aluminium
•
Copper — Superior electrical conductivity (~40%
better than aluminium), higher tensile strength, and better resistance to
corrosion. Copper windings allow smaller, more compact transformer designs for
the same power rating. Most of Makpower's industrial transformers use
high-grade copper windings.
•
Aluminium — Lighter and more affordable, but
requires a larger cross-section to carry the same current. More prone to
oxidation at connection points. Widely used in distribution transformers where
cost is a critical factor.
Both are viable — the right choice depends on your
application, budget, and space constraints. Read our detailed breakdown: Copper vs. Aluminium Winding in Distribution Transformers.
3. Insulation — What Keeps the Transformer Safe
Insulation in a transformer does two things: it prevents
electrical faults between components, and it protects against thermal damage.
Poor insulation is one of the leading causes of transformer failure — and it's
entirely preventable with the right materials.
Insulation materials used in transformer construction:
•
Kraft Paper and Pressboard — Wrapped between
winding layers to prevent inter-turn short circuits. These are oil-impregnated
in oil-type transformers to further improve dielectric strength.
•
Epoxy Resin — Used in dry-type transformers as a
cast insulation. Provides excellent dielectric properties and protects against
moisture.
•
Nomex (Aramid Paper) — High-temperature
insulation used in demanding applications. Can handle continuous temperatures
up to 220°C.
•
Transformer Varnishes and Impregnating Resins —
Applied to windings to improve moisture resistance and mechanical stability.
The insulation system's thermal class (like Class A, B, F, H)
defines the maximum permissible operating temperature — directly affecting
transformer lifespan. For dry-type designs, see Makpower's Dry Type Transformers which use Class F or H
insulation systems as standard.
4. Transformer Oil — Cooling and Insulation in One
In oil-filled transformers (the most common type in industrial
use), transformer oil serves a dual function: it is both a coolant and an
insulating medium. The oil circulates around the core and windings, absorbing
heat and carrying it away to the tank walls and radiators.
Types of transformer oil:
•
Mineral Oil (Naphthenic or Paraffinic) — The
industry standard. Cost-effective, excellent insulating properties, widely
available. Used in most of Makpower's oil-cooled transformers.
•
Silicone Oil — Fire-resistant. Ideal for indoor
or environmentally sensitive applications where mineral oil poses a risk.
•
Natural Ester (Bio-Based) Oil — Biodegradable
and environmentally friendly. Excellent moisture tolerance, making it suitable
for transformers in humid or ecologically sensitive areas.
Oil quality degrades over time due to oxidation, moisture
ingress, and thermal stress. Regular testing and timely changes are critical.
Learn more: Why Transformer Oil Changes Matter and Transformer Oil Testing: What to Test and When.
5. Transformer Tank — The Protective Enclosure
The transformer body material — primarily its tank — is
the outer enclosure that houses the core, windings, and oil. It isn't just a
box; the tank is engineered to withstand internal oil pressure, thermal
expansion, and harsh outdoor environments.
What is the transformer tank made up of?
•
Mild Steel (MS Plate) — The most common tank
material. It's strong, weldable, and cost-effective. Tank walls are typically
4–8mm thick depending on the transformer rating.
•
Stainless Steel — Used in applications requiring
high corrosion resistance, such as coastal or chemical industry installations.
•
Surface Treatment — Tanks are shot-blasted,
primed, and finished with anti-corrosion paint (typically two coats of epoxy
primer + topcoat). This is critical for long-term outdoor durability.
The tank also features welded cooling radiators or
corrugations that increase surface area for heat dissipation. The design of the
tank directly affects the transformer's thermal performance and protection rating
(IP class).
6. Bushings — The High-Voltage Entry Points
Bushings are insulated connectors that allow the high-voltage
and low-voltage conductors to pass safely through the transformer tank without
electrical contact with the tank wall. They are one of the most vulnerable
components — bushing failures account for a significant proportion of
transformer outages.
•
Porcelain Bushings — Traditional, robust, and
widely used for outdoor HV applications. High mechanical strength.
•
Epoxy Resin Bushings — Compact and
moisture-resistant. Preferred for indoor transformers and modern designs.
•
Oil-Impregnated Paper (OIP) Bushings — Used in
high-voltage power transformers. Provide excellent dielectric performance.
7. Tap Changer — Voltage Fine-Tuning
A tap changer allows the transformer's voltage ratio to be
adjusted without taking it out of service (on-load) or during shutdown
(off-load). It does this by selecting different numbers of winding turns.
•
Off-Circuit Tap Changer (OCTC) — Simple,
low-cost. Adjustment requires de-energising the transformer. Suitable for
systems where voltage adjustments are infrequent.
•
On-Load Tap Changer (OLTC) — Complex mechanism
that switches taps while the transformer is energised. Essential for power
transformers feeding variable industrial loads.
Key materials: high-conductivity copper contacts, robust
insulating materials, and in OLTCs, precision-engineered diverter switches
immersed in separate oil compartments.
Makpower's Power
Transformers can be equipped with OLTCs for demanding industrial
applications. Our Automatic Step Voltage Regulators offer a
related solution for continuous voltage regulation.
8. Cooling Systems — Managing Heat for Long Life
Every transformer generates heat — the goal is to remove it
effectively enough that the insulation and oil never exceed their rated
temperature limits. Cooling system design is one of the most important structural
decisions in transformer construction.
Common cooling methods and materials:
•
ONAN (Oil Natural Air Natural) — Radiators and
fins made from aluminium or steel dissipate heat passively. No moving parts.
Used in most standard distribution and power transformers.
•
ONAF (Oil Natural Air Forced) — Fans attached to
the radiators force air across them, increasing cooling capacity without
cooling the oil directly.
•
OFAF (Oil Forced Air Forced) — Pumps circulate
oil through external coolers while fans force air. Used in large power
transformers where space is limited and ratings are high.
•
AN (Air Natural) — Used in dry-type
transformers. No oil required; ventilation channels within the windings allow
natural air convection.
For a deeper look at cooling approaches and temperature
management, read our blog: Cooling Methods for Transformers. Also see: How Temperature Affects Transformer Lifespan.
9. Gaskets, Fittings, and Accessories
These smaller components are often overlooked, but they're
critical to safe, long-term operation:
•
Gaskets — Made from oil-resistant rubber (NBR)
or neoprene. Seal flanges, cover plates, and bushings to prevent oil leaks.
•
Breathers — Contain silica gel that absorbs
moisture from the air entering the conservator tank as oil expands and
contracts.
•
Buchholz Relay — Detects gas accumulation inside
the transformer, providing early warning of internal faults.
•
Temperature Indicators and PRDs — Monitor oil
and winding temperatures, and pressure relief devices prevent tank rupture in
fault conditions.
•
Steel Fasteners and Lifting Lugs — High-strength
galvanised or stainless steel hardware for structural assembly and
installation.
Quick Reference: Transformer Material List
Here's a summarised transformer raw material list at a
glance:
•
Core: CRGO Silicon Steel / Amorphous Steel
(laminated sheets)
•
Windings: Electrolytic Copper or Aluminium
conductor
•
Insulation: Kraft Paper, Pressboard, Epoxy
Resin, Nomex
•
Cooling Medium: Mineral Oil / Silicone Oil /
Natural Ester Oil
•
Tank (Body): Mild Steel plate with epoxy coating
•
Bushings: Porcelain / Epoxy Resin / OIP
•
Tap Changer: Copper contacts + insulating
materials
•
Radiators & Fins: Aluminium or Steel
•
Gaskets & Seals: NBR Rubber / Neoprene
•
Accessories: Silica Gel Breather, Buchholz
Relay, Thermometers, PRDs
Frequently Asked Questions
What is the transformer core made of?
The core of a transformer is made up of Cold Rolled Grain
Oriented (CRGO) Silicon Steel in most modern industrial and distribution
transformers. It is laminated — stacked in thin sheets insulated from each
other — to minimise eddy current losses. Amorphous steel is used in
high-efficiency designs where core losses need to be minimised.
Which material is most widely used in the core of a transformer?
CRGO (Cold Rolled Grain Oriented) Silicon Steel is the most
widely used transformer core material. Its grain-oriented structure gives it
high permeability in the rolling direction, which aligns with the direction of
magnetic flux in a well-designed core. This minimises core loss and keeps the
transformer running cool and efficiently.
What are transformers made of on the outside (body/tank)?
The transformer body — or tank — is typically fabricated from
mild steel (MS) plate, usually 4–8mm thick depending on the rating. It is
shot-blasted, cleaned, and coated with a corrosion-resistant primer and topcoat
paint system. Radiator fins or panels, also made from steel or aluminium, are
welded to the tank to increase the cooling surface area.
What are the parts of a transformer and their materials?
A transformer's main parts and their materials are: Core (CRGO
silicon steel), Windings (copper or aluminium), Insulation (kraft paper,
pressboard, epoxy), Tank (mild steel), Bushings (porcelain or epoxy),
Transformer Oil (mineral, silicone, or ester-based), Tap Changer (copper
contacts, insulation), Cooling fins (aluminium/steel), Gaskets
(rubber/neoprene), and accessories like breathers and relays.
What is transformer engineered materials — how does material choice affect
performance?
Transformer engineered materials refers to the deliberate
selection and engineering of every material in the transformer to meet specific
performance, safety, and efficiency targets. For example: using CRGO steel
reduces no-load losses; using copper instead of aluminium reduces winding
resistance and heat; using Class H insulation allows the transformer to operate
at higher ambient temperatures without degradation. Every material decision is
an engineering trade-off.
What is inside a transformer?
Inside an oil-filled transformer, you'll find: the laminated
steel core at the centre; copper or aluminium windings wrapped around the core
limbs; layers of insulating paper and pressboard separating the windings; oil
filling the tank to cool and insulate; a tap changer mechanism; and various
monitoring devices like temperature gauges and a Buchholz relay. In dry-type
transformers, there is no oil — epoxy resin-cast windings and air cooling
replace it.
How are transformers rated — what does kVA mean?
Transformers are rated in kVA (kilovolt-amperes), which
represents the apparent power the transformer can handle continuously without
exceeding its temperature limits. The rating depends on the cross-sectional
area of the core and windings, the quality of the insulation system, and the
cooling arrangement. A higher-rated transformer uses more conductor material and
a larger core. Voltage ratios (e.g. 11kV/433V) define the transformation ratio;
current capacity determines the physical size.
Explore Makpower's Transformer Range
Now that you understand the materials behind a transformer,
here's a look at how Makpower puts them to work across different product lines:
•
Distribution Transformers — BIS-certified,
copper or aluminium winding options, built to IS:1180.
•
Power Transformers — For high-voltage
industrial and substation applications.
•
Dry Type Transformers — Epoxy-cast windings, no oil, safe
for indoor environments.
•
Electric Arc Furnace Transformers — Heavy-duty
construction using premium-grade materials for extreme industrial conditions.
•
Induction Melting Furnace Transformers —
Designed for continuous heavy-duty furnace loads.
•
Automatic Step Voltage Regulators — For stable
voltage delivery across variable load conditions.
Related reading:
•
Construction of Transformer — Manufacturing Process
•
Dry Type vs. Oil Type Transformers: What You Need to Know
•
How to Improve Transformer Efficiency by Reducing Energy
Losses
•
Signs Your Transformer Needs Servicing or Replacement
•
Copper vs. Aluminium Winding in Distribution Transformers
Need a Transformer Built with the Right Materials?
At Makpower Trans-Systems Pvt. Ltd., every
transformer we build — from a 25 kVA distribution unit to a 60 MVA arc furnace
transformer — is engineered with carefully sourced, certified materials. Our
team can advise on the right material configuration for your application,
climate, and budget.
