Introduction
Choosing the right transformer construction can feel overwhelming when you’re facing dozens of technical
specifications and design options. After spending over 10 years on our factory floor building transformers
ranging from small EI units to large power transformers, I’ve seen firsthand how construction choices impact
performance, efficiency, and reliability.
In this comprehensive guide, I’ll walk you through everything you need to know about transformer
construction, focusing on core and windings—the two most critical components
that determine how well your transformer performs. Whether you’re an engineer selecting a transformer for a
new project or a technician troubleshooting issues, understanding these fundamentals will help you make
better decisions.
We’ll cover the differences between core-type
and shell-type transformers, explore various winding configurations, and share some hard-won lessons
from the factory floor that you won’t find in textbooks.
What is Transformer Construction?
Transformer construction refers to the physical design and assembly of the components that
enable a transformer to convert electrical energy from one voltage level to another through electromagnetic
induction. According to Wikipedia, a transformer is “a passive component that transfers electrical
energy from one electrical circuit to another circuit, or multiple circuits.”
The construction of any transformer revolves around three essential components:
- Magnetic Core (Iron Core): Provides a low-reluctance path for magnetic flux to flow
between the primary and secondary windings - Windings (Primary and Secondary): Copper or aluminum coils that carry current and
create/receive the magnetic field - Insulation System: Materials that prevent electrical contact between windings and the
core
When alternating current flows through the primary winding, it creates a varying magnetic
field that induces a magnetic flux in the core. This flux links with the
secondary winding, inducing a voltage through electromagnetic induction. The efficiency of this energy
transfer depends heavily on how well the transformer cores are designed to maximize
magnetic coupling between the windings.
Transformer Core Construction
The transformer core is the heart of any transformer. Its primary function is to concentrate
the magnetic flux and provide an efficient path for the magnetic field to link the primary
and secondary windings. Let’s explore the materials and designs that make this possible.
Core Materials: Why Silicon Steel?
Most modern transformer cores are constructed from Cold Rolled Grain Oriented Silicon Steel,
commonly known as CRGO steel. This material offers:
- High Permeability: About 1,500 times better than air at conducting magnetic flux
- Low Hysteresis Loss: Silicon content reduces molecular friction during magnetization
cycles - Reduced Eddy Currents: High resistivity limits circulating currents within the core
The silicon steel is manufactured as thin laminated sheets, typically 0.25mm to 0.5mm thick.
Each lamination is insulated from its neighbors by a thin coating of varnish or oxide layer. This
construction drastically reduces eddy current losses—something I learned the hard way early in my career
when we received a batch of poorly insulated laminations that caused a transformer to overheat during
testing.
Common Lamination Shapes
| Shape | Description | Common Application |
|---|---|---|
| E-I | E-shaped + I-shaped pieces | Standard shell-type cores |
| E-E | Two E-shaped pieces | Smaller transformers |
| L-shaped | L-shaped laminations | Special configurations |
| U-I | U-shaped + I-shaped | Core-type designs |
Core Types Comparison: Core-Type vs Shell-Type
The way the core and windings are arranged defines whether a transformer is “core-type” or
“shell-type.” This fundamental distinction affects performance, cost, and application suitability.
| Characteristic | Core-Type Transformer | Shell-Type Transformer |
|---|---|---|
| Structure | Windings surround the core | Core surrounds the windings |
| Magnetic Path | Single magnetic circuit | Dual magnetic path (Φ/2 in each outer limb) |
| Leakage Flux | Higher | Lower |
| Cooling Efficiency | Better (windings exposed) | Moderate |
| Maintenance Access | Easier | More difficult |
| Mechanical Strength | Lower | Higher |
| Manufacturing Cost | Lower | Higher |
| Best For | Distribution transformers, smaller units | Large power transformers, high-voltage applications |
In a core-type transformer, the primary and secondary windings are wound
concentrically around the vertical limbs of a rectangular core. This design is economical and provides
excellent cooling because the windings are exposed on the outer surface.
A shell type transformer has both windings wound on a central limb, with the core forming a
“shell” around them. The magnetic flux divides equally between the two outer limbs, reducing leakage flux
and improving efficiency—but at a higher manufacturing cost.
Core Joint Techniques
How the limbs and yokes of a transformer core are joined together significantly impacts performance:
Interleaved Joints: The simplest approach where laminations are stacked with flush butt
joints. The magnetic flux enters and exits perpendicular to the grain orientation, causing higher
“cross-grain” losses. We typically use this for smaller, cost-sensitive transformers.
Mitred Joints (45° Cut): Laminations are cut at 45° angles, allowing flux to enter and exit
more smoothly in the direction of flow. This minimizes cross-grain losses but increases manufacturing
complexity. For large power transformers where loss minimization is critical, mitred joints
are the standard choice.
0.5mm gap can noticeably increase magnetizing current and core losses.
Transformer Winding Construction
While the core provides the magnetic path, the transformer winding is where the electrical
action happens. The windings are the current-carrying conductors that create and capture the electromagnetic
field.
Winding Materials: Copper vs Aluminum
The choice between copper and aluminum conductors involves trade-offs:
| Property | Copper | Aluminum |
|---|---|---|
| Resistivity @20°C | 0.0175 Ω·mm²/m | 0.0285 Ω·mm²/m |
| Density | 8,900 kg/m³ | 2,700 kg/m³ |
| Tensile Strength | 24 kgf/mm² | 10 kgf/mm² |
| Relative Cost | Higher | Lower |
| Thermal Conductivity | 393 W/m·°C | 203 W/m·°C |
Copper conductors are preferred for small to medium type transformers (up
to several hundred kVA) due to their superior conductivity, mechanical strength, and compact size. The
primary winding and secondary winding in our EI transformers are almost exclusively
wound with copper.
Aluminum conductors require about 60% larger cross-sectional area to carry the same current
as copper, but weigh only 48% as much. This makes aluminum attractive for large power
transformer applications where weight and cost savings outweigh the size increase.
Types of Transformer Windings
Different applications require different winding configurations. Here are the four main types used in
cores types construction:
1. Cylindrical Winding
The simplest winding design, consisting of layers of conductors wound helically around a cylindrical former.
- Conductor Type: Round or rectangular (strip)
- Application: LV windings up to 6.6 kV, ratings up to 600-750 kVA
- HV Application: Up to 33 kV, 800 kVA, 80 A maximum
- Features: Multi-layer designs with oil ducts (5-8mm) between layers for cooling
2. Helical Winding (Spiral Winding)
Designed for low-voltage, high-current applications where the number of turns is small but current flow is
substantial.
- Current Capacity: Up to 2,000 A
- Parallel Conductors: 6-20 strips in parallel
- Rating Range: 160 kVA to 30 MVA, 400 V to 33 kV
- Features: Axial spacers between turns improve oil circulation; excellent mechanical
strength
We use helical windings extensively in our toroidal transformers for audio
applications where low DCR and high current capacity are essential.
3. Crossover Winding
Suited for high-voltage windings in smaller transformers where current is limited.
- Current Limit: ≤20 A
- Voltage Application: High-voltage windings
- Construction: Divided into multiple coils with 0.5-1mm axial separation
- Inter-coil Voltage: Limited to 800-1,000 V
- Trade-off: Higher normal strength but lower impulse (surge) strength
4. Continuous Disc Winding
The workhorse winding for large capacity transformers, consisting of flat coils (discs) wound spirally from
center outward.
- Conductor Area: 4-50 mm²
- Current Range: 12-600 A
- Minimum Oil Duct Width: 6mm for 35 kV ratings
- Key Advantage: High axial mechanical strength and economical construction
- Flexibility: Can be used as both LV and high voltage windings
Winding Arrangements
Beyond the winding type, the arrangement of primary and secondary windings relative to each
other and the core matters significantly:
Concentric Arrangement: Used in core-type transformers. The LV winding is placed closest to
the core (requires less insulation from the grounded core), with the HV winding wound concentrically around
it. Half of each winding is typically placed on each limb to maximize magnetic coupling.
Sandwich (Interleaved) Arrangement: Predominant in shell-type transformers. HV and LV coils
alternate along the core limb, with end LV sections having half the normal turns. This arrangement allows
precise control of leakage reactance—the more subdivisions, the lower the leakage.
Insulating Materials in Transformer Construction
Proper insulation is critical for safety, longevity, and performance. The insulating
materials used in transformers must:
- Withstand operating voltages without breakdown
- Resist thermal degradation at operating temperatures
- Provide mechanical support for windings
Insulation Classes
| Class | Maximum Temperature | Typical Materials |
|---|---|---|
| A | 105°C | Cotton, paper, varnish |
| B | 130°C | Mica, glass fiber |
| F | 155°C | Modified polyester, epoxy |
| H | 180°C | Silicone, polyimide |
Dry-Type vs Oil-Immersed Insulation
Dry-type
transformers use solid insulation systems:
- Varnish-coated magnet wire for small units
- Epoxy-cast windings for medium-voltage applications
- Vacuum pressure impregnated (VPI) systems for harsh environments
Oil-immersed
transformers use mineral oil that serves dual purposes:
- Electrical insulation (high dielectric strength)
- Cooling medium (circulates via natural convection or pumps)
- Paper-insulated conductors immersed in oil
The high voltage winding always requires more robust insulation, typically multiple layers
of oil-impregnated paper or crepe tape in oil-filled designs.
Cooling Systems for Transformer Core and Windings
Heat is the enemy of transformer longevity. A rule of thumb: every 7-10°C increase in operating temperature
halves insulation life. Effective cooling is essential.
Cooling Designations
| Code | Description |
|---|---|
| ONAN | Oil Natural, Air Natural (oil circulation by convection, air cooling by natural ventilation) |
| ONAF | Oil Natural, Air Forced (adds cooling fans) |
| OFAF | Oil Forced, Air Forced (oil pumps + fans) |
| OFWF | Oil Forced, Water Forced (oil pumps + water cooling) |
| AN | Air Natural (dry-type, natural convection) |
| AF | Air Forced (dry-type with forced air fans) |
For our medium-sized power transformer customers, ONAN/ONAF designs are most common—they
provide good cooling performance without the maintenance burden of pumps.
Small dry-type units typically use
natural air convection (AN), with fans added (AF) for higher capacity or enclosed installations.
Core Losses: Hysteresis and Eddy Currents
Understanding hysteresis and eddy current losses is crucial for specifying efficient
transformers. These losses occur whenever the transformer is energized, regardless of load.
Hysteresis Loss
When alternating current magnetizes the iron core, the magnetic domains must realign each
cycle. This “molecular friction” generates heat.
Hysteresis Loss Formula
Ph = Kh × Bmax1.6 × f × V
Where:
- Kh = Hysteresis constant (material dependent)
- Bmax = Maximum flux density (Tesla)
- f = Frequency (Hz)
- V = Core volume (m³)
Lower-frequency operation (50 Hz vs 60 Hz) actually increases hysteresis per cycle, which is why transformers
designed for 60 Hz may overheat at 50 Hz if not properly derated.
Eddy Current Loss
Alternating magnetic flux induces circulating currents within the conductive core material
itself. These eddy currents dissipate energy as resistive heating.
Eddy Current Loss Formula
Pe = Ke × Bmax² × f² × t²
Where:
- Ke = Eddy current constant
- t = Lamination thickness
This is precisely why we use thin, insulated laminations—reducing thickness dramatically cuts eddy current
losses. Our standard 0.35mm CRGO laminations reduce eddy losses by over 80% compared to solid iron cores.
For more on how these losses affect overall transformer performance, see our guide on Ideal
Transformer vs Real Transformer.
Real-World Factory Experience
Core Assembly Quality Checks
When assembling laminated cores, we inspect every unit for:
- Rust or oxidation spots: Even small areas of corrosion can create hot spots and
increased losses - Bent or distorted laminations: Must be rejected—they create air gaps that increase
magnetizing current - Proper stacking tightness: Loose laminations will vibrate at twice the line
frequency (100/120 Hz), generating the characteristic transformer “hum”
assembly. If you hear rattling or feel vibration, the laminations aren’t tight enough.
Winding Quality Indicators
During winding operations, we watch for:
- Consistent tension: Uneven winding tension leads to mechanical stress during
thermal cycling - Proper insulation between layers: Even one bare spot can cause an inter-turn short
under surge conditions - Correct lead routing: Leads must exit without creating stress points
I recall a case early in my career where we had repeated failures in a batch of small type
transformers. The root cause? The winding former had developed a sharp edge that was
nicking the enamel coating on the primary winding during the winding process. The
damage wasn’t visible during production but caused field failures within weeks.
What a Healthy Transformer Sounds Like
A properly constructed transformer will produce a steady, quiet hum at 100 Hz (50 Hz mains) or 120 Hz (60
Hz mains). Warning signs include:
- Loud humming or buzzing: Often indicates loose core laminations or mounting
hardware - Crackling or snapping: May indicate insulation breakdown—immediate investigation
required - Unusual temperature: Core running significantly hotter than windings suggests core
material or joint problems
Selection Guide: Choosing the Right Construction
Selecting the optimal transformer construction depends on your specific application
requirements:
│
├─ Priority: Cost Efficiency?
│ └─ ✓ Core-Type + Cylindrical Winding
│
├─ High Voltage, Large Capacity?
│ └─ ✓ Shell-Type + Disc Winding
│
├─ Low Voltage, High Current?
│ └─ ✓ Helical Winding
│
├─ Compact Size, Low Noise?
│ └─ ✓ Toroidal Core Design
│
└─ Custom Requirements?
└─ Contact Transformer4U
For standard applications, core-type construction with cylindrical or helical windings offers the best
balance of performance and cost. For demanding high voltage or high-reliability
applications, shell-type construction with disc windings provides superior mechanical strength and lower
leakage inductance.
Not sure which approach is right for your project? Contact our
engineering team for personalized recommendations.
Frequently Asked Questions
What is the main purpose of a transformer core?
The transformer core provides a low-reluctance path for magnetic flux to flow
between the primary and secondary windings. This concentrates the magnetic field
and maximizes magnetic coupling, enabling efficient transfer of electrical
energy from the primary to secondary circuit. Without the core, most of the magnetic
flux would disperse into the air, drastically reducing transformer efficiency.
Why are transformer cores laminated?
Transformer cores are laminated (made of thin insulated sheets) to reduce eddy current losses. When
magnetic flux passes through a solid conducting core, it induces circulating
currents that generate heat and waste energy. Thin laminations (typically 0.25-0.5mm) break up these
eddy current paths, dramatically reducing losses. A laminated core can reduce eddy current losses by
80% or more compared to a solid iron core.
What is the difference between core-type and shell-type transformer construction?
In core-type transformer construction, the windings surround the core, with half of
each winding placed on each vertical limb. In shell type transformer construction,
the core surrounds the windings, forming a protective “shell.” Core-type offers better cooling and
easier maintenance access, while shell-type provides lower leakage flux, higher mechanical strength,
and better short-circuit withstand capability—making it preferred for large power
transformer applications.
What materials are used for transformer windings?
The two primary materials for transformer winding construction are:
- Copper: Higher conductivity, better mechanical strength, more compact—preferred
for small to medium transformers - Aluminum: Lighter weight (1/3 of copper), lower cost—requires larger
cross-section for same current capacity, used mainly in large power transformers
Both materials are used with insulation coatings (enamel for small units, paper for large oil-filled
units) to prevent inter-turn short circuits.
How do insulating materials protect transformer windings?
Insulating materials in transformers serve three critical functions:
- Electrical Isolation: Prevent short circuits between turns, between windings,
and between windings and the grounded core - Dielectric Strength: Withstand operating voltages and transient surges without
breakdown - Thermal Protection: Many insulation systems also help dissipate heat while
maintaining their integrity at operating temperatures
The insulation class (A, B, F, or H) indicates the maximum continuous operating temperature the
materials can withstand.
Key Takeaways
- Silicon steel laminations reduce core losses by limiting eddy currents while
providing excellent magnetic properties - Core-type construction offers cost efficiency and easy maintenance;
shell-type provides superior mechanical strength and lower leakage - Winding selection (cylindrical, helical, crossover, or disc) should match your
voltage, current, and capacity requirements - Proper insulation is critical for safety and longevity—always specify the
appropriate thermal class for your application - Cooling design directly impacts transformer life expectancy—every 7-10°C
temperature reduction extends insulation life
Need a Custom Transformer Solution?
Our engineering team at Transformer4U brings decades of hands-on manufacturing experience to help you
select the optimal transformer construction for your application.