Transformer Construction

Part of Power Transmission

Transformers change AC voltage levels, enabling efficient long-distance power transmission. Building a functional transformer from scrap iron and copper wire is one of the most valuable electrical skills in a rebuilding scenario.

Why Transformers Are Critical

Without transformers, electrical power cannot travel more than a few hundred meters before most of it is wasted as heat in the wires. A generator producing 100 volts would lose over 90% of its power to resistance in a 1-kilometer transmission line. By stepping voltage up to 1,000 volts (reducing current by 10x), the same wire carries the same power with only 1% loss.

Transformers are the reason AC won the “war of currents.” DC cannot be easily voltage-transformed (it requires motor-generator sets), but AC transforms with a simple device containing no moving parts: two coils of wire on a shared iron core.

How Transformers Work

A transformer has two windings (coils) on a shared magnetic core. AC current in the primary winding creates a changing magnetic field in the core. This changing field induces voltage in the secondary winding. The voltage ratio equals the turns ratio:

V_secondary / V_primary = N_secondary / N_primary

Transformer Type	Turns Ratio	Purpose
Step-up	Secondary > Primary	Increase voltage for transmission
Step-down	Secondary < Primary	Decrease voltage for use
1:1 (isolation)	Equal turns	Safety isolation, no voltage change

The Power Rule

Transformers do not create energy. Power in equals power out (minus losses). If you step voltage up by 10x, current drops by 10x. V1 x I1 = V2 x I2. This is why high-voltage transmission works: high voltage means low current means low I2R losses in the wire.

Core Design

The iron core provides a low-resistance path for magnetic flux between the primary and secondary windings. Core quality directly determines transformer efficiency.

Core Shapes

Shape	Description	Efficiency	Difficulty
I-core (rod)	Straight iron rod, coils at each end	Poor (50-70%)	Easy
U-core	U-shaped iron, coils on straight section	Good (80-90%)	Moderate
E-I core	E-shaped + I-shaped pieces interleaved	Very good (90-95%)	Moderate
Toroidal	Ring-shaped core	Excellent (95-98%)	Hard to wind

Building a Laminated E-I Core

The E-I core is the best balance of performance and buildability:

1. Cut E-shaped and I-shaped pieces from 0.3-0.5mm soft iron sheet
2. File or grind all edges smooth to remove burrs
3. Coat one side of each lamination with shellac, varnish, or thin paper
4. Stack E-pieces alternating direction (reversing each one) to distribute the air gap
5. Stack I-pieces on top, also alternating
6. Clamp the entire assembly with bolts through the corners (insulate bolts from laminations with paper washers)

Why Laminate

Solid iron cores create massive eddy currents — circulating currents within the iron that waste energy as heat. Thin laminations (0.3-0.5mm) insulated from each other reduce eddy current losses by over 95%. This is not optional for any transformer operating above a few watts.

Using Scavenged Cores

If you have access to old transformers, motors, or other electrical equipment:

1. Disassemble carefully, noting the lamination pattern
2. Remove old windings (soak in hot water to soften varnish, then unwind)
3. Clean laminations, re-coat with fresh shellac if the old insulation is damaged
4. Reassemble in the same pattern
5. A reused core is often better than a hand-cut one, as factory laminations are thinner and more uniform

Winding the Coils

Wire Selection

Wire Gauge	Current Capacity	Best For
0.3-0.5mm	0.1-0.5A	High-voltage secondary, low-power signal
0.5-1.0mm	0.5-3A	Medium power, lighting
1.0-2.0mm	3-10A	Heavy loads, battery charging
2.0-3.0mm	10-30A	High-current industrial

Calculating Turns

To design a transformer for a specific voltage ratio:

1. Determine the primary voltage (from your generator)
2. Determine the desired secondary voltage
3. Calculate turns ratio: N2/N1 = V2/V1
4. Choose a “volts per turn” value based on core size (typically 0.5-2V per turn for small transformers)
5. Calculate primary turns: N1 = V1 / (volts per turn)
6. Calculate secondary turns: N2 = N1 x (V2/V1)

Example: Generator produces 120V AC, you need 12V for lighting.

• Turns ratio: 12/120 = 1:10
• At 1V per turn: Primary = 120 turns, Secondary = 12 turns
• Secondary wire must handle the full load current (10x the primary current)

Winding Procedure

1. Insulate the core: Wrap the core leg with several layers of cloth tape or paper
2. Wind the primary first (closer to the core for better coupling):
   • Feed wire through the core window, wrap around the leg
   • Wind evenly, layer by layer, left to right then right to left
   • Place a layer of paper or tape between each layer
   • Count turns carefully — errors compound
3. Insulate between windings: Apply 3-4 layers of tape or cloth between primary and secondary
4. Wind the secondary on top of the primary, using the same careful layering technique
5. Bring out leads: Mark and protect all four wire ends (primary in/out, secondary in/out)
6. Final insulation: Wrap the entire assembly in cloth tape and apply varnish or shellac

Insulation Is Critical

Primary and secondary windings must never touch each other or the core. In a step-up transformer, the secondary carries high voltage that can arc through weak insulation, causing shorts, fires, and lethal shock hazards. Use multiple insulation layers and test with a battery before applying full power.

Testing Your Transformer

Before Energizing

1. Measure primary winding resistance with a multimeter — it should be low (a few ohms) but not zero
2. Measure secondary winding resistance — proportional to wire length
3. Test insulation between primary and secondary — should read open circuit (infinite resistance)
4. Test insulation between each winding and the core — should read open circuit

Under Power

1. Connect primary to a low-voltage AC source first (if possible) to verify the ratio before applying full voltage
2. Measure secondary voltage — it should match the turns ratio
3. Apply rated load gradually and monitor temperature
4. The transformer should be warm but not hot to touch after 30 minutes of rated operation
5. Listen for buzzing — excessive buzz means loose laminations (tighten core clamps)

Efficiency and Losses

Loss Type	Cause	Mitigation
Copper loss (I2R)	Resistance in wire	Use thicker wire, shorter runs
Core loss (hysteresis)	Magnetization cycling	Use soft iron, not steel
Eddy current loss	Circulating currents in core	Thinner laminations
Leakage flux	Flux not linking both coils	Wind coils tightly on same leg
Stray loss	Heating of frame, bolts	Insulate bolts from core

A well-built small transformer achieves 85-95% efficiency. A crude first attempt might achieve 60-75%. The biggest gains come from proper lamination and tight winding.

Cooling

Transformers generate heat proportional to their losses. For small transformers (under 100W), natural air cooling suffices. For larger units:

1. Mount the transformer where air circulates freely
2. For 100-500W, add fins or a fan
3. For 500W+, immerse the transformer in oil (mineral oil or clean vegetable oil). The oil conducts heat better than air and also provides additional insulation
4. For oil-filled transformers, use a sealed metal tank with external cooling fins

Common Mistakes

1. Using solid iron instead of laminations: A solid core wastes enormous energy in eddy currents. Even crude hand-cut laminations vastly outperform solid iron.
2. Wrong turns count: One extra or missing turn per hundred is acceptable. But miscounting by 10% produces wrong output voltage. Count carefully and verify.
3. Undersized wire for the secondary: The secondary carries higher current in a step-down transformer. If the secondary wire is too thin, it overheats and the insulation melts. Size wire for the expected current.
4. Poor winding technique: Loose, uneven windings create air pockets that trap heat and allow vibration. Wind tightly and evenly.
5. Forgetting the turns ratio applies to current too: Stepping voltage down by 10x means current goes up by 10x. Your secondary wire and connections must handle this higher current.

Summary

Transformer Construction -- At a Glance

• Transformers change AC voltage using two coils on a shared iron core; voltage ratio equals turns ratio
• Always laminate cores from thin (0.3-0.5mm) insulated iron sheets to prevent eddy current losses
• E-I core shape offers the best balance of efficiency and ease of construction
• Wind primary first (closest to core), insulate heavily between windings, then wind secondary
• Power in equals power out: stepping voltage up reduces current proportionally, and vice versa
• Test insulation between windings and core before energizing — high-voltage shorts are lethal