Transformers

Why it matters

Transformers are everywhere in power systems: the pole or pad-mount transformer outside a building, the control transformer inside a machine's panel that turns 480 V into 120 V or 24 V, the doorbell transformer, and the isolation transformers used on test benches. Electricians size and connect them, maintenance mechanics replace control transformers and fuses, and engineers choose ratings. All of that starts with one idea, the turns ratio.

How a transformer works

A transformer has two (or more) coils of wire wound on a shared iron core:

• The primary winding is connected to the source.
• The secondary winding is connected to the load.

AC current in the primary creates a changing magnetic field in the core. That changing field passes through the secondary and induces a voltage in it (Faraday's law of induction). There is no electrical connection between the windings, only a magnetic one, which is why transformers also provide isolation.

Because induction needs a changing field, a transformer does not work on steady DC. Connect a transformer primary to DC and it acts almost like a short circuit of wire resistance: it overheats. This is one reason AC became the standard for power grids: transformers make it easy to step voltage up for transmission (less current, so less loss in the lines) and down for use.

The turns ratio

For an ideal transformer, the voltage per turn is the same on both windings:

Vs / Vp = Ns / Np
Vs = Vp × Ns / Np

where Np and Ns are the number of turns on the primary and secondary.

• Step-down: fewer secondary turns, lower secondary voltage.
• Step-up: more secondary turns, higher secondary voltage.

Currents: power in equals power out

An ideal transformer has no losses, so apparent power is the same on both sides:

Vp × Ip = Vs × Is
Is / Ip = Np / Ns

Stepping voltage down steps current up by the same ratio, and vice versa.

Worked examples

Example 1: a control transformer. A 480 V primary with 2,000 turns and a secondary with 500 turns.

Turns ratio Np : Ns = 2,000 : 500 = 4 : 1
Vs = 480 × 500 / 2,000 = 120 V

If the 120 V control circuit draws 2 A:

Ip = Is × Ns / Np = 2 × 500 / 2,000 = 0.5 A
Check: 480 × 0.5 = 240 VA = 120 × 2 ✓

Example 2: sizing by VA. A 24 V secondary must supply a load of 1.5 A. The transformer's VA rating must be at least 24 × 1.5 = 36 VA; a designer would choose a 40 VA or 50 VA unit for margin and inrush current.

Example 3: step-up. A 12 V primary, 100 turns, and a secondary of 1,000 turns.

Vs = 12 × 1,000 / 100 = 120 V

(This is the principle of an inverter's output stage, not something to build.)

Real transformers

• Efficiency is high (often 95–99% for power transformers), but there are copper losses (I²R heating in the windings) and core losses (eddy currents and hysteresis in the iron).
• Regulation: secondary voltage sags slightly as load increases.
• Nameplates show kVA, primary and secondary voltages, frequency, impedance (%Z) and connection diagrams. Many control transformers have multiple primary taps, such as 240/480 V, selected by jumpers.
• Three-phase transformers are connected delta or wye on each side; you will study connections and code requirements in the electrical code and installations course.

Common mistakes

• Inverting the ratio. Always ask: is this step-up or step-down? The answer should match.
• Forgetting the current goes the other way. Lower voltage means higher current on that side, so conductors and fuses on the low side are often larger.
• Using transformers on DC.
• Ignoring the VA rating. Overloading causes overheating and shortens the insulation's life.

Safety and real-world requirements

Transformers can deliver very high fault currents on their secondaries, and some can back-feed a "dead" circuit to a dangerous voltage. Connecting, testing or replacing a transformer is done only after employer training, under lockout/tagout procedures (https://www.osha.gov/control-hazardous-energy) and an electrical safety program based on NFPA 70E (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e), and installation work requires a licensed electrician. Checked on 2026-10-23.

Recap

• AC in the primary makes a changing magnetic field that induces voltage in the secondary.
• Vs = Vp × Ns / Np; current scales the opposite way: Is = Ip × Np / Ns.
• VA in ≈ VA out; size transformers by VA with margin.

Try it

Open the Do tab and write the secondary-voltage formula in a transformer calculator.