Lo básico de los transformadores

A transformer is two coils of wire on one iron core, with no wire joining them: alternating current in one coil makes a changing magnetic field in the core, and that changing field pushes a voltage into the other coil. That is the whole device. Everything else — step-up, step-down, isolation, the little control transformer in every starter — is that one idea with different numbers of turns. The numbers on this page are practice numbers.

How it works, in one paragraph

Push alternating current through the primary coil and it makes a magnetic field in the core that grows, collapses and reverses — completing sixty full cycles a second on a 60 Hz supply, which is two reversals per cycle. The core carries that changing field through the secondary coil, and a changing field through a coil pushes a voltage into it — the same thing that happens inside a generator. No current crosses from one coil to the other; the energy goes across as magnetism. That is why it only works on AC: a steady DC current makes a steady field, and a steady field pushes nothing.

The turns ratio

Each turn of wire on the core sees the same changing field, so each turn gets the same little slice of voltage. A coil with more turns collects more slices. The voltage ratio is the turns ratio. Four times the turns on the primary means the secondary gets a quarter of the voltage — step-down. Fewer turns on the primary and it goes up — step-up.

Power has to come out roughly equal to what went in (the losses are small and become heat). Volts times amps on one side equals volts times amps on the other. So if the voltage goes down by four, the current goes up by four. A step-down transformer is a current step-up. This is the fact that catches people: the low-voltage side of a big transformer can carry a very large current, and its conductors are sized for that.

Worked: the ratio, both ways

1. Plate says 480 V primary, 120 V secondary. Ratio = 480 ÷ 120 = 4 : 1.
2. The secondary is delivering 10 A to its load. Power out ≈ 120 × 10 = 1,200 VA.
3. Power in ≈ power out, so primary current ≈ 1,200 ÷ 480 = 2.5 A — the secondary current divided by the ratio, ignoring losses.
4. Check: 2.5 × 4 = 10. Current went up by exactly the amount voltage came down.

Worked: what a kVA rating buys you

1. A 3 kVA control transformer, 480 V to 120 V. kVA = V × I ÷ 1,000, so I = kVA × 1,000 ÷ V.
2. Full-load secondary current = 3,000 ÷ 120 = 25 A.
3. Full-load primary current = 3,000 ÷ 480 = 6.25 A. Same power, four times the volts, a quarter of the amps.
4. What that tells you on the job: the low side can push 25 A into a fault. The conductor and the protection on that side are sized for it — the sizes are in the book, not here.

Why kVA and not kW

The transformer heats up from the amps it carries and the volts it drops, whatever the load does with them. A motor with a poor power factor pulls more amps for the same watts, and the transformer has to carry every one of them. So the plate rates apparent power — volts × amps — and leaves the watts to the load.

H1, H2, X1, X2

The high-voltage winding is marked H, the low-voltage winding X, and the numbers count the leads. That is why the control circuit in every motor control lab runs between rails labelled X1 and X2: they are the two ends of the control transformer's low side. The H side is fed from two of the motor's line conductors. Which of H or X is the "primary" is only a matter of which one you feed — feed the H side and it steps down, which is the usual case. On a real transformer the relative polarity of H1 and X1 is fixed by the maker and matters when you parallel two units or build a bank; the plate and the maker's diagram say how.

Isolation, and the control transformer

Because no wire joins the two sides, the secondary is a fresh circuit. It has no electrical connection to the primary's neutral or ground unless somebody makes one. That is isolation, and it is useful: a fault on the low side does not go looking for the supply's neutral, and the low side can be referenced to ground on purpose, at one point, so the control circuit behaves predictably and its protection can clear a fault. Which side of the secondary gets grounded, and how the secondary is protected, are code rules — the book has them, and it has opinions.

A starter runs its control circuit at a lower voltage than the motor for two reasons: the coil, the pushbuttons and the pilot lights are cheaper and safer at the lower voltage, and a long run of control wire to a remote station is less of a shock hazard. The seal-in lesson is that control circuit; the transformer feeding it is drawn as the X1–X2 rails.

Single-phase and three-phase

Everything above is one transformer, one pair of coils. A three-phase supply uses three of them — three separate cans, or three pairs of coils on one core — connected in one of two patterns on each side. Wye and delta are those patterns, and the choice on the secondary decides whether you get a neutral and what the line-to-line and line-to-neutral voltages come out as. The phase formula sheet has the √3 that turns up in the three-phase current arithmetic.

What the secondary does when you short or open it

Short the secondary of a normal transformer and the current is limited only by the windings' own impedance — it will be large, and the protection on the primary side is what stops it. That is the case the book's rules on secondary and primary protection are written for.

A current transformer is the reverse case, and it is the one that hurts people. A CT is a transformer whose primary is the conductor you are measuring — one turn through a doughnut — and whose secondary feeds a meter or relay through a low-resistance loop. It is built to be run shorted. Never open a CT secondary while the primary carries current. With nothing to carry the secondary current, the transformer tries anyway: the voltage across the open secondary climbs until something conducts — an arc, the insulation, or you. CT secondaries get a shorting block, and you close it before you touch the wiring. Say it out loud on the job until it is a reflex.

Inrush

Energising a transformer with no load on it can still draw a brief, large current — the core has to be magnetised from wherever the last cycle left it, and for a few cycles it pulls hard. It is over in a fraction of a second, and it is why the primary protection has to tolerate a short surge without tripping — the same lesson as motor inrush. If a transformer trips its breaker at the instant of switching on and then holds fine on a second try, that is probably what you saw.

En la obra

Preguntas frecuentes

Ver también

• Motor control course — the X1–X2 control circuit in every lab is a transformer secondary.
• Wye and delta — the two ways a three-phase set of windings is joined.
• Power factor — why the plate says kVA.
• Single & three phase formulas — the kVA arithmetic, printable.
• Contactor wiring diagram — the coil the control transformer feeds.