Single-phase transformers
What you’ll learn
- The turns ratio, and how it sets both the voltage and the current.
- Full-load current on each side from the kVA rating.
- Additive and subtractive polarity — and the meter test that tells them apart.
- Series and parallel connections of dual-voltage windings, including 120/240 V three-wire.
A single-phase transformer steps AC voltage up or down by the ratio of the turns on its two windings — and steps the current the opposite way by the same ratio. That one rule, plus reading the H and X markings and knowing which way round the windings are wound (polarity), covers most of what you will do with one: pick the voltage, work out the current, and connect it right.
The turns ratio
Every turn on the core sees the same changing magnetic field, so every turn gets the same slice of voltage. Voltage divides in the ratio of the turns. Power in is (nearly) power out, so current divides the other way. The transformer basics page tells the physics story; here is the arithmetic:
- a = Np ÷ Nsturns ratio
- Vp ÷ Vs = avoltage follows turns
- Is ÷ Ip = acurrent goes the other way
- I = VA ÷ Vfull-load amps, each side
1. A 10 kVA dry-type, 600 V to 240 V
- Ratio a = 600 ÷ 240 = 2.5 : 1. If the primary has 1,500 turns, the secondary has 1,500 ÷ 2.5 = 600.
- Primary full-load current: 10,000 ÷ 600 = 16.67 A.
- Secondary full-load current: 10,000 ÷ 240 = 41.67 A. Check: 16.67 × 2.5 = 41.67. ✓
- The same unit with a 480 V primary (US): 10,000 ÷ 480 = 20.83 A on the primary.
2. A pole-top transformer
- 50 kVA, 7,200 V primary, 120/240 V secondary.
- Ratio to the full secondary: 7,200 ÷ 240 = 30 : 1.
- Primary current: 50,000 ÷ 7,200 = 6.94 A. Secondary (at 240 V): 50,000 ÷ 240 = 208.3 A.
- A small wire on the pole, a big service drop to the house — same power, 30 times the current.
Polarity: which ends are in step
The high-voltage leads are marked H1, H2; the low-voltage leads X1, X2. The numbering tells you that at the instant H1 is positive with respect to H2, X1 is positive with respect to X2. Where X1 comes out of the case relative to H1 is the transformer’s polarity:
3. The polarity test
- A 480:120 V transformer (ratio 4). Jumper H1 to the X terminal directly across from it.
- Apply a safe test voltage to the H winding — say 120 V. The X winding then makes 120 ÷ 4 = 30 V.
- Meter across H2 and the other X terminal.
- Reads 120 + 30 = 150 V: the voltages add — additive. Reads 120 − 30 = 90 V: subtractive.
Dual-voltage windings: series and parallel
Many transformers have each winding split in two so one unit can serve two voltages. A common secondary is two 120 V coils, X1–X2 and X3–X4. Joined in series (X2 to X3) they give 240 V across X1–X4, and the joint becomes the midpoint — the 120/240 V three-wire supply every house runs on. Joined in parallel (X1 to X3 and X2 to X4) they give 120 V with twice the current of one coil. Primaries are split the same way — two 240 V halves in series for 480 V, in parallel for 240 V.
For parallel, X1 goes to X3 and X2 to X4 — like ends together, so the two coils push the same way. Cross them (X1 to X4) and each winding drives straight into the other: a short circuit the moment it is energized.
Losses and efficiency
Real transformers lose a little power as heat: copper losses in the windings (I² × R, so they grow fast with load) and core losses in the iron (present whenever it is energized, even with no load). Well-loaded transformers are commonly well above 95% efficient, which is why “power in ≈ power out” is good enough for the arithmetic above.
Try one: 25 kVA, 600 V to 120/240 V — both full-load currents
Primary: 25,000 ÷ 600 ≈ 41.7 A. Secondary at 240 V: 25,000 ÷ 240 ≈ 104.2 A. Ratio 600 ÷ 240 = 2.5, and 41.7 × 2.5 ≈ 104.2. ✓
En la obra
Tips from the job
- Read the nameplate before you land anything: kVA, both voltages, the connection diagram for each voltage, taps, polarity and impedance.
- Taps change the ratio a little (often in 2.5% steps) to correct a supply that runs high or low. Measure the supply first, then set the tap the plate says.
- Primary and secondary protection, conductor sizes and where the secondary is grounded are code questions — check your code book, and your journeyman.
- Before paralleling two windings, confirm polarity. Joining unlike ends puts the two windings in a dead short against each other.
Common mistakes
- Treating the low-voltage side as low current. It is the high-current side.
- Jumpering a dual-voltage secondary for parallel with the wrong pairs (X1 to X4) — a short across both windings.
- Feeding the wrong winding: a step-down fed from its X side becomes a step-up.
- Doing a polarity test at full line voltage. Use a lower test voltage on the H winding.
Check yourself
8 questions. Tap an answer — it is marked straight away and the reason opens under it. Get 80% or better and the lesson is ticked off on the year page.
Not started yet.
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A transformer has 1,000 turns on the primary and 250 on the secondary. With 480 V on the primary, the secondary gives…
Answer and why
C. Voltage follows the turns ratio: 1,000 ÷ 250 = 4, so 480 ÷ 4 = 120 V.
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Full-load secondary current of a 10 kVA transformer with a 240 V secondary?
Answer and why
B. I = VA ÷ V = 10,000 ÷ 240 ≈ 41.7 A.
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Same 10 kVA transformer, 600 V primary. Full-load primary current?
Answer and why
A. 10,000 ÷ 600 ≈ 16.7 A. Same power both sides (ignoring small losses): the higher-voltage side carries less current.
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Stepping voltage DOWN by a ratio of 4 makes the current…
Answer and why
C. Power in ≈ power out, so if voltage falls by 4 the current rises by 4. The low-voltage side is the high-current side.
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On a transformer with SUBTRACTIVE polarity, H1 and X1 are…
Answer and why
B. Subtractive: H1 and X1 sit directly across from each other. Additive: they are diagonally opposite. The nameplate diagram is the final word.
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Polarity test on a 480:120 V transformer: H1 is jumpered to the adjacent X terminal and 120 V is applied to the H winding. The meter across the other two terminals reads 150 V. The polarity is…
Answer and why
B. The H side gets 120 V, so the X side makes 120 ÷ 4 = 30 V. 120 + 30 = 150 V means the voltages add: additive. Subtractive would read 120 − 30 = 90 V.
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A secondary has two 120 V windings, X1–X2 and X3–X4. To get 120/240 V three-wire you…
Answer and why
B. Series: X2–X3 jumpered becomes the midpoint (the neutral), with 240 V across X1–X4 and 120 V from each end to the midpoint. X1–X3 plus X2–X4 is the parallel (120 V only) connection.
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Why is a transformer rated in kVA rather than kW?
Answer and why
B. The windings heat from current and the core from voltage. Neither cares what the power factor is, so the rating is volts × amps.
Preguntas frecuentes
What is transformer polarity?
It describes which secondary lead is “in step” with which primary lead at any instant. It only matters when you connect windings or transformers together — in series, in parallel or in three-phase banks. Get it wrong and voltages cancel instead of adding, or windings short against each other.
Can I run a transformer backwards as a step-up?
Physically most small dry-type transformers will work that way, but the maker may not rate it for it, and inrush and protection are different when fed from the low side. Check the maker’s instructions and your code book before doing it on purpose.
How do I work out a transformer’s full-load current?
Single-phase: amps = VA ÷ volts, on each side. A 25 kVA transformer at 240 V is 25,000 ÷ 240 ≈ 104 A. The transformer calculator does both sides at once.
Ver también
- Transformer basics — how two coils share one field, and the CT warning.
- Power factor and the power triangle — the lesson before this one.
- Intro to three-phase — three of these working together.
- Estrella y delta
- Single & three phase formulas
- All second year lessons
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