Journeyman refresher Refresher 3 of 10 · about 9 min
Rusty on transformer connections?
What you’ll learn
- The four bank connections and what each one gives you on the secondary.
- The voltage across each winding, and the difference between line ratio and turns ratio.
- The 30° shift through a delta-wye, and why it forbids some parallels.
- The measurements to take before you put load on a new or re-connected bank.
Before this: Three-phase transformer connections
The four connections at a glance
| Connection (primary–secondary) | Secondary gives you | Phase shift | Where you meet it |
|---|---|---|---|
| Delta–wye | Three phases + neutral (208Y/120, 480Y/277, 600Y/347) | 30° | The usual building step-down |
| Wye–wye | Three phases + neutral | 0° | Utility distribution with a grounded common neutral |
| Delta–delta | Three phases, no neutral (or a centre-tapped high-leg winding) | 0° | Industrial power; can run open delta on two cans |
| Wye–delta | Three phases, no neutral | 30° | Step-up at generation; some industrial supplies |
| Open delta (V–V) | Three phases from two cans, about 58% of a three-can bank | 0° | Emergency running; light three-phase loads |
The numbers
- I = kVA × 1000 ÷ (√3 × V)full-load current, each side
- Wye winding: V ÷ √3carries line current
- Delta winding: V linecarries line current ÷ √3
- Open delta ≈ 57.7%of the closed three-can bank
Worked check: a 75 kVA, 600 V delta to 208Y/120 bank
- Primary FLA = 75,000 ÷ (1.732 × 600) = 72.2 A. Secondary FLA = 75,000 ÷ (1.732 × 208) = 208.2 A.
- Current in each primary (delta) winding = 72.2 ÷ 1.732 = 41.7 A.
- Turns ratio = 600 ÷ 120 = 5 : 1. Line ratio = 600 ÷ 208 = 2.88.
- Check with power: 3 windings × 120 V × 208.2 A = 74,950 VA. Closes at 75 kVA.
Before you put load on it
- Match the plate. Connection diagram, taps and voltages on the plate agree with what you built and the supply you measured.
- Check polarity marks. H1/X1 on each can match the drawing — one reversed can is the classic bank fault.
- Energize unloaded and measure. All three line-to-line and all three line-to-neutral readings. Equal and as expected? Yes — go on. No — de-energize and find the reversed winding or wrong tap.
- Check rotation before connecting motors, if the bank is new or has been re-terminated.
On the job
Tips
- On a new 208Y/120 secondary, two of the three line-to-line readings coming out near 120 V instead of 208 V is the signature of one reversed winding — the two readings that involve it.
- Say the connection as primary–secondary. “Delta–wye” is a building step-down; “wye–delta” is the other way round.
- Keep the grounding and bonding of a new secondary in mind — it is a separately derived system, and your code book says how it is grounded.
Common mistakes
- Treating the line ratio as the turns ratio on a delta–wye bank.
- Paralleling banks with different phase shifts.
- Loading an open delta as if it still had two-thirds of its capacity.
- Feeding a 120 V load from the high leg of a four-wire delta.
Check yourself
-
1.A bank is fed 600 V delta and delivers 208Y/120. What voltage is across each secondary winding?
Each wye winding sits between a line and the neutral point, so it sees the line-to-neutral voltage: 208 ÷ 1.732 = 120 V.
-
2.Which common connection gives a neutral for 120 V or 277/347 V loads on the secondary?
A wye secondary has a centre point to bring out as a neutral. Delta-wye is the usual building step-down for exactly that reason.
-
3.Three single-phase cans in a closed delta; one fails and is removed. The bank keeps running as an open delta. Roughly what share of the original three-can capacity is left?
Two cans in open delta can carry about 1/√3 = 57.7% of what three carried in closed delta — not the 2/3 you might guess, because the remaining two windings are no longer sharing the load the same way.
-
4.Why can you not parallel a delta-wye bank with a delta-delta bank, even with matching voltages?
A delta-wye connection shifts the secondary 30° relative to the primary; a delta-delta does not. Two sources 30° apart tied together is effectively a fault between them. Paralleling needs matching phase shift, voltage, polarity and similar impedance.
-
5.Before energizing a newly connected three-phase bank, what is the most useful single check once it is energized unloaded?
A reversed winding or a wrong tap shows immediately as one or two odd voltages. Measure all three line-to-line and all three line-to-neutral readings before any load is connected. (Insulation testing is done de-energized, never live.)
-
6.A 45 kVA three-phase transformer, 208 V secondary. Full-load secondary current?
I = kVA × 1000 ÷ (√3 × V) = 45,000 ÷ (1.732 × 208) = 45,000 ÷ 360.3 = 124.9 A.
Questions people ask
Which way does a delta-wye bank shift the phase?
By 30°. In the usual North American convention the low side lags the high side by 30°. You rarely need the direction on the job — you need to know the shift is there, so a delta-wye and a delta-delta or wye-wye bank are never paralleled.
Why is wye-wye less common for building services?
A plain wye-wye with an ungrounded primary neutral has trouble with unbalanced loads and third-harmonic voltages, because nothing gives those currents a path. Utilities do use grounded wye-wye with a common neutral. For a building step-down, delta-wye is the usual answer because the delta primary absorbs those problems and the wye secondary gives the neutral.
Is the turns ratio the same as the voltage ratio?
Per winding, yes. Line to line, only if both sides are the same connection. A 480 V delta to 208Y/120 bank has a line ratio of 2.31 but a winding (turns) ratio of 4 : 1 — 480 V across a delta winding to 120 V across a wye winding.
Related
- Three-phase transformer connections — the full lesson
- Polarity, phase shift and open delta — the full lesson
- Transformer full-load current & ratio — the tool
- Transformer fault current (infinite bus)
- Transformer basics
- Rusty on three-phase math?
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