Polarity test on a 480 : 120 V transformer: 120 V applied to H1–H2, H1 jumpered to the X terminal beside it, meter from H2 to the other X terminal. It reads 90 V. The polarity is…

C. Reversed — the transformer is faulty

D. Cannot tell without the load

Why does polarity matter when you build a three-phase bank from single-phase cans?

B. One can connected with the wrong polarity puts its voltage backwards, so the line voltages come out wrong — and a closed delta can short on itself

C. It only matters on the primary

Can a delta-wye transformer be paralleled with a wye-wye transformer of the same ratio?

B. No — their secondaries are 30° apart, so large current circulates between them

A. Yes, if the kVA ratings match

Third year Lesson 5 of 13 · about 14 min

Polarity, phase shift and open delta

What you’ll learn

What additive and subtractive polarity mean, and the meter test that tells them apart.
Why polarity decides whether a bank of single-phase cans comes out right — and the check before you close a delta.
The 30° phase shift through a delta-wye, and why two transformers with different shifts cannot be paralleled.
How an open delta keeps running on two cans, and why it only carries 57.7% of the full bank.

Before this: Transformer connections

Every winding’s voltage has a direction at any instant, and polarity tells you which terminals are “in step”: when H1 is going positive, so is X1. One transformer on its own does not care. The moment you connect windings together — three cans into a bank, two transformers in parallel — getting that direction wrong is the difference between the right voltage and a short circuit.

Additive and subtractive

Look at a single-phase transformer from the low-voltage side. H1 is on your left. If X1 is directly opposite H1, the polarity is subtractive; if X1 is diagonally opposite, it is additive. The names come from the test: tie one H terminal to the X terminal beside it and measure across the other two. The two winding voltages either subtract or add.

The polarity test. Feed a low, safe voltage to the H winding, jumper H1 to the X terminal beside it, and read across H2 and the other X. Less than the applied voltage: subtractive. More: additive.

Worked: testing a 480 : 120 V transformer at 120 V

The ratio is 480 ÷ 120 = 4 : 1, so 120 V on the H side puts 120 ÷ 4 = 30 V on the X side.
Subtractive: the meter reads 120 − 30 = 90 V. The terminal beside H1 is X1.
Additive: the meter reads 120 + 30 = 150 V. The terminal beside H1 is X2.
Why test at 120 V and not 480 V? The same result, at a voltage that is safer to handle with test leads on open terminals — and nothing on the X side is left floating at a surprise voltage.

Polarity in a bank

Build three cans into a wye and one reversed can pulls two of the three line voltages down to about the phase voltage. Build them into a delta and it is worse: around a correct delta the three voltages add to zero, so closing the loop joins points at the same voltage. Reverse one can and the loop voltage is twice a winding voltage — close that last corner and you have shorted the bank through its own windings.

Before you close the last corner of a delta, leave it open, energise, and measure across the gap. Near zero: close it. Anything close to twice the winding voltage: de-energise and find the reversed can.

The 30° phase shift

A delta winding sits across two lines, so its voltage lines up with a line-to-line voltage. A wye winding sits line to neutral, so its voltage lines up with a line-to-neutral voltage. Those two are 30° apart — the same 30° you met in the √3 triangle. So the secondary line voltages of a delta-wye (or wye-delta) come out 30° away from the primary’s. In the usual North American arrangement, the low side lags the high side by 30°. Delta-delta and wye-wye have no shift.

Your loads never notice. It matters when two supplies meet: two transformers in parallel must have the same ratio, polarity, rotation and phase shift, so a delta-wye cannot be paralleled with a wye-wye. Relays that compare current on both sides of a transformer must allow for it too.

Open delta: running on two cans

Two cans still make three line voltages: A-B and A-C across the windings, and B-C as the difference of the two. The third can was helping carry current, not making the voltage.

Take one can out of a closed delta and you still get three balanced line voltages, because the third side of a delta is always the difference of the other two. What you lose is capacity. In an open delta the line current flows straight through each remaining can, so the bank can carry only √3 × one can’s kVA.

Worked: losing one can of three

Three 50 kVA cans in closed delta: 150 kVA.
One fails. Open delta capacity = 1.732 × 50 = 86.6 kVA.
That is 86.6 ÷ 150 = 57.7% of the full bank, or 86.6 ÷ 100 = 86.6% of the two cans left.
If the plant was drawing more than 86.6 kVA, it has to shed load until the third can is replaced.

On the job

Tips

Mark X1 on every can before you start building a bank. Test if the plate is gone or unreadable.
Test polarity and ratio at a low, safe voltage with the transformer otherwise disconnected.
Check rotation on a new or rebuilt bank before motors run — a swapped pair runs every motor backwards.
Open delta is a legitimate way to keep a plant running, and some services are built that way on purpose. Know its limit.

Common mistakes

Closing a delta without measuring across the open corner first.
Assuming every can in a bank has the same polarity because they look the same.
Paralleling transformers with different connections or phase shifts.
Running an open delta at two-thirds of the old bank rating — it is only 57.7%.
Working on the H side of a can with the X side still tied into a live bank — it back-feeds.

Check yourself

0 right of 0 answered 7 questions · pick one answer each

1.Polarity test on a 480 : 120 V transformer: 120 V applied to H1–H2, H1 jumpered to the X terminal beside it, meter from H2 to the other X terminal. It reads 90 V. The polarity is…
2.Same test, but the meter reads 150 V. What does that tell you?
3.Why does polarity matter when you build a three-phase bank from single-phase cans?
4.Three 50 kVA cans form a closed delta bank. One fails and is removed, leaving an open delta. About how much load can the bank carry now?
5.How big is the phase shift between the primary and secondary line voltages of a standard delta-wye transformer?
6.Can a delta-wye transformer be paralleled with a wye-wye transformer of the same ratio?
7.You are about to close the last corner of a delta secondary on a new bank. What should your meter read across the gap?

Answers

B. Subtractive — The 4 : 1 ratio puts 30 V on the X side. Subtractive polarity reads the difference, 120 − 30 = 90 V. Additive would read the sum, 150 V.
B. Additive polarity — 120 + 30 = 150 V: the two windings’ voltages add, so the polarity is additive. Nothing is wrong with the transformer — you now know which X terminal behaves like X1.
B. One can connected with the wrong polarity puts its voltage backwards, so the line voltages come out wrong — and a closed delta can short on itself — Each can’s voltage has a direction. Reverse one in a wye and two line voltages collapse to phase value; reverse one in a delta and the loop voltage is double instead of zero, driving a large circulating current when you close it. Measure across the last open corner before closing a delta — it should read near zero.
B. 86.6 kVA — An open delta can carry √3 × one can’s rating: 1.732 × 50 = 86.6 kVA. That is 57.7% of the 150 kVA closed bank, or 86.6% of the 100 kVA of cans still in it.
B. 30° — Delta-wye (and wye-delta) shift the voltages by 30°. Delta-delta and wye-wye have no shift. In the usual North American convention the low-voltage side lags the high side by 30°.
B. No — their secondaries are 30° apart, so large current circulates between them — Paralleled transformers must have the same voltages, polarity, rotation AND phase shift. Two secondaries 30° apart are pushing against each other with a big voltage difference and nothing but winding impedance to limit it.
A. About zero — Three voltages 120° apart add to zero around a correctly connected delta. A reading of about twice the winding voltage means one can is reversed — closing it would be a short circuit through the windings.

Questions people ask

Is additive or subtractive polarity better?

Neither. Polarity is just which way round the windings are brought out to the terminals. What matters is that you know it — from the nameplate, the terminal layout or a test — so that X1 behaves like X1 when you connect cans together. Small distribution transformers are often additive and larger ones subtractive, but the plate is the authority.

Why is an open delta only 57.7% and not 66.7% of a full bank?

In an open delta each remaining can carries the full line current, but the two cans are no longer working in the neat 120° partnership of a closed delta — their current and voltage are 30° out of step. The bank’s capacity becomes √3 × one can’s kVA, which is 1 ÷ √3 = 57.7% of three cans.

Does the 30° phase shift affect my loads?

Not by itself — a motor or a heater on the secondary neither knows nor cares. It matters when two sources meet: paralleling transformers, a tie breaker between two services, a generator synchronising, or a protective relay comparing currents on both sides of a transformer.

Three-phase transformer connections — the lesson before
Transformer basics — H and X markings
Transformer full-load current & ratio
Protective relaying concepts — where the 30° shift bites
Using a multimeter
Rusty on transformer connections?

Spot something wrong on this page? Report it

Thanks — your report was sent.

That didn’t send. Please try again later.