Delta Transformers

[jmellc]

In all my years of electrical work, I have never thought to ask why delta transformers were ever used. I knew about them, saw just a few in the 70's & was warned about the high leg/stinger. I have only seen about 4-5 in the last 20 years or so.

I had a young coworker burn up a charger this past week. He knew about deltas but only from class. I had totally forgotten to warn him about running into them. He wired up a temp circuit and plugged in his charger. He now knows to check all labeling, look for skipped spaces and any orange tape or wires.

But why were they ever needed? Couldn't 240 be achieved on a wye transformer? Seems a few coils could be added to each phase if 240 were specially needed. Nearly all equipment I've ever seen was listed for either 208 or 240. I've called factories if 240 were labeled and been told 208 was also OK.

How is 240 double pole to a house achieved? 2 120 volt legs of a delta transformer or some other way?

I should have asked these questions long long ago. I've been getting older but only slightly wiser.

 

[Jraef]

Long story short, it stems from a "Damn the torpedoes, full steam ahead!" attitude. When you have an ungrounded Delta supply feeding your plant or machine, the first time there is a grounded fault, the ungrounded Delta becomes a corner grounded Delta and the plant or machine keeps on going (other than perhaps the failed piece of equipment). This was done quite a bit during WWII when factories were deemed critical for war production and the practice continued on long afterward. But starting in around the 70s, people started to get more nervous about the implications and the fact that circuit breakers were not necessarily clearing grounded faults. So going to a grounded Wye system ensures that any grounded fault will result in enough current flowing to ground to open the (hopefully) nearest circuit protection device.

 

[jim dungar]

I have never thought to ask why delta transformers were ever used. I

Do you mean all delta transformers or you using that generic term to cover the specific 240/120 3Ph 4W connection that provides a 'high' leg of 208V L-G due to a center tapped winding?

Yes, you can get 240V from a wye connected transformer, but you L-N voltage would be 136V not 120V.

 

[texie]

In all my years of electrical work, I have never thought to ask why delta transformers were ever used. I knew about them, saw just a few in the 70's & was warned about the high leg/stinger. I have only seen about 4-5 in the last 20 years or so.

I had a young coworker burn up a charger this past week. He knew about deltas but only from class. I had totally forgotten to warn him about running into them. He wired up a temp circuit and plugged in his charger. He now knows to check all labeling, look for skipped spaces and any orange tape or wires.

But why were they ever needed? Couldn't 240 be achieved on a wye transformer? Seems a few coils could be added to each phase if 240 were specially needed. Nearly all equipment I've ever seen was listed for either 208 or 240. I've called factories if 240 were labeled and been told 208 was also OK.

How is 240 double pole to a house achieved? 2 120 volt legs of a delta transformer or some other way?

I should have asked these questions long long ago. I've been getting older but only slightly wiser.

A 240/120 4 wire delta service is an economical way to provide service to light commercial customers that only need 3 phase for a small part of the service load. Typically they are open delta on the primary that is fed from a Y distribution system. This then requires the POCO to only have 2 of the primary phases and the neutral present at the transformer bank and use only 2 transformers.
In some areas where only 2 phases are available on the POCO system this is the only way to get 3 phase service to a customer.

 

[LarryFine]

But why were they ever needed?

That's like asking why we ever had 4-way intersections without stop-lights.

You have to remember how electrical systems evolved. A high-leg delta is basically a 3ph delta supply superimposed over a 120/240v 1ph supply. It's effectively both at the same time, in the same space. They're mostly found in older parts of town supplying older services and some older homes.

The high-leg delta originated as a modification to an existing 120/240v 1 ph service to support the addition of 3ph equipment. Add one transformer, one conductor, a 3ph meter, and what was called a delta breaker, and you can supply one 3ph device, typically air-conditioning or refrigeration.

A delta breaker was a 3ph breaker that received two lines from the panel stabs like a 2p breaker, and the third line from a separate input terminal on a fourth body section. (picture a shunt-trip 3p breaker that takes up four spaces). The added phase wire ran directly to this input terminal.

Couldn't 240 be achieved on a wye transformer?

Yes, if the individual secondary voltages were correct, but then the L-N voltages would be higher than 120v.

How is 240 double pole to a house achieved? 2 120 volt legs of a delta transformer or some other way?

Two 120v legs of a single secondary 240v winding that has a grounded center tap, aka the neutral.

 

[GoldDigger]

Yes, if the individual secondary voltages were correct, but then the L-N voltages would be higher than 120v.

Two 120v legs of a single secondary 240v winding that has a grounded center tap, aka the neutral.

I think the OP may have been talking about adding additional secondary taps to the wye windings so that they could produce both 120 and 137. The problem with that is that you have now gone from a four wire system (three hot plus neutral) to a seven wire system (six hot plus neutral) and that overwhelms any potential advantage from having both a true 240 delta and a true 120 wye from the same transformer.

 

[LarryFine]

I think the OP may have been talking about adding additional secondary taps to the wye windings so that they could produce both 120 and 137.

I think he was really asking theoretical questions about why the high-leg system was ever employed, not to fill a particular need.

Even if he were, for a relatively small amount of 240v load, two or three buck-boost transformers would be a viable alternative.

Added: By the way, I recently learned that you can not use three BB units in a delta configuration on a wye system.