Center tap delta 120/240V

[Johnaorr]

For many small shops, the high leg delta system is preferred over the wye system because the machines get 240 volts three phase and not 208. But the delta should be a full three-transformer delta connection, and not a two-transformer open delta or Vee connection, which gives quite poor voltage regulation under either unbalanced loads or high loads.

Why does an open delta setup give you poor voltage regulation and nuisance tripping due to higher loads?

 

[GoldDigger]

For many small shops, the high leg delta system is preferred over the wye system because the machines get 240 volts three phase and not 208. But the delta should be a full three-transformer delta connection, and not a two-transformer open delta or Vee connection, which gives quite poor voltage regulation under either unbalanced loads or high loads.

Why does an open delta setup give you poor voltage regulation and nuisance tripping due to higher loads?

One reason that an open delta gives you poor voltage regulation is that any current on the open line-to-line phase must go through the series impedance of the secondary windings on the other two sides, rather than one secondary if there were a third pot.
The nuisance tripping part does not make as much sense to me unless you are talking about OCPD on the primary side of the two pots.

 

[Iron_Ben]

For many small shops, the high leg delta system is preferred over the wye system because the machines get 240 volts three phase and not 208. But the delta should be a full three-transformer delta connection, and not a two-transformer open delta or Vee connection, which gives quite poor voltage regulation under either unbalanced loads or high loads.

Why does an open delta setup give you poor voltage regulation and nuisance tripping due to higher loads?

I never saw an inordinate number of nuisance tripping or voltage regulation issues with this service. The open delta 240 service or 120/240 with wild leg or whatever you want to call it has got a proven track record. At the POCO I worked for we had many thousands of them in service. Although - full disclosure here - I prefer a wye connected service, 208Y/120 or 480Y/277, for many reasons.

We had some closed delta three pot banks in service too (typically much older), but our spec for new delta services was open delta (2 pot). Two transformers are cheaper than three. There are some potential safety and operational issues with the closed configuration as well. Our spec also limited the service size to 400 amps max, with no individual motor larger than 20 HP (although I've seen motors as large as 40 HP run for years and years on these services). Does the unbalanced nature of the open delta service cause overheating and premature failure in a motor that size? I don't know for sure, but my guess is yes, in many cases it will.

My years of experience show it's a fine service when properly applied. Hundreds and hundreds of lift stations in our area are served in this fashion. Our larger fabrication and machine shops are 480Y/277 of course, but we've got many smaller, mom and pop types with 200 and 400 amp open delta services, and really, no more problems than with any other type of service.

Ben

 

[starbolin]

...
Why does an open delta setup give you poor voltage regulation and nuisance tripping due to higher loads?

Does it? Is that out of some book? I am with Iron_Ben, not a problem if the plant is wired correctly.

 

[GoldDigger]

Does it? Is that out of some book? I am with Iron_Ben, not a problem if the plant is wired correctly.

Will you at least agree that it is a poorer idea to put a single phase line to line load on the open leg of an open delta than on either of the other two legs?

 

[mivey]

I never saw an inordinate number of nuisance tripping or voltage regulation issues with this service.

The voltage regulation gets worse as the transformer is loaded up. If there is a tendency to oversize transformers then you might not notice the poor regulation with open deltas.

Although - full disclosure here - I prefer a wye connected service, 208Y/120 or 480Y/277, for many reasons.

I like the wye-delta for many reasons, including the zero sequence blocking capability.

Does the unbalanced nature of the open delta service cause overheating and premature failure in a motor that size? I don't know for sure, but my guess is yes, in many cases it will.

It can if the transformer is heavily loaded and the motors are maxed out. We tend to over size the transformer banks in some cases to reduce the voltage unbalance to reasonable levels.

 

[mivey]

...
Why does an open delta setup give you poor voltage regulation and nuisance tripping due to higher loads?

Does it? Is that out of some book? I am with Iron_Ben, not a problem if the plant is wired correctly.

Does it?

Yes.

Is that out of some book?

It is out of many books. Any good transformer book covers it.

I am with Iron_Ben, not a problem if the plant is wired correctly.

Even a perfectly balanced load will experience voltage unbalance on an open delta. Proper transformer sizing and equipment sizing is key.

Most equipment should allow for unbalances on the utility system but this fact often gets overlooked. Instead of de-rating, sometimes a motor, chiller, etc. will be loaded to max capacity then overheat. On a three-phase line operating under normal circumstances the voltage unbalance can be minimal and stay well below acceptable limits most of the time. If an installation is on a two-phase utility circuit, then you need to be sure to de-rate properly as the voltage unbalance can vary more.

 

[paul]

The phase angle shrinks the more you put load on the system, which in turn drops the voltage between C&A. Without going into the math calculations behind the FM, just know that because you are missing a transformer there nothing there to hold the voltage/phase angle steady. By making one of the pots larger than the other, it helps keep this to a minimum, but does not stop it from happening. You tend to only see it in older parts of town where light industrial is located.

 

[starbolin]

...Even a perfectly balanced load will experience voltage unbalance on an open delta. Proper transformer sizing and equipment sizing is key.

...

I was being sarcastic. Never works online. Sorry. Proper sizing and equipment was my point. If the EC did his job the systems can be trouble free. Most tenants will never know. Problems with red-leg deltas are usually due to EC inexperience or plant managers doing their own engineering.

Friend of mine has a machine shop in a red-leg building. Voltage regulation actually improved with the 3-phase motors online. Full load to the motors is only periodic and they act like phase converters when idling.

The only problem was he decided to turn an empty bay into a DIY laundry room and he didn't realize the orange tag was meant to mark the whole bus. Result was a fried clothes drier. ( Add machine shop owners to the list of people will never hire an EC. )