Open Delta new question

[stantonlw]

I've read the old thread on open "Y" Open "Delta" (ie., using two distribution phases to provide "3 ph" power) but still have a question.

Here in Northern Fla we have a power company using open delta to provide small amounts of 3 ph power. PEPCO, in my area, says they don't provide it. Dominion Vepco says they do but only small services (< 200A), with warning that it may damage equipment such as motors and compressors. Progress energy (the supplier here) says it works for small 3 ph motors but cannot tell me how large and wonders why I have any questions.

Question is: What is "small" ie., how large a motor can be used safely?

I have been asked to help a small business (Wakulla Dive Center) get up and running in a new facility which has a 225A "Open Delta" "three phase" . This business will be using a 20hp 3ph air compressor and a 10 hp 3ph air compressor which might occasionally start up at the same time. They will frequently be both on line.

Will this be a problem (with a 250A service)? The upgrade cost is rather high for a small business.

Laird Stanton

If this question is unsuited to this forum, perhaps one of your members could respond via PM

Thanks,

 

[benaround]

Laird,

You need to talk to the serving power co. Have your load calcs for them to look at and

meet them at the location.

Some poco install open delta for residential services, the 3 phase is for the a/c unit, maybe

5 ton, then 120/240v for the house loads.

 

[gar]

100121-2334 EST

We have a lot of open deltas around here. At my son's building there is one. The building is about 40,000 sq-ft with about 6 tenants. My son has a 200 A service for his machine shop. This includes 5 CNC machines, nominal rating 20 HP or so, a 5 HP air compressor, a 20 HP air compressor, a 20 HP belt sander, air conditioning, and other smaller machines. Lighting is a moderate continuous load. Obviously these are not all run under full load much of the time, or even all running.

I believe the pole transformers are 100 KVA for the single phase, and 50 KVA for the wild leg. If greater capacity is needed larger or an additional transformer can be added.

.

 

[stantonlw]

Ben,

Your reply makes me a little more sanguine. I do have to wrap my old brain cells around those vector diagrams from the older post, plot them out even!

I understand that the pole transformers only provide a percentage of their rated power with this connection but that is the poco's side. HOWEVER, On another site (one showing the use of an idler motor to make the conversion), it was mentioned that the three phase motors, running on an open delta service were not capable of delivering full rated power. Can you shed light on this and on what the horsepower derating factor might be?

Laird
(already too many "larrys" around here)
N3HLL
NATCAP 4

 

[jim dungar]

HOWEVER, On another site (one showing the use of an idler motor to make the conversion), it was mentioned that the three phase motors, running on an open delta service were not capable of delivering full rated power. Can you shed light on this and on what the horsepower derating factor might be?

If there is an idler motor, that system is a 'manufactured' three phase. This is not the same as an open delta being supplied by the utility.

There is absolutely no problem with running three phase equipment, including motors, on an open-delta as long as you are with-in the rating of the transformer bank. I have installed open-delta systems in excess of 800A to feed printing presses.

 

[stantonlw]

Jim,

Thanks for the info.

I understand the "manufactured 3 ph" concept with the idler motor, that I don't have.

When I use a 10hp 3ph motor on an open delta (not what the Navy calls open delta, rather what the poco's deliver) do I need to derate the motor power? Ie., can a "10hp" motor deliver 10hp?

Laird
:-?

 

[gar]

100125-1940 EST

stantonlw:

Jim already answered your question.

To a large extend an open delta will run your three phase motor as well as a full delta. There is nothing phony about an open delta.

If the transformer capability was marginal relative to the motor load, then there would be some effect. Usually the open delta systems in our area have a much larger transformer for the single phase supply, and therefore the source impedance on the wild leg is higher than on the other two legs.

.

 

[LarryFine]

To a large extend an open delta will run your three phase motor as well as a full delta. There is nothing phony about an open delta.

Agreed. I've even supplied 2-wire loads from all three phase pairs on an open Delta.

 

[jtester]

The biggest problem with open delta's is that the voltage drop across the open leg will be different than that across the two with one transformer coil across them. This is particularly a problem when you have a motor that starts under load vs one that can start easily. You will get voltage imbalance between phases and this will smoke 3 phase motors.

I often limit the maximum hp on an open delta to 10 hp, although a larger lightly loaded motor won't be a problem.

Jim T

 

[wirenut1980]

As has been said, the main problem with open delta services and 3 phase motors is they usually have higher voltage unbalance, which can lead to heating of the motors. My personal opinion: it would be a good idea to de-rate the motors to about 0.8 to 0.9 to allow for up to about 3-4% voltage unbalance.

I do not think there needs to be a limit on how large of a main or motor should be supplied by open delta.