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Boost transformer check 1

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itsmoked

Electrical
Feb 18, 2005
19,114
Please check this. I know Bill has suggested open-delta to me a few times and now I want to try it.

I have a 30A machine tool load designed for 230V.
At the site I have about 200V showing up from the 208 service.

I think I want to boost 32V.

This means 32V x 30A ~ 900, call it 1kVA of boost.

This is three-phase DELTA.

If I go with open delta do I still use the same two 32V 1kVA boost transformers?

Does the center of the three phases lean-off a bit from the theoretical center it would be with non-open delta?

Since the one leg isn't being boosted does the 32V boost of only two legs still represent 232V between all three legs or is there now an odd leg? Should I care?

Keith Cress
kcress -
 
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Hi Keith.
Glad to help.
I size the boost transformers based on motor full load current and the boost voltage.
The boost winding is connected to a motor lead. Therefore the motor current must pass through the boost winding.
So,
Keith said:
This means 32V x 30A ~ 900, call it 1kVA of boost.
Perfect and to my comfort level of precision as well.
Each of the two transformers will be 1 KVA or more.
Keith said:
If I go with open delta do I still use the same two 32V 1kVA boost transformers?
Yes
Keith said:
Does the center of the three phases lean-off a bit from the theoretical center it would be with non-open delta?
Good catch. Yes the center point will be offset. Never had a problem.
I still see this connection used from time to time. I recognize it but most others don't care as long as it's working.
Keith said:
Since the one leg isn't being boosted does the 32V boost of only two legs still represent 232V between all three legs or is there now an odd leg? Should I care?
With a wye supply, you will have two "high" legs to the neutral. I don't care.
Connections:
You can do a single phase autotransformer boost, this is basically two single phase boosts with A phase common.
Let's call the transformer leads 0 Volts, 200 Volts and 232 Volts.
Transformer #1
Connect 0 Volts to A phase.
Connect 200 Volts to B phase.
Connect 232 Volts to the load.
Transformer #2
Connect 0 Volts to A phase.
Connect 200 Volts to C phase.
Connect 232 Volts to the load.
If you draw the vectors, A-B will now be extended to B', The magnitude will be 232 units.
A-C will now be extended to C', The magnitude will be 232 units.
You now have an isosceles triangle with the sides at 232 units.
But, an isosceles triangle with an included angle of 60 degrees is actually an eqilateral triangle so the voltage on the third side equals the voltage on the other two sides.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
PS. How are you healing Keith? I hope you're up and around, if not now then soon.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
Thanks bunch Bill!

Funny you should ask. I just now came back from walking around my circle. My knee allowed me actually walk around the circle last night for the first time in three months! I had to walk slowly and work hard to not think about my gait or it would go pathological on me. I got back and vacuumed our small livingroom since my dog drops buckfulls of white 2 inch hairs every day. Once I finished I noticed my legs were shaking from the exertion! Before I hurt my knee -on a three mile fast tramp thru hilly woods that starts with a 200ft -both feet and hands vertical climb- I used to walk my dog about 2 miles every single evening. Depressing. You want to put on a bunch of weight? Crap-out your knee so you can't burn ANY calories.

My knee has gone thru about four distinct injuries since my fall. With each problem arising then slowly healing to reveal the next. At the 'moment' I 'think' I can start some very light walking and that's what I'm doing.

Thanks again for the check above.

Keith Cress
kcress -
 
Good to hear that you're on the mend.
I forgot to mention, even though you are aware of the issue, others following the post may find it valuable information. There will be two legs with higher current in about the ratio of the boost. Motor current should be balanced but supply currents ahead of the boost will not be. I have never had a problem on the grid.
I was on a project where the open delta connection was used for banks of temporary fans fed from a generator. I suggested that it may be well to "roll" the connections so as to better balance the loads on the generator. The suggestion did not go over well. Supervision was not in the habit of re-visiting designs. Even on the generator it worked fine. The adjustment was from 600 VAC down to 480 VAC. When the voltage is dropped, two legs have less current instead of two legs with more current.
Three reasons why I like this connection versus a three autotransformer connection.
1> Less connections to make, less work and less chance of error.
2> Even though the transformers must be slightly larger than with the three transformer solution, very often it works out that the next available size is the same for both connections and the price of the third transformer is saved.
3> This connection does not require a neutral conductor for three phase loads. When using the wye autotransformer connection, for full compliance, 5 wires should be used. Three phase wires, a neutral and a ground conductor.

On the site mentioned, there were also three transformer, wye connected autotransformers. The ground was used as the neutral connection. I suggested that there may be unintended complications in the event of a ground fault. I was still the know-it-all and was ignored. Then there was a fault on a 30 amp circuit that fed through to the main and the main breaker tripped on ground fault. I went from know it all to flippin-know-it-all. A few weeks later I was reassigned to another project. No loss of pay, and I ended up still employed after the original project wrapped up and most of the others were gone, so it worked out in the end. And when that project was wrapping up I was kept on and moved to another project, and then again. I guess someone upstairs appreciated some knowledge in the lower ranks. I finally left for personal business. There was a no re-hire policy for quitting. When I checked my documents I found that they had arranged a "shortage of work" layoff to circumvent the no rehire policy if I should decide to return. Although my suggestions were ignored, all in all I was treated very well there. Not always the case with short term construction projects.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
Keith, we found a couple roombas pick the two dogs hair up really good. with good battery they go 1-1.3 hours. totally full of dog hair when done! maybe time for a roomba or two so your knee doesn't get overworked!
 
LoL Rombas... I have a bright red one! It was neglected during some remodeling and it's battery pack choked. I disassembled it finding half the batteries were sub 1/10V and the others were typical NiMH voltage. I researched rebuilding it about a month ago and discovered they're like a lot of things. Buying the batteries yourself and rebuilding a pack costs slightly more than buying a new aftermarket pack from the same place.

I've been contemplating purchasing a new pack but was concerned the dog fur would jam it up anyway. I'm SO glad you've put that to rest Mike! I shall proceed post-haste with getting a new pack. Currently we vacuum about every other day to keep the floor sane. Be nice to avoid a few of those.

Keith Cress
kcress -
 
I have a firm rule that's only been broken once, when I was out of town. No animals in the house.
We don't have any dogs or cats but we do have three horses.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
Did this today and was disturbed by the feeble wires the boost transformers had. 1.5KVA and they had frikken 18AWG primary wires! Rather annoying having to mate up #6s with #18s. I guess they don't care about the drops caused by the transformer itself.

Keith Cress
kcress -
 
You will have about 7 Amps on the primary at full load.


Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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