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Old 10-03-2019, 10:51 AM   #1
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Default Transformer & Generator Dilemma

Hey all,

We have a client that bought a 40kw 600 volt 3 phase generator for his property off grid (local poco wanted +/- $180 000 to run poles and lines into the property).

Problem is, he wants to run his woodworking shop/business off this land. All his woodworking machines are 220-240 volts, so we cannot use a 120/208 transformer, as the motors are not rated for that voltage. (we're talking about $15k in woodworking equipment).

So now we are trying to figure out what to do. We cannot use a single phase transformer, because the generator will not be balanced and we will burn the generator up. So that leaves us with only three phase.

Should we use a 120/208 transformer and install an auto-transformer/buck-boost transformer to get the voltage we need? These machines are not hardwired, so we would have to install a panel for these loads if we do this. The client has a 120/208 transformer already (He drove 8 hours round trip to buy a used one...)

Or should we use a delta high leg transformer (240 delta with center tap). We are not sure how much 120 volt stuff he wants to use, but we know he will have some. The one quote on a delta high leg stated the center tap is only good for 1.5kva... So we're not even sure if this is an option.

If I had it my way, I'd install 2 transformers, but we have to work within a budget.

Any input is appreciated,
Thanks in advance.
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Old 10-03-2019, 11:18 AM   #2
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There is probably a lot more info we are not seeing but here are some general comments.


Running the generator loading only 2 phases is not going to burn it up but capacity will be limited. 58%, 67%, you have to consult the manufacturer.


Are the 240V loads all single phase?


Choices are a 600 - 120/240 delta center tap tx like you stated, that would be my first choice aside from the C.T. limitation you stated, maybe you can find a different mfr. with something better.


Another option could be 600 volts to 220/127Y . A little low for the motors, a little high for the light bulbs, but "probably" ok.


Customers that buy the wrong generator and expect you to make it better are sure a pain.
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Old 10-03-2019, 11:28 AM   #3
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Quote:
Originally Posted by Jerome208 View Post
There is probably a lot more info we are not seeing but here are some general comments.


Running the generator loading only 2 phases is not going to burn it up but capacity will be limited. 58%, 67%, you have to consult the manufacturer.


Are the 240V loads all single phase?


Choices are a 600 - 120/240 delta center tap tx like you stated, that would be my first choice aside from the C.T. limitation you stated, maybe you can find a different mfr. with something better.


Another option could be 600 volts to 220/127Y . A little low for the motors, a little high for the light bulbs, but "probably" ok.


Customers that buy the wrong generator and expect you to make it better are sure a pain.
All loads are single phase.

We are not even sure how many tools he will have running at once... he's more worried about planet X...

I'll have to get the client to contact the manufacturer to see if they still have documentation. He bought an old onan generator from the 80's with only 120 hours! Such a great "deal"!

Capacity being reduced is a given if we use a single phase transformer. There were no markings on the generator stating the balancing (I've seen some that only like 25% imbalance... hence out worries)

I'll run the 127/220Y past the boss and see what he says. Could we use the 570 volt tap on the 120/208 transformer to achieve 127/220?

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Old 10-03-2019, 01:39 PM   #4
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Seems like the 570 volt tap would work, and you can twiddle with the adjustment on the generator itself to get it perfect but still it is a split-the-difference kind of solution.


I hope the others have more to say about this. I only make the power, not so much into the things that use it.


If they guy bought the generator to "save money" make sure you are not also doing all kinds of work to save him money, your job is to take the money that he saved by making you do all this work.
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Old 10-03-2019, 02:00 PM   #5
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If they guy bought the generator to "save money" make sure you are not also doing all kinds of work to save him money, your job is to take the money that he saved by making you do all this work.
Kevin,
If the customer came to you before he bought the genset, what would the design look like? That will give you a target to aim for. Btw, he may figure out the genset maintenance costs will eat him up.
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Old 10-03-2019, 02:34 PM   #6
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Kevin,
If the customer came to you before he bought the genset, what would the design look like? That will give you a target to aim for. Btw, he may figure out the genset maintenance costs will eat him up.
If he had come to me (or my boss) before buying the genset, we would have told him to buy a single phase generator. He paid 5 figures for this genset, then had the radiator rebuilt, and had it fully overhauled to make sure it's in good working order.

Problem is he is trying to do this on the cheap. (Like spending 800 dollars on fuel driving to buy a used 25kva 120/208 transformer).

I'll try sourcing out another 240 volt delta high leg transformer. This is the only way to do this with 1 transformer.

Maybe a buck-boost transformer to drop the voltage would work... then I only need one.

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Old 10-03-2019, 02:42 PM   #7
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Are you busy with other [properly planned] jobs? Do you have time for this hassle? Bid the hassle in.
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Old 10-03-2019, 02:47 PM   #8
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Have you actually looked over the wood working equipment & will it run on single phase, 208v?
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Old 10-03-2019, 02:51 PM   #9
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Get a 75 kava 240 V delta high leg secondary transformer. More than enough capacity. Balance the single phase loads around the 3 phases. A-B, A-C, B-C. Tell him to buy 3 phase tools when he upgrades or replaces.
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Old 10-03-2019, 03:11 PM   #10
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The NEMA standard dry type transformer has multiple taps.

Meaning: you can lift the secondary voltage well above 208 -- taking 120 VAC north at the same time.

Plan B:

So the solution for your pal: two transformers.

One set at true 208Y120 for all 120 VAC loads.

One re-tapped to 220 VAC or better Line to Line. This unit would not feed ANY 120 VAC loads.

Use different colored hots and neutrals when you wire this puppy up.



208Y120 is merely the Factory setting with a NEMA transformer.
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Old 10-03-2019, 03:12 PM   #11
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Changing the taps is simple Simon. They're right in front -- and the face of the transformer has all of the dope on which to pair up. Obviously, on a 3-phase unit, all taps must be the same.
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Old 10-03-2019, 04:52 PM   #12
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Have you actually looked over the wood working equipment & will it run on single phase, 208v?
Yes, we checked the name plates on most of the motors. Some we could reconfigure to 120 if we wanted but the rest are strictly 240 volts.

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Old 10-03-2019, 04:54 PM   #13
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Are you busy with other [properly planned] jobs? Do you have time for this hassle? Bid the hassle in.
Yes. This is a side job for a friend. He's in a rush, but we aren't. This has been talked about for 5 years with no official plans and all of the sudden he bought a generator wants it wired.

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Old 10-03-2019, 04:56 PM   #14
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Originally Posted by telsa View Post
The NEMA standard dry type transformer has multiple taps.

Meaning: you can lift the secondary voltage well above 208 -- taking 120 VAC north at the same time.

Plan B:

So the solution for your pal: two transformers.

One set at true 208Y120 for all 120 VAC loads.

One re-tapped to 220 VAC or better Line to Line. This unit would not feed ANY 120 VAC loads.

Use different colored hots and neutrals when you wire this puppy up.



208Y120 is merely the Factory setting with a NEMA transformer.
Might end up doing this. It's easy to find used 120/208 transformers.

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Old 10-03-2019, 05:00 PM   #15
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Get a 75 kava 240 V delta high leg secondary transformer. More than enough capacity. Balance the single phase loads around the 3 phases. A-B, A-C, B-C. Tell him to buy 3 phase tools when he upgrades or replaces.
Will the inrush current stall this generator though?

I should have taken a photo of the nameplate on the generator. It's 40kw at 600 volts. That gives me 66 amps. Generator has a 50 amp breaker.

75kva at 600 volts is 72.2 amps.

50kva at 600 volts is 58.1 amps.

I do like this plan but my boss said the generator can handle 30kva max.

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Old 10-03-2019, 06:32 PM   #16
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I’ve run into this before. You can usually adjust the generator regulator up so you are a little hot on the 120 output and a little low on the 230 side but enough to split the difference. Be careful with buck boost because you might be reading specs wrong. You only need kVA to cover the boost which is 230-208=22 V. So if you have 100 kVA off a generator the boost only needs 22/208 of that or 11 kVA, or something like that. I don’t have my transformer book handy.


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Old 10-03-2019, 06:41 PM   #17
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Will the inrush current stall this generator though?

I should have taken a photo of the nameplate on the generator. It's 40kw at 600 volts. That gives me 66 amps. Generator has a 50 amp breaker.

75kva at 600 volts is 72.2 amps.

50kva at 600 volts is 58.1 amps.

I do like this plan but my boss said the generator can handle 30kva max.

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Yeah, may have to be a 45 kVA, but my gut feeling is that the generator should be able to handle it.
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Old 10-03-2019, 06:45 PM   #18
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Yeah, may have to be a 45 kVA, but my gut feeling is that the generator should be able to handle it.
Thanks CoolWill.

Correction: 50kva is 48.1 amps. I made a typo.

I'll price out a 45kva. They're spec'd at 43.3 amps. It'll give me 108 amps at 240 volts.

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Old 10-04-2019, 04:30 AM   #19
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Another option is a homebrew open delta using 2 single phase xformers.
Another is a 600 to 120-240 and a 347 to 120-240 xformers connected like a 2 phase transformation. The secondaries of each in 120 vac in series make 240 volt. Kinda old school connection, not used much any more.
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Old 10-04-2019, 06:07 AM   #20
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Another option is a homebrew open delta using 2 single phase xformers.
Another is a 600 to 120-240 and a 347 to 120-240 xformers connected like a 2 phase transformation. The secondaries of each in 120 vac in series make 240 volt. Kinda old school connection, not used much any more.
I was wondering if there was something creative you could do to turn three phase into single phase, like a rotary phase converter in reverse, or convert to DC and back to AC like a UPS or a drive, but every idea I had would be more expensive than trading in for the right generator.

I think the balance will be a challenge here no matter how you slice it, I don't know how important it is to keep that generator balanced, but in a wood shop you don't run all those loads together. The wood shop loads won't run at the same time the house loads run. The house loads will vary quite a bit seasonally. (I am assuming heat, range, hot water, dryer, etc. are all propane but AC is electric.)
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