Will a 225 kva boost xformer trip my 100 amp breaker at no-load startup?

I'd like to buy a 225 kva boost xformer, 208 delta to 480/277 wye, which is much bigger than I need now but allows for future upgrades. This used xformer is cheaper than much smaller new ones. Wall service now is 100 amps 208v 3 phase, and I need 480v. 3 phase for various machines.

Will I run into any electrical problems running this xformer off my 100 amp breaker, particularly regarding no-load startup surge, if there is one?

Probably not, unless the breaker is very sensitive. In my experience most transformers cause breaker problema when they are first connected to the supply circuit rather than when the load is switched on. It is usually not a problem with breakers rated for motors and other inductive loads.

"Probably not"???

Uh, the 208V full load cuirrent draw of a 225kVA transformer is rated at 625A. Sure, the current draw equates to the load, but the INRUSH is related to the transformer impedance and conductor resistance. You can expect the current inrush to be 10-12x the FLC, so in this case the MINIMUM inrush will likely be 6250A! A 100A circuit breaker will have, AT BEST, instantaneous trips that can take 1000A. That's 1/6th of what it will see.

So the correct answer is, NO, you cannot even turn that transformer on...
The biggest 3 phase 208V transformer you can expect to connect to a 208V service with a 100A breaker is 30kVA. Anything bigger is dangerous.

Jraef said:

"Probably not"???

Uh, the 208V full load cuirrent draw of a 225kVA transformer is rated at 625A. Sure, the current draw equates to the load, but the INRUSH is related to the transformer impedance and conductor resistance. You can expect the current inrush to be 10-12x the FLC, so in this case the MINIMUM inrush will likely be 6250A! A 100A circuit breaker will have, AT BEST, instantaneous trips that can take 1000A. That's 1/6th of what it will see.

So the correct answer is, NO, you cannot even turn that transformer on...
The biggest 3 phase 208V transformer you can expect to connect to a 208V service with a 100A breaker is 30kVA. Anything bigger is dangerous.

Click to expand...

Thanks and I myself have little knowledge of this area, but on advice of others I got an experienced electrician involved, who advised me to get a 3 phase boost transformer in the 37.5-45 kva range. He worked some numbers to get there. You obviously came to a much different conclusion, which actually means I won't be able to get enough amps at 480 volts to run the machines I was planning to run (even one at a time per my plan.). Would you take a quick look again before I go back to my electrician and tell him we probably can't do 480 volts at all? He really didn't get involved with my interest in the 225 kva unit, I'm kind of out on a limb there.

100 amp 208v service and you can't get enough amps to run the 480v machines with a 37 to 45kw transformer?

You can't even get more than 35 KVA out of your 3 phase service continuously. 80% is the continuous limit far as i know. you could run a single 30HP motor in theory, but you probably won't be permitted to do a direct on line start of a motor that large.

I have not used a soft start to run a transformer, but it might work... But why anyone would use a HUGE transformer with a big and expensive idle draw at horrible power factor, I do not know.

Size it right for the load....

Around here, any motor over 2 or 3 HP has to have a soft start, according to the powerco. That would generally be 5 HP and up.

The OP has lots of questions, and off-the wall ideas. A fact someone already mentioned elsewhere. I don;t know WHAT he wants.

JST said:

I have not used a soft start to run a transformer, but it might work... But why anyone would use a HUGE transformer with a big and expensive idle draw at horrible power factor, I do not know.

Size it right for the load....

Around here, any motor over 2 or 3 HP has to have a soft start, according to the powerco. That would generally be 5 HP and up.

The OP has lots of questions, and off-the wall ideas. A fact someone already mentioned elsewhere. I don;t know WHAT he wants.

Click to expand...

Thanks. I want to run some older machines I have that need 480 volts. Happy to run them one at a time. I was badly misinformed about how easy it is to make 480v. From 208. I asked about backfeeding a buck transformer I had and although I know how to do it now (make a corner ground) the electrician I consult isn't comfortable with that, so I'm not either now. He said get a 37.5-45 kva boost transformer, but earlier in this thread I'm told anything above 30 kva is dangerous given my 100 amp, 208v. Power supply in the building. So all I can do is keep working on this project until a viable solution pops out, and I greatly appreciate the advice.

I'll have to look up what soft-start means, how it is done really.

I've actually started looking into renting/buying a used diesel generator; an 80 kva unit only weighs about 2.5 tons, get it on a trailer and wheel it outside the building when the power is needed (yes those fat cables would be expensive!) It'd have to be standalone, powering machines directly, I wouldn't think of trying to integrate them with the building's power. Has anyone you know given up on transformers and gone that way?

Might even try renting some space in a building already equipped with major power. The marble and granite guy down the street had his service upgraded to 800 amps, for example, but he still has to use a transformer for the one 480v. Stone saw he has (his other saws are 208v.). But he's got no space left and his whole shop is always very wet.

soft start: 2 second time delay with resistors. 2 ohms is probably about right for your 225kw unit. need some kind of intelligence so you can't power the load while the resistors are feeding the transformer, otherwise you'll burn up the resistors.

for instance two sets of relays, one on input and output of transformer. a third relay energizes the transformer through two or three resistors, after a 2 second time delay you close the other two, which shorts out the resistors and they are out of circuit.

you can do all this with manually with two, 3 phase circuit breakers. just remember which one to turn on first.

JST said:

I have not used a soft start to run a transformer, but it might work... But why anyone would use a HUGE transformer with a big and expensive idle draw at horrible power factor, I do not know.

Size it right for the load....

Around here, any motor over 2 or 3 HP has to have a soft start, according to the powerco. That would generally be 5 HP and up.

The OP has lots of questions, and off-the wall ideas. A fact someone already mentioned elsewhere. I don;t know WHAT he wants.

Click to expand...

I'm sure soft start would work on a transformer. Resistors in series with the line would limit current and as the magnetic flux got going in step with the line the current drawn would fall off so the resistor drop would be less and you could short them out.

A while back I did some tests during a debate here about inrush but I doubt anyone read the results I posted. Because I don't have a ballistic galvanometer, I read the residual flux the same way you read computer memory cores, with a DC pulse that was enough to drive the core to saturation. The secondary output when the second pulse was in the same or opposite polarity was a measure of the residue. I was amazed at how great it was. I had assumed that the flux would fall back close to zero. If it did, you could connect at a voltage peak (not zero crossing, think about it) and the surge would be minor. With the residual flux at some indeterminate point, you would have to match the voltage on the sine wave to that flux point. Very far off that point and the core would hit saturation somewhere in the cycle and it effectively disappear, leaving a copper resistor to limit the current.

Bill

johansen said:

soft start: 2 second time delay with resistors. 2 ohms is probably about right for your 225kw unit. need some kind of intelligence so you can't power the load while the resistors are feeding the transformer, otherwise you'll burn up the resistors.

for instance two sets of relays, one on input and output of transformer. a third relay energizes the transformer through two or three resistors, after a 2 second time delay you close the other two, which shorts out the resistors and they are out of circuit.

you can do all this with manually with two, 3 phase circuit breakers. just remember which one to turn on first.

Click to expand...

Is this about right for the two breaker method? Excuse the sloppiness. Another breaker is needed between xformer and load.

9100 said:

I'm sure soft start would work on a transformer. Resistors in series with the line would limit current and as the magnetic flux got going in step with the line the current drawn would fall off so the resistor drop would be less and you could short them out.

.....
Bill

Click to expand...

It would reduce the surge. I don't know if it would reduce enough to get you there in one step with that big transformer and the little breaker. Might need a couple steps. The resistor may spread out the surge time-wise, so what it loses in peak, it might have in duration, and still pop the breaker. (They respond to time and current. less time more current, and more time less current can be equal as far as the breaker is concerned.) Then you'd need bigger resistors and more steps to bring down the peak.

It also can depend on breaker type, but I don;t know what options exist with that brand of panel, plus the options may vary with rating. For low current and single phase you can get high inrush breakers intended for HID lighting. I suspect most do not have that available in 3 phase and high current, but I have never looked for it.

The remanent field may be 40 to 60% of peak. Quite a bit.

9100 said:

I'm sure soft start would work on a transformer. Resistors in series with the line would limit current and as the magnetic flux got going in step with the line the current drawn would fall off so the resistor drop would be less and you could short them out.

A while back I did some tests during a debate here about inrush but I doubt anyone read the results I posted.
Bill

Click to expand...

What do you mean no one read it? I did!



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277/480 Y Output versus 480V Corner Grounded Delta Output for Machine Loads

Cannonmn,

Your cause might be better served if you put together a list of equipment specs that you plan to run on 480V.
From your original thread, you mentioned a 2HP pedestal grinder, and a battery charger.

Its most likely that the battery charger can be reconnected for operation at 208V, by changing some jumpers inside the unit. Check the nameplate for compatibility. Instruction for the jumpers should be inside the cabinet.

If that is correct, that leaves you with a 2HP grinder, that we know of.

The grinder may also be re-connectable for 230/460V operation. You never showed us the motor wiring connections, so that we could be sure. If there are 9 or 6 wires coming from the motor, its likely connectable for 230V operation. The motor connected for 230V and operated from 208V will work fine, as long as you upgrade the motor starter and heaters for the increased current at the lower voltage. If you really wanted to be picky, a couple of small buck boost transformers could be used to boost the voltage from 208 to 240. Really not necessary on a single grinder motor though.

Then you told us that the sparky you have is hesitant to use a standard 30KVA 480/208-120V 3Φ transformer in reverse, and corner ground the 480V delta output. So you switched gears and went for a huge 225KVA transformer that had a 277/480V Y output, so it would be a standard Y output and normal bonding and grounding. This lead to the fact that you can't reliably energize that monster on a 208V 100A supply circuit.

For machine tools with standard motors and electro-mechanical controls, corner grounding is no problem when implemented correctly. What can be a problem is equipment that uses electronic drives. VFD's normally want to see a Y input, or damage can occur.

At this point I'm not convinced that you even need 277/480V Y, to power the equipment you have. And if you do, we have no idea what current capacity you require. I did some looking and found one other step up transformer with a 208Pri and a 277/480V Sec in a 15KVA size from New York, if your up for a drive. The 15KVA would be good for an 18A output @ 480V

If it were me, I would step back and assess your actual needs, before you buy any more gear that is really not required. Do the homework first. Check the equipment you have, to see if it's convertible for the voltage you have. Simple is good.

SAF Ω

Thanks, very appropriate comments, indeed I'm at the re-assessment point, but I don't think the half-cocked threads were wasted, I've learned a huge amount in the past month here. I am sorry that they contained incomplete info. I still have pics of the 3 hp grinder's wiring scheduled, hope this week. I'm also looking at the Big Picture and have the power company's contractor coming for a site inspection to cost-out an upgrade to 200 amps at least. I have that much avail. already but unfortunately it is 100a. On each of two separate services/meters/panels about 100 feet apart. If doubling my amps on one panel comes in at $5K or less I'll probably do it. If much more I may buy a used diesel generator and keep it on a trailer outside. I asked power company about 480 volts, and they'd love to buy a new transformer and build a pad outside etc. but we're looking at some 5-figure bill and I'm not going to do that in a leased space. I appreciate the info on the small xformer, I will follow that up. I found some perfect ones listed in the Surplus Record yesterday but when I called the seller he had none.

Here's a fairly complete list of stuff I've collected over the years pending availability of 480 volts, and I'm sure you can see how some single-Motor items may be easy to rewire and one (the mill with 6 motors) would be near impossible/not worth effort. We have other machines that do basically same things as these do, which is why getting these running hasn't been top priority here.

480 volt 3-phase machines:

K&T mod 2D rotary-head mill
3/4 hp pedestal grinder
Snow Tapper
3 hp Queen City? Pedestal grinder
Oster Industrial threader model 784
Powermatic Drill Press
Taft-Peirce Surface Grinder
Boice Gear head Drill Press
Industrial Dust collector
I.R. 3-cyl. Air compressor 15 hp
Geometric Threader
Gigantic Canuck battery charger

mike_kilroy said:

What do you mean no one read it? I did!



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Click to expand...

I had made the last post in the thread, then did some tests and posted the results, so I had the last 2 and no one responded.

Jerry, you could use resistors that limited current to the breaker's limit, but with the big transformer's idle current, I agree that it might take multiple steps. My former boss referred to such machinations as "Going way around Robin Hood's barn to get a result."

Bill

No one commented on something I asked so lemme ask again: Has anyone you know needed more electric power to run machine tools and couldn't or wouldn't get it from The Power Company, so they resorted to renting or buying diesel generators?

Jraef said:

"Probably not"???

Uh, the 208V full load cuirrent draw of a 225kVA transformer is rated at 625A. Sure, the current draw equates to the load, but the INRUSH is related to the transformer impedance and conductor resistance. You can expect the current inrush to be 10-12x the FLC, so in this case the MINIMUM inrush will likely be 6250A! A 100A circuit breaker will have, AT BEST, instantaneous trips that can take 1000A. That's 1/6th of what it will see.

So the correct answer is, NO, you cannot even turn that transformer on...
The biggest 3 phase 208V transformer you can expect to connect to a 208V service with a 100A breaker is 30kVA. Anything bigger is dangerous.

Click to expand...

Whew! Thanks for correcting that. I somehow got into my head 25kVA instead of the 225kVA actually written in the original post. Yesterday was a very busy day and I guess multitasking doesn't always work so well for us human beans.

Cannonmn said:

Here's a fairly complete list of stuff I've collected over the years pending availability of 480 volts, and I'm sure you can see how some single-Motor items may be easy to rewire and one (the mill with 6 motors) would be near impossible/not worth effort. We have other machines that do basically same things as these do, which is why getting these running hasn't been top priority here.

480 volt 3-phase machines:

K&T mod 2D rotary-head mill
3/4 hp pedestal grinder
Snow Tapper
3 hp Queen City? Pedestal grinder
Oster Industrial threader model 784
Powermatic Drill Press
Taft-Peirce Surface Grinder
Boice Gear head Drill Press
Industrial Dust collector
I.R. 3-cyl. Air compressor 15 hp
Geometric Threader
Gigantic Canuck battery charger

Click to expand...

A list is a good start, but now you need to add some numbers to it.
HP, Volts, #Phases, Amps or KW, Is It convertible to the service voltage or 230V, Largest motor started.

You need some real numbers to be able to size any service, generator or transformer. If your going to run off of the existing split service locations, then the layout becomes a factor. What loads go with each service panel.

What loads are used the most and duration, has a good bearing on what minimum size you can get away with.
Setting things up for the full load of everything, all the time, is just plain expensive. Guessing can be costly as well. You might want to have a look at your lease as well. Most stipulate that equipment(electrical) added attached to the structure must remain with the building at the end of the lease.

SAF Ω

Will you folks let me know if this xformer comes close-I know the kva is a bit high, and I'm working on a deal for a 15 kva one, but if the voltage specs on this are acceptable, it is at least an improvement over the 225 kva monster. My concern on this one deals with the voltages given-240 to 480/277. I see there's a tap as low as 228 v. This is foreign-made and although now I'm used to seeing "nominal" voltages on us stuff I have no experience with foreign practices. Edit:
Delta Transformateur is in Quebec, so foreign, yes, but I'd doubt their standards are much different from US, since US is probably a good part of their market.

SAF said:

A list is a good start, but now you need to add some numbers to it.
HP, Volts, #Phases, Amps or KW, Is It convertible to the service voltage or 230V, Largest motor started.

You need some real numbers to be able to size any service, generator or transformer. If your going to run off of the existing split service locations, then the layout becomes a factor. What loads go with each service panel.

What loads are used the most and duration, has a good bearing on what minimum size you can get away with.
Setting things up for the full load of everything, all the time, is just plain expensive. Guessing can be costly as well. You might want to have a look at your lease as well. Most stipulate that equipment(electrical) added attached to the structure must remain with the building at the end of the lease.

SAF Ω

Click to expand...

Thanks, sounds like a spreadsheet is in order, will add that to the worklist while I continue working things already in progress such as getting the Power Company out for a visit. They mentioned the same thing, a requirements analysis. The big simplifying assumption for my shop is that for all the 480v. machines, there's only one operator, me. The hired help only uses a few 120 v. Machines that are all 20 amps or less, and no more than one is in use at any time. That and the lighting, that's it. I haven't got the exact numbers yet but I'd be happy to live within an additional 100 amps on either one of the presently-100 amp panels, for the next year, if that upgrade doesn't break the budget. We don't use the two installed gas heater-blowers no matter how cold it gets, never had the gas turned on, the gas company insists on us paying for all the gas that leaked out and spun their meter before we signed the lease and we're just not interested.