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Transformer Question #193736 04/16/10 11:02 PM
Joined: Jun 2005
Posts: 124
poorboy Offline OP
I have this transformer to boost a shop's 208 volt 3 phase to run a 460-480 volt table saw (customer bought it without paying close attention to the voltage---can't return it because it was used).

I am afraid that because of the 240 volt rating I will only get a voltage in the low 400's range, too little for the 5hp saw. This used X'former was 100 bucks (yes I can return it) and a new one was $725 (my contractor cost). New one was 3 weeks out, as well. They aren't stocking very much at the factories in the current economy---they build it when you order it.

Can I get away with it?

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Re: Transformer Question [Re: poorboy] #193737 04/17/10 02:05 AM
Joined: Jul 2004
Posts: 9,571
gfretwell Offline
It looks like the wrong part to me. You could hook it up and try it but I would be ready to just measure the voltage and turn it off.

Greg Fretwell
Re: Transformer Question [Re: gfretwell] #193739 04/17/10 03:30 AM
Joined: Jul 2002
Posts: 8,379
Trumpy Offline
Something doesn't seem right about using this transformer for this particular application.

Re: Transformer Question [Re: Trumpy] #193743 04/17/10 04:43 AM
Joined: Mar 2005
Posts: 1,803
Alan Belson Offline
I'd say it will go,even though it's wired 'backwards', on the basis that you have a situation somewhere between star and delta as regards the volts hitting the motor windings.
I'm running a planer permanently wired to star to reduce the amps with no probs long term.
Rated at 6kva, it should handle 5hp.
Run it and saw some stock! The operator will tell you if its any good!

Wood work but can't!
Re: Transformer Question [Re: Alan Belson] #193747 04/17/10 06:31 AM
Joined: Jun 2004
Posts: 1,273
Tesla Offline
This XFMR can do the job as 6 kVA should be more than enough for a 5 hp motor....

The taps provide you with an opportunity to adjust down to a reverse transform from 208 up to 460 to 480...

I'm not willing to punch the math...

But the basic equation -- per the diagram -- seems to be a delta- delta transform; or perhaps an ungrounded wye-wye transform...


Based upon their label notation...

Direct inspection would determine which is which.

Note the lack of ANY PHASE ANGLE SHIFT...

So I'd treat the reverse transform as if the primary voltage was HIGH and the desired voltage was 240 3-phase.

By which I mean that the reverse transform comes from only 208 3-phase and I want a high result.

Test unloaded, check the output voltage, you should be good to go.

Last edited by Tesla; 04/17/10 06:34 AM.

Re: Transformer Question [Re: Alan Belson] #193749 04/17/10 06:53 AM
Joined: Jun 2005
Posts: 124
poorboy Offline OP
My worry was based on the thought that the ratio of 208 to 240 was the same as the ratio between 416 and 480. 416 volts sounds too low.

The 208 volt systems here actually measure 215-216 volts, which would give me 430 volts on the high side. The nameplate actually calls for 460-480 volts. So 30 volts low sounds better.

The "backwards" thing didn't concern me, as I thought it would work fine in either direction. Was I wrong on that?

Re: Transformer Question [Re: poorboy] #193750 04/17/10 08:26 AM
Joined: Jun 2005
Posts: 124
poorboy Offline OP
Testa, your post was not visible to me yet when I posted last, so forgive me if I did not address any info in it.

The tap adjustments on the high side do not help me here, I believe, as the 100 percent taps give me the most I can get.

Since this is a 3 wire circuit and needs no neutral, do I care if it is wye or delta? This shop is 208/120 WYE, but I don't know about the motor.

If I seem a little short on knowledge it is because, like many electricians, I have worked at the trade for years (35) with little need to deal with transformers like this. Industrial and mill work have been a small percentage of my work which has been lots of commercial and residential stuff.

At times like these it is important to know what questions to ask, and other than this site there are really no resource people I can go to. The inside salesmen usually know only enough to order specifically and EXACTLY what YOU tell THEM. I threw a few questions at the guy at the salvage yard (tons of mill "take outs" and used mechanical and electrical equipment and the prices are RIGHT) and he looked at the logo on the side of my truck ("XXXX ELECTRIC, INC.") and said, "If you don't know, I don't know!".

The table saw in question is one of those which stops the blade instantly if you touch it (quite a finger saver) and thus has a circuit board in it which will have a power supply which uses 2 legs of the 460 to create its working voltage, so this variance in the voltage may actually be quite important.

I am always amazed the havoc customers can cause when they do not consult an electrician before ordering a piece of equipment.

Re: Transformer Question [Re: poorboy] #193754 04/17/10 10:48 AM
Joined: Dec 2002
Posts: 317
sabrown Offline
It also appears to me that the most you can get towards 480v is from the 100% taps. On the other hand equipment tend to have an operating range of +10 to -30% of the stated voltage. Mind you that operating in these ranges may shorten the equipments life. You may be able to verify this range with the manufacturer.

As far as the emergency stop, the ones I am familiar with would not care about the voltage for this portion of the circuit. They are a one time deal firing a cartridge into the blade, they have to be factory rebuilt afterwords. Costly but far less so than even the loss of one finger. Again verifying with the designer would be smart.

When said and done, it is better just to do it right.

Re: Transformer Question [Re: sabrown] #193761 04/17/10 03:37 PM
Joined: Mar 2005
Posts: 1,803
Alan Belson Offline
Practicalities. The most arduous duty when using a tablesaw is ripping thick stock. When ripping, you adjust the feed, [which sets the power consumed], by the weight of push applied on the stick and by listening to the motor revs and the 'singing' of the blade accordingly. My Lurem was fitted with a 4.5hp x 3ph motor when I bought it from a bankrupted carpenter's shop in 2002. The maker is only 40 miles north of here, & I was able to buy a new special custom framed single-phase 230v x 2.5hp motor that allows 45deg. blade tilt inside the machine frame. This cost me almost as much as the saw! This was before I got a 3 phase supply to my shop BTW. With a regular 305mm [12"] carbide tipped rip blade, 4" oak stock can be ripped without hassle, no doubt slower than if the 4.5hp motor was fitted, but not a problem, not very often and not exactly slow either. In a pro shop, I'd expect the CORRECT blades for the work in hand to be kept sharp and regularly inspected for damage - because it's blunt sawblades with chipped or missing teeth that soak up the watts. No operator worth his corn will push a machine to its limits anyway, and for most of it's work a saw will be running light, cutting thinner stock, crosscutting, tenoning, dadoing, etc. using the right change-out blades. As to volts, the motor will run at the correct rpm, even at 208v - [the effective 'star' volts used to start big motors]. Only if and when a saw is pushed to its limits or hits a bit of wild grain/stress area will the motor slip exceed the rated phase shift. The result is usually that it comes to a sudden stop, due to the lack of inertia in the machine's rotating parts, swiftly followed by a wild lunge for the red button! Personally, I doubt the user would even miss the slight power loss from a 30volt drop for most of the jobs ever put through the saw. The transfomer needs cooling air round it, of course. Just make sure you put down in writing what's proposed and why in your quote. These favors you do for folks to get them out of the tarpits they dug themselves can blow up in your face, if no records exist, when the biological fertiliser hits the rotating ventilation device and the blame dodge-fest commences!

Wood work but can't!
Re: Transformer Question [Re: Alan Belson] #193767 04/18/10 12:31 AM
Joined: Sep 2002
Posts: 908
NORCAL Offline
Since the only tablesaw w/ that feature is a "Sawstop", & they are built in Taiwan,+ IMO electrical gear of Asian origin is not really desirable I would suggest contacting them to see if it is feasible, Off topic,wet wood can trigger the cartridge which destroys the blade & a replacement cartridge is about $75.00.

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