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?

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.

Something doesn't seem right about using this transformer for this particular application.

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!

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...

Again...

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.

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?

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.

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.

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!

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.


http://www.sawstop.com/

I was hoping to see at least (2) 5% FCAN Taps listed on the Nameplate, but there are only the (2) 5% FCBN Taps.

*** BTW, Did anyone else notice this Transformer is an Open Delta "TEE" arrangement? ***

As suggested by others, hook this Transformer's 240V Windings to a 208V 3 Phase - 3 Wire Circuit, then measure the Voltage between Terminals "H1-H2", "H2-H3" and "H1-H3", using a low Impedance Voltmeter.
Do not worry about L-G Voltage Tests, as they will be inaccurate and irrelevant.

It may be best to "Load-Down" the Voltmeter by placing a "Wiggy" (Solenoid-Type Voltage Tester) in Parallel with the Voltmeter, in order to stabilize Voltage readings.

You might be able to use this Transformer without modifications to the Input Circuit. Verify what you have "At-Idle" to see where to go next - either use it "as-Is", alter the Input Circuit (described next), or to ditch the thing and get a Transformer with 208V x 480V 3 Phase.

If the output Voltage at idle is anything less than 450 Volts, it is highly possible for the output Voltage to drop below 432 Volts (480V - 10%) when the Connected Loads are running.
In that case, a possible fix would involve connecting the 208V Input Circuitry to the Transformer, through an Open Delta setup of Two 500VA "Buck-Boost Transformers", with 32 Volt Secondary Windings.

Although this is a "Redundant" setup of Transformers, it would apply 240V to the Transformer.

*** NOTE: Seeing that the Motor's Nameplate designates the Supply as "460-480V", this would allow a Full-Load Voltage as low as 414V - since that is 90% of 460V (460V - 10%).
In this situation, the Motor may be run from the Transformer "As-Is", without any Input Voltage alterations.

Lastly, unless your Client plans to frequently push this Saw to Maximum Load, and sustain it for more than 5 Minutes, the Transformer's output Voltage + KVA capacity should not cause the Motor to slip too low in Speed.

The Wattage drawn by the Motor's Rotor is directly related to the Mechanical Load placed on the Rotor.
Simply this means the more cutting required, the more True Power (Wattage) required.
The "Amount" of True Power which the Motor's Rotor may "Draw", depends on the "Apparent Power" (Volt-Amps, or KVA) the Motor can draw in.
This is where the Volts -vs- Amps issue arises.

As the FLA (Full-Load Amperage) of a typical 3 Phase 5 HP 460V (480V System) Squirrel Cage Induction Motor is 7.6 Amperes, this equates to 6316 Volt-Amps, and is around 106% the rated capacity of the Transformer.

That amount of Apparent Power would only be drawn when the Motor is developing 5.0 Horsepower.

Drawing 106% of the Transformer's rating for periods of time not exceeding 3+ hours, will not (should not) cause issues - so keep that in mind.

Good luck and keep us posted to the outcome!!!


Scott

I completed the pipe and wire portion of this job late today. Before we mounted the x'former we hooked it up temporarily to test it and, just like the math said it would, it put out 430 volts on H1-H2-H3.

H7-H8-H9 gave us in the 390 volt range, also just as the math said it would.

A nice quiet hum accompanied it, and had I been able to get the salvage boys to have dug me out a real 208 to 480 volt transformer I would have been happy. I was unable to feel good about the low voltage to run the guy's new pride and joy, however, and will make him wait til I return from an out of town job next week before he can play with his new toy.

The salvage place I got this one at is near this out of town job and I will go there with some time to spend and make sure they look around for a better fit this time.

When all is said and done it is better to be able to relax mentally after completing a job than to hope and pray something will hold out.

Its always satisfying to get hands on experience with something and widen your horizons a little.

I will post the final chapter in this saga next weekend, if all goes well. Thanks for the replies. This site is just so great with its cross section of knowledge and wide array of experience to draw from. No such think tank exists for me around town where helpful brainstorming takes place so quickly.

Thanks, Ken

Scott35, your post was not visible to me til after I finished and posted this reply. Just as you say, the line to ground voltages were weird and I ignored them. The thought of adding transformers to step up the incoming to 240 also crossed my mind and was nixed in favor of just getting the right one for the job.

The comments about it being an "open Delta tee " and Tesla's earlier mention of the lack of any "phase angle shift" will have me running the "Google" drill for a few sessions now.

I went to the salvage place and we totally struck out on any better transformer than this. I was ready to tell my friend he would have to bite the bullet and order a new 208-480 one at $725 plus shipping---prob 150 bucks or more for a 150 lb. item like this. At least I did find one on the shelf and only 2 days out (instead of 2-3 weeks).

However,after talking with a guy at the local 3-phase motor sales/service shop a couple days ago and him telling me that most motors manufactured nowadays can take a 10 percent variance in voltage, I decided to try this transformer out, check some Amp readings and see how the saw performed thru some cutting.

He also said the nature of a table saw being intermittent duty (as opposed to, say, a gang rip saw)would probably mean they would never heat it up.

It idled at 1.4 Amps, drew 2-2.5 Amps on some easy cuts, and then with an old dull rip blade ripping a piece of hardwood topped out at 4-4.3 Amps. Nameplate says FLA is 5.8.

Looks good for now, we'll see how it does this week in its regular all day duty. I really am expecting no problems, though.

Appreciate all the input.

He dodged a $1500 bullet (minimum, if contractor rates and markups were applied) and paid $300 for my materials and my labor was donated. Hope it helps him thru these hard times!

I did note that a couple of you actually told me of the 10 percent deal as well.

I fully agree that as long as the actual current is lower than that given on the name plate it is not worth worrying too much.

But I'd like to ask some additional questions concerning the North American safety approach. If I set up an arrangement like that I would have to decide whether the secondary windings are to be grounded or not. If ungrounded this would from an "IT-system" and I would have to install a device performing a permanent check of insulation mandatorily.

Otherwise If grounded (here corner grounded?) then I would have to measure the insulation of course, but furthermore the maximum overcurrent to ground or its equivalent, the impedance of the so called "ground fault loop". With this value I would have to prove that the saw is switched off within 0.2 seconds. Otherwise an rcd (European sort of GFI) is mandatory. The above mentioned measurements are (at least theoretically) absolutely mandatory.

How do you proceed in the US according to NEC or other regulations.

Very interesting stuff, Wolfgang. I'm leaving the house for a few hours, but will be composing a list of a few simple questions this raises for me.

Periodically we are called on, in this trade, to perform tasks we really have no business performing. Knowing, however, that if the customer (or friend) can't get it from us they will go to a possibly even less qualified person for help.

I am not in the business of sacrificing safety to do things cheaply but may, thru ignorance, do just that. I am willing to pursue this further, or at least gain some knowledge of some of your points.

Those of us who work from engineered prints for years do not develop our engineering skills too well. Neither can we afford to hire an engineer to help with these things---thus the beauty of sites like this.

Thanks for the continuing feedback!

Sounds good! When they get a sharpened or new blade fitted, try to get some amps figures then. My saw cuts like a dream with a new blade, and regular resharpenings actually save money in the long run by hugely prolonging blade life, giving increased safety and reduced poco bills.

It is hard to "Google" one's way to an engineering degree, Wolfgang. LOL My search of terms has not led to my understanding of your points.

Is your point that a fault to ground in one of my 3 phase conductors would not clear via the fuses in the disconnect on the output (480 volt) side of the transformer?

@ poorboy:
My curious questions from outside (Europe) were here,
whether and how you grounded it, as the plate shows separate windings (if I interprete it correctly)
and
how you prove that the fuses or whatever is used to prevent danger by a live to ground fault will work fast enough.

Isn't a bit early for Maine, 7:40 UTC?

Wolfgang, I am usually up at 3 or 4 AM EDT and on the computer for a bit. Yes, it's early, but I am afflicted with old man's disease (early to bed---early to rise). Just had my 60th birthday!

The only grounding I did was to connect my 208 volt system ground to the ground lug on the transformer case, and also to put the ground going to my saw under the same lug.

Since I was not creating a neutral, I did not re-ground it.

How does one go about grounding the high side windings?

If I sound dumb, it is only because I am...LOL...about transformers, that is. There is a lot to know and a my work has never involved them very much other than lots of 45-75 KVA dry types in large schools. Those are 277/480 to 120/208 and are pretty straightforward.

I just found this thread and will be checking it out for a while: MIKE HOLT

I would be curious to know the nameplate data on the table saw, to rate the circuit protection correctly. Any info. on amps?

*** THIS RESPONSE APPLIES TO BOTH "POORBOY" AND "WOLFGANG" ***

In regards to Grounded -vs- Ungrounded Separately Derived System ("SDS") referring to the Secondary, or Output of the Transformer...

Personally I would Ground the Secondary side (the Output, or the SDS), as leaving it Ungrounded places too much stress on the Conductors' and Motor Windings' Insulation, due to unstabilized Voltage-To-Ground per Capacitive Coupling effect.

Simply stated, even though the SDS is not Physically Ground Bonded, there still is a path from each Line Conductor to the Metallic Equipment + Earth Ground, via Capacitive couplings.
Think of this as thousands of small Capacitors connected between each Line (Phase) Conductor, plus between each Line and any Grounded Materials (Conduits, Enclosures, The Earth, etc.)

The issue here is that with high Impedance, the Voltage measured across the Capacitor is also high.
L-G Voltages of 1500 Volts are normal on an Ungrounded 480V Delta System.

These unstable Voltages tend to break down the integrity of Winding and Conductor Insulation, resulting in Arc leakage and eventually a Ground Fault issue.

Additionally, Filters, VFDs and other Electronic Circuitry are prone to premature failure due to the unstable L-G Voltage.

Lastly, NEC 250.21 requires Ground Fault Detection, which is something I would rather stay away from unless absolutely necessary!
(Read: Client will never look at the Indicator Lamps, or respond if an issue arises; also becomes something else to mess with...).

---- Grounded SDS ----

This refers to "Bonding" the Separately Derived System ("SDS"), to the local Grounding Electrode System ("GES").

Grounding the SDS (Output) may be done in several ways.

• For a Wye Connected SDS, Bond the Star / Common Point to the GES,
• For a Delta Connected SDS with a Center Tap, bond the Center Tap to the GES,
• For a Delta Connected SDS without a Center Tap, bond the output designated as "Phase B" to the GES.

Per the OP's Delta "TEE" Connected SDS, there are two possible ways for bonding the SDS to the GES:

1. "Corner Ground",
   or
2. "Center Tap" Ground

Option #1 is the simplest, and creates a System Grounded Conductor.
Connect Line / Phase "B" to the GES at the Transformer.
Bond Line "B" to the Transformer's enclosure, and to any Equipment Grounding Conductors (EGCs) used on the SDS / Output side.
Color code for Line "B" is White (or Gray) - as it is the System's Grounded Conductor.
No Fuses in Line "B", however 3 Pole Circuit Breakers with common trip & handles may be used, since all three lines are opened simultaneously.

The "Cons" of the Corner-Grounded System include:

• Full System Voltage to Ground,
• Need devices rated for Full System Voltage (480V),
• Tagging the Grounded Conductor at all accessible points,
• Solid Links in 3 Pole Disconnects on Line B (replaces Fuse).

Option #2:

If it is possible to tap into the Jumper between the Center Tap of the "Main Winding", and the Winding End-Lead on the "Kicker Winding", this would result in a Grounded SDS which does not include a Grounded System Conductor.

Voltage to Ground would be reduced to 240V, and Slash Rated devices may be used, instead of Fully Rated Devices.
Slash Rated Devices = 277/480V
Fully Rated Devices = 480V

Bonding would be as normal:
* Bond Center Tap to the GES,
* Connect System Bonding Jumper (SBJ) from Center Tap to Transformer Enclosure,
* Terminate EGCs to Center Tap via terminations to SBJ.

No need to identify one Conductor with White / Gray.
No need to place solid links in Disconnect Switches - all 3 Poles may be Fused.

"Con":
* May not be able to connect to Center Tap without modifications.

Good luck everyone.

Scott

Scott, thank you for the very detailed info. I need to ask a couple very specific questions as soon as I get a little time, so keep your eye on this thread.

The saw is running fine but the possibilities you raise about L-G voltages make me want to address the situation further if possible. At least I want to come to a good understanding of what I have done vs. what I should have done.

There are areas of this very diversified trade we work in that are not encountered even a very long career such as I have had. One can be an expert in several facets of the trade and still run into jobs which drop them back to novice (or less) status in an instant. Gotta know how (and where) to ask questions to get through many tasks.

Once again, Scott, you are a saint for taking the time to lay this info out for me...Ken