ECN Forum Forums General Topics of the Trade General Discussion Area Transformer Question

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.