Run 415 V machines on 480 V?

I see no reason why it cannot be used as a step up transformer.

I think that it would be much more efficient to just buy a nice used

220V motor, like 7.5 HP or some such, than mess with very high voltages, heavy transformers, etc etc etc, all to power a puny 2.5 HP motor, which in the end will end up costing a lot and won't provide much power. It's 40 pounds starting bid, who knows what it will sell for, etc.

Let's do the math:

A. Making RPC with your current motor and transformer - very heavy motor - not much power - pay money for transformer - very heavy setup - very high (easily lethal) voltages

B. Buying a used 5-7 HP motor - light motor - very small size and weight of setup - not so deadly voltages - not much more money, all you need is caps and a contactor.

Really, having a 2.5 HP idler will severely limit your options as of what to run. Like, yesterday I bought a 2 HP compressor, I could not run it from a 2.5 HP idler, not very well anyway. I am glad that I have a 10 HP idler. (I am aware that your electric supply does not permit 10 HP idlers).

I am sorry if this message appears to "rain on your parade", I admire your skills and determination and the motor you have is beautiful and historical. But, at the same time, I think that you get a "better bang for pound sterling", if you use your skills to play with machinery that you can run off the phase converter. RPC is just a tool to get other tools running.

i

It depends how you see the project. Either you just see the RPC as a tool, or you see it as a fun science project, into which you can incorporate a cool motor you got free. For me it's the latter. I don't need the RPC to drive particularly big loads, and I'd like to use it to do some electrical engineering experiments. I'm also rather fussy about the way my projects look, and a modern TEFC motor just doesn't match the looks of that old motor.

You also perhaps underestimate the need for a 415 V convertor in the UK. If I have a 240 V convertor, every piece of equipment I want to hook up will need to be rewired for 240 V. And, as I've discovered, it isn't possible with all motors. I have a good friend who's very serious about electrical projects (Tesla coil enthusiast, etc.) and he's building a convertor too. He wants a 415 V convertor so he can reproduce an industrial supply in his workshop.

I saw a couple of 7.5 kW (10 hp) motors on the demolition site when I collected the 2.5 hp motor last week. I could probably get one, but again they're delta connected for 415 V, so I would need a transformer.

Basically I'm in it for the science project, whereas you're in it for the tool. Just different priorities.

Chris

Chris

The single phase 10 KVA transformer looks like it can be wired to take 240 vac up to 480 vac. Transformers dont mind being reverse connected, as you probably know, even though the primary is conventially wound closest to the core. The eBay transformer would be a good choice for your need to use a

480 vac idler to make 480 vac 3 phase from a 240 vac power source.. I asume that you are more comfortable with 480 vac than I am. I hate 480.

Jerry

Thanks for the advice. Out of curiousity, why is the primary traditionally wound closest to the core? This transformer is tempting, but my concern is that the "official" voltage in the UK is 415 V, so I will be supplying a somewhat excessive voltage to the motors. What do you think?

I'm pretty careful. I only ever once had a shock, and that was off a capacitor which had held a charge for a while. After that I was very wary of capacitors.

Chris

What you are trying to do sounds like the Westinghouse 4kva convertor I have. It has a step-up transformer built in and various taps to allow

380V, 415V, 440V etc. Makes us> Still thinking about a phase convertor...

Chris

The only reason I know about for winding the primary close to the core is to give it maximum capability to couple to the magnet core. That would ninimize the losses even while no load is applied (idling time)

The ratio of IN to OUT voltage as determined by the jumpers on the eBay transformer is the imprtant factor for you. Can you put 208 VAC single phase into the eBay transformer?

There is a problem with 480 other than electric shock. 480 has the ability to vaporize metals and spray it around. I have seen the results of dropping things across 480 and I'm probably overly 'scared of it.

Jerry

I'd never heard that before, but it explains why a short in a 480 box tends to blow the cover off, while a 240 "event" isn't usually nearly as exciting. Personally, I'm respectful of 120, very cautious around 240, and just plain skittish around 480. I have one customer that has a bunch of 550 volt imported from Canada equipment that I did control retrofits on that makes me feel much as you - probably more nervous than I should be.

Ned Simmons

I would not try to run anything 480v. Especially with a kid at home.

i

Yeah... Check out ogrish.com for some industrial accident pictures.

i

Industrial fault currents can be a lot higher than what you otherwise run into. 10kA can do a lot of damage. Even 240V in an industrial situation can do impressive things if you short it, unlike in a home.

Best regards, Spehro Pefhany

[...]

Eww, last time I went there I saw a dude squeezed in between dual tires of a truck. His innards had become outards, and it looked like they went through the top of his skull to get there. Lovely stuff.

Just one word :

Buck / boost..

Google is your friend.

It's more than tradition. The winding closest to the core has the shortest mean turn length so the copper loss of an inner winding is less than that of an identically wound outer winding.

Best efficiency occurs if the winding handling the highest power (the primary) is the inner winding.

Because this results in the lowest total copper loss it also yields the best voltage regulation.

Power transformers are efficient devices and it's usually OK to use an outer winding as a primary and accept a pretty minor drop in efficiency.

However it reverses the design corrections for the voltage drop resulting from winding resistance. This will alter the apparent full load voltage ratio. For example:-

With 1% copper loss in an inner winding and 1 1/2% copper loss in an outer winding a 1:1 full load rated transformer would need 2 1/2% extra turns on the secondary to compensate for the voltage drop.

If this outer winding is now used as a primary the OPEN CIRCUIT output voltage is now only 97 1/2 %. The full load voltage drop will reduce this to 95% so that the nominally 1:1 transformer is now only

1:0.95

Perhaps a more useful way of assessing this is to allow for a voltage error of twice the rated no load to full load voltage drop

Jim

Its not the shock so much as the tendency to form a nice plasma cloud which can really f*ck up your whole day.

Anyone need transformers, I have em up to 45kva. Single and 3 phase.

And 3 phase motors up to 5 hp coming out my ass.

Gunner, California