What size motor can I start with a 45 kVA transformer

does this help?

Not in the normal sense, no.

One horsepower is around 750 watts (there's more than one definition of horsepower, and I don't keep them all memorized).

The MOST electrical power that you can get from a 25kVA transformer is

25kW -- and that's if your load is perfectly resistive, which a motor isn't.

So 55HP is a bit over 41kW, and definitely more than 41kVA.

Somewhere out there in web-land there must be at least one set of guidelines for the size of transformer to get to match a certain size motor -- why not do a search?

Tim Wescott fired this volley in news: snipped-for-privacy@giganews.com:

Just might be a few power conversion calculators, too. This is another one of those people unwilling to do any research on his own -- wants us to answer (and ostensibly be responsible for the results).

Like that mook who comes on here periodically to ask questions like, "Should I move my car jack handle up and down to lift my car, left-to- right, or twist it? And if I have to twist it, which direction?"

Lloyd

That would be Spawtted right? Should we

a)Answer him. I. Give honest helpful answer II. Give dishonest and harmful answer.

b) Ignore him. I. Silently II. Announce he is being ignored.

c) Be sarcastic too him. I. Derisively. II. Humorously. III. Obtusely.

"Bob La Londe" fired this volley in news:lp47qi$9a4$1 @dont-email.me:

Just tell him where to get the information, and let him look it up himself... which is simple:

Go find a motor power to KVA calculator, and do some thinking on your own.

Lloyd

Obviously, you can't run the motor at full rated load from the undersized transformer.

But, induction motors draw a very lagging power factor at light load. Thus, their current doesn't change very much from idle to full load, but the phase angle of the current changes dramatically. At idle, the current is at least 75% of full rated load current, but at almost exactly 90 degrees lagging. At full load, the current is almost completely in phase with the voltage. That is what causes the increase in POWER demand with increasing load.

So, under light load, the POWER demand of the motor will be fairly small, but the current will exceed the current rating of the transformer. Power transformers have some margin so they can deliver rated POWER with a less than unity power factor, but they are usually capable of only handling about 15% above rated current to account for the power factor. Therefore, if there is a large power factor correction capacitor bank placed across the transformer secondary, it might make it possible to test the motor at minimal load with the 25 KVA transformer. But, you won't be able to get more than 50% of rated power from the motor, even WITH the phase correction capacitors.

Jon

Interesting. It takes 166.187kvA to directly run a 55hp motor. The 26kva xfmr couldn't quite run a 7hp motor.

Good page, bookmarked!

That sounds like your friend, Stryped's, question.

Yoda said "This guy's GoogleFu is weak as hell, mon." (translated)

Well, in THAT case...a 25kva xformer will run THREE 55hp motors! Especially in critical applications under heavy load.

Don't forget "Nonsensical"!

Tom Gardner fired this volley in news:cN6dnS2ofYlAeyjOnZ2dnUVZ snipped-for-privacy@giganews.com:

What itches in my mind about this guy is this: With his broken English and desire to run a 55HP electric motor (which has somewhat limited uses in residential applications), I fear he's trying to figure how to start up some sort of factory, and he'll burn it down with all the 'thinking' he's doing on his own.

Lloyd

According to the bookmarked site's calc, it takes 166.187kva to run that motor properly. Where did you get the 45kva number?

That IS a mighty big motor! Any application of that much power has responsibilities of understanding.

He should pay reparations!

Larry Jaques fired this volley in news: snipped-for-privacy@4ax.com:

Seriously, I see this guy setting up a line-shaft factory on a motor that size. Then

8-year-old little girls get tangled up in the belts and pulleys, and the rest of them on the 2nd and 3rd story of the building die when it burns down because he's pulling that load over #10 wires.

Lloyd

The calculator gives 166.187 KVA for a GENSET that will start the motor. A transformer has the capacity of the grid behind it to provide the starting surge, a genny needs to provide the peak current all by itself.

-jsw

Oh, you're right. I focused too much on the translation figures.

I wonder if an AC drive would be of any benefit? I'm guessing the used market for 55HP drives may not have as much demand as smaller home shop size drives.

My 10HP drive on my 7.5HP lathe runs around 13A at idle on the motor side but only draws 5A from the outlet. If my understanding is correct, the 13A is mostly apparent power while the 5A is more representative of the true power. Perhaps a drive would do better at getting close to 25KW from the

25KVA transformer and handle the VAR on the motor side.

RogerN