transformer sizing

Is there a general rule of thumb for how big a transformer will handle a certain size motor.

I need to step up 220 volt to 440 volt for a 30 horse motor. Haven't purchased the motor yet so I don't have any amp specs.

There's a 25 KVA unit on Ebay right now, (114 amps at 220, 57 amps at 440). Big enough?


Reply to
Karl Townsend
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According to the calculator, 25 kva isnt quite big enough--then again, do you ever expect to draw the full shaft horsepower from said motor ???

Reply to

If there isn't some other compelling reason to go to 440V you'll probably do better to use a motor (and starter, etc.) for your line voltage -- in principal the cost of the motor won't vary much with the required line voltage, so the transformer is just wasted money. If you can't seem to find a 220V motor in that power range it may be worthwhile digging a bit for it, or inquiring with your power company about a 440V feed.

Reply to
Tim Wescott

THe first thing you got to determine is that your voltage is 208 or

240 vac... delta or Y (also called star). If you use a Y 208 volt transformer on a delta 240 volt input the core will saturate and will draw excessive current since it presents a highly reactive load to the electric company.(they dont like that) It will also hum like crazy. The best thing is to get a 240 volt motor. Usually they come with a way to change the motor input voltage to either 480 or 240. IF you have nine wires coming out of the peckerhead then it will go both ways. A transformer will only cost you extra losses and money.


Reply to

Thanks for the calculator. I'm sure it will only see full load for short periods, I'll bid on the 25KVA unit.


Reply to
Karl Townsend

I'd be carefull of using a 25KVA. transformer to feed a 30HP. motor even if you don't expect more than 1/2 load. No-one has addressed the question of inrush current and tansformer impedance, which might be troublesome especially if the motor starts under load. Without the motor rating letter and the transformer 'Z` I can't calculate it, but the impedance of the transformer might lead to an extended starting period and overheating of both units. MadDog

Reply to

This problem is far too complicated and dangerous for off the cuff answers in a world wide news group. First it needs a real electrical engineeer on the spot.

But to even offer any choices , Where in the world are you? What style is the motor? 3phase Y 3phase Delta. What supply do you have? Real 3phase, US scrambled who knows what phase, etc. Etc, Etc.

Reply to
John G

Karl knows Don Foreman. There are *no* better technical engineering referances than Don.

Reply to
Jerry Martes

Yes, all true. But I hate to wear him out with all my questions


Reply to
Karl Townsend


Here are some back of envelope calculations for determining the KVA rating you need...

In general, the formula for THREE PHASE KVA is:

(volts time amperes x 1.73)/1000

You can calculate HP from 1 hp = ~746 watts so 30 HP = 22380 watts or


Power = Volts x Amperes so substituting:

22.4 KW =440 x Amps

Amps = 50.86 or 51 Amps is what your motor can load-- not countin starting current inrushes.

Now substituting 51 amps at full load into the KVA formula ,you get:

(440V times 51Amps times 1.73 power correction factor )1000 = 38.8KVA.

25 KVA from ebay is not enough.....

And the usual disclaimers appy... I am an EE but not a power generaton expert, (EE degree 25 years ago and software is my game) and mixing Delta and Y three phase systems is an absolute no no....

Good luck.

Steve Koschmann

----- Original Message ----- From: "Karl Townsend" Newsgroups: rec.crafts.metalworking Sent: Sunday, February 20, 2005 12:34 PM Subject: transformer sizing

Reply to
Steve Koschmann

What the heck is this? The 1.73 is not a power (correction) factor, it is the square root of 3, and allows you to figure line current. If a certain KVA would work out in a single phase system to X amps, you DIVIDE by 1.73 to figure line current in a 3-phase system. Your V * A * 1.73 = KVA is a total mystery.

What you want is : Single phase line current would be 22400 VA / 440 V = 51 A.

51 / 1.73 = 29.4 line amps on the 440 V 3-phase system.

Now, of course, motors have losses, so pure power draw of the motor will be higher than the HP out of the motor. Also, induction motors have a power factor that varies with load. At no load, the motor current is near the rated load current, but the phase angle is very close to 90 degrees, lagging. At full load, the phase angle is close to 0, just slightly lagging. So, the VA demand from the transformer is close to the expected full load at all times, even if almost none of that is real power. So, although you can run the motor at light load and not have a high real power demand, you can't get by on a 10 KVA transformer if you will only load the motor to a 9 KW load! (Talking about the

30 Hp motor, here.) Actually, you CAN, but only if you put a power factor correcting capacitor bank right at the motor, and drop out capacitors as the motor is loaded from min to max.

Yeah, we can tell.

(EE degree 25 years ago and software is my game) and mixing Delta

it is done all the time, within certain rules. A balanced load, such as a motor, can be put on any delta or Wye system. An unbalanced load must be run line-to-line on a delta system, as there is no neutral. Some delta transformers are not designed for unbalanced loads, and must only have motors or other balanced loads applied. But, a delta motor can be run on a Wye power source, and it is done all the time. And, a Wye motor can be run on a Delta power source, but you don't connect the neutral tap, if the motor has one. (One trick to start large motors is the "delta-Wye starting system" where the motor is started in the Wye connection, and when near synchronous speed contactors switch the connection to Delta, which is the normal rating of the motor for that voltage.)

It is true you don't connect computer gear or CNC machines to a delta source without careful investigation to make sure the load is balanced and there is no neutral connection hiding inside.

Jon without

Reply to
Jon Elson

Yes 1 KVA per. HP Don

Reply to
Don Murray

Have you talked to your favorite electrical contractor? I had to have the same conversion done to run a 440 volt convection oven for soldering circuit boards. We only have 208 three phase in the plant. They had a transformer almost large enough, all paid for, but excess from a job several years ago. Could have had it for very few dollars.

They found a new transformer in Portland that was big enough, 75 kva, and I think it was only about $150, including freight. The total, including instalation was less than the price of a new transformer from Granger, etc.

So, be careful buying such a heavy item on Ebay. Check locally, first!

Paul, KD7HB Redmond, Oregon

Reply to

True enough for running loads, but you need about 5X for unreduced starting torque(also normal accleration time) This means about 150KVA as a worst case. The usual choice is less than this because full starting torque is usually not required. However, the actual value is dependant on motor locked rotor current and the load's speed-torque characteristics. Without those parameters, you are guessing.


Reply to
R. O'Brian


I really wasn't guessing. Karl asked for a rule of thumb and that's it. :)

You did make me think though, I'm sure Karl is talking about a dry pack, and most of my experience is with oil filled transformers. So I got out my GE Distribution Transformer Manual, and for a 30Hp motor, they recommend 3-10KVA tranformers. "This is only an approximation to be used when specific data on motor characteristics and loading is unobtainable." Since Karl hadn't purchased his motor yet, the information was "unobtainable".

Another bit of information from the Transformer Manual, short time overloads for oil immersed, self cooled transformers:

Time ----- Times Rated Current:

2 seconds-----25.0 10 sec--------11.3 30 sec--------6.7 60 sec---------4.75 5 min----------3.0 30 min---------2.0

I don't have any idea what these numbers would be for dry pack transformers. But they must be available somewhere.

One last note, sometime ago there was a thread that discussed transformer oil and PCBs. I was busy and unable to respond at the time. Around here the program to remove PCBs in the '70's dealt mostly with capacitors and some transformers that were in high risk areas around waterways or food-processing plants. The way we deal with PCBs today; when there is a spill of a gallon or more and the transformer is not labelled Non-PCB, we take a sample to a lab, which we have on call, with a 2-hour turn around time. The type of cleanup used depends on the results, and less than 50PPM is considered not contaminated.


Reply to
Don Murray

I know a little about a few subjects, thank you, and so do lots of other guys.

It's certainly true that there is nothing like real experience, of which I have none with 30 HP motors or 25 KVA xfmrs.

I agree with Precision Machinist. The xfmr cited is a bit lightweight "by the numbers", but would probably work OK unless the machine really needs 30 HP for extended periods -- like more than 20 or 30 minutes at a stretch. It might hack it even then with a big fan cooling it -- and it might not even need that since Karl's "shop season" is during cooler months in MN.

Reply to
Don Foreman

5X nominal on start current is on the optimistic side. I've observed 200 amp transients on a 2 HP 115-volt singlephase motor with nothing but buffing wheels as a load. With decent wiring, the transient was short enough that it didn't trip a 20 amp QD breaker.

Starting is a brief transient, thermal time constants of 30 HP motors and 25KVA xfmrs are in the tens of minutes at least. Breakers would (or should) protect against sustained locked-rotor condx while tolerating start transients. Transformers can shrug off other overloads of duration short enough that they don't overheat.

Reply to
Don Foreman

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