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
Without the motor rating letter and the transformer 'Z` I can't
but the impedance of the transformer might lead to an extended starting
period and overheating of both units.
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
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.
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,
I know a little about a few subjects, thank you, and so do lots of
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.
Here are some back of envelope calculations for determining the KVA rating
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 D0 x Amps
Amps = 50.86 or 51 Amps is what your motor can load-- not countin starting
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....
----- Original Message -----
Sent: Sunday, February 20, 2005 12:34 PM
Subject: transformer sizing
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
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.
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
So, be careful buying such a heavy item on Ebay. Check locally, first!
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.
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:
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.
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.
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