R. O'Brian wrote:

"Don Murray" wrote in message
m...


Karl Townsend wrote:

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

Yes 1 KVA per. HP
Don

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.

Randy

Randy,

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.

Don