On 2012-02-03, Jon Elson wrote:
Ignoramus10043 wrote:


Hammond (one of the "better" transfomer makers)
say that their 45kVA 3phase transormer is good for :

125 amps @ 208VAC (= approx. 50hp )
108 amps @ 240VAC (= approx. 40hp )
54.2 amps @ 480VAC (= approx. 20hp )
43.4 amps @ 600VAC (= approx. 15hp )

The more I think about the above, the less sense it makes.

54 amps at 480VAC is the same HP as 108 amps at 240 VAC, no?

i
These numbers must refer to one specific multi-tap transformer,
which has some current limits on the primary winding. When you
change the primary taps for different output voltage, the
rated current available at the secondary would change.

Of course, on YOUR transformer, which you are running backwards,
you'd just have to make sure you stay within the ratings of
each winding, and mostly forget the KVA rating.

So, in the above table, the 43.4 Amps at 600 V is at the secondary.
When you change the taps, the current has to go down. On the other
hand, the KVA should still end up being the same, so the HP also ought
to be the same. Also, 43.4 * 600 * 1.732 = 45 KVA, which should be good
for 50+ shaft output HP from a motor. So, maybe the HP calculation
above was added by somebody who doesn't know what they are doing.

So, I guess I agree with you, the HP part does NOT make sense.

Jon

Yep, no sense.

As for the 45 kVA reverse wired transformer, I am using it multiple
times every day now, it works very well, I am happy. I tested five 1
ton Dayton hoists yesterday.

i