Well, 15 Hp is about 11 KW continuous, maybe 15 KVA including the power
factor. But, such a transformer should be able to handle a larger motor.
You should be able to start a 50 Hp motor, although the line voltage might
dip just a bit. Typical transformers are designed to permit starting
motors near the full running KVA of the motor. Not sure how your running
the transformer in reverse and altering the primary turns have affected
Your building metering probably has a meter that records "demand" and
will give you a BIG hit on your electrical bill every time you start a
big motor. Be sure to calculate that into the price of your big motor.
Well, if your meter says "demand" anywhere on it, then it is.
For instance, our university has demand metering, a cassette tape
(probably replaced with something more modern, now) records power
demand every 15 seconds, all year long. Our power bill is just
the worst peak of the entire year, multiplied out to the full
year! Suddenly, you can understand why they have this huge energy
monitoring and motor control system, to make sure that peak is
flattened way out.
But, that is for a large business, with 2 transformers as big as
If you are lucky, you don't have a demand meter, and are billed by
just plain consumption. If your meter has an LCD display, you may be
able to see all the parameters it is recording and radioing to the
power company. They usually cycle through several parameters every
If you don't know about this smart meter stuff, the meters have
a spread-spectrum radio transmitter in them. On poles scattered
around, there are boxes with Yagi antennas pointing at a tall building
somewhere and a whip antenna pointing straight down. The whip picks
up the transmissions from all the meters in a couple block area and
then sends them through the Yagi to a collection point (a couple
of these in each town) that then packs this all up and sends it to the power
company's data center. They use it both for network monitoring
(if a bunch of meters in an area stop reporting power, they know
they have an outage) and for billing. In some areas they can interface
to air conditioners and such to shut them off during peak consumption
and give you a discount.
All depends on the rate schedule your building is on. Could be the
instantaneous peak KW reading over a 15 minute period. that is what mine
is. OR could be the peak averaged for one second ever so many minutes.
To be sure, get your bill and call the customer service rep and ask them
about it based on your rate schedule.
Every state is different.
kva stands for kilo 00 volts * amps
now any body would say that volts times amps is watts which is true in
a dc circuit but in ac there is factors such as the the windings in the
motor which when layer against the next winding induces an opposite
current flow which opposed the current flow in the first winding that is
why we use VA volts amps or KVA 1000volts amps
now the motor load allowed would would be maxed by the main circuit
breaker after the transformer a 45 kva would be proving about a 400 amp
service in my industry. now if you figure that 50hp is 37300watts 1hp is
746w 50 times 746 7300w divide that by the voltage rating usually
240 = 155amps. this is all single phase power if you are talking 3phase
power that is another monster and alot of other calculations because
there is many different voltages 120/208 120/240 277/480 347/600 and so
clear as mud
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.
Screw and reciprocating, yes. I used this transformer to test a Quincy
reciprocating compressor with two 10HP heads, one head at a time. I
do not think that the transformer is bigger than I need, I would
really like it to be 110 kVA.
On Sat, 04 Feb 2012 05:21:12 -0600, Ignoramus27096
Hammond, Acme, and Hevi-Duty all say a 45kVA transformer is the proper
size for a 30HP motor with frequent starting (more than once per
hour). 40HP is OK if the motor is started less than once per hour.
Remember that the bigger the transformer, the greater the fault current.
And, one of these days, you WILL run into some piece of gear that
does have an electrical defect. I'm pretty bold fooling with electrical
gear, but 480 with serious current behind it does scare me!
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