What size motor can I start with a 45 kVA transformer

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 this.

Jon

I think that my custom tap reduced the kVA a little bit, that's all.

i

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.

Paul

How does that metering work?

i

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 my house.

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 minute.

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.

Jon

kva stands for kilo =1000 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 =37300w 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 on

clear as mud

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.

Paul

Provided an unloaded shaft, 45 kw from a standstill I would suspect.

Hey Iggy,

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 )

Take care.

Brian Lawson

Is that starting amps or running amps?

i

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

Well, I think it is bigger than you need, although I guess you want to be able to test screw compressors, which might be the only thing you would be dealing in that have such big motors.

Jon

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.

i

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.

On Thu, 02 Feb 2012 18:24:31 -0600, Ignoramus10043 wrote: SNIP

Noted as "FULL LOAD CURRENTS"

Brian Lawson

That's not bad. Thanks for this very useful info. Right now I have a Quincy 75 HP screw compressor. I would love to run it.

i

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!

Jon