| I have a transformer and it has no datasheet, but i know it is a DY
| and 380/220 3 phases. How can i determine the KVA for this thing? Can
| i measure the iron core and get a referential value?
There is no nameplate, either?
Mechanical measurements could give you a good start. Is it an enclosed
dry-type? What are the dimension of the enclosure. What is the weight?
What is the gauge size of the wires from terminals to primary? Secondary?
What are the dimensions of the core itself?

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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |

Phil Howard asked some good questions. KVA can be approximated by
knowing the size of wire used for either the primary or secondary
windings. We approximate by calcuating the circular mils per amp
(CM/I). When we design transformer coils we try to make the CM/I come
out between 600 and 800 for copper conductor, 1000 to 1200 for aluminum
conductor. Some well-known manufacturers use values in the low 400's
for copper which is designed to last just beyond the warranty period.
If you can determine the size and number of conductors coming out of
the coils, here is the formula to calculate approximate KVA:
Conductor: Width x Height = Square mils
NOTE: If the conductor is round you will have to determine the size by
measuring the diameter and comparing it to a chart which shows the
circular mils, i.e., 10 round copper is 10380 circular mils. If you
have circular mils you can skip the next calculation.
Square Mils x 1.273 = Circular mils
Then let's take a good average value for CM/I, like 1100, assuming the
conductor is aluminum.
Also, let's say there are two square conductors at 204 x 204 mils each.
204 x 204 = 41616 square mils per conductor
2 conductors x 41616 square mils = 83232 square mils
83232 square mils x 1.273 = 105594 circular mils
Now you can calculate the winding amperage of the coil.
105594 circular mils divided by 1100 circular mils per amp = 96.3 amps
(this is winding amps)
Since your transformer has a secondary voltage of 220Y/127 and we have
calculated the winding amps for one coil, you have to calculate the KVA
of that one coil only and multiply it by three for the full KVA of the
transformer.
KVA = Kilovolts x Amps
KVA = 0.127 kilovolts x 96.3 amps = 12.23 KVA (this is one coil only)
12.23 KVA x 3 coils = 36.69 KVA (our estimate)
Since 36.69 KVA is not a standard rating, and because many
manufacturers try to use as little material as possible, I would
estimate this transformer to have a KVA rating of 45 KVA which would
have a CM/I rating of 894.86.
If my presentation of these calculations isn't clear, you can send me
your conductor sizes and the number of conductors in one of the leads
coming from the coil, I can estimate the KVA for you. Also, be aware
that some manufacturers use multiple conductors and they may not be the
same size. Measure each one individually. If it is a small
transformer, there may be only one conductor.
Steve Lockridge
Manager, Alfa Transformer

Chris -
Based on the information you gave me on the conductor size and
material, here is how to estimate the KVA of the transformer. Remember,
this is an estimate only.
Secondary Wire Size: 6mm x 10mm (I have to convert these to inches)
6mm = 0.236220472 inches
10 mm = 0.393700787 inches
Since rectangular wire is sized in mils (a mil equals 1/1000th of an
inch), we can use 236 mils x 394 mils.
236 mils x 394 mils = 92,984 square mils
Convert to circular mils:
92,984 square mils x 1.273 = 118,369 circular mils
Since the conductor is copper, we would like to have a circular mil/amp
(CM/I) rating that is a minimum of 600. There is no maximum but you
would be underrating the transformer if you went too high with this
number. Let's use 700 CM/I as our target value.
118,369 circular mils / 700 = 169 amps (this is winding amps, not line
amps)
Your secondary voltage appears to be 220Y/127. That means the voltage
of each coil is 127V.
Now we can calculate the KVA of each coil. Since we are calculating
KVA, we need to use the KV in our voltage.
169 amps x 0.127 KV = 21.46 KVA per coil
Since you have three equal coils:
21.46 KVA x 3 = 64.38 KVA estimate
Here is my conclusion but again remember, this is an estimate. If you
use a CM/I rating of 600, this comes out to 75 KVA which is a standard
rating. Since many manufacturers design these transformers to use as
little material as possible, they run the CM/I as low as they can.
Some well-known manufacturers have been known to run units as low as
400 CM/I which is dangerously low. We call those units "disposable
transformers". Their life-expectancy is very low and once it is
designed and built, you don't have room to enlarge the coils when
rewinding it to increase the CM/I and the life of the unit.
So my guess is that this unit was designed to be a 75 KVA transformer.
This is based on the limited information available to me and I cannot
guarantee that this is accurate.
Steve Lockridge
Manager
Alfa Transformer
http://www.alfatransformer.com

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