Non-DOE Compliant Transformer Efficiency

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Superdemon

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Location
New York
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Electrical Engineer
Assuming minimal connected load required to reach optimal efficiency, is there a general rule of thumb for the transformer efficiency of non-DOE compliant dry type transformers such as non-ventilated ones smaller than 15 KVA?
I'd like something to use when doing voltage drop calculations involving transformers. It would also be great if there was some generally accepted documentation for this but I have not found anything so I assume there isn't any.
 
You probably need the impedance, not the efficiency. There's no ANSI standard for impedances for transformers this small.

If you Google "GET-3550F" you can download an old GE guide that has typical transformer impedances for smaller transformers in the back.
 
Let's assume I have a 240/480V single phase 10 KVA transformer supporting a 9 KVA load. The transformer is serviced via 1000' of #1 AWG copper conductor.

Can the 2.5% impedance stated in the GET-3550F be treated as the same as saying that the transformer has 97.5% efficiency?

Effectively increasing the load on the conductor to 9 KVA / 97.5 % = 9.23 KVA ?
 
%Z is not the same as Efficiency, especially when you are not at full load. You also need to consider the X/R ratio of the transformer.
You need to know the Lw (losses due to windings) and the Lfe ( losses due to the iron core). The Lw are usually due to the resistive component of the impedance.
 
Unfortunately, I don't have those losses since these are typically planning stage calculations prior to any equipment procurement.
I was hoping there was some generally accepted low ball efficiency for non-DOE compliant transformers to utilize in calculations.
 
If the goal is to calculate voltage drop across the transformer, the efficiency is not really required, just the impedance. For highest accuracy, the X/R is used to separate out the resistive and reactance components of the impedance. If you do know the efficiency, it can be used to determine the X/R ratio. The voltage drop is heavily dependent on the transformer load.
 
Here in post #11 I measured some transformer no load losses:

Please discuss transformer losses vs %Z

I know what transformer impedance is and how it is calculated. It is, of course, a somewhat unique use of the word "impedance" How does %Z relate to load and no load losses? I suspect there are other factors at play where we could have transformer A that has a lower %Z yet higher no load losses...
forums.mikeholt.com

From what I have measured, no load losses can really vary a lot. New 2016 DOE can be as low as .18% with her older units being well up above 1%.
 
Not the the voltage drop across the transformer but rather the additional voltage drop on the conductors between the source and the transformer due to the small amount of power consumed by the transformer itself, a.k.a., its efficiency. I'm looking for a reasonable low ball number I can use in conductor voltage drop calcs where non-DOE compliant transformers are utilized.
 
True, it's fairly negligible but it's not zero as seen with the list of efficiencies ror DOE compliant transformers. I'd like to account for it in my calcs if possible. Is there really no general low ball transformer efficiency assumption?
 
Superdemon said:
True, it's fairly negligible but it's not zero as seen with the list of efficiencies ror DOE compliant transformers. I'd like to account for it in my calcs if possible. Is there really no general low ball transformer efficiency assumption?
Click to expand...
Am I understanding right you don't want the transformer no load losses, you want the losses in the supply conductors from the magnetizing current?
 
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