No-load losses in mains transformers

[rs20]

Simple question: how am I supposed to know/calculate the no-load losses (i.e., the power and/or VAs drawn by the transformer if nothing is connected to the secondary) of transformers based on datasheets like these?:

• https://www.zettlermagnetics.com/pdfs/EI30_1.pdf
• https://catalog.triadmagnetics.com/Asset/TCT3-04E07AE.pdf
• https://www.hammfg.com/electronics/transformers/class-2/ba-c-d-e.pdf

They don't seem to mention anything useful for estimating power losses; they don't even mention an efficiency figure at all!? What am I missing?

[Kleinstein]

The datasheets don't give the values, which is pretty poor.  For efficiency with load one can at least a crude estimate from the no load voltage. The efficiency is at most the nominal voltage divided by the no load voltage.
If in doubt one may have to get a sample an measure yourself. If not given I would not expect good performance, so expect significant no load loss (e.g. 1-1.5 W range for the 1 VA transformer).

[jonpaul]

No transformer mfg will specify this: The designer or engineer must calculate using Bsat, core area, # turns.

Loss varies non-linear with applied pri V and F.

(flux density)

Known and calculkated since 1880s, Steinmetz.

Use classic magnetizing inductance and hysterisis loss calculations.

With load add copper losses.

Jon

[coppercone2]

lol ask for a budget, buy a sample of each and test them out. get a variable frequency voltage AC source too.

A designer that is asked to figure out the answer to that question, imo, theoretically, does not have sufficient resources. I feel like that kind of calculation would occur in the soviet union. Its too much work for the ubiquitous piece of iron.

[jonpaul]

Estimated rule of thumb losses at full load.

small low W mains trsf, like mass mkt Chinese wall plug trsf: 3..20W   ~ 10..20%
Medium 30..70W 7..15%
100..500W 3..7%
over 500W 3..5%

Special cases like Micro Wave oven (super low cost, short duty cycle ) are run near  to Bsat, thus much hogher Pcu, Pfe losses ~ 20%

Jon

[Kleinstein]

With the small transformers ( < 2 VA) calculating the no load loss gets tricky. In addition to the iron core loss the magnetizing current add significan copper losses. Here the magnetizing current depend a lot on the residual air gaps in the 2 part core and quality of the core material.  I don't see a good way to calculate the losses up front- these small transformers are just too far away from the usual.
A a consequence the loss from the magentizing current the primary coil often has more volume (e.g. 2 x) than the seconday with such small transformers.
There are even cases where the loss at no load is higher than the loss at the nominal current and theoretical the transformer would run too hot at no load.
It would absolutely make sense to have the data in the data-sheet.  It may be worth asking for a better, more detailed datasheet as an alternative to measuring yourself.
The transformers can be quite different, depending on the priorties (cost,size, isolation, voltage drop on loading).  There are a few with reduced no load loss (e.g. small toroids depite the difficulty winding them and with isolation). Other may intentionally run the core to saturation to stabilize the voltage.