# Transformer efficiency

Discussion in 'Electrical Engineering' started by Den, Aug 17, 2004.

1. ### DenGuest

Group:

Can anyone give me an idea of typical transformer efficiencies.

I've got a 120V input / 240V output 1kVA transformer.

I assume that sticking a 1kW resistive load won't just draw 2kW on the input
side, but 2*(1+x) kW. How big, typically, will x be?

Cheers

D

2. ### Don KellyGuest

Sticking a 1KW resitive load on the output side will result in (1+x ) KW on
the input side. Even for a cheap 1KVA transformer the efficiency should be
in the order of 90 % (i.e. x=0.1). If you put enough money into it (as would
be the case for a 1MVA transformer) then efficiencies should peak in the
Ivory soap level. (99 44/100 % pure (x<0.006). I have tested 3KVA
transformers which peaked at about 98%- these are run of the mill production
devices.

If the input is 2+X - you have a piece of junk. Note that when you step up
from 120V to 240V the current steps down and power in, ignoring losses will
be the same as power out.

3. ### DenGuest

Don

Thanks for this. You've answered my question

And I have made a public fool of myself by forgetting that V(1)I(1) =
V(2)I(2) - I had somehow forgotten that the power is constant! Oh well it
won't be the first time I've looked dumb!

Cheers

D  