Hello,
The transformer has primary voltage Vp at the terminals of 400v, secondary voltage Vs of 80v with full load, and primary current with full load Ip and secondary current Is with full load.
One way to handle the transformer with primary and secondary resistances is to simply subtract the voltage dropped by the primary current and primary resistance from the primary voltage and use that as the new primary voltage, then add the voltage dropped by the secondary resistance and secondary current to the secondary voltage and call that the new secondary voltage, then take the ratio of the new secondary voltage to the new primary voltage and that will be the turns ratio required.
If we formulize this process it comes out to this:
A=(Vs+Is*Rs)/(Vp-Ip*Rp)
where
A is the turns ratio primary to secondary, and because this is a step down transformer you can take 1/A to be the turns ratio here, and
Vs is the secondary voltage, and
Vp is the primary voltage, and
Is is the secondary current, and
Ip is the primary current, and
Rp is the primary resistance, and
Rs is the secondary resistance.
Example:
We have 400v input and we want 80v output with load, and the primary resistance is 1.5 ohms and secondary resistance is 0.1 ohm, and the full load VA is 4kVA.
The primary current is Ip=4000/400.
The secondary current is Is=4000/80.
The primary resistance Rp=1.5 ohms.
The secondary resistance Rs=0.1 ohms.
The primary voltage Vp is 400 volts.
The secondary voltage Vs is 80 volts.
Plugging these into the formula:
A=(Vs+Is*Rs)/(Vp-Ip*Rp)
we come out with:
A=(80+50*0.1)/(400-10*1.5)
and after doing the math we get:
A=0.22077922
which is the primary to secondary turns ratio, and the secondary to primary turns ratio is simply 1/A which is:
1/A=4.52941176
There is one more little catch here however, and that is that there is always some primary excitation current. To get more accurate results, we'd have to look up the excitation current from the manufacturers data sheet and figure that in as an additional voltage drop on the primary side.
Colin:
The turns ratio actually is usually made a little higher to make up for the loss of the primary and secondary resistances. The formula above takes these two resistances into account.