Transformer Loading: Relationships & Calculations

[temujin]

dear all,

When loading the secondary winding of a transformer, my textbook gives the following relationships:
$$\frac{V_{sec}}{V_{pri}}=\frac{N_{sec}}{N_{pri}}\\$$
and
$$\frac{I_{sec}}{I_{pri}}=\frac{N_{sec}}{N_{pri}}\\$$

Do these relationships hold also when the diameter of the windings are different?
I suspect they don´t...If they do not, are there any other relationships I can use to calculate I and V in the secondary winding?


regards
t.

 

[berkeman]

Those are the simple first-order relationships, and as you suspect, they ignore parasitic inductance, capacitance and resistance, as well as core material issues. I did a quick google search, and pages like this one are pretty common:

http://www.beigebag.com/case_xfrmer_2.htm

Depending on lots of stuff, you can make your model a lot more accurate by including at least the leakage inductance and winding resistance (both are in series with the magnetizing inductance), and also the inter-winding capacitance of each winding (put in parallel with the magnetizing inductance of each winding). The N1:N2 transformation only applies to the magnetizing inductance components of this model.

Hope that helps, -Mike-