# Homework Help: Quick transformer regulation question - Please help

1. Apr 29, 2010

### MRSquared

1. The problem statement, all variables and given/known data

Tests are performed on a 50Hz transformer which yield the following results:
Short circuit test: V1 = 40V, I1 = 10A, Power input = 20W
Open circuit test: V1 = 240V, I1 = 1.2A, Power input = 70W

Here are the equivalent circuit parameters from the above tests.
Rc = 823 Ω, (R1 + R2') = 0.2 Ω, (l1 + l2') = 12.7mH, Lm = 0.656H

Determine the regulation of the transformer (using the approximate equivalent cct), assuming that the rated primary voltage is 240V (50Hz) and that the rated secondary current referred to the primary is 10A. (Assume pure resistive load).

2. Relevant equations

Per unit voltage regulation = $$\frac{{{V_{{\rm{sc}}}}\cos \left( {{\phi _e} - {\phi _2}} \right)}}{{{V_1}}}$$

3. The attempt at a solution

This is the problem given by my lecturer. My textbook says the voltage regulation can be calculated using the above formula from the short circuit test. I'm now going to substitute the variables into the equation.
Vsc = 40 V (from short-circuit test)
Φc = 0.243 (calculated)
V1 = 240 V (given)
Φ2 = ???

From all the examples I've seen, Φ2 is given. Am I missing something or there's a problem with the question? Thank you in advance.