On a trafo i just bought, it says COS Ø = 0,99. What does this mean? Is there some angle involved in changing the voltage?
That is the 'Power factor'. To be simple, the ratio of actual power consumed to the product of voltage and current through the trafo is 0.99, it means. See this: http://en.wikipedia.org/wiki/Power_factor
trafo ? http://www.trafo.co.in/ To your trig question: cos^{-1}(0.99) = 8.1 degrees and that's the angle between primary side volts and amps at rated resistive load. draw a right triangle with a hypotenuse of 1, base of 0.99 and an opposite side of 0.1424 this represents primary current at rated conditions with resistive load small angle is 8.1 degrees base is 0.99 units and is load current corrected for turns ratio opposite side is 0.1424 units and is magnetizing current hypotenuse is total current flowing into primary so this transformer draws 14.24% magnetizing current.