# Transformer Basics

According to my thinking, the answer should be 400-200+200=400V, but the answer given in my workbook is 0V. Can you please point out my mistake ?

• Put brackets around 200+200 as both are negative ... Commented Dec 31, 2017 at 10:33

The two 200v windings have their 'finish' (not dotted) terminals connected together. This means that there is 0v between the two start (dotted) terminals, so the two 200v windings by themselves contribute zero to the measurement.

The voltage is being measured between the start and finish of the 400v winding, so should be 400v. I agree with you, and disagree with a 0v answer.

Ah! Unless it's a DC voltmeter. Does your book ask trick questions? A DC voltmeter will read zero!

• No, it will be 400V only as nothing was mentioned about DC voltmeter. Thanks @Neil_UK !! Commented Dec 31, 2017 at 12:50

As the two 200 turns inductors are in series opposition to each other, their net effect can be nullified. So we are left with only one inductor with 400 V across it. So 400V is the answer. 0V would have been the answer if the second 200turns inductor was in series aiding to the first. I.e, with the dot in the other end.

Only the 400V winding has active input, the others are unloaded.

Think a +400V peak inputted just now to the dotted end of the 400V winding. 200V peaks are offered out from both 200V windings, + is at the dotted ends. Thus the 200V windings cancel each other, you get 400V to the meter.

User Neil_UK proposed a good explanation for the zero reading. If that's not the case, your book has an error.