center tapped xfmr question

Thread Starter


Joined Sep 23, 2004
I'm an apprentice electrician and I have a question that I have been wondering for a long time...

How does a single phase center-tapped transformer produce two voltages that are 180 degrees apart from each other on each line?

I understand that voltage and current are induced into the secondary winding from the primary, and that at any instant the current flow is such that line A might be flowing "into" the coil while line B is flowing "away" from the coil, but since the lines are tied together onto a load to create 240V, how could two sine waves 180 degrees "out of phase" with each other add to create 240V?

If they were 180 degrees apart, wouldn't one wave be peaking at its positive value while the other wave was peaking negatively? Wouldn't the net result of the two waves be zero?

If the lines are tied together to form 240V, wouldn't there only be one voltage pushing across the entire coil and through both lines and the load?

Also, since the primary of the transformer is connected to a single phase source, wouldn't the secondary also produce current and voltage that are in phase? I've read books that state that the voltage between the two lines are 180 degrees apart, and also books which state that the two lines are in phase with each other.

Could you please clarify what happens in the secondary of a center-tapped transformer, and also what the resultant sine wave looks like? It is a wave with 240V rms- how could two sinewaves 180 degrees apart form such a wave?

Thank you