In centre tapped transformer (12v-0-12v) when i measured voltage between phase to neutral that is 12v,and line to line 24v but line to line voltage was to be zero volts (voltage were 180°opposite to each other between line to neutral) why voltage between line to line not zero?
Consider the center tapping point as O, the upper terminal as A and the lower terminal as B. Now, in a center tapped transformer Voa is 180 degrees out of phase with Vob. But, the Vao and Vob are in phase with each other. So, the voltage between terminals A and B is additive.