The Dot Notation
Generally,
when we study about Transformers, we assume that the primary and
secondary voltage and currents are in phase. But, such is not always the
case. In Transformer, The phase relation between primary and secondary
currents and voltages depends on how each winding is wrapped around the
core.
But
in fig (2) you may notice that the secondary winding is wound around
the core in the opposite direction from the secondary winding in fig
(1).
Consequently,
the voltage induced in the Secondary winding in fig (2) is 180° out of
phase as compared with the induced voltage in secondary in fig (1) and
the direction of secondary current (IS) is opposite from the primary current (IP)
So we see that
- The primary and secondary voltage and current are in phase in fig (1)
- The primary and secondary voltage and current are 180° out of phase in fig (2)
Dot Convention
To
eliminate any confusion in the phase relation between primary and
secondary voltage and current, a dot convention has been adopted for
transformer schematic diagrams. Dots are placed on the top of primary
and secondary terminals as shown in fig (3) and (4)
In
fig (3), we see that dots are placed at the top in both primary and
secondary terminals. It shows that the primary and secondary current and
voltages are in phase. Moreover, the primary and secondary voltages (VP and VS) have similar sine wave, also the primary and secondary (IP and IS) currents are same in direction.
The
story is opposite in fig (4). We can see that one dot is positioned at
the top in primary terminal and the other one (dot) is placed at bottom
of secondary terminal. It shows that the primary and secondary current
and voltages are 180° out of phase. In addition, the primary and
secondary voltages (VP and VS) sine waves are opposite to each other. Also the primary and secondary currents (IPand IS) are opposite in direction.
