what does the dot in a transformer symbol represents?

Answer 1

It represents how the windings are arranged with reference to current flow.

For most applications (linear type AC-DC power supplies) it doesn't matter that when the AC signal on one side of the transformer is going from zero volts up to the peak voltage and back down, the secondary is doing the same thing or is 180 degrees out-of-phase.

For some applications it does matter that the secondary is in-phase with the primary (or 180 degrees out-of-phase). The dots tell you that when current is flowing "in" to the dot on the primary side, that current is also flowing "in" to the dot on the secondary side as well, or if current is flowing "out" on one side it is flowing "out" on the other side as well.

two examples:
1. Flyback type switch-mode power supplies. The primary current is a pulse switched by a transistor, but the secondary current is out-of-phase with the primary so secondary current flows in the rectifier only when the transistor is 'off' and the primary is dumping its stored energy. The dots on the transformer tell you how to wire the circuit properly.

2. Sometimes SCRs and Triacs are turned on by trigger pulses, which need to be isolated. The phase of the trigger current is important with respect to the driving voltage, so you need to know how the transformer is wound. The dots indicate this.

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Answer 2

Transformer Symbol

Answer 3

by convention, the dot symbolises the side of the transformer winding from where the current is supposed to enter in that winding. if the current leaves from the dot, it has a negative value...