You may find transformers just about anywhere, from massive power plants to tiny SMPS circuits. While the specifics of a transformer’s operation will vary depending on its intended use, all transformers operate in a similar fashion. We may have seen “dot” like symbols on one end of the transformer windings when studying a design with a transformer in it. The dot convention rule governs the positioning of these symbols. What is it, though? And for what function does it exist?
What is Dot Convention?
To indicate which end of a transformer winding should be connected to which of the other windings, the Dot convention is used. Dots placed above the primary and secondary terminals serve as an indication of phase relationships in transformer schematic representations.
The instantaneous voltage between the primary winding and the secondary winding will have the same polarity if the dots are placed adjacent to the top ends of the windings as shown below. This indicates that the currents Is and Ip, which flow through the secondary and primary windings, respectively, are in phase with one another and flow in the same direction.
If, however, the dots are switched around (up on primary and down on secondary, or vice versa), as in the image below, this means that the primary and secondary currents (IP and IS) are 180 degrees out of phase and flow in the opposite directions.
Now that they are aware of this practice and the transformer’s polarity, the engineers are free to reverse the phase relationships in any way they see fit simply by switching which ends of their circuit are connected to the transformer’s terminals. By reversing the terminal connections in the out-of-phase transformer shown above, the secondary winding can be brought into phase with the primary.
Why is Dot Convention Important?
At least for resistive loads, it is commonly assumed in transformer research that the secondary and primary windings’ voltage and current are in phase. Assuming that the transformer’s primary and secondary windings are wound in the same direction is the norm. Each winding’s direction of wrapping around the core determines the phase relation between the primary and secondary currents and voltages; thus, if the winding is wound around the core in the same way as indicated below, the voltage and current on both sides should be in phase.
However, this isn’t always the case, as the windings’ directions might be reversed (as depicted in the image above), in which case the secondary winding’s voltage (VS) and current (Is) would be out of phase with the primary current if they were connected to the same terminals.
Despite appearances, phase loss and inverse polarity cause significant issues in power system protection, measurement, and control. Reversing the polarity of a winding on an instrument transformer, for instance, might cause protective relays to fail, skewed readings of power and energy, and the display of a negative power factor. In signal circuits, it can cause amplifiers and speakers to malfunction or cancel out signals that are supposed to add. In parallel transformer windings, it can create an effective short circuit.
Since transformers are opaque, it is impossible to tell which way to connect a circuit to it to get in-phase (or out-of-phase) voltage and current. Therefore, transformer manufacturers devised a polarity indication standard called; the “Dot Convention” to reduce the risks associated with reverse polarity connection and phase loss.
Alphanumeric Labels in Transformers
In addition to the dot convention, alphanumeric labels, consisting of the letters “H” and “X” followed by subscript numbers that signify winding polarity, are also used to indicate polarity on transformers. Wires labeled “1” (H1 and X1) stand in for the dots used to indicate polarity. Below is an illustration of a common transformer with its numeric label.