THE DOT CONVENTION
Figure 3.11 illustrates a load supplied from a transformer. The dots on the windings of the transformer help to determine the polarity of the voltages and currents in the core without performing a physical examination. The dot convention states that a positive magnetomo- tive force 2F is produced when the current flows into the dotted end of the winding. A neg- ative magnetomotive force is produced when the current flows into the undotted end of the winding. Therefore, the magnetomotive forces will be subtracted if one current flows into the dotted end of a winding and the second flows out of the dotted end.
The primary current shown in Fig. 3.11 produces a positive magnetomotive force 2FP = NPiP. A negative magnetomotive force 2FS = –NSiS is produced by the secondary current. Therefore, the net magnetomotive force in the core is given by
The current then must flow into one dotted end and out of the other to eliminate the net magnetomotive force in the core.