Phase Spread

In the star vector diagram of a unit electric machine, the range occupied by the vectors of each phase under one pole is termed phase spread, expressed in electric degrees or number of slots. For a single-layer winding, the phase spread is 180°/m (m – the number of phases). The phase spread of a double-layer half-coiled winding is 360°/m, and the phase spread of a double-layer full-coiled winding is 180°/m.

The phase spread of a 2-phase winding is always 90° (= 180°/m). Therefore, a 2-phase winding cannot take the double-layer half-coiled winding type. The windings for single-phase induction motor are also considered as 2-phase windings.

When the number of phases is an even number of greater than or equal to 4, the phase spread is always 360°/m. Therefore, a winding with even number of phases (4, 6, …) can take only the double-layer half-coiled winding type.

When the number of phases is an odd number of greater than or equal to 3, the phase spread can be either 360°/m or 180°/m. Therefore, a winding with odd number of phases (3, 5, …) can take any winding types.