Jim Bryan
EASA Technical Support Specialist (retired)

The connection of a three-phase motor is one of the many variables a motor designer can use to optimize the performance and life of the machine. The designer determines whether to use a wye or delta connection and how many parallel circuits to maximize current density (circular mils per amp or cm/A) while optimizing flux densities and manufacturability.

In three-phase motors, the square root of three is an important number. Because of the phase relationships of the three windings shown in Figure 1, the voltage and current are intertwined with this factor. In the delta winding, the phase voltage is applied to each phase winding but the current has two possible paths. Due to the phase relationship of the winding, the current is not split in two but by the square root of three (1.73). The opposite is true for the wye connection; the phase voltage impressed on each phase is the line voltage divided by 1.73, and the phase current equals the current in each coil. This is the reason that wye wound motors have fewer turns of heavier wire than do delta-connected motors.

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