You have probably noticed that three-phase motors can have a varying number of leads exiting the junction box. The most common numbers are three, six, nine or twelve.
Note that these numbers are all multiples of three since their combinations must accommodate three incoming phases. These lead combinations are designed to accommodate single or dual voltages and Wye, Delta or Wye/Delta winding connections. The twelve-lead motor will accommodate a combination of dual-voltage and a Wye/Delta connection. We will take a detailed look at each of these a little later.
What is the purpose of these two connections, and why are motors wound as Wye, Delta or a combination of the two? The Wye/Delta combination provides a couple advantages, and we will address them in this column.
Why are single- and dual-voltage motors wound as either Wye or Delta? Why not just standardize on one or the other? Although the Wye and Delta connection diagrams are quite simple, the actual motor windings are far more complex. Often, the connection will depend on the manufacturing process.
For example, the Wye connection requires fewer turns than the Delta connection (1.732:2) to achieve the same electrical characteristics. This makes machine winding smaller motors with narrow stator slots easier. On the other hand, a portion of the leads in a dual-voltage, Delta connection can be a smaller diameter than those of a Wye. This reduces the wire cost and often simplifies manufacturing. An engineer for a major motor manufacturer recently told me, “It is a juggling act between the number of turns, number of circuits and the wire size.”
The stator windings of a three-lead motor may be connected as either Delta or Wye (see Figure 1). These motors are wound for a single voltage, and the windings are connected as either Wye or Delta during the manufacturing process.