Last of Three Parts

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.

Incoming power is connected to terminals T1, T2 and T3. An advantage of this design is that wiring mistakes during installation are usually avoided because of the pre-connected windings. Proper rotation direction still must be checked.

The six-lead motor is wound in a manner that allows the windings to be connected in a Wye or Delta configuration (see Figure 2). If leads T4, T5 and T6 are wired together and power is applied to leads T1, T2 and T3, a Wye connection is achieved. If leads T1 & T6, T2 & T4 and T3 & T5 are wired together and power is applied at the vertices, the connection is Delta.

In the U.S., the ratio of high and low voltage is 2:1 (460 volts:230 volts), but in Europe, it is √3:1 (380 volts:220 volts). This allows Europe to take advantage of the 1.732 voltage relationship between the Wye and Delta connections (discussed in Part 1) and use them for dual voltage. Since the impedance of the Wye connection is three times that of the Delta connection, high voltage is connected Wye and low voltage is connected Delta.

Another application of the six-lead motor, used in the U.S. and Europe, is a low-voltage starting technique known as Wye start/Delta run. This application uses a special starter to Wye connect the windings during starting and shift them to Delta after the motor reaches a particular speed.

The lower starting voltage reduces the starting current to about 1/3 of normal. Starting torque is also reduced substantially, so the speed at which the transition from Wye to Delta occurs will depend on the inertia of the load. Centrifugal pumps and fans can often reach full speed before switching to the Delta run mode.