Connecting the Stator Leads
Wye-delta starting is accomplished by connecting the stator windings in wye during the starting period and then reconnecting them in delta for normal run operation. For simplicity, it will be assumed that the motor illustrated is designed for single voltage operation and has leads T1 through T6 brought out at the terminal connection box. If a dual voltage motor is to be connected, make the proper stator winding connections for high or low voltage operation and then change T4, T5, and T6 to T10, T11, and T12 in the following connections. A basic control circuit for wye-delta starting is shown in Figure 39 – 9. This circuit employs time delay to determine when the windings will change from wye to delta. Starting circuits that sense motor speed or motor current to determine when to change the stator windings from wye to delta are also common.
When the Start button is pressed, control relay CR energizes, causing all CR contacts to close. This immediately energizes contactors 1M and S. The motor stator windings are now connected in wye, as shown in Figure 39 – 10. The 1M load contacts connect power to the motor, and the S contacts form a wye connection tor windings are now connected in delta, Figure 39 – 11. Note that the 2M load contacts are used to make the delta connection. A diagram showing the connection of all load contacts is shown in Figure 39 – 12.
The most critical part of connecting a wye-delta starter is making the actual load connections to the motor. An improper connection generally results in the motor stopping and reversing direction when transition is made from wye to delta. It is recommended that the circuit and components be numbered to help avoid mistakes in connection (Figure 39–13).