END BELLS The end bells, or brackets, are bolted to each end of the stator frame and contain the bearings in which the shaft revolves. Either ball bearings or sleeve bearings are used for this purpose.
CONNECTING THREE-PHASE MOTORS Connecting a three-phase motor is a simple operation. All three-phase motors are wound with a number of coils, with a 2-to-1 ratio of slots to coils.
These coils are connected to produce three separate windings called phases, and each must have the same number of coils. The number of coils in each phase must be one-third the total number of coils in the stator. Therefore, if a three-phase motor has 36 coils, each phase will have 12 coils. These phases are usually called Phase A, Phase B, and Phase C. All three-phase motors have their phases arranged in either a wye connection or a delta connection.
WYE CONNECTION A wye-connected three-phase motor is one in which the ends of each phase are joined together paralleling the windings. The beginning of each phase is connected to the line. Figure 7-42 shows the wye connection.
DELTA CONNECTION A delta connection is one in which the end of each phase is connected in series with the next phase. Figure 7-43 shows the end of Phase A connected to the beginning of Phase B. The end of Phase B is connected to the beginning of Phase C, and the end of Phase C is connected to the beginning of Phase A. At each connection, a wire is brought out to the line.
Figure 7-42.- Star, or wye, connection.
VOLTAGES Most small-and medium-sized three-phase motors are made so that they can be connected for two voltages.
The purpose in making dual-voltage motors is to enable the same motor to be used in facilities with different service voltages. Figure 7-44 shows four coils which, if connected in series, may be used on a 460-volt ac power
Figure 7-43.- Delta connection.
Figure 7-44.- Four 115-volt coil connected in series to produce 460 volts
Figure 7-45.- Four 115-volt coil connected in parallel for 230 volts; each coil still receives only 115 volts. supply. Each coil receives 115 volts. If the four coils were connected in two parallel sets of coils to a 230-volt line, as shown in figure 7-45, each coil would still receive 115 volts. So, regardless of the line voltage, the coil voltage is the same. This is the principle used in all dual-voltage machines. Therefore, if four leads are brought out of a single-phase motor designed for 460/ 230 or 230/ 115-volt operation, the motor can be readily connected for either voltage.
Dual-Voltage Wye Motor When you are connecting a dual-voltage wye motor, remember practically all three-phase dual-
Figure 7-46.- Terminal markings and connection for a wye-connected dual-voltage motor.
voltage motors have nine leads brought out of the motor from the winding. These are marked TI through T9, so that they may be connected externally for either of the two voltages. These are standard terminal markings and are shown in figure 7-46 for wye-connected motors.
HIGH VOLTAGE.- to connect for high voltage, you should connect groups in series, as shown in figure 7-47. Use the following procedure:
1. Connect T6 and T9; twist and wire nut.
2. Connect leads T4 and T7; twist and wire nut.
3. Connect T5 and T8; twist and wire nut.
4. Connect leads TI, T2, and T3 to the three-phase line.