Connecting Three-Phase Motors

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Figure  7-41.—Three-phase  wound  rotor. rotor (fig. 7-41) has a winding on the core that is connected to three slip rings mounted on the shaft. 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. 7-36

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