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.
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 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.
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.
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.
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.