386 Electrical Power Systems Technology
correct the power factor of three-phase systems. Synchronous motors are
usually very large in size and horsepower rating.
Figure 14-27 shows a pictorial diagram of the construction of a three-
phase synchronous motor. Physically, this motor is constructed like a
three-phase alternator (see Chapter 6). DC is applied to the rotor to pro-
duce a rotating electromagnetic field; the stator windings are connected in
either a wye or delta configuration. The only difference is that three-phase
AC power is applied to the synchronous motor, while three-phase power
is extracted from the alternator. Thus, the motor acts as an electrical load,
while the alternator functions as a source of three-phase power. This rela-
tionship should be kept in mind during the following discussion.
The three-phase synchronous motor differs from the three-phase
induction motor in that the rotor is wound and is connected through a
slip ring/brush assembly to a DC power source. Three-phase synchro-
nous motors, in their pure form, have no starting torque. Some external
means must be used to initially start the motor. Synchronous motors are
constructed so that they will rotate at the same speed as the revolving sta-
tor field. We can say that at synchronous speed, rotor speed equals stator
Figure 14-27. Pictorial diagram of three-phase synchronous motor construction