Mechanical Systems 387
speed, and the motor has zero slip. Thus, we can determine the speed of a
synchronous motor by using the following formula:f × 120
S = ———
n/3
where:
S = the speed of a synchronous motor in r/min,
f = the frequency of the applied AC voltage in hertz,
n/3 = the number of stator poles per phase, and
120 = a conversion constant.Note that this is the same as the formula used to determine the sta-
tor speed of a single-phase motor, except that the number of poles must be
divided by three (the number of phases). A three-phase motor with twelve
actual poles will have four poles per phase. Therefore, its stator speed will
be 1800 rpm. Synchronous motors have operating speeds that are based
on the number of stator poles they have.
Thr ee-phase synchronous motors usually are employed in very large
horse power ratings. One method of starting a large synchronous motor
is to use a smaller auxiliary DC machine connected to the shaft of the syn-
chronous motor, as illustrated in Figure 14-28. The method of starting is as
follows:Step 1. DC power is applied to the auxiliary motor, causing it to increase
in speed. Three-phase AC power is applied to the stator.
Step 2. When the speed of rotation reaches a value near the synchronous
speed of the motor, the DC power circuit is opened and, at the
same time, the terminals of the auxiliary machine are connected
across the slip ring/brush assembly of the rotor.
Step 3. The auxiliary machine now converts to generator operation and
supplies exciter current to the rotor of the synchronous motor, us-
ing the motor as its prime mover.
Step 4. Once the rotor is magnetized, it will “lock” in step, or synchro-
nize, with the revolving stator field.
Step 5. The speed of rotation will remain constant under changes in load
condition.Another starting method is shown in Figure 14-28. This method uti-
lizes damper windings, which are similar to the conductors of a squirrel-