Mechanical Systems 391
Note the variation of stator current drawn by the synchronous motor
as the rotor current varies. You should also see that stator current is mini-
mum when the power factor equals 1.0, or 100%. The situations shown on
the graph in Figure 14-31A indicate stator and rotor currents under no-
load, half-load, and full-load conditions. Current values when the power
factors are equal to 1.0, 0.8-leading, and 0.8—lagging conditions are also
shown. These curves are sometimes referred to as V-curves for a synchro-
nous machine. The graph in Figure 14-29B shows the variation of power
factor with changes in rotor current under three different load conditions.
Thus, a three-phase synchronous motor, when over-excited, can improve
the overall power factor of a three-phase system.
As the load increases, the angle between the stator pole and the cor-
responding rotor pole on the synchronous machine increases. The stator
current will also increase. However, the motor will remain synchronized
unless the load causes “pull-out” to take place. The motor would then
stop rotating because of the excessive torque required to rotate the load.
Most synchronous motors are rated greater than 100 horsepower and are
used for many industrial applications requiring constant-speed drives.Wound-Rotor Induction Motor
The wound-rotor induction motor (wrim), shown in Figure 14-30 is
a specialized type of three-phase motor. This motor may be controlled ex-
ternally by placing resistances in series with its rotor circuit. The starting
torque of a wrim motor can be varied by the value of external resistance.
The advantages of this type of motor are a lower starting current, a high
starting torque, smooth acceleration, and ease of control. The major dis-
advantage of this type of motor is that it costs a great deal more than an
equivalent three-phase induction motor using a squirrel-cage rotor. Thus,
they are not used as extensively as other three-phase motors.SPECIALIZED MECHANICAL POWER SYSTEMS
There is a need for specialized mechanical systems that can produce
a rotary motion that is somewhat different from that produced by most
electric motors. This type of system employs rotary motion to control the
angular position of a shaft that is used to position the shaft of a second de-
vice. Synchro systems and servomechanisms are used to achieve this basic
operation. With these devices, it becomes possible to transmit a rotary mo-