Mechanical Systems 389
cage rotor. These windings are placed within the laminated iron of the
rotor assembly. No auxiliary machine is required when damper windings
are used. The starting method used is as follows:Step 1. Three-phase AC power is applied to the stator windings.
Step 2. The motor will operate as an induction motor, because of the
“transformer action” of the damper windings.
Step 3. The motor speed will build up, so that the rotor speed is some-
what less than the speed of the revolving stator field.
Step 4. DC power from a rotating DC machine, or more commonly from
a rectification system, is applied to the slip ring/brush assembly
of the rotor.
Step 5. The rotor becomes magnetized and builds up speed until rotor
speed is equal to stator speed.
Step 6. The speed of rotation remains constant regardless of the load
placed on the shaft of the motor.An outstanding advantage of the three-phase synchronous motor is
that it can be connected to a three-phase power system to increase the
overall power factor of the system. Power factor correction was discussed
previously. Three-phase synchronous motors are sometimes used only to
correct the system power factor. If no load is to be connected to the shaft
of a three-phase synchronous motor, it is called a synchronous capacitor. It
is designed to act only as a power factor corrective machine. Of course, it
might be beneficial to use this motor as a constant-speed drive connected
to a load, as well as for power factor correction.
We know from previous discussions that a low power factor cannot
be tolerated by an electrical power system. Thus, the expense of installing
three-phase synchronous machines can be justified in industrial use, as a
means of appreciably increasing the system power factor. To understand
how a three-phase synchronous machine operates as a power factor cor-
rective machine, refer to the curves of Figure 14-29. We know that the syn-
chronous motor operates at a constant speed. Variation in rotor DC exci-
tation current has no effect on speed. The excitation level will change the
power factor at which the machine operates. Three operational conditions
may exist, depending on the amount of DC excitation applied to the rotor.
These conditions are:- Normal excitation—operates at a power factor of 1.0.