Three-phase Power:

         When only one armature coil is used to generate power through
one set of brushes. The generator produces one waveform called single
phase. When a generator uses more than one armature coil, the output
is multiphased. A three phase ac generator uses three armature coils
and 3 sets or 6 brushes to produce three current waveforms. These 
waveforms indicate the rise and fall of the voltage and current. Each
wave lags the other indicating the use of three different coils of
the armature. This is called three phase power. Three phase power is
used extensively in industry for operating motors and machinery.

Synchronous AC Motors:

        Three-phase power can be used to operate a motor by producing
a Rotating Magnetic Field. The stator windings 1,2, and 3 are
connected to the incoming power phases 1,2, and 3 respectively. Since
each phase reaches peak amplitude at successively later times, the 
electromagnetic field becomes the strongest in each winding in succession.
This creates the effect of and electromagnetic field with its N and
S poles continually shifting around the stator hull. The field is, in
addition, rotating one complete revolution for each cycle of phase
1,2, and 3. This means that the field is rotating in Synchronism
with the incoming line frequency. The rotor rotates in synchronism with
the S and N poles of the stator. Thus the rotor and the output shaft
are also synchronized to the line frequency, and the motor rotates at
Synchronous Speed. This is the only speed at which a synchronous motor
can be used. Synchronous motors are used in applications requiring 
precise speed control.

Three-Phase Induction Motors:

        A three-phase induction motor requires a revolving stator field
derived from the three-phase power source just as does a sync motor. The
main difference is that an induction motor does not require dc excitation
in the rotor. Therefore, a basic induction motor uses no slip rings or
commutator assembly. Instead, current is induced in the rotor by the 
cutting of electromagnetic flux lines (from the rotating stator field)
across the rotor windings. This voltage induced in the rotor causes a
current to flow which sets up an electromagnetic field in the rotor.
Thus, while the sync motor uses external power to supply the rotor
current, the induction motor generates its own rotor current. Induction
motors can never operate at synchronous speed. If the rotor were to
turn at the same speed as the rotating field, no lines of force would
be cut by the rotor conductors, and no rotor field would be produced.
An induction motor must, Therefore, rotate at something less than 
synchronous speed. The difference between the synchronous speed and the
actual rotor rpm is called Slip.

        Slip is found by subtracting the rotor speed from the synchronous
speed. The percentage of slip can be found by using the formula:

                          Ss - Sr
                % Slip = ------------  x 100
                             Ss

        Ss = The synchronous speed in rpm

        Sr = The actual rotor speed in rpm


        The full load slip in most induction motors varies between 4
and 6 percent. Should an induction motor become heavily overloaded or
stalled (the slip would be 100 %), the rotor would be damaged. In 
general, slip in an induction motor should not exceed 10 %.

        Induction motors also have wirewound rotors. In this case, the
coil ends are shorted together, and the operation of the motor is the
same as for the squirrel cage rotor.
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