Unit 31
Three-Phase Alternators
Objectives:
• Discuss the operation of a three-phase
alternator.
• Explain the effect of rotation speed on
frequency.
• Explain the effect of field excitation on
output voltage.
Unit 31
Three-Phase Alternators
Power Generation
• Three-phase AC generators called
alternators provide most of the electrical
power we use today.
• Electrical power companies use
alternators rated in gigawatts.
• 1 gigawatt = 1,000,000,000 watts
Unit 31
Three-Phase Alternators
Power Generation
• The entire North American continent is
powered by AC generators connected
together in parallel.
• These alternators are powered by steam
turbines.
• The turbines called prime movers use oil,
coal, natural gas, or nuclear energy.
Unit 31
Three-Phase Alternators
Construction
• Alternators use the same operating
principle as direct-current generators.
• However, alternators have no commutator
to change the armature AC into DC.
• Most alternators are three-phase.
Unit 31
Three-Phase Alternators
Basic design of a three-phase armature.
Unit 31
Three-Phase Alternators
Construction
• There are two basic types of alternators:
– revolving-armature-type alternators
– Revolving-field-type alternators
Unit 31
Three-Phase Alternators
Revolving armature design.
Unit 31
Three-Phase Alternators
Revolving field design.
Unit 31
Three-Phase Alternators
•
•
•
•
Revolving-Armature-Type Alternators
The revolving-armature type is the least
used of the two basic types.
This type uses sliprings instead of a
commutator.
The armature windings are rotated inside
a magnetic field.
This type has very limited output power.
Unit 31
Three-Phase Alternators
Revolving-Field-Type Alternators
• The revolving-field type uses a stationary
armature called a stator and a rotating
magnetic field.
• This design permits much higher power
output.
Unit 31
Three-Phase Alternators
Rotor
• The rotor is the rotating part of an
alternator.
• The rotor is an electromagnet that
provides the magnetic field needed to
induce a voltage into the stator windings.
• Excitation current (DC) in the rotor is
required to establish the magnetic field.
Unit 31
Three-Phase Alternators
The alternator produces three sine wave
voltages 120 out of phase with each other.
Unit 31
Three-Phase Alternators
Rotator pole pieces become electromagnets.
Unit 31
Three-Phase Alternators
•
•
•
Brushless Exciter
Most large alternators use an exciter that
contains no brushes.
A separate small alternator of the
armature type is added to the shaft of the
rotor. The armature rotates between
wound electromagnets.
This current is then rectified and used as
excitation current.
Unit 31
Three-Phase Alternators
The brushless exciter uses stationary
electromagnets.
Unit 31
Three-Phase Alternators
Brushless exciter schematic.
Unit 31
Three-Phase Alternators
Frequency
• The output frequency (Hz) of an
alternator is determined by two factors:
1. the number of stator poles
2. the speed of rotation
Unit 31
Three-Phase Alternators
RPM
STATOR POLES
3600
2
1800
4
1200
6
900
8
Unit 31
Three-Phase Alternators
Output Voltage
• Three factors that determine the output
voltage of an alternator are:
1. the conductor length of the armature or
stator winding.
2. the strength of the rotator magnetic
field.
3. the speed of the rotor.
Unit 31
Three-Phase Alternators
Paralleling Alternators
• Before two alternators can be connected
in parallel:
– the output voltage of the two machines
should be the same.
– the phase rotation of the machines must
be the same.
– the output voltages must be in phase.
Unit 31
Three-Phase Alternators
•
•
Phase Rotation
Three lamps connected between the two
alternators can be used to test for phase
rotation.
A synchroscope can be used to determine
phase rotation and difference of frequency
between two alternators.
Unit 31
Three-Phase Alternators
Determining phase rotation using lights.
Unit 31
Three-Phase Alternators
Checking phase alignment using a voltmeter.
Unit 31
Three-Phase Alternators
•
•
Field Discharge Protection
When the DC excitation current is
disconnected, the collapsing field can
cause contacts to arc and damages to the
rotor windings.
Two devices used to prevent a high
voltage being induced in the rotor when
the DC excitation current is stopped are a
field-discharge resistor and a diode.
Unit 31
Three-Phase Alternators
Field-discharge resistor schematic switch open.
Unit 31
Three-Phase Alternators
Field-discharge resistor schematic switch
closed.
Unit 31
Three-Phase Alternators
Field-discharge diode schematic normal flow.
Unit 31
Three-Phase Alternators
Field-discharge diode schematic induced flow.
Unit 31
Three-Phase Alternators
Review:
1. The two basic types of three phase
alternators are: the rotating-armature type
and the rotating-field type.
2. The rotating-armature type is the least
used because of its limited voltage and
power rating.
Unit 31
Three-Phase Alternators
Review:
3. The rotating-field-type alternator contains
electromagnets.
4. Direct current must be supplied to the field
before the alternator can produce an
output voltage.
5. The direct current supplied to the field is
called excitation current.
Unit 31
Three-Phase Alternators
Review:
6. The output frequency of an alternator is
determined by the number of stator poles
and the speed of rotation.
7. The output voltage is controlled by the
amount of DC excitation current.
Unit 31
Three-Phase Alternators
Review:
8. Three factors that determine the output
voltage of an alternator are:
a. the conductor length of the armature or
stator winding.
b. the strength of the rotator magnetic
field.
c. the speed of the rotor.
Unit 31
Three-Phase Alternators
Review:
9. Before two alternators can be connected
in parallel, the output voltage of the two
machines should be the same, the phase
rotation of the machines must be the
same, and the output voltages must be in
phase.
Unit 31
Three-Phase Alternators
Review:
10. Three lamps connected between the two
alternators can be used to test for phase
rotation.
11. A synchroscope can be used to determine
phase rotation and difference of frequency
between two alternators.
Unit 31
Three-Phase Alternators
Review:
12. Two devices used to prevent a high
voltage being induced in the rotor when
the DC excitation current is stopped are a
field-discharge resistor and a diode.
13. Many large alternators use a brushless
exciter to supply direct current to the rotor
winding.