3 phase alternator

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.


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


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.


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.


Basic design of a three-phase armature.


Construction• There are two basic types of alternators:

– revolving-armature-type alternators– Revolving-field-type alternators


Revolving armature design.


Revolving field design.


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.


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.


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.


The alternator produces three sine wave voltages 120 out of phase with each other.


Rotator pole pieces become electromagnets.


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.


The brushless exciter uses stationary electromagnets.


Brushless exciter schematic.


Frequency• The output frequency (Hz) of an

alternator is determined by two factors:1. the number of stator poles2. the speed of rotation


RPM STATOR POLES

3600 2

1800 4

1200 6

900 8


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.


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.


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.


Determining phase rotation using lights.


Checking phase alignment using a voltmeter.


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.


Field-discharge resistor schematic switch open.


Field-discharge resistor schematic switch closed.


Field-discharge diode schematic normal flow.


Field-discharge diode schematic induced flow.


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.


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.


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.


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.


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.


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.


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.

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3 phase alternator