Unit 32 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 poles 2. 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: Unit 32 Three-Phase Alternators 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: Unit 32 Three-Phase Alternators 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: Unit 32 Three-Phase Alternators 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: Unit 32 Three-Phase Alternators 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: Unit 32 Three-Phase Alternators 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.