Unit 31 Three-Phase Alternators
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.
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