1. AC Generators (Alternators)
Chapter 18
AC Generators (Alternators)

2. Objectives After studying this chapter, you will be able to:
Describe the construction and operating characteristics of various types of alternating current generators
Discuss the methods for controlling the output voltage and frequency of alternating current generators

3. Objectives (cont’d.)
Discuss the methods for producing single-phase and multiphase voltages
Describe the procedures for connecting alternating current generators in parallel

4. AC Generators Versus DC Generators
Alternating current is less expensive to produce and transmit than DC
DC generators are used for metal refining, electroplating and battery charging
DC generators have limitations that restrict their power output
AC generators are typically more practical

5. Alternator Construction
AC generator construction is similar to that of a DC generator except commutator is replaced with slip rings

6. Alternator Construction (cont’d.)
Large alternators have a stationary armature (stator) and a rotating field (rotor)
The field receives its energy from the DC exciter, usually at 250 volts
Steam turbines can be used to drive alternators constructed for high-speed operation

7. Alternator Voltage Output
Single-phase alternators
AC generators that produce only one voltage
Small in size; used for temporary power
Two-phase alternators
Produce two separate voltages 90 degrees out of phase with each other
Three-phase alternators
Produce three voltages 120 degrees apart

8. Alternator Voltage Output (cont’d.)
Wye (star) connection
See Figure 18-13A
Delta connection
See Figure 18-16
Only one value of voltage appears for both single phase and three phase

9. Alternator Voltage Output (cont’d.)

10. Alternator Voltage Output (cont’d.)
Phase sequence
The order in which the voltages reach their maximum values

11. Alternator Voltage Output (cont’d.)
Voltage and current in a delta-connected alternator
The line voltage is equal to the phase voltage
The line current (for balanced loads) is equal to 1.73 times the phase current
Voltage and current in a wye-connected alternator
The line voltage is 1.73 times the phase voltage

12. Alternator Voltage Output (cont’d.)
The line current is equal to the phase current
Power in a three-phase system
Power for a balanced three-phase system can be calculated from the formula P=1.73IEcosf
If load is unbalanced, do a vectorial analysis of each phase, then combine the phases

13. Voltage and Frequency Control
Standard frequency for power distribution in the US is 60Hz
Europe, Asia and South America generally operate on 50Hz, and in some cases 25Hz
To calculate frequency, use the following equation:

14. Alternator Characteristics
Three factors affect alternator voltage output
Resistance of stator windings cause a voltage drop within the generator
Self-induction takes place within the stator windings, causing a voltage drop
Power factor of the load affects output voltage

15. Paralleling Alternators
AC power systems often consist of several alternators connected in parallel
Synchronizing: certain conditions must be fulfilled
Effect of varying field strength
When alternators are synchronized, they are in phase relative to the load and 180 degrees out of phase with each other

16. Paralleling Alternators (cont’d.)
If magnetic field of one alternator increases, output voltage also increases, causing current to flow
Because of the alternators’ low resistance and high inductance, the current tends to keep field strength of all the alternators equal

17. Paralleling Alternators (cont’d.)
Effect of increased driving force
If two alternators in parallel supply equal currents to a load, and the driving torque of one machine increases
Power supplied by that machine increases
Power supplied by other machine decreases
Change does not materially affect power factor of either machine

18. Motor-generator sets Combination of electric motor and generator
Motor serves as prime mover
Serve many different purposes
Example: AC motor may drive a DC generator to provide power for a specific load

19. Rotary Converters (Dynamotors)
Motor-generator set combined into a single housing
Armature for the motor wound on same shaft as generator armature
Rotary converters used for purposes similar to motor-generator sets
Supply DC power in areas where only AC is available

20. Summary Alternators are similar in construction to DC generators.
May be single-phase, two-phase or three-phase
Coils connected in delta or wye arrangements
Factors affecting alternator voltage output:
Resistance, self-induction and power factor of the load

21. Summary (cont’d.) Alternators may be connected in parallel
To synchronize, must follow certain steps
Motor-generator sets combine two machines: motor and generator
Rotary converters (also called dynamotors) are motor-generator sets combined into a single housing