Chapter 18 AC Generators (Alternators)
Chapter 18
AC Generators (Alternators)
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
Objectives (cont’d.)
• Discuss the methods for producing single-phase and multiphase voltages
• Describe the procedures for connecting alternating current generators in parallel
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
Alternator Construction
• AC generator construction is similar to that of a DC generator except commutator is replaced with slip rings
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
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
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
Alternator Voltage Output (cont’d.)
Alternator Voltage Output (cont’d.)
• Phase sequence– The order in which
the voltages reach their maximum values
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
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=1.73IEcos– If load is unbalanced, do a vectorial analysis
of each phase, then combine the phases
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:
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
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
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
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
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
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
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
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