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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 singlephase 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=1.73IEcosf
– 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 threephase
• 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