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