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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