OBJECTIVES After studying Chapter 20, the reader should be able to: • Prepare for ASE Electrical/Electronic Systems (A6) certification test content area “D” (Charging System Diagnosis and Repair). • List the parts of a typical generator (alternator). • Describe how a generator works. • Explain how the voltage regulator works to control generator (alternator) output voltage.
PRINCIPLES OF GENERATOR OPERATION • All vehicles operate electrical components by taking current from the battery. • The amount of current generated can be increased by the following factors. 1. Increasing the speed of the conductor through the magnetic field 2. Increasing the number of conductors passing through the magnetic field 3. Increasing the strength of the magnetic field
ALTERNATING-CURRENT GENERATORS (ALTERNATORS) • An alternator (AC generator) generates an alternating current when the current changes polarity during the generator’s rotation. • Most manufacturers call an AC generator a generator, but the most commonly used term in the industry is alternator.
GENERATOR CONSTRUCTION • An alternator is constructed of a two-piece cast-aluminum housing. • The rear housing, or the slip-ring end (SRE housing), usually contains a roller-bearing support for the rotor and mounting for the brushes, diodes, and internal voltage regulator (if the generator is so equipped).
GENERATOR CONSTRUCTION FIGURE 20-1 A typical generator (alternator) on a Chevrolet V8 engine. FIGURE 20-2 The end frame toward the drive belt is called the drive-end housing and the rear section is called the slip-ring-end housing.
ALTERNATOR OVERRUNNING PULLEYPurpose and Function • Using a one-way clutch in the alternator pulley allows the belt to apply power to the alternator in only one direction thereby reducing the fluctuations in the belt. FIGURE 20-3 An OAP on a Jeep generator with a diesel engine.
ALTERNATOR OVERRUNNING PULLEYPurpose and Function • Another type of alternator pulley uses a dampener spring inside, plus a one-way clutch. • This unit is called an overrunning alternator dampener (OAD).
ALTERNATOR OVERRUNNING PULLEYPurpose and Function FIGURE 20-4B An overrunning alternator dampener (OAD) disassembled, showing all of its internal parts. FIGURE 20-4A An OAD on a Chrysler vehicle generator.
ALTERNATOR OVERRUNNING PULLEYDiagnosis and Service • Overrunning alternator pulleys and overrunning alternator dampeners can fail. • In most cases, the entire alternator assembly will be replaced because each OAP or OAD is unique for each application and both require special tools to remove and replace.
ROTORS • At both ends of the rotor windings are heavy-gauge metal plates bent over the windings with triangular fingers called claw poles. • These pole fingers do not touch, but alternate or interlace FIGURE 20-5 A cutaway of a General Motors CS-130D generator showing the rotor and cooling fans that are used to force air through the unit to remove the heat created when it is charging the battery and supplying electrical power for the vehicle.
ROTORS • The magnetic fields are created between the alternating pole piece fingers. • These individual magnetic fields produce a current by electromagnetic induction in the stationary stator windings. FIGURE 20-6 Rotor assembly of a typical alternator (AC generator). Current through the slip rings causes the “fingers” of the rotor to become alternating north and south magnetic poles. As the rotor revolves, these magnetic lines of force induce a current in the stator windings.
STATORS • The stator is the stationary coil winding inside the alternator. • The stator is supported between the two halves of the generator housing and there are three copper wire windings wound on a laminated metal core. FIGURE 20-7 A cutaway view of a typical AC generator (alternator).
STATORS • As the rotor revolves, its moving magnetic field induces a current in the stator windings. FIGURE 20-8 An exploded view of a typical generator (alternator) showing all of its internal parts.
DIODES • AC generators use six diodes (one positive and one negative set for each of the three stator windings) to convert alternating current to direct current. • The symbol for a diode is shown. FIGURE 20-9 A diode symbol.
HOW A GENERATOR WORKS • A rotor inside a generator is turned by a belt and drive pulley which are turned by the engine. • The magnetic field of the rotor generates a current in the stator windings by electromagnetic induction. FIGURE 20-10 Magnetic lines of force cutting across a conductor induce a voltage and current in the conductor.
HOW A GENERATOR WORKS • Field current flowing through the slip rings to the rotor creates an alternating north and south pole on the rotor, with a magnetic field between each finger of the rotor. • Therefore, the current generated is described as being of a sine wave pattern.
HOW A GENERATOR WORKS FIGURE 20-11 Sine wave voltage curve created by one revolution of a winding rotating in a magnetic field.
HOW A GENERATOR WORKS • As the rotor continues to rotate, this sine wave current is induced in each of the three windings of the stator. FIGURE 20-12 When three windings (A, B, and C) are present in a stator, the resulting current generation is represented by the three sine waves. The voltages are 120° out of phase. The connection of the individual phases produces a three-phase alternating voltage.
WYE-CONNECTED STATORS • The Y (pronounced “wye” and generally so written) type or star pattern is the most commonly used generator stator winding connection. FIGURE 20-13 Wye-connected stator winding.
WYE-CONNECTED STATORS • In a wye-type stator connection, the currents must combine because two windings are always connected in series. FIGURE 20-14 As the magnetic field, created in the rotor, cuts across the windings of the stator, a current is induced. Notice that the current path includes passing through one positive (+) diode on the way to the battery and one negative (-) diode as a complete circuit is completed through the rectifier and stator.
DELTA-CONNECTED STATORS • The delta winding is connected in a triangular shape. • The delta-connected generator produces lower current at low speed and must be operated at high speed to produce its maximum output. FIGURE 20-15 Delta-connected stator winding.
GENERATOR OUTPUT FACTORS • The output voltage and current of a generator depend on several factors: 1. Speed of rotation. 2. Number of conductors. 3. Strength of the magnetic field.
GENERATOR VOLTAGE REGULATION • An automotive generator must be able to produce electrical pressure (voltage) higher than battery voltage to charge the battery. • After generator output begins, the voltage regulator controls the current flow through the rotor. FIGURE 20-17 Typical voltage regulator voltage range.
BATTERY CONDITION AND CHARGING VOLTAGE • If the automotive battery is discharged, its voltage will be lower than the voltage of a fully charged battery. • If a discharged battery is used during charging system testing, tests could mistakenly indicate a defective generator and/or voltage regulator.
TEMPERATURE COMPENSATION • A thermistor is used in the electronic circuits of the voltage regulator to control charging voltage over a wide range of under-the hood temperatures. FIGURE 20-18 A typical electronic voltage regulator showing the connections and the circuits involved.
A AND B FIELD CIRCUITS • When testing the generator, using a bench-mounted test unit, most test equipment requires the technician to select either A or B field type. • An A circuit is the most commonly used. FIGURE 20-19 Diagram of a A-type field circuit.
A AND B FIELD CIRCUITS • In a B-circuit field, the voltage regulator controls (opens and closes) the power side of the field circuit, and the circuit is grounded inside the generator. FIGURE 20-20 Diagram of an B-type field circuit.
ELECTRONIC VOLTAGE REGULATORS • The electronic circuit of the voltage regulator cycles between 10 and 7,000 times per second as needed to accurately control the field current through the rotor, and therefore control the generator output. • Whether mounted inside the generator or externally under the hood, electronic voltage regulators are mounted where normal airflow can keep the electronic components cool.
HOW AN ELECTRONIC VOLTAGE REGULATOR WORKS • Depending on the generator RPM, vehicle electrical load, and state of charge of the battery, this controlled switching on and off can occur between 10 and 7,000 times per second. FIGURE 20-21 Typical General Motors SI-style AC generator with an integral voltage regulator. Voltage present at terminal 2 is used to reverse bias the zener diode (D2) that controls TR2. The hot brush is fed by the ignition current (terminal 1) plus current from the diode trio.
COMPUTER-CONTROLLED GENERATORS • Computers can interface with the charging system in three ways. 1. The computer can activate the charging system by turning the field current to the rotor on and off. 2. The computer can monitor the operation of the alternator and increase engine speed if needed during conditions where a heavy load is demanded by the alternator. 3. The computer can control the generator by controlling alternator output to match the needs of the electrical system.
COMPUTER-CONTROLLED GENERATORS • A typical system used on some General Motors vehicles is called electrical power management (EPM). • It uses a Hall effect sensor attached to the negative battery cable to measure the current leaving and entering the battery. FIGURE 20-22 A Hall-effect current sensor attached to the negative battery cable is used as part of the EPM system.
COMPUTER-CONTROLLED GENERATORS • The engine control module (ECM) controls the alternator by changing the on-time of the current through the rotor. • The on-time, called duty cycle, is varied from 5% to 95%.
COMPUTER-CONTROLLED GENERATORS • This system has six modes of operation, including: • Charge mode. • Fuel economy mode. • Voltage reduction mode. • Start-up mode. • Battery sulfation mode. • Headlight mode.
GM CS GENERATORS • Beginning in the mid-1980s, General Motors introduced a smaller, yet high-output series of generators. FIGURE 20-23 General Motors CS generator. Notice the use of zener diodes in the rectifier to help control any high-voltage surges that could affect delicate computer circuits. If a high-voltage surge does occur, the zener diode(s) will be reverse biased and the potentially harmful voltage will be safely conducted to ground. Voltage must be preset at the L terminal to allow the generator to start producing current.
GM CS GENERATORS FIGURE 20-24 The components inside a GM CS generator.
GM CS GENERATORS • The voltage regulator switches the field voltage on and off at a fixed frequency of about 400 times per second. FIGURE 20-25 The alternator field (rotor) current is controlled by the computer. SMEC stands for single module engine controller.
GM CS GENERATORSTypes of Computer Controlled Charging Systems • Computer control of the charging system has the following advantages. 1. The computer controls the field of the generator, which can pulse it on or off as needed for maximum efficiency, thereby saving fuel. 2. Engine idle can also be improved by turning on the generator slowly, rather than all at once, if an electrical load is switched on, such as the air conditioning system.
GM CS GENERATORSTypes of Computer Controlled Charging Systems 3. Most computers can also reduce the electrical load on the electrical system if the demand exceeds the capacity of the charging system by reducing fan speed, shutting off rear window defoggers, or increasing engine speed to cause the alternator to increase the amperage output. 4. The computer can monitor the charging system and set diagnostic trouble codes (DTCs) if a fault is detected. 5. Because the charging system is computer controlled, it can be checked using a scan tool.
SUMMARY • Generator output is increased if the speed of the generator is increased. • The parts of a typical generator (alternator) include the drive-end housing (DE), slip-ring-end (SRE) housing, rotor assembly, stator, rectifier bridge, brushes, and voltage regulator. • The magnetic field is created in the rotor. • The current is created in the stator windings. • The voltage regulator controls the current flow through the rotor.
REVIEW QUESTIONS • Describe how a small electronic voltage regulator can control the output of a typical 100 ampere generator. • List the component parts of a typical generator (alternator). • Explain how the computer is used to control a generator (alternator). • Why do voltage regulators include temperature compensation?
CHAPTER QUIZ • Technician A says that the diodes regulate the generator output voltage. Technician B says that the field current can be computer controlled. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B
CHAPTER QUIZ 2. A magnetic field is created in the _____ in a generator (AC generator). • Stator • Diodes • Rotor • Drive-end frame
CHAPTER QUIZ 3. The voltage regulator controls the current through _____. • The generator brushes • The rotor • The generator field • All of the above
CHAPTER QUIZ 4. Technician A says that two diodes are required for each stator winding lead. Technician B says that diodes change alternating current into direct current. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B
CHAPTER QUIZ 5. The generator output current is produced in the _____. • Stator • Rotor • Brushes • Diodes (rectifier bridge)
CHAPTER QUIZ 6. Generator brushes are constructed from _____. • Copper • Aluminum • Carbon • Silver-copper alloy
CHAPTER QUIZ 7. How much current flows through the generator brushes? • All of the generator output flows through the brushes • 25 to 35 A, depending on the vehicle • 10 to 15 A • 2 to 5 A
CHAPTER QUIZ 8. Computers can be used in charging system in which way(s)? • Activate the charging system • Monitoring the operation of the charging system • Control the charging system • Any of the above
CHAPTER QUIZ 9. Operating a generator (alternator) in a vehicle with a defective battery can harm the _____. • Diodes (rectifier bridge) • Stator • Voltage regulator • Brushes