Transcript Chapter 8
Chapter 8 Charging Systems Objectives (1 of 2) • Identify charging circuit components. • Navigate a charging circuit schematic. • Voltage drop-test charging circuit wiring and components. • Describe the construction of an alternator. • Explain full-wave rectification. Objectives (2 of 2) • • • • Full-field an alternator. Measure AC leakage in the charging circuit. Verify the performance of an alternator. Use Intelli-check to assess charging circuit performance. • Disassemble and reassemble a Delcotron 40SI alternator. Charging Systems Alternator Construction (1 of 2) Alternator Construction (2 of 2) • To generate electricity, the alternator uses this basic law of physics: – When magnetic lines of force move across a conductor (such as a wire or bundle of wires), an electrical current is produced in the conductor. • Actual current flow induced depends on several factors: – The strength of the magnetic field – The speed of the wire passing through the field – The size and number of wires Rotor • The rotor is the only moving component within the alternator. • It is responsible for producing the rotating magnetic field. • The rotor consists of a coil, two pole pieces, and a shaft. • The magnetic field is produced when current flows through the coil; this coil is simply a series of windings wrapped around an iron core. • Increasing or decreasing the current flow through the coil varies the strength of the magnetic field, which in turn defines alternator output. Slip Rings and Brushes • The wiring of the rotor coil is connected to slip rings. • The slip rings and brushes conduct current to the rotor. • Most alternators have two slip rings mounted directly on the rotor shaft; they are insulated from the shaft and from each other. – A spring-loaded carbon brush is located on each slip ring to carry the current to and from the rotor windings. Stator • The stator is made up of many conductors, or wires, into which the spinning rotor induces voltage. • The wires are wound into slots in the alternator frame, with each wire forming several coils spaced evenly around the frame. • The wires are grouped into three separate bundles, or windings. • The coils of the three windings are staggered in the alternator frame so that the electrical pulses created in each coil will also be staggered. • This produces an even flow of current out of the alternator. Alternator Operation Voltage Regulators • Microprocessors and electronic sensors and switches are easily damaged by voltage spikes and high voltage levels. • The voltage regulator receives battery voltage as an input. – This is called the sensing voltage; it allows the regulator to sense and monitor the battery voltage level. – When the battery voltage rises to a particular level (approximately 13.5 volts), the regulator will turn the field current off. Types of Field Circuits • The field circuit, which is controlled by the voltage regulator, might be one of two types. – “A” circuit – “B” circuit • With an “A” circuit, the regulator is on the ground side of the rotor. The regulator turns the field circuit off and on by controlling a ground. • With a “B” type, voltage regulator is positioned on the feed side of the alternator. Battery voltage is fed through the regulator to the field circuit, which is then grounded in the alternator. Charging System Failures and Testing • A malfunction in the charging system results in either: – An overcharged battery – An undercharged battery Overcharging • An overcharged battery will produce water loss, eventually resulting in hardened plates and the inability to accept a charge. • Overcharging can be caused by one or a combination of the following: – Defective battery – Defective or improperly adjusted regulator – Poor sensing lead contact to the regulator or rectifier assembly Undercharging (1 of 2) • An undercharged battery will result in slow cranking speeds and a low specific gravity of the electrolyte. – A loose drive belt – Loose, broken, corroded, or dirty terminals on either the battery or alternator – Undersized wiring between the alternator and the battery – Defective battery that will not accept a charge Undercharging (2 of 2) • Undercharging can also be caused by one or a combination of the following defects in the alternator field circuit: – Poor contacts between the regulator and brushes – Defective diode trio – No residual magnetism in the rotor/shorted, open, or grounded rotor coil – Defective or improperly adjusted regulator – Damaged or worn brushes/damaged or worn slip rings – Poor connection between the slip rings and field coils • Undercharging can also be the result of a malfunction in the generating circuits: – One or more of the stator windings (phases) can be shorted, open, or grounded. – The rectifier assembly might be grounded. – One or more of the diodes might be shorted or open. Charging System Testing • The battery must be at least 75 percent charged before the alternator will perform to specifications. • The output of the alternator is first tested. • If the output is below specifications, the voltage regulator is bypassed and battery current is wired directly to the field circuit of the rotor. • This is called full-fielding the rotor. – If this corrects the problem, the fault is in the regulator. – If the output remains low with the regulator bypassed by full-fielding, the alternator might be defective. Full-field Testing the Alternator (1 of 2) • By applying full battery voltage directly to the field windings in the rotor, it can be determined whether or not the regulator is the cause of the undercharging condition. • There are two variations of this procedure that apply to alternators with external regulators. Caution • When testing the output of a full-fielded alternator, carefully observe the rise in system voltage. • Because the current output is not regulated, battery voltage can quickly rise to an excessive level, high enough to overheat the batteries, causing electrolyte to spew from the vent holes, and possibly damage sensitive electronic components. • Do not allow system voltage to rise above 15V. Shop Talk • Some alternators with remote-mounted electronic regulators are connected to the regulators by a wiring harness and a multi-pin connector. • Full-fielding the field circuit is accomplished by removing the connector from the regulator and connecting a jumper wire between two pins (terminals) in the harness connector (consult OEM service literature to correctly identify the pin assignments). Doing so bypasses the regulator, sending battery current directly to the field circuit. Caution • Never full-field an alternator without applying an electrical load to the batteries. • Whenever the voltage regulator is bypassed, there is nothing to control peak alternator output; this can cause voltage spikes that can damage both electrical and electronic components. Full-field Testing the Alternator (2 of 2) • If the field circuit is grounded through the regulator (an A circuit), the regulator is disconnected from the field terminal on the alternator and a jumper is connected between the terminal and a ground. • If the alternator receives battery voltage through the regulator (a B circuit), the regulator is disconnected from the field (F) terminal and a jumper is connected to the terminal and to the insulated battery terminal. Full-fielding Internally Regulated Alternators • Full-field testing is not possible on some alternators with internal voltage regulators. • To isolate and test the regulator on these types of alternators, the alternator must be removed from the truck and disassembled. • Other internally regulated alternators have a hole through which the field circuit can be tested. AC Leakage Test • Over time, the diode bridge can start to “leak” AC current to ground. • Typically AC leakage should not exceed 0.3 volts AC. • Use a DMM set to the AC voltage scale. • Place one test lead on the alternator insulated terminal and connect the other lead to chassis ground. Intelli-Check™ • Intelli-check™ should first be plugged into the twoway connector attached to the alternator output and ground terminal. • Next, the engine should be started and run with no chassis electrical loads turned on. • Rev up the engine once to high idle. • Then turn on the lights and the heater blower to high, and once again rev up the engine again to high idle. • This completes the test, and the charging circuit condition will be displayed by illuminating one of five LEDs. Scope Testing (1 of 2) • Oscilloscopes can be used to diagnose alternator conditions. • The scope used may be either a full-sized oscilloscope or a good-quality, hand-held lab scope. • Use the scope manufacturer’s instructions to make the connections. Scope Testing (2 of 2) Caution • Truck alternators are not provided with reverse-polarity protection. – Reverse-polarity connections such as those caused by jump-starting can destroy alternator diodes and numerous other chassis solidstate devices. Shop Talk • Alternators that energize the field circuit with current produced by the stator windings rely on residual magnetism in the rotor to initially energize the stator when the engine is being started. • During handling or repair, this residual magnetism can be lost. – It must be restored before testing the system. This is done by connecting a jumper wire between the diode trio terminal and the alternator output terminal as shown in the following figure. Summary (1 of 4) • A truck charging system consists of batteries, alternator, voltage regulator, associated wiring, and the electrical loads of the chassis. • The purpose of the charging system is to recharge the batteries whenever necessary and to provide the current required to power the electrical components on the truck chassis. • A malfunction in the charging system results in either undercharged or overcharged batteries. • The subcomponents of an alternator consist of stator, rotor, slip rings, brushes, and rectifier. Summary (2 of 4) • A magnetic field is established in the rotor windings, and this is used to induce current flow in a stationary stator. • Slip rings are used to conduct current to the rotor to establish a magnetic field. • Because the brushes in slip rings conduct very low current to the rotor windings, they significantly outlast the brushes used in now-obsolete generators. • The alternator rectifier is located in the end frame assembly. Summary (3 of 4) • Most current truck alternators use a delta configuration of diodes in the rectifier to achieve full wave rectification. • Current truck alternators use solid-state, electronic voltage regulators. • TR-type alternators use electronic switching in 12/24V systems, eliminating the need for electromechanical series-parallel switches. • When the voltage regulator is shorted out of the circuit, the alternator is full-fielded and will produce maximum output. Summary (4 of 4) • The sensing voltage of the charging system should be battery voltage at any given moment of operation. • AC leakage testing can verify the performance of the diodes in the rectifier and help predict imminent alternator failures. • Scope testing can graphically identify rectifier solidstate component failures. • Delco-Remy’s Intelli-check™ tool enables technicians to make a rapid test of charging system performance.