AC Generators

Components of an AC generator
Note: The first charging systems used a DC generator that had two field coils that created a magnetic field. Output voltage was generated in the wire loops of the armature as it rotated inside the magnetic field. Current sent to the battery was through the commutator and the generator's brushes. FIGURE. Components of an AC generator. The DC generator was unable to produce the sufficient amount of current required when the engine was operating at low speeds. With the addition of more electrical accessories and components, the AC (alternating current) generator, or alternator, replaced the DC generator. The main components of the AC generator are: The rotor. Brushes. The stator. The rectifier bridge. The housing. Cooling fan. Rotors FIGURE. Components of a typical AC generator rotor. The rotor creates the rotating magnetic field of the AC generator. It is the portion of the AC generator that is rotated by the drive belt. The rotor is constructed of many turns of copper wire around an iron core. There are metal plates bent over the windings at both ends of the rotor windings. The poles (metal plates) do not come into contact with each other, but they are interlaced. When current passes through the coil (1.5 to 3.0 amperes), a magnetic field is produced. The strength of the magnetic field is dependent on the amount of current flowing through the coil and the number of windings. FIGURE. The north and south poles of a rotor's field alternate. The poles will take on the polarity (north or south) of the side of the coil they touch. The right-hand rule will show whether a north or south pole magnet is created. When the rotor is assembled, the poles alternate from north to south around the rotor. As a result of this alternating arrangement of poles, the magnetic flux lines will move in opposite directions between adjacent poles. This arrangement provides for several alternating magnetic fields to intersect the stator as the rotor is turning. These individual magnetic fields produce a voltage by induction in the stationary stator windings. FIGURE. Magnetic flux lines move in opposite directions between the rotor poles. The wires from the rotor coil are attached to two slip rings that are insulated from the rotor shaft. The slip rings function much like the armature commutator in the starter motor, except they are smooth. The insulated stationary carbon brush passes field current into a slip ring, then through the field coil,...


Charging: Principle of Operation

Simplified AC generator indicating electromagnetic induction
FIGURE. Simplified AC generator indicating electromagnetic induction. All charging systems use the principle of electromagnetic induction to generate electrical power. Electromagnetic principle states that a voltage will be produced if motion between a conductor and a magnetic field occurs. The amount of voltage produced is affected by: The speed at which the conductor passes through the magnetic field. The strength of the magnetic field. The number of conductors passing through the magnetic field. FIGURE. Alternating current is produced as the magnetic field is rotated. To see how electromagnetic induction produces an AC voltage by rotating a magnetic field inside a fixed conductor (stator), refer to the illustration. When the conductor is parallel with the magnetic field, the conductor is not cut by any flux lines (A). At this point in the revolution, zero voltage and current are being produced. As the magnetic field is rotated 90 degrees, the magnetic field is at a right angle to the conductor (B). At this point in the revolution, the maximum number of flux lines cut the conductor at the north pole. With the maximum amount of flux lines cutting the conductor, voltage is at its maximum positive value. When the magnetic field is rotated an additional 90 degrees, the conductor returns to being parallel with the magnetic field (C). Once again, no flux lines cut the conductor and voltage drops to zero. An additional 90-degree revolution of the magnetic field results in the magnetic field being reversed at the top conductor (D). At this point in the revolution, the maximum number of flux lines cut the conductor at the south pole. Voltage is now at maximum negative value. FIGURE. Sine wave produced in one revolution of the conductor or magnetic field. When the magnetic field completes one full revolution, it returns to a parallel position with the magnetic field. Voltage returns to zero. The sine wave is determined by the angle between the magnetic field and the conductor. It is based on the trigonometry sine function of angles. The sine wave shown plots the voltage generated during one revolution. It is the function of the drive belt to turn the magnetic field. Drive belt tension should be checked periodically to assure proper charging system operation. A loose belt can inhibit charging system efficiency, and a belt that is too tight can cause early bearing failure.


Charging Systems

Current flow when the charging system is operating
The automotive storage battery is not capable of supplying the demands of the electrical system for an extended period of time. Every vehicle must be equipped with a means of replacing the current being drawn from the battery. A charging system is used to restore the electrical power to the battery that was used during engine starting. In addition, the charging system must be able to react quickly to high load demands required of the electrical system. It is the vehicle's charging system that generates the current to operate all of the electrical accessories while the engine is running. Two basic types of charging systems have been used. Hie first was a DC generator, which was discontinued in the 1960s. Since that time the AC generator has been the predominant charging device. The DC generator and the AC generator both use similar operating principles. The purpose of the conventional charging system is to convert the mechanical energy of the engine into electrical energy to recharge the battery and run the electrical accessories. When the engine is first started, the battery supplies all the current required by the starting and ignition systems. As the battery drain continues, and engine speed increases, the charging system is able to produce more voltage than the battery can deliver. When this occurs, the electrons from the charging device are able to flow in a reverse direction through the battery's positive terminal. The charging device is now supplying the electrical system's load requirements; the reserve electrons build up and recharge the battery. FIGURE. Current flow when the charging system is operating. If there is an increase in the electrical demand and a drop in the charging system's output equal to the voltage of the battery, the battery and charging system work together to supply the required current. The entire conventional charging system consists of the following components: Battery. Generator. Drive belt. Voltage regulator. Charge indicator (lamp or gauge). Ignition switch. Cables and wiring harness. Starter relay (some systems). Fusible link (some systems). This chapter also covers the operation of the charging systems used on HEVs. The HEV can recharge the HV battery by running the engine and using the ISG or AC motors as generators. They can also use regenerative braking. To charge the auxiliary battery they may use a DC/DC converter.