AC Generators

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.

Components of an AC generator

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:

  1. The rotor.
  2. Brushes.
  3. The stator.
  4. The rectifier bridge.
  5. The housing.
  6. Cooling fan.

Rotors

Components of a typical AC generator rotor

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.

The north and south poles of a rotor's field alternate

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.

Magnetic flux lines move in opposite directions between the rotor poles

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, and back to the other slip ring. Current then passes through a grounded stationary brush or to a voltage regulator.

The slip rings and brushes provide a current path to the rotor coil

FIGURE. The slip rings and brushes provide a current path to the rotor coil.

Brushes

Brushes are the stationary electrical contact to the rotor's slip rings

FIGURE. Brushes are the stationary electrical contact to the rotor’s slip rings

The field winding of the rotor receives current through a pair of brushes that ride against the slip rings. The brushes and slip rings provide a means of maintaining electrical continuity between stationary and rotating components. The brushes ride the surface of the slip rings on the rotor and are held tight against the slip rings by spring tension provided by the brush holders. The brushes conduct only the field current (2 to 5 amperes). The low current that the brushes must carry contributes to their longer life.

Direct current from the battery is supplied to the rotating field through the field terminal and the insulated brush. The second brush may be the ground brush, which is attached to the AC generator housing or to a voltage regulator.

Stators

Components of a typical stator

FIGURE. Components of a typical stator.

The stator contains three main sets of windings wrapped in slots around a laminated, circular iron frame. The stator is the stationary coil in which electricity is produced. Each of the three windings has the same number of coils as the rotor has pairs of north and south poles. The coils of each winding are evenly spaced around the core. The three sets of windings alternate and overlap as they pass through the core. The overlapping is needed to produce the required phase angles.

Overlapping stator windings produce the required phase angles

FIGURE. Overlapping stator windings produce the required phase angles.

The rotor is fitted inside the stator. A small air gap (approximately 0.015 inch or 0.381 mm) is maintained between the rotor and the stator. This gap allows the rotor’s magnetic field to energize all of the windings of the stator at the same time and to maximize the magnetic force.

A small air gap between the rotor and the stator maximizes the magnetic force

FIGURE. A small air gap between the rotor and the stator maximizes the magnetic force.

Wye-connected stator winding

FIGURE. Wye-connected stator winding.

Each group of windings has two leads. The first lead is for the current entering the winding. The second lead is for current leaving. There are two basic means of connecting the leads.

Delta-connected stator winding

FIGURE. Delta-connected stator winding.

The first method is the wye wound connection. In the wye connection, one lead from each winding is connected to one common junction. From this junction, the other leads branch out in a Y pattern. A wye wound AC generator is usually found in applications that do not require high amperage output.

The second method of connecting the windings is called the delta connection. The delta connection attaches the lead of one end of the winding to the lead at the other end of the next winding. The delta connection is commonly used in applications that require high amperage output.

In a wye wound or delta wound stator winding, each group of windings occupies one third of the stator, or 120 degrees of the circle. As the rotor revolves in the stator, a voltage is produced in each loop of the stator at different phase angles. The resulting overlap of sine waves that is produced is shown. Each of the sine waves is at a different phase of its cycle at any given time. As a result, the output from the stator is divided into three phases.

The voltage produced in each stator winding is added together to create a three-phase voltage

FIGURE. The voltage produced in each stator winding is added together to create a three-phase voltage.

Diode Rectifier Bridge

The battery and the electrical system cannot accept or store AC voltage. For the vehicle’s electrical system to be able to use the voltage and current generated in the AC generator, the AC current needs to be converted to DC current. This process is called rectification. A split-ring commutator cannot be used to rectify AC current to DC current because the stator is stationary in an AC generator. Instead, a diode rectifier bridge is used to change the current in an AC generator. Acting as a one-way check valve, the diodes switch the current flow back and forth so that it flows from the AC generator in only one direction.

 General Motors' rectifier bridge

FIGURE. General Motors’ rectifier bridge.

When AC current reverses itself, the diode blocks and no current flows. If AC current passes through a positively biased diode, the diode will block off the negative pulse. The result is the scope pattern shown in Figure. Hie AC current has been changed to a pulsing DC current. This process is called half-wave rectification.

AC current rectified to a pulsating DC current after passing through a positive-biased diode. This is called half-wave rectification

FIGURE. AC current rectified to a pulsating DC current after passing through a positive-biased diode. This is called half-wave rectification.

A simplified schematic of the AC generator windings connected to the diode rectifier bridge

FIGURE. A simplified schematic of the AC generator windings connected to the diode rectifier bridge.

The positive-biased diodes are mounted into a heat sink to provide protection

FIGURE. The positive-biased diodes are mounted into a heat sink to provide protection.

Negative-biased diodes pressed into the AC generator housing

FIGURE. Negative-biased diodes pressed into the AC generator housing.

An AC generator usually uses a pair of diodes for each stator winding, for a total of six diodes. Three of the diodes are positive biased and are mounted in a heat sink to dissipate the heat. The three remaining diodes are negative biased and are attached directly to the frame of the AC generator. By using a pair of diodes that are reverse-biased to each other, rectification of both sides of the AC sine wave is achieved. The process of converting both sides of the sine wave to a DC voltage is called full-wave rectification. The negative-biased diodes allow for conducting current from the negative side of the AC sine wave and putting this current into the circuit. Diode rectification changes the negative current into positive output.

Full-wave rectification uses both sides of the AC sine wave to create a pulsating DC current

FIGURE. Full-wave rectification uses both sides of the AC sine wave to create a pulsating DC current.

With each stator winding connected to a pair of diodes, the resultant waveform of the rectified voltage would be similar to that shown. With six peaks per revolution, the voltage will vary only slightly during each cycle.

With three-phase rectification, the DC voltage level is uniform

FIGURE. With three-phase rectification, the DC voltage level is uniform.

The examples used so far have been for single-pole rotors in a three-winding stator. Most AC generators use either a twelve- or fourteen-pole rotor. Each pair of poles produces one complete sine wave in each winding per revolution. During one revolution, a fourteen-pole rotor will produce seven sine waves. The rotor generates three overlapping sine wave voltage cycles in the stator. Hie total output of a fourteen-pole rotor per revolution would be twenty-one sine wave cycles. With final rectification, the waveform would be similar to the one shown.

Sine wave cycle of a 14-pole rotor and three-phase stator

FIGURE. Sine wave cycle of a 14-pole rotor and three-phase stator.

Rectified AC output has a ripple pattern that can be shown on an oscilloscope

FIGURE. Rectified AC output has a ripple pattern that can be shown on an oscilloscope.

Current flow through a wye-wound stator

FIGURE. Current flow through a wye-wound stator.

Full-wave rectification is desired because using only half-wave rectification wastes the other half of the AC current. Full-wave rectification of the stator output uses the total potential by redirecting the current from the stator windings so that all current is in one direction. A wye wound stator with each winding connected to a pair of diodes is shown.

Each pair of diodes has one negative and one positive diode. During rotor movement, two stator windings will be in series and the third winding will be neutral. As the rotor revolves, it will energize a different set of windings. Also, current flow through the windings is reversed as the rotor passes. Current in any direction through two windings in series will produce DC current.

The action that occurs when the delta wound stator is used is shown. Instead of two windings in series, the three windings of the delta stator are in parallel. This makes more current available because the parallel paths allow more current to flow through the diodes. Since the three outputs of the delta winding are in parallel, current flows from each winding continuously.

Note: Not only do the diodes rectify stator output, but they also block battery drain back when the engine is not running.

Current flow through a delta-wound stator

FIGURE. Current flow through a delta-wound stator.

AC Generator Housing and Cooling Fan

Typical two-piece AC generator housing

FIGURE. Typical two-piece AC generator housing.

Most AC generator housings are a two-piece construction, made from cast aluminum. Hie two end frames provide support of the rotor and the stator. In addition, the end frames contain the diodes, regulator, heat sinks, terminals, and other components of the AC generator. The two end pieces are referred to as:

  1. The drive end housing: This housing holds a bearing to support the front of the rotor shaft. The rotor shaft extends through the drive end housing and holds the drive pulley and cooling fan.
  2. The slip ring end housing: This housing also holds a rotor shaft that supports a bearing. In addition, it contains the brushes and has all of the electrical terminals. If the AC generator has an integral regulator, it is also contained in this housing.

The cooling fan draws air into the housing through the openings at the rear of the housing. The air leaves through openings behind the cooling fan.

The cooling fan draws air in from the rear of the AC generator to keep the diodes cool

FIGURE. The cooling fan draws air in from the rear of the AC generator to keep the diodes cool.

Liquid-Cooled Generators

Water-cooled generator

FIGURE. Water-cooled generator.

High output generators have a tendency to have higher internal temperatures that can shorten the life of the diodes. To help reduce diode temperatures, some manufacturers are using a liquid cooled generator. In addition, since these generators do not use a fan, underhood noises are reduced. The water-cooled generator has water jackets cast into their housing and is connected to the engine’s cooling system by hoses.