chap41 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 41, the reader should
be able to:

•
•
•
•

Describe how a generator works.
Discuss the various generator test procedures.

Explain how to disassemble a generator and test its
component parts.
Discuss how to check the wiring from the generator to the
battery.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
AC voltage • brushes
charging voltage
delta-connected stator • delta winding • diodes • diode check •
diode trio • drive-end (DE) housing • duty cycle
Electrical power management (EPM)

fusible link
generator output test
Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
overrunning alternator dampener (OAD) • overrunning
alternator pulley (OAP)
poles
rectifier bridge • rotors
sine wave • slip ring end (SRE) • stators • stator tap
temperature compensation • thermistor
wye-connected stator

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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All vehicles operate electrical components by taking
current from the battery. It is the purpose and function
of the charging system to keep the battery fully charged.
The Society of Automotive Engineers (SAE) term for
the unit that generates electricity is the generator.
The term alternator is also commonly used, especially
in service manuals before 1993 when the SAE term
was adopted by most vehicle manufacturers.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition

By James D. Halderman

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PRINCIPLES OF GENERATOR OPERATION
All electrical generators use the principle of electromagnetic
induction to convert mechanical energy into electrical energy.
Electromagnetic induction involves the generation of an electrical
current in a conductor when the conductor is moved through a
magnetic field. 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
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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ALTERNATING CURRENT GENERATORS
(ALTERNATORS)
An AC generator generates an alternating current when the current
changes polarity during the generator’s rotation.
A battery cannot “store” alternating current; therefore, this
alternating current is changed to direct current (DC) by diodes
inside the generator.
Diodes are one-way electrical check valves that permit current to
flow in only one direction. Most manufacturers call an AC
generator an alternator.

Continued
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GENERATOR CONSTRUCTION
A generator is constructed of a two-piece cast-aluminum housing.
Aluminum is used because of its lightweight, nonmagnetic
properties and heat transfer properties needed to help keep the
generator cool.
A front ball bearing is pressed into the front housing (called the
drive-end [DE] housing) to provide the support and friction
reduction necessary for the belt-driven rotor assembly.
The rear housing, or the slip ring end (SRE), 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). See Figures 41–1 and 41–2.
Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Figure 41–1 A typical generator (alternator)
on a Chevrolet V-8 engine.

Figure 41–2
The end frame toward the drive belt is called
the drive-end housing and the rear section
is called the slip-ring-end housing.

Continued
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Generator Horsepower and Engine
Operation - Part 1
Many technicians are asked how much power certain accessories require.
A 100-ampere generator requires about 2 horsepower from the engine.
One horsepower is equal to 746 watts. Watts are calculated by multiplying
amperes times volts:

Power in watts 100A x 14.5V = 1450 1hp = 746 W
1450 watts is about 2 horsepower and the generator uses about 2 hp to
generate 100 A. Allowing about 20% for mechanical and electrical losses

adds another 0.4 horsepower. When someone asks how much power it
takes to produce 100 amperes from a generator, the answer is 2.4 hp.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Generator Horsepower and Engine
Operation - Part 2
Power in watts 100A x 14.5V = 1450 1hp = 746 W
Many generators delay output to prevent the engine from stumbling when
a heavy electrical load is applied. The voltage regulator or vehicle
computer is capable of gradually increasing the generator output over a
period of up to several minutes.
Though it does not sound like much, a sudden demand for 2 horsepower

from an idling engine can cause the engine to run rough or stall. The
difference in part numbers of various generators is often an indication of
the time interval over which the load is applied. Therefore, the use of the
wrong replacement generator could cause the engine to stall!

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ALTERNATOR OVERUNNING PULLEY
Purpose and Function Many generators are equipped with an
overrunning alternator pulley (OAP), called a clutch pulley.
This eliminates noise and vibration in the accessory drive belt
system, especially when at idle, when engine impulses are
transmitted to the alternator through the accessory drive belt.

The mass of the rotor of the alternator tends to want to keep
spinning, but the engine crankshaft speeds up and slows down
slightly due to the power impulses.
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. See Figure 41–3.
Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Figure 41–3 An OAP on a generator on a Chevrolet Corvette.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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A conventional drive pulley attaches to the alternator (rotor) shaft
with a nut and lock washer.
In the overrunning clutch pulley, the inner race of the clutch acts as
the nut and screws onto the shaft. Special tools are required to
remove and install this type of pulley.
Another type of alternator pulley uses a dampener spring inside plus
a one-way clutch. This unit is called an overrunning alternator
dampener (OAD).
An OAD is larger than an OAP and is used on the Pontiac G8 and
other vehicles.
See Figure 41–4.
Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition

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Figure 41–4 An overrunning alternator dampener (OAD) disassembled, showing all of its
internal parts.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Diagnosis and Service Overrunning alternator pulleys and
alternator dampeners can fail, with the most common factor the
one-way clutch.
If it fails, it can freewheel and not power the alternator or it can lock
up and not provide dampening as designed. If the charging system
is not working, the OAP or OAD could be the cause rather than a
fault in the alternator itself.
In most cases, the entire alternator assembly will be replaced
because each OAP or OAD is unique for each application and they
require special tools to remove and replace.

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Can I Install an OAP or OAD to My Alternator?
No. An alternator needs to be equipped with the proper shaft to allow the
installation of an OAP or OAD. This also means a conventional pulley
cannot be used to replace a defective overrunning alternator pulley or
dampener.

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ROTORS
The rotor creates the magnetic field of the generator and produces
a current by electromagnetic induction in the stationary stator
windings.
The generator rotor is constructed of many turns of copper wire
coated with a varnish insulation wound over an iron core. The iron
core is attached to the rotor shaft. At both ends of the rotor
windings are heavy-gauge metal plates bent over the windings
with triangular fingers called poles.
These pole fingers do not touch, but rather they alternate or
interlace as shown in Figure 41–5.
Continued
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Figure 41–5 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.

Continued
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If current flows through the rotor windings, the metal pole pieces at each end of the rotor become
electromagnets. Whether a north or a south pole magnet is created depends on the direction in which the
wire coil is wound.
Because the pole pieces are attached to each end of the rotor, one pole piece will be a north pole magnet.
The other pole piece is on the opposite end of the rotor and therefore is viewed as being wound in the
opposite direction, creating a south pole. The rotor fingers are alternating north and south magnetic poles.
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.
See Figure 41–6.

Continued
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The current necessary for the
field (rotor) windings is
conducted through carbon
brushes to the slip rings.
Maximum-rated generator
output in amperes is largely
dependent on the number
and gauge of the windings
of the rotor.
Substituting rotors from one
generator into another can
greatly affect maximum output
either positive or negative.
Figure 41–6 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.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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GENERATOR BRUSHES
The current for the field is controlled by the voltage regulator and
is conducted to the slip rings through carbon brushes.
The brushes conduct only the field current (approximately 2 to 5
amps), and therefore tend to last longer than the brushes used on a
DC generator, where all the current generated in the generator must
flow through the brushes.

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STATORS
Supported between the two halves of the generator housing are
three copper wire windings wound on a laminated metal core.
See Figure 41–7.
As the rotor revolves, its moving magnetic field induces a current
in the windings of the stator.
See Figure 41–8.

Continued
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Figure 41–7 A cutaway view of a typical AC generator (alternator)

Continued
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Figure 41–8 An exploded view of a typical generator (alternator) showing all of its internal
parts.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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