Automotive alternator
PARTS AND MATERIALS
Old alternators may be obtained for low
prices at automobile wrecking yards. Many yards have
alternators already removed from the automobile, for your
convenience. I do not recommend paying full price for
a new alternator, as used units cost far less money and
function just as well for the purposes of this experiment.
I highly recommend using a Delco-Remy brand
of alternator. This is the type used on General Motors (GMC,
Chevrolet, Cadillac, Buick, Oldsmobile) vehicles. One
particular model has been produced by Delco-Remy since the
early 1960's with little design change. It is a very
common unit to locate in a wrecking yard, and very easy to
work with.
If you obtain two alternators, you may use
one as a generator and the other as a motor. The steps
needed to prepare an alternator as a three-phase generator
and as a three-phase motor are the same.
CROSS-REFERENCES
Lessons In Electric Circuits, Volume
1, chapter 14: "Magnetism and Electromagnetism"
Lessons In Electric Circuits, Volume
2, chapter 10: "Polyphase AC Circuits"
LEARNING OBJECTIVES
-
Effects of electromagnetism
-
Effects of electromagnetic induction
-
Construction of real electromagnetic
machines
-
Construction and application of
three-phase windings
SCHEMATIC DIAGRAM
An automotive alternator is a three-phase
generator with a built-in rectifier circuit consisting of
six diodes. As the sheave (most people call it a "pulley")
is rotated by a belt connected to the automobile engine's
crankshaft, a magnet is spun past a stationary set of
three-phase windings (called the stator), usually
connected in a Y configuration. The spinning magnet is
actually an electromagnet, not a permanent magnet.
Alternators are designed this way so that the magnetic field
strength can be controlled, in order that output voltage may
be controlled independently of rotor speed. This rotor
magnet coil (called the field coil, or simply
field) is energized by battery power, so that it takes a
small amount of electrical power input to the alternator to
get it to generate a lot of output power.
Electrical power is conducted to the
rotating field coil through a pair of copper "slip rings"
mounted concentrically on the shaft, contacted by stationary
carbon "brushes." The brushes are held in firm contact with
the slip rings by spring pressure.
Many modern alternators are equipped with
built-in "regulator" circuits that automatically switch
battery power on and off to the rotor coil to regulate
output voltage. This circuit, if present in the alternator
you choose for the experiment, is unnecessary and will only
impede your study if left in place. Feel free to "surgically
remove" it, just make sure you leave access to the brush
terminals so that you can power the field coil with the
alternator fully assembled.
ILLUSTRATION
INSTRUCTIONS
First, consult an automotive repair manual
on the specific details of your alternator. The
documentation provided in the book you're reading now is as
general as possible to accommodate different brands of
alternators. You may need more specific information, and a
service manual is the best place to obtain it.
For this experiment, you'll be connecting
wires to the coils inside the alternator and extending them
outside the alternator case, for easy connection to test
equipment and circuits. Unfortunately, the connection
terminals provided by the manufacturer won't suit our needs
here, so you need to make your own connections.
Disassemble the unit and locate terminals
for connecting to the two carbon brushes. Solder a pair of
wires to these terminals (at least 20 gauge in size) and
extend these wires through vent holes in the alternator
case, making sure they won't get snagged on the spinning
rotor when the alternator is re-assembled and used.
Locate the three-phase line connections
coming from the stator windings and connect wires to them as
well, extending these wires outside the alternator case
through some vent holes. Use the largest gauge wire that is
convenient to work with for these wires, as they may be
carrying substantial current. As with the field wires, route
them in such a way that the rotor will turn freely with the
alternator reassembled. The stator winding line terminals
are easy to locate: the three of them connect to three
terminals on the diode assembly, usually with "ring-lug"
terminals soldered to the ends of the wires.
I recommend that you solder ring-lug
terminals to your wires, and attach them underneath the
terminal nuts along with the stator wire ends, so that each
diode block terminal is securing two ring lugs.
Re-assemble the alternator, taking care to
secure the carbon brushes in a retracted position so that
the rotor doesn't damage them upon re-insertion. On
Delco-Remy alternators, a small hole is provided on the back
case half, and also at the front of the brush holder
assembly, through which a paper clip or thin-gauge wire may
be inserted to hold the brushes back against their spring
pressure. Consult the service manual for more details on
alternator assembly.
When the alternator has been assembled, try
spinning the shaft and listen for any sounds indicative of
colliding parts or snagged wires. If there is any such
trouble, take it apart again and correct whatever is wrong.
If and when it spins freely as it should,
connect the two "field" wires to a 6-volt battery. Connect
an voltmeter to any two of the three-phase line connections:
With the multimeter set to the "DC volts"
function, slowly rotate the alternator shaft. The
voltmeter reading should alternate between positive and
negative as the shaft it turned: a demonstration of very
slow alternating voltage (AC voltage) being generated. If
this test is successful, switch the multimeter to the "AC
volts" setting and try again. Try spinning the shaft slow
and fast, comparing voltmeter readings between the two
conditions.
Short-circuit any two of the three-phase
line wires and try spinning the alternator. What you should
notice is that the alternator shaft becomes more difficult
to spin. The heavy electrical load you've created via the
short circuit causes a heavy mechanical load on the
alternator, as mechanical energy is converted into
electrical energy.
Now, try connecting 12 volts DC to the field
wires. Repeat the DC voltmeter, AC voltmeter, and
short-circuit tests described above. What difference(s) do
you notice?
Find some sort of polarity-insensitive 6 or
12 volts loads, such as small incandescent lamps, and
connect them to the three-phase line wires. Wrap a thin rope
or heavy string around the groove of the sheave ("pulley")
and spin the alternator rapidly, and the loads should
function.
If you have a second alternator, modify it
as you modified the first one, connecting five of your own
wires to the field brushes and stator line terminals,
respectively. You can then use it as a three-phase motor,
powered by the first alternator.
Connect each of the three-phase line wires
of the first alternator to the respective wires of the
second alternator. Connect the field wires of one alternator
to a 6 volt battery. This alternator will be the generator.
Wrap rope around the sheave in preparation to spin it. Take
the two field wires of the second alternator and short them
together. This alternator will be the motor:
Spin the generator shaft while watching the
motor shaft's rotation. Try reversing any two of the
three-phase line connections between the two units and spin
the generator again. What is different this time?
Connect the field wires of the motor unit to
the a 6 volt battery (you may parallel-connect this field
with the field of the generator unit, across the same
battery terminals, if the battery is strong enough to
deliver the several amps of current both coils will draw
together). This will magnetize the rotor of the motor. Try
spinning the generator again and note any differences in
operation.
In the first motor setup, where the field
wires were simple shorted together, the motor was
functioning as an induction motor. In the second
setup, where the motor's rotor was magnetized, it functioned
as a synchronous motor.
If you are feeling particularly ambitious
and are skilled in metal fabrication techniques, you may
make your own high-power generator platform by connecting
the modified alternator to a bicycle. I've built an
arrangement that looks like this:
The rear wheel drives the generator sheave
with a long v-belt. This belt also supports the rear
of the bicycle, maintaining a constant tension when a rider
is pedaling the bicycle. The generator hangs from a steel
support structure (I used welded 2-inch square tubing, but a
frame could be made out of lumber). Not only is this machine
practical, but it is reliable enough to be used as an
exercise machine, and it is inexpensive to make:
You can see a bank of three 12-volt "RV"
light bulbs behind the bicycle unit (in the lower-left
corner of the photograph), which I use for a load when
riding the bicycle as an exercise machine. A set of three
switches is mounted at the front of the bicycle, where I can
turn loads on and off while riding.
By rectifying the three-phase AC power
produced, it is possible to have the alternator power its
own field coil with DC voltage, eliminating the need for a
battery. However, some independent source of DC voltage will
still be necessary for start-up, as the field coil must be
energized before any AC power can be produced. |
