alternators and relays
- A bit of clarification on the operation of an automobile alternator. The
outer coils are called the stator, because they are stationary, i.e., they
do not rotate. There are three of these, albeit very intertwined, and
they're connected through a three-phase silicon rectifier to your car's
The rotor that rotates within the stator coils contains a single coil. This
is an electromagnet that's supplied with DC from your car's battery. If the
current through this rotor coil (also called the 'field coil' is turned off,
no voltage will be developed across the stator (also called the 'armature
coils.') If the current through the rotor coil is turned on, the resulting
magnetic field sweeps through the stator coils and develops a voltage across
them, thus charging the battery.
The voltage regulator turns the rotor coil current on and off as
appropriate: when the battery voltage falls below a certain value, the rotor
current is turned on, thus charging the battery. Once the battery voltage
rises to a sufficient voltage, the regulator shuts the rotor current off.
The rotor current is quite small--only a few amperes--so only a small switch
is necessary to turn it on and off.
There are other variations in voltage regulator designs, but this is roughly
how the arrangement works. Big electric power generators work in a similar
manner, but in these the rotor current is changed in very small increments
in order to keep the voltage of the stator coils constant. Since the
automobile alternator is used exclusively to charge a battery, it's only
necessary to turn it either ON or OFF: the battery itself will even out the
Electromechanical relays are, in general, a big pain. Nobody, for example,
missed them when they were replaced by solid-state devices in automobile
voltage regulators. You're forever fooling around with the electrical
contacts: cleaning, polishing, bending, etc. Relays are slow, they gather
dirt, they're noisy, the coils burn out, and they're expensive.
Having said all that, I should add that electromechanical relays are a great
deal of fun to fool around with while you're learning electronics. I love
them for one project or another: they give a satisfying click so you know
they've been energized, their contacts can take a tremendous amount of
abuse--say, from an inductive circuit--and a multiple-contact relay can
switch several circuits at once and enable you to make some extraordinarily
complex stuff with only a few bits of wire and a lot of thought.
My favorite relay trick involves the use of a single-pole, single-throw 12v
relay. Connect in parallel the contacts, the coil, and an automotive
ignition capacitor. Connect this parallel combination in series with a car
battery and an automobile ignition coil. Result: a 15kV high-frequency
power supply that won't kill you. Light up fluorescent lights, make sparks,
and generally have fun. I use one in my How Things Work demonstrations to
run spark plugs and create radio signals.
512 E Mulberry St. Lancaster, Ohio USA 740 687 6368
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