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alternators and relays

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  • Mark Kinsler
    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
    Message 1 of 1 , Nov 5, 2001
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      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
      voltage variations.

      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.

      M Kinsler
      512 E Mulberry St. Lancaster, Ohio USA 740 687 6368

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