breedon@ucdhep.ucdavis.edu writes
>I want to add one bit more to Dan Masters' clear explanation to the
following
>question posed by Warren.Allen@infores.com:
>> Here goes: A generator consists of an
>> armature (a metal shaft covered with wires) rotating between
>> two magnets. There's no physical contact (other than
>> brushes). How, therefore, can increased load make the
>> armature harder to rotate?
>I do not know much about generators in particular, but I am a physicist so
I do
>know about magnetic fields. The electric polarity of the aramature
changes
>every time it rotates so that the magnetic field induced by the current
also
>switches so that it always opposes that of the permanent magnets (in other
>words N faces N and S faces S). As the current in the armature increases
due
>to a load, the magnetic field it creates also increases, requiring greater
>force to bring the opposing magnetic poles into proximity. Intuitively,
think
>of trying to bring the N poles of two magnets together. Even though there
is
>no physical contact, if you try it again with a stronger magnet, it is
going to
>require more force. This is why the aramature is harder to turn when it
is
>required to generate more current.
>Richard Breedon
>Dept. of Physics, University of California, Davis
>'74 TVR 2500M (TR6 engine)
>'36 Ford V8
Richard,
I don't know much about Physics (they help relieve constipation, don't
they?)
but your basic theory is correct but replace permanent magnets with
electromagnets and also note that an alternator is a generator turned
inside out where the field poles rotate around and the electricity
is generated in the stator.
Electromagnets are used because the voltage would vary wildly with
the varying engine speed if permanent magnets were used and I
could write 4 paragraphs about why an alternator works better than
a generator.
Dave Massey, BSEE
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