alt.energy.homepower

On 16 Sep 2007 15:34:25 -0400, nicksanspam@ wrote:


>>>12V fans have brushes that are slanted in the normal rotation direction
>>>in a way that lets the commutators quickly grind them up in reverse?
>
>My mechanic friend says no. Reversing should be OK.

You're mechanic friend is wrong for most automotive DC motors. The issue isn't brush
slant because most small motors don't use brush slant. The issue is brush timing.

The reaction of the field and armature magnetic fields forces the field toward the
edge of the pole. It is desirable to commutate at a zero potential point and that
means where the field is weakest. Therefore brushes are advanced over the static
timing to the new dynamic neutral position. If the motor is reversed without
re-timing the brushes, there will be heavy sparking, rapid commutator and brush wear,
high current draw, overheating and low power. The timing changes with load but since
a fan is a more or less constant load, static timing works.

Larger more expensive DC motors frequently have "interpoles", also known as
"commutation windings" to stabilize the commutation point but small cheap automotive
motors don't.

The old EV warrior electric bicycle used two Ford radiator fan motors for propulsion.
One on either side of the wheel, shafts pointing toward each other with a rubber
roller connecting the two. The roller contacted and drove the tire. These motors
came in left and right-hand rotation versions.

If one attempts to run one backwards it does everything I described above. When the
company went bankrupt I bought a box full of the motors and have used them on a
variety of small EV projects so I'm quite familiar with the characteristics.

On the web there are various descriptions of techniques people have used to reverse
these motors. For some reason there were many more of one type than the other on the
surplus market so there was a demand for reversing techniques. It looked like a lot
of work to me, for this motor uses a radial commutator and the brushes are riveted to
the end bell plus the motor is assembled with crimped joints.

John
--
John De Armond
See my website for my current email address

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Tellico Plains, Occupied TN
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