<< Additionally, can you explain to me how the electric stop
 >  > > mechanism on an
 >  > > > electric lawn mower works? I have an electric mower and it kills
 >  > > the blade
 >  > > > as soon as the power is cut. Could such a circuit be easily and
 >  > > cheaply
 >  > > > adapted to an after-market electric cooling fan?
 >  > > >
 >  > > > Barry Gardner
 >  > > > Wheaton, IL >>

The two I have repaired have been powered by permanent magnet DC motors,
running off of a full wave bridge. The stop mechanism is just the brushes
being shorted (shunted) by the off-on switch (double pole double throw) in
the off position.
If you turn the blades, the motor becomes a generator and produces current
that is in reverse polarity to the normal direction of rotation of the
blades. (the left hand rule for generators) This tends to make the blade
appear to be locked. Like the brakes on a slot car. The motor is not
mechanically locked, and will turn slowly.
If you let the fan blades motor from air flow through the radiator, this will
produce some amount of DC current. Can you use an extra source of DC power?

Lynn E Hanover

 
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