Re: (ET) corded mower motor

["Bob Murcek" <rmurcek geisinger edu>]

I knew there would be someone out there with the exact info!

>>> Rhett George <rtg ee duke edu> 5/13/2005 12:50:51 PM >>>
 - Greetings -

Bob Murcek asked about permag motors in corded lawn mowers.  Methinks
there
are two reasons for using the permag motor with its bridge rectifier.

  First - apparently lower cost in that the magnets are cheaper to mold
and
        magnetize  than the cost of winding a field electromagnetic. 
The 
        1989 Ford (Mazda) Probe I used a permag starter motor.  This is
a 
        high torque application.

  Second - in coasting, back EMF is generated.  By shorting this out,
i.e.,
        shorting the motor when the switch is open, a large torque in
the 
        direction of slowing the motor is developed.  This is simpler
than 
        using a friction brake with some linkage to apply it when the
blade
        should stop.

With regard to the induction motor, the torque vs speed curve is not
favor-
able when starting with the blade in tall grass.  It does not have
overload
characteristics that yield performance most users want in a mower.

Hope this helps.

                                        Rhett

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