On 6/18/2014 1:21 PM, micky wrote:
The electric lawn mower I bought over the weekend had its handle folded
over and something heavy piled on it, and it crushed and cut some of the
electric cord going from the on/off switch to the motor, specifically
the red wire.

Red seems pretty important so I was surprised the mower started, for the
1 second I let it run.

http://nickviera.com/electrical/lawn_mower/bandd/ If you go a third
of the way down there is a diagram of a typical B&Decker lawn mower.

It turns out the red wire (and the blue one) connect the output of the
bridge rectifier WHEN THE POWER SWITCH IS ALL THE WAY OFF. They also
connect the inputs of the motor.

How important is this? If these two places are not shorted out, when
power is removed (when the switch has left the on position) but the
blade is still spinning, will it make electricity? If so the
commutator will cause it to make DC.

(Side question. Will the what was the positive input be the positive
output or the negative output, when the spinning armature is generating
electricity?)

Assuming there is voltage, regardless of which direction the voltage is,
it will make it through the bridge rectifier, and up the orange and
black wires. Does that matter?


If none of this matters, why are there red and blue wires in the first
place? They must have had a motive.


If the drawing is exactly what you have then the red and blue wire have
nothing to do with the motor running -- you could remove them completely
and the motor would still run. Problem is that those two wires 'apply the
brakes' on the motor when the switch is released. Basically the permanent
magnet DC motor acts a generator when unpowered and spinning and shorting
it out sucks up the power it is making bringing it to a semi-screeching
halt. Operating with a freewheeling motor/blade would make the mower less
safe and I certainly wouldn't do it although it would still cut grass.