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
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.
Thanks, and thanks, Guv, for the warnings. I'll fix it then. (Since
posting, I noticed the orange wire is partially cut, so I'd have to fix
Your description reminds me of the 7th grade electronics club, when
someone found a couple generators, probably what were used in crank
telephones, and we played with them. IIRC, They were easy to turn when
not connected to anything, but when one of us put our fingers across the
output, they were much harder to turn.
I thought it would be the other way around, that with no connections,
electrons would pile up at one output screw and somehow make it harder
to turn, and with a connection it woudl be easier. But that clearly
** If it weren't so close to the housing, it would be easy, but
otoh, I have one or two broken B&Decker mowers so I can take the part
from them if that turns out to be easier than fixing the wires it has.
I have to strip the other one or two lawn mowers anyhow, before trashing
I say "one or two" because tThe original broken mower might just need a
new bridge rectifier. It still runs, just slowly, maybe on one side of
the AC curve and one pair of rectfiers.
For this sort of application all that is required is that the voltage be
DC. Having no filtering simply means that the motor is given a hefty kick
120 times per second rather than the more consistent, but smaller, push
that a filtered supply would provide. In this case the inertia of the the
rotating motor and blade are their own filter.
Nope. In diesel-electric locomotives, when they are stopping the traction
motors are connected to a large resistor bank to aid in braking. Some of the
more modern systems store the energy or feed it back into the power system
if it's an electrifed train but the old junk I worked on just made very hot
Dynamic braking. When the power is shut off, the motor is shorted to
stop the motor almost instantly. Shutting off power without shorting
the motor allows the motor to freewheel and produces high CEMF on the
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