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.

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.

"micky" wrote in message ...
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.

Be careful working on this without the wiring diagram or schematic for this exact model. You probably know this, but be sure to unplug the AC cord and remove the blade first.

On Wed, 18 Jun 2014 15:44:45 -0400, BenignBodger
wrote:

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.

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
it anyway.)**

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
wasn't it


** 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
t hem.

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.

micky wrote:
On Wed, 18 Jun 2014 15:44:45 -0400, BenignBodger
wrote:

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.

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
it anyway.)**

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
wasn't it


** 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
t hem.

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.

Hi,
I think that shown is block diagram(simplified version) Full wave
rectifier bridge does not produce pure DC, it has to be further
filtered. There must be a filtering caps in the mower too. NO?

On 6/18/2014 6:48 PM, Tony Hwang wrote:
snip...
Hi,
I think that shown is block diagram(simplified version) Full wave rectifier
bridge does not produce pure DC, it has to be further filtered. There must
be a filtering caps in the mower too. NO?

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.

micky wrote:

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
wasn't it

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
resistors.

On Wed, 18 Jun 2014 13:21:50 -0400, 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.
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
motor leads.

On Wed, 18 Jun 2014 16:48:45 -0600, Tony Hwang
wrote:

micky wrote:
On Wed, 18 Jun 2014 15:44:45 -0400, BenignBodger
wrote:

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.

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
it anyway.)**

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
wasn't it


** 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
t hem.

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.

Hi,
I think that shown is block diagram(simplified version) Full wave
rectifier bridge does not produce pure DC, it has to be further
filtered. There must be a filtering caps in the mower too. NO?
The motor does not require "pure" DC - and the inductance of the
motor also acts as a "filter " (choke)

On Wed, 18 Jun 2014 16:48:45 -0600, Tony Hwang wrote:


** 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
t hem.

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.

Hi,
I think that shown is block diagram(simplified version) Full wave
rectifier bridge does not produce pure DC, it has to be further
filtered. There must be a filtering caps in the mower too. NO?

No. I took apart one of the three this evening, and no caps at all. It
was just like the drawing.

I guess the spinning blade and armature are like a flywheel, evening out
the spinning.

In BenignBodger writes:

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.

Ah, you've just answered a related question I had with my (bought
used a year ago) Sears rechargeable battery mower.

(4 by 12V batteries in series - 48 volt)

The batteries caught on fire (yes, really) because.. (insert
a forehead slap here) the wires connecting the batteries
and leading off from them used... friction/slide on tabs.

A lawnmower is pretty much the poster child for vibration.

One wire came just a bit loose, was sparking... hot enough
to set that battery corner on fire...

No major damage, ust that first battery and the one next
to it, and also... the wiring between them and above.

So I put it back together with new batteries but there was
this one "extra" wire I sticking out that I couldn't
figure out where to place. Anyway, I turned on the unit
and it worked fine. Charges up a-ok, too. (oh, I
left the wire in place and capped off the open end).

But now that you've written your note, I realized
that, eyup, the blade keeps spinning a lot longer
now when I release the handle.

- now have to figure out where that wire should have
gone, or maybe just live with it now as is...

--
__________________________________________________ ___
Knowledge may be power, but communications is the key

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