Why is the bridge rectifier output shorted when the swtich is not On.

[micky]

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.

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.

[BenignBodger]

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.

[Guv Bob]

generating

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.

[micky]

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.

[Tony Hwang]

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?

[BenignBodger]

On 6/18/2014 6:48 PM, Tony Hwang wrote: snip...

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.

[rbowman]

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.

[clare]

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.

[clare]

The motor does not require "pure" DC - and the inductance of the motor also acts as a "filter " (choke)

[micky]

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.