Circular Saw failure

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I pulled my failed circular saw apart to see whether it was repairable.

Seems a rather dramatic failure of the commutator.

https://www.dropbox.com/s/gzp9tqd14nd9ot8/20170525_200615.jpg?dl=0

So, no :(

Sylvia.


Re: Circular Saw failure
On 25-May-17 6:11 PM, Sylvia Else wrote:
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Antifungal cream fixed my circular sore but I doubt it will do much for  
that commutator.

Re: Circular Saw failure
 Aussie wrote:

---------------

Sylvia Else wrote:
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 **  ROTFLMAO  ...  




....  Phil  

Re: Circular Saw failure
Sylvia Else wrote:
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Back in the day we used to repair or replace commutators but not in this  
day and age especially ones with this method of construction.

I made this one with a lathe and files over 50 years ago as part of a  
course, bit tarnished wit age.
http://imgur.com/a/SHLw1

Re: Circular Saw failure
On 25/05/2017 10:35 PM, FMurtz wrote:
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Are you an armature winder by trade Fred?  Maybe an electrical fitter?


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Xeno

Re: Circular Saw failure
Xeno wrote:
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Not an armature winder but have done so.

Re: Circular Saw failure
On 25/05/2017 10:37 PM, FMurtz wrote:
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More of an amateur winder then? :-)

Re: Circular Saw failure
On 25-May-17 11:27 PM, Clocky wrote:
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Very good!

Re: Circular Saw failure
On 26/05/2017 12:37 AM, FMurtz wrote:
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It was the only reason you would be tinkering with commutators I would  
have thought. My electrical mate at the railways in Melbourne started in  
the trade of armature winder (motor winder?) but gravitated to the  
electrical fitting side of things. That's how he ended up working on the  
metro trains in Vic. I suppose an electrical fitter would be tinkering  
with commutators a lot too.


--  

Xeno

Re: Circular Saw failure
On 26/05/2017 3:47 PM, Xeno wrote:
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  Especially on the metro trains. Thousands of commutatators using the  
rail network every morning and evening.

Re: Circular Saw failure
On 5/25/17 8:35 PM, FMurtz wrote:
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Ahhh the old mica insulation. Things have changed all right.

Re: Circular Saw failure
On 25/05/2017 6:11 PM, Sylvia Else wrote:
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Yup, she's cactus.

Re: Circular Saw failure
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true.


false.  ( see "back EMF" )

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nope.

you get electric braking, but not electric energy recovery  



In general motors are like capacitors, energy goes in and out as
currents and is stored as a voltage (called back emf) in a permanent
magnet motor back emf is proportional to the spin speed and direction.

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Re: Circular Saw failure
On 29/05/17 06:09, Jasen Betts wrote:
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In general, you are wrong. There are two kinds of stored energy,
rotational and magnetic. Different approaches are used to recover
them, and depend on the type of motor. For the most common PM
type, the armature has enough resistance that you can short it
and stop it quickly - which is what Phil was talking about.

With a very high-performance (high L/R ratio) motor, shorting it
keeps the current circulating in the motor inductance, not being
lost in its resistance, so you don't get strong braking and it
won't stop very quickly.

So instead, you short it to allow the rotational energy to pump
up the current (against the motor inductance) then switch to route
the current into the battery. Because this is pushing current
against a significant voltage, it does work (extracts energy)
(based on L*I^2), but you're only extracting the electromagnetic
energy. So the current falls, and to get more of the rotational
energy out, you commutate back to a short circuit until the current
rises again, pumping more energy into the magnetic field, which you
then extract into the battery.

Regeneration requires this kind of commutation.

Clifford Heath

Re: Circular Saw failure
On 29/05/2017 6:09 AM, Jasen Betts wrote:
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Back EMF results from changing magnetic flux as a result of the rotor's  
motion. It depends on that change, and has nothing to do with the  
applied voltage. It's only referred to as "back EMF" because in a motor  
that's running normally, it is opposite to the applied voltage. If you  
instantaneous reverse the applied voltage, the back EMF doesn't change,  
because the rotor is still rotating in the same direction. So the back  
EMF now adds to the applied voltage.

Sylvia.


Re: Circular Saw failure
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Voltage is speed. Current it torque. It stops fast.
(this only works with permanent magnet motors)

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