Mr Macaw wrote:
On Mon, 16 May 2016 03:37:19 +0100, Rod Speed
wrote:



"Mr Macaw" wrote in message
news
On Sat, 14 May 2016 04:33:16 +0100, Rod Speed
wrote:



"Mr Macaw" wrote in message
news On Fri, 13 May 2016 06:44:02 +0100, Rod Speed

wrote:



"Mr Macaw" wrote in message
news On Sun, 08 May 2016 23:03:48 +0100, Fredxxx
wrote:

On 08/05/2016 15:23, Mr Macaw wrote:
My jigsaw started sparking and throwing out smoke, so I took it to
bits,
found a shorted coil in the motor, and disconnected it. But it's
still
doing this (sew video). Is there anything that can be done, or
does
it
go in the bucket?

https://www.dropbox.com/s/75z1zyve3jajgg7/Saw.AVI?dl=0

Consider the armature approximating a rotating transformer.

One segment that has a short is like shorting a winding in a
transformer.

Either get a new armature, or get a new jigsaw. I would favour the
latter.

Couldn't I short a winding 180 degrees round from the already
shorted
one?

You could, but you'd still get that arcing.

Why?

Because that is how those motors work.

Changing 12 coils in series to 11 coils in series doesn't change much
surely?

It isn't 11 coils in series. A much smaller number is electrically
connected
at a particular
time and the arcing is due to the fact that the one coil is a sort
currently.

I can't see how they're wired, but I can tell you that the resistance
between the brushes is always around 15 ohms, and 1.1 ohm between
neighbouring contacts on the commutator.

If that was true in all positions of what rotates, you wouldn't be
getting
arcing.

Indeed, I don't understand what's causing the arcing. I'll check the
brush to brush resistances again then try new brushes (even though they
look ok).

You will not be able to tell what is going on with dc resistance checks.
The problem seems to be shorted turns and even if you disconnect that
part of the winding the short is still there and high current will flow
when that part of the winding has a current induced in it.

Either just give up and throw it away or use it as an exercise to
properly understand how motors and their fault conditions work.

If you can find someone with a machine called a "growler" this can be
used to show shorted turns. Google.