"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.