"Mr Macaw" wrote in message news
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.

Yes, that has always been obvious. The problem is that coil is a dead short.

I'll check the brush to brush resistances again then try new brushes (even
though they look ok).

The problem is that that coil is a dead short.

On Tue, 17 May 2016 11:18:26 +0100, Rod Speed wrote:



"Mr Macaw" wrote in message news
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.

Yes, that has always been obvious. The problem is that coil is a dead short.

I'll check the brush to brush resistances again then try new brushes (even
though they look ok).

The problem is that that coil is a dead short.

But the coil is not connected directly to the brushes at any point.

--
"Boy, will I give YOU a haircut!" said Tom barbarously.

"Mr Macaw" wrote in message news
On Tue, 17 May 2016 11:18:26 +0100, Rod Speed
wrote:



"Mr Macaw" wrote in message
news
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.

Yes, that has always been obvious. The problem is that coil is a dead
short.

I'll check the brush to brush resistances again then try new brushes
(even
though they look ok).

The problem is that that coil is a dead short.

But the coil is not connected directly to the brushes at any point.

It must do before it failed unless you mean you have disconnected it now.

If you mean you have disconnected it now, it's the lack of a coil
in that position of the rotation that is producing the arcing now.

But from memory you said it was arcing badly and that is why you
decided that it had failed and even when you disconnected the coil
that had failed, it still kept arcing as badly as it did just after it had
failed. In which case it's the failed coil that is why it is arcing.

On Wed, 18 May 2016 05:14:29 +0100, Rod Speed wrote:



"Mr Macaw" wrote in message news
On Tue, 17 May 2016 11:18:26 +0100, Rod Speed
wrote:



"Mr Macaw" wrote in message
news 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.

Yes, that has always been obvious. The problem is that coil is a dead
short.

I'll check the brush to brush resistances again then try new brushes
(even
though they look ok).

The problem is that that coil is a dead short.

But the coil is not connected directly to the brushes at any point.

It must do before it failed unless you mean you have disconnected it now.

Several coils are in series connected to the brushes.

If you mean you have disconnected it now, it's the lack of a coil
in that position of the rotation that is producing the arcing now.

But from memory you said it was arcing badly and that is why you
decided that it had failed and even when you disconnected the coil
that had failed, it still kept arcing as badly as it did just after it had
failed. In which case it's the failed coil that is why it is arcing.

--
1 in 6 men in Iowa have had sex with a chicken -- The Kinsey Report