Motor Casings Contacting Frame: Potential Short Source for Decoders?

[tehachapifan]

This is probably another "it depends" type question, but do the motor casings themselves ever need to be isolated from the frame in addition to the brush leads? I'm about to tackle my first non-DCC ready install in an older ConCor SW1500 frame that has a 5 pole motor (not the can motor). My guess is they do not need to be isolated, as a short in DCC seems like it would be a short in DC too....although I suppose the casing could be in contact with one side of the frame in DC and be OK.

[Ken Rice]

As long as the motor brushes aren't electrically connected to anything but the decoder you'll be fine.

[peteski]

Like Ken stated, today's motors have both brushes in a plastic housing, insulated from the rest of the motor. In the earlier days sometimes one of the brushes was electrically connected with the motor's housing. Sometimes the connection was created on purpose (many Rivarossi motors had a metal contact installed to connect one of the brushes to the motor's housing).  That was done because in the early locos the solid metal frame was electrically connected to the electric pickup on one side of the model.

If you have a modern motor which shows any connectivity (normally is is an open circuit) between the metal motor housing and the brushes that would mean that one of the windings inside the motor is shorting out to the metal armature.  That would be a bad thing (and I have never encountered this).

[victor miranda]

Hi tehachapifan,
I think not really needed.
when you are installing check that the brushes have no connection to the motor case.

ummmm.
when you can't find the cause of a short in a loco....

Decoders have no way to protect themselves against motor to track shorts
and often the decoder will fail in that case.
As peteski points out if a winding breaks down it can short to the frame.
often the motor will run poorly. the results of that kind of failure can be a little costly.

I tend to put a layer of kapton tape in the kato Mikado
because I think that motor is a hair from a short in any case.

most motors are isolated from the frame so you don't need to insulate.

victor