Help with Diagnosis: Bad Motor Pole?

[tehachapifan]

So, I have an N scale SW1200 that has the newer-but-essentially-same MT SW1500 mechanism under it and a Digitrax decoder. For the past couple days, it's been intermittently stalling when going from stopped to speed step 1 and mostly when switching directions. It will start moving, then stop...at which time a faint shorting-type noise can be heard coming from the loco. When this is happening, it will not accept any commands at all and the noise won't stop after returning the throttle to 0. The only thing that will stop the shorting sound is lifting one side of the loco off the rails briefly, then it may or may not run fine for a bit. Beyond speed step 1, the loco runs great (well, one time it did it at a higher speed step). This has me scratching my head a bit. A bad motor pole comes to mind but I'm not sure all the symptoms fit that. Maybe a loose piece of (conductive) debris in the motor? Thoughts? I should also mention that the loco starts heating up quite a bit when this is happening.

[tehachapifan]

...I should mention that one time this apparent short in the loco caused my DCC system to go into overload briefly (edit: maybe), but every other time it has not. Also, sometimes the loco lights will dim and flicker when this is happening and other times they will not. This loco is probably going to get the Jim Starbuck coreless motor and LokSound retrofit now, but I would still like to find the problem for sanity sake.

[peteski]

I doubt that a bad (shorting) motor would cause the DCC booster to shut down. If the decoder has its own over-current protection for the motor output, those should cut off the power to the motor. If the decoder has no internal protection I would think that the DCC booster could supply high enough current to burn up either the bridge rectifier on the decoder, or some of the transistors in the H-bridge which supplied power to the motor.

So the loco acts up just on speed step 1, but runs fine when you crank the throttle past step 1?  Flickering lights indicate a problem with track power pickup, or possibly with the decoder itself.  What exactly is heating up in the loco?  The motor or the decoder, or something else?

The more I think about it, that sounds like a short (well, maybe not a dead, zero ohms but low resistance) short somewhere, but I don't think it is in the motor itself. But maybe one of the motor leads is shorting somewhere (like the chassis)?  Or some metal debris shorting between the chassis halves?  If you can better pinpoint the source of the heat (with the shell removed to help troubleshooting), that should aid in troubleshooting.

Thinking some more, bad (open) motor pole should cause the motor to stall in certain position at very slow speeds.  I suppose that another possibility is that one of the pole winding is shorted to the metal core, then the shaft, then (if the bearings are not isolated from the motor's frame, to the motor frame itself. If the frame is then in contact with one of the chassis halves, that could cause odd behavior at very slow speeds.

[tehachapifan]

Thanks, Peteski! While I have made and installed some hardier pickup strips, These locos are the ones that have the body-mount pickup strips that require the shell to be on to run, which doesn't help with pinpointing issues. Regarding the DCC system going into overload briefly....there's a chance it might have been unrelated as I've had this happen before for various reasons. So, since it only happened the one time, maybe it would be best to uncomplicate the issue and remove that from the equation for now. Regarding some type of short between the motor and the frame, wouldn't that show signs of some sort at any/every speed step? Again, this almost exclusively happens at speed step 1 and almost always going from a stop and also almost always after a direction change.

[tehachapifan]

Well, now the problem has degraded into a full-time failure. Motor will no longer run at all, even after manually turning the poles. have everything apart and can't spot any shorts. Going to cut my losses and do the coreless motor and sound retrofit now. Gives my an excuse to go forth on that.

[peteski]

Well, now the problem has degraded into a full-time failure. Motor will no longer run at all, even after manually turning the poles. have everything apart and can't spot any shorts. Going to cut my losses and do the coreless motor and sound retrofit now. Gives my an excuse to go forth on that.

Since you have the model apart, test the motor.  If you have a DC throttle, run the motor from the track output and see how it behaves.  If you don't at least try running it using a 9V battery.  Motors are usually very robust (not many things to break in them), so it might be the decoder.

[tehachapifan]

I might do that. That said, I have had a bad run with motors lately (well, over the past 5 years or so). I think I've had about 3 or 4 motors from various mfr's become erratic or stop working in that time. One was in another (LL) SW1200 and had an almost identical issue. Swapped it with another SW1200 motor and it's been running great since (same decoder).

[peteski]

I might do that. That said, I have had a bad run with motors lately (well, over the past 5 years or so). I think I've had about 3 or 4 motors from various mfr's become erratic or stop working in that time. One was in another (LL) SW1200 and had an almost identical issue. Swapped it with another SW1200 motor and it's been running great since (same decoder).

I would love to (as my time allows) to  dissect some of those failing motors.

[mmagliaro]

The loco heating up and the other behaviors you describe really sound like a short somewhere other than the motor.
I really think you should check the motor on the bench and confirm whether it's good or bad before you do anything else.  Otherwise, you'll be spinning your wheels with other motors or another decoder. 

And I would go beyond just testing it with a 9v battery.  I'd want to check it over a range of voltages and make sure it starts and runs correctly in both directions at low voltages.  (or see if it shows this bad behavior all on its own when starting at low voltages)

[tehachapifan]

You may be right. However, I'm going to do a complete remotoring plus a new sound decoder install, leaving just the frame halves as a source of trouble...which I'll take a close look at when everything is disassembled. Not going to waste any more time testing/troubleshooting the motor until a time comes when I might want to try to use it again.

Thanks!

[peteski]

You may be right. However, I'm going to do a complete remotoring plus a new sound decoder install, leaving just the frame halves as a source of trouble...which I'll take a close look at when everything is disassembled. Not going to waste any more time testing/troubleshooting the motor until a time comes when I might want to try to use it again.

Thanks!

Whatever works for you.
But it appears that Max and I are in agreement that the motor is the least likely culprit here.  To me it is always important to determine what caused the problem (even if I decide to make other changes to the model).  But my full time job is troubleshooting things, so I might be a bit more inquisitive than others.

[mmagliaro]

... because problems that go away by themselves
     always come BACK by themselves! ...
Not knowing the source of the problem can really hit you in the rear end later.
Besides, how long would it take to put some clip leads on that motor and just fool around with it at different speeds and voltages to see what it does?  If it just plain works, well then, okay.  The problem is a short somewhere else.
If it has problems, then, okay, the problem was the motor.  But either way, you have more information, which is always good.

[tehachapifan]

All great points! I'll mess around with it in the next few days and see what happens.