RailPro > RailPro Specific Help & Discussion

Motor continuity/short problem

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G8B4Life:
We have discussed this a few times here, searching Motor Over Current would have given you a few threads but none of them came to any closure on the reason for the OP but there was some good discussion on possible reasons.

Anyway, Motor Over Peak Current does not automatically mean a short. A short might give that error but I think the "Motor Overload" error is given for a short instead of Motor Over Peak Current and neither should show until you apply power to the motor so you are sure it was as soon as you placed the loco onto the track? It's also strange that you couldn't set the Motor Full Load Current.

It was pointed out by another member in one of the other threads that trying to measure the resistance across a motor isn't going to tell you anything useful as you'll get wildly varying results.

To try and work out what was going on you'll need to test the old motor, preferably cleaning it first with some contact cleaner and then measuring the current draw with your multimeter while running on pure DC.

As for things connected across the motor as KB02 suggested, if you found a resistor connected across the motor I'd be worried about whoever thought to do that and you'd need to remove it but it is possible to find capacitors and inductors wired across the motor. These should have no effect on current draw but are best removed as it can interfere with motor operation.

- Tim

Pilar Valley Railway:
Tim, I did search that exact thing with no results to my question. As I already said, there was nothing hooked to the motor as far as resistors, capacitors or whatever. YES, it did happen as soon as I put it on the test track. This is not my first install like I said, I've did at least a hundred of them.

This engine was brand new but run on DC for a bit until I got around to installing RailPro in it. So no one has been in it changing or adding anything. I was the first one to open the shell.

I did check the ohms but I was not talking about that. I was just checking continuity between the 2 terminals on the motor.

gregeusa:
Kenny, I did understand your post, but there are several things going on here, and I did not wish to go too crazy on speculation, but to try to elminate the things that do not make sense, i.e. "red herrings".

I'll give more detailed responses in line below.



--- Quote from: Pilar Valley Railway on March 20, 2022, 09:47:26 PM ---I have searched on here for this topic with no luck.

I just ran into this problem after about a hundred RailPro installs in all different makes and models of engines.

I installed an LM3S in a Stewart AS-16. As soon as I put the engine on the test track to program it the warning ⚠️ came up for "motor over peak current". I couldn't even set the load.
>>>>>> so that would indicate either something has shorted in the motor, or that you have somehow caused excessive load to the motor driver. Most of these designs are "H bridge" with 4 FETs, and if you have some errant connection between an input to the H bridge and an output of the H bridge.

I checked to make sure the frame and motor were isolated,
>>>>> so that eliminates "H bridge problems"
 made sure all wire connections were good. Everything was as it should be. Tried it again and the same problem.
>>>> that would logically point to the motor of course

So I unhooked the motor and removed it. Tried just the motor on DC and it worked great and before this install the engine ran on DC for a while.
>>>>> so now it seems to make no sense, although, it does indicate the AVERAGE current seems ok, even though you do not report current measurements here, which would help. Now I would be thinking perhaps either excessive current draw, or more likely one segment/pole has shorted windings, so the peak power could be over, but the average ok.

So I took a multimeter and set it to continuity (the beep), and put a lead on each wire of the motor as it was sitting on my bench unhooked from everything. The motor leads had continuity between the positive and negative leads. If I'm thinking right that means the motor has a short.
>>>>> well that is not true, continuity is what? how many ohms? Motors have a low resistance when static/unpowered. what would make sense is to read the various poles individually with an ohmmeter, and make sure they are all very close. Continuity is just that, current flowed, but won't tell you the difference between a shorted pole and the normal low resistance of an individual pole.

So I had a brand new Atlas motor which has never been installed. I tested this motor in the same manner. It also had the same result. I then tested 2 different brand new Athearn motors and they DID NOT have electrical continuity. I then tested a couple more odd ball used motors I have lying around. Those also did not have continuity.
>>>>> again, you need to read the actual ohms, not "continuity" which can be differently defined for different meters. When a motor is new, and the brushes have not full seated, the resistance can be higher. I would have run these motors for 15 minutes or so, I would bet the resistance would change.

I ended up installing one of the new Athearn motors in the engine and everything is working fine.
>>>> as above, they were probably fine.


So my question is.... Has anyone else experienced this problem??
>>> yes I have seen exactly what you saw. What is common in motors is that one way to make them cheaper is to use a poorer insulation on the windings. When the insulation breaks down (mechanically, heat, or just poor coating thickness), the windings can short to each other. It's not immediately apparent what has happened, because that damaged "pole/winding" will draw more current to compensate. So the motor might run OK on DC with a slow acting circuit breaker, or lots of current, but every time that damaged "pole" came around, the peak current would be way higher, and a solid state current monitor might flag it.

So, I have seen a situation like you describe, and it has been what I have described. (please use ohms next time too! you might have found the issue more specifically, but you did determine the motor was no good)


--- End quote ---

Pilar Valley Railway:
Greg, thank you. That is what I was wondering, if a motor could work on DC while being shorted out and not on DCC/RailPro. That makes sense.

Also...I did check the motor with ohms and it tested fine. I did not find out that it was shorted out until just checking the continuity.

gregeusa:
When you check with an ohmmeter, test it on different segments (of the commutator), i.e. measure the ohms, then turn the motor (usually 1/5 of a revolution).

You do not want a wide variation in resistance (ohms), and never zero ohms on segments.

Note that when turning it slowly, it won't always be perfect, the brushes are not perfect electrically

Greg

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