Re-motoring a few of my Athearn RTR engines. I am using older BB box motors, installing the correct Hex head flywheels. I am running DCC.
One engine I have just plain seems cursed, SP SD-50. It ate like 3 decoders before I said time to change motors. I might add that was a few years ago, and did not have a Multimeter.
Installed a replacement motor, motor was noisy but amp readings where good. Installed in the chassis with drive shafts hooked up it pulled pulled .50 amps with me holding it in places and stalled at .90 amps.
I speed matched it to it’s consist and started running it. Ran well for about say 6 hours and motor did start quieting down. Yesterday I started the train up and it was moving about 1/3 of the speed it should. Broke up the consist, the 2 Dash 9’s where still running at it’s normal speed, SD-50 on the hand was moving at about a 1/2 speed at best.
Today I opened the engine and saw nothing odd. Went ahead and polished the commutator, oiled the bearings, lubed the trucks and cleaned the brass wheel bearings. Checked the amps the motor was pulling and was still .50 and stall at .90 amps.
Only thing I can think of is I lost a winding in the armature. What is the correct way to test for a bad winding? I have some extra armature’s. Like to test the current one, and the extras I have use on in the motor.
By the way, I have read the Athearn Tune Up guide. Here is a link.
To check your armature windings, measure the resistance across the entire motor, from one brush to the other brush. Then rotate the armature by hand and watch the meter. As the armature rotates, the bushes connect to one winding after another. A good winding will read a few ohms, open or short is a bad winding. For open windings, inspect the solder joints near the commutator. Windings often break right at the solder joint, and can be repaired with solder.
In addition to open and short, a winding’s insulation can fail, which shorts out turns. Windings with shorted turns draw more current, which tends to heat them, which tends to cook the insulation resulting in yet more shorted turns. Some times you can see shorted turns with an ohmmeter and sometimes you can’t. The winding resistance is low to start with, and a few shorted turns don’t make it enough lower to see on the meter.
If you are feeling deft, it is perfectly possible to rewind bad windings. Use the same size wire and the same number of turns as the original and everything comes out right. Remember slot cars? Every kid into slot car racing rewound his own armatures. T’aint hard.
Thanks for the answer Dave, but I am still unclear on a few things.
Measure the resistance across the entire motor, from one brush to the other brush.
I am guessing top and bottom motor clip will be OK.
As the armature rotates, the bushes connect to one winding after another. A good winding will read a few ohms.
What is a few ohm’s, 2? All so as I spin the motor faster, I am getting higher readings. All so, how can I tell which winding I am testing? Would it be where the Top Brush contacts the commutator, or the bottom one.
Open or short is a bad winding.
When you say a open or a short, would the reading be 0 then?
What setting would I use on the meter, I currently have it sat on 200k.
Is there a way to test the windings with out turning them?
Sorry I have so many questions, this is the first time I have tried to fix a motor.
Yes. My elderly Athearn blue box locomotives have a top strap going to one brush and a metal clippy thing on the bottom that connects the other brush to the frame. Another way of putting it, is ohm out between the two points the motor gets power from. When in doubt, connect the power pack and make sure the motor turns. If it’s turning, its drawing current.
A few ohms. Maybe ten. More than one ohm and less than 100 ohms. It varies from motor to motor. Spin the motor very slowly for this test. All motors will act as generators if spun fast enough. When you spin the motor rapidly it generates electricity and feeds it back into the ohmmeter, which confuses the ohmmeter. Turn slowly .
Short reads 0 ohms, same as you get by touching the ohmmeter probes together. Open reads either very high, or blank display on some digital meters or sometimes a mathematical infinity symbol. Depends upon the make of the meter. Easy way to check is just put the meter on ohms and don’t touch the probes to anything. That’s an open, by definition. What ever the meter read
If the blow up picture works, the section I am using is in around the 4 O Clock area. Lowest setting I see says 20k. Would that work?
If you have the armature out on the bench you can just ohm out from the commutator. Just place the ohmmeter leads on the commutator the way the brushes go (180 degrees apart). Putting the armature in a vise makes it easier. Work around the armature to test all the windings.
That was what I was thinking when I started the project. But here why I am confused. My commutator has 5 sections. If the top brush was in the center of the top (winding contact?) the bottom brush would be touching 2 of the other contact points. Same if I use my meter leads, bottom one would between the opening of the two bottom contact points.
So I will get a reading with no power to the armature and with it not moving, correct?
Boy Ken, that’s a bright colored meter! That ought to show up on the workbench real good. I just nailed one just like it on Ebay for $6, free shipping. I should have it about the end of next week. My existing meter works on ohms only about half the time and the stores here are really proud of them.
From the picture it appears that your meter has a 200 ohm setting. That is the 200 right above the yellow button switch. The 200 ohm setting will probably be sensitive enough for what you are trying to do.
That Harbor Freight picture shows you have ohms ranges of 2000K, 2000K, 20K, 2000 and 200 ohms. You will get best results using the 200 ohm scale. (2000 is another way of writing 2K, 200 is another way of writing 0.2K. )
And yes, the commutator slugs are always kinda staggered. But one and only one winding (or winding end really, each winding has two ends) is soldered to each slug. So you put one ohmmeter lead on the top centered slug. Try both of the bottom slugs, the one that is offset left and the other one offset right. One will read a few ohms, the other will read open. Go with the few ohms reading. The one that reads open is clearly the other end of some other winding. Then move on and ohm out the rest of the slugs.
And yes, you get a good ohms reading (resistance reading) with no power applied and the armature not moving, or at least moving very slowly.