# Meter



## alaft61ri (Oct 11, 2019)

I have a 290 and iam measuring the ohms just want to make sure iam doing it right. Measure the plates iam getting 1.5not sure if that is low average


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## AmFlyer (Mar 16, 2012)

Yes, you are conducting the measurements correctly. That looks like the XA9569 armature w/o the oil slinger. it was used in 1949. The resistance should be 1.4 to 1.6 Ohms. The meter shows 1.4 Ohms, within specification.


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## alaft61ri (Oct 11, 2019)

Ok thankyou just wanted to make sure.
Al


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## TimmyD (Dec 6, 2021)

alaft61ri said:


> Ok thankyou just wanted to make sure.
> Al


I do believe that you want to take the measurement three times, each commutator section to the adjacent. I believe you can have an issue between 2 sections and not the others. Forgive me if you already knew that.
TimmyD


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## alaft61ri (Oct 11, 2019)

Yes u are no apologies necessary my electrical knowledge not great so any info is appreciated. 
Thanks al


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## mopac (Feb 24, 2011)

I know many measure the ohms between segments.. I don't do that anymore and here is why.
My 282 that ran slow and motor got very hot, the armature tested very well within specs. The
armature was still bad. That motor gave me fits for many many years. Changed the armature 
and it is a race car now and runs cool. I firmly believe if motor runs fine all parts are good. And
if motor runs bad start changing parts till culprit is found. Just my opinion.


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## TimmyD (Dec 6, 2021)

mopac said:


> I know many measure the ohms between segments.. I don't do that anymore and here is why.
> My 282 that ran slow and motor got very hot, the armature tested very well within specs. The
> armature was still bad. That motor gave me fits for many many years. Changed the armature
> and it is a race car now and runs cool. I firmly believe if motor runs fine all parts are good. And
> if motor runs bad start changing parts till culprit is found. Just my opinion.


MOPAC, I think another important measurement is from each commutator segment to the shaft, which should be open. A breakdown of insulation within the coil wrap could lead to a low resistance from one segment to the other. A breakdown of insulation between a coil and the shaft would essentially be a short, and I think that would also likely create a lot of heat. Since that is a separate path, I think if you were shorted to the shaft, you could still get a reasonable measurement from segment to segment. 
I would be interested to know if that bad commutator had a measurable resistance from any of the commutator segments to the shaft.
TimmyD


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## AmFlyer (Mar 16, 2012)

Shorts to the shaft are possible but rare. Most common is a broken wire or heat damage to the enamel on the winding wire. Mopac likely had internal heat damage on the bad armature. The armature can still show a DC resistance within or close to spec but be a poor runner. The reason is as follows, slightly simplified.
The DC resistance is 1.6 Ohms, but the AC impedance during steady operation is about 10 Ohms, due to the back EMF created by the spinning armature inside the energized field assembly. When some of the armature winding enamel is damaged it can greatly reduce the electromagnetic field generated by the armature and also cause it to be imbalanced between the three poles. This results in reduced rotational torque at all RPM's, causing the motor to operate poorly. DC resistance measurements are useful but not always conclusive.


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## mopac (Feb 24, 2011)

I have to say I did not measure to the shaft. If I had it would probably shown a short. I bought a 282 chassis with motor and started changing parts one at a time. After changing the armature all was well in my 282 world.
Shortly after I found 2 brand new armatures from a train repair shop that had gone out of business. I did not put one of the new armatures in my 282. I can not imagine my 282 running any better. So I have the new ones in a baggie marked NEW and I put the bad one in a baggie marked BAD. Armatures seldom go bad but you never know.

I did not get good advice here on the forum. I was only told about the testing between segments. This is the first I have heard about the shaft. But it makes sense. Some day
I will check the shaft on the bad armature.

I was so happy to get my 282 running good. I got it when I was 5 years old. For at least
25 years every Christmas I would get it out, took it apart, and hoped when I reassembled
it would be fixed. Never happened. My 282 is from 1952. After my uncle was done with it
he gave it to me in 1955. He was kinda rough with it. Lots of scratches. But to me it was the greatest train in the world.


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## TimmyD (Dec 6, 2021)

mopac said:


> I have to say I did not measure to the shaft. If I had it would probably shown a short. I bought a 282 chassis with motor and started changing parts one at a time. After changing the armature all was well in my 282 world.
> Shortly after I found 2 brand new armatures from a train repair shop that had gone out of business. I did not put one of the new armatures in my 282. I can not imagine my 282 running any better. So I have the new ones in a baggie marked NEW and I put the bad one in a baggie marked BAD. Armatures seldom go bad but you never know.
> 
> I did not get good advice here on the forum. I was only told about the testing between segments. This is the first I have heard about the shaft. But it makes sense. Some day
> ...


That sounds like a beautiful train and a is a nice story, glad it is finally working well again! I mentioned the second test based on thinking about electricity, not based on knowledge of AF failure mechanisms, it just makes sense to me. It sounds like the failure that AmFlyer suggested is also highly probable. I would be interested to hear though if there is a path to the shaft on your old armature if you ever break out the baggie! Hope you had a pleasant weekend,
TimmyD


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## AmFlyer (Mar 16, 2012)

Perhaps some additional background on my thinking about armature shorts to the shaft will help. In my experience those shorts are quite rare so not the usual cause of a poorly running motor. It never hurts to meter from each commutator segment to the shaft to confirm a reading close to infinite. Specifically for a 282 and similar 4 wire engines the chassis is not used as a conductor like in later engines. Therefor, one end of one winding touching the shaft will not affect motor operation.
It is always dangerous to diagnose problems from a distance but based on the above it is likely the poor running armature in the 282 is bad laminations from overheating.


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## TimmyD (Dec 6, 2021)

AmFlyer said:


> Perhaps some additional background on my thinking about armature shorts to the shaft will help. In my experience those shorts are quite rare so not the usual cause of a poorly running motor. It never hurts to meter from each commutator segment to the shaft to confirm a reading close to infinite. Specifically for a 282 and similar 4 wire engines the chassis is not used as a conductor like in later engines. Therefor, one end of one winding touching the shaft will not affect motor operation.
> It is always dangerous to diagnose problems from a distance but based on the above it is likely the poor running armature in the 282 is bad laminations from overheating.


Thanks for the explanation Tom, I am currently (sporadically) putting a 370 back together, so my brain is thinking about flow through the truck. I hadn't realized that any of the AF motors were insulated from the Chassis, I thought they were insulated at the drive wheels. I appreciate the clarification! This is also the fifth engine I have touched, so I have a lot to learn! 
Cheers,
Tim


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## AmFlyer (Mar 16, 2012)

You are correct, the diesels, like the 370, are different animals because the power from the track passes through the trucks. We do not want an armature short to the motor shaft in the diesels. 
In the Link coupler and early KC era all wheels on the steam engines and the motor were insulated from the chassis.Later in the KC era Gilbert began using the chassis as a conductor in place of one wire. These steam engines present different failure modes.


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