# Identifying Interpoles on DC Motor Leads



## BrettC (Apr 10, 2016)

I've got a practical bench test coming up and I know I'll have to megger a DC motor which (I have heard from prior test takers) has 9 unidentified leads coming out of it. No big deal but I was wondering if anyone had practical advice on differentiating the interpole leads from the series and shunt field windings? My old lab partner is of the opinion that if there are 9 unidentified and unconnected leads coming out of a DC motor (including one for grounding purposes) then 4 of them will be for 2 shunt field windings whereas I contend it is more likely that one of those sets is an interpole field winding. 

I guess what I'm really asking is: we know the series field winding resistance is low and the shunt field winding is high, but where would the interpole winding resistance lie on that spectrum, between the two or lower than the series winding?

Thanks in advance. I can find no practical advice on this specific matter in my motor repair handbook or on the internet. Rosenburg and Hand recommend using a lamp test I will not be able to perform as I just have my DMM as a test instrument. FWIW my best guess is that it will be between the two and probably lower than the armature winding.


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## telsa (May 22, 2015)

Is the commutator not exposed ?


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## BrettC (Apr 10, 2016)

telsa said:


> Is the commutator not exposed ?


On the practice motors no, not always. It's usually just the wires coming out. 

I have no idea on the motor in the test.


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## telsa (May 22, 2015)

You haven't spelled out how big this test motor is.

Residual magnetism from prior runs is often more that enough to produce EMF at the leads -- even if you're spinning the motor by hand. 

And a DMM is very sensitive.

Since the circuits pass through the commutator -- or don't -- that's a big 'tell' right there.


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## BrettC (Apr 10, 2016)

telsa said:


> You haven't spelled out how big this test motor is.
> 
> Residual magnetism from prior runs is often more that enough to produce EMF at the leads -- even if you're spinning the motor by hand.


Larger than 1/2 hp? Unfortunately the size information wasn't provided. 

That is good to note re: residual magnetism. Since it's a traveling test motor though I don't think it's been run for a while, if ever. 



telsa said:


> Since the circuits pass through the commutator -- or don't -- that's a big 'tell' right there.


If the second interpole lead is disconnected from the armature would I still get a reading through the commutator between the interpole terminals? I thought only readings between A1 and A2 would involve the commutator and the interpole field winding (between S3 and S4) would be left out of it.


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## John Valdes (May 17, 2007)

I have never seen 9 leads coming out of a DC motor.
I guess they are bring the internal connections out for you to connect.

So look at this on page 10. And maybe page 11.
http://www.goevans.com/filesSite/EHB_pgs0803.pdf


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## BrettC (Apr 10, 2016)

John Valdes said:


> I have never seen 9 leads coming out of a DC motor.
> I guess they are bring the internal connections out for you to connect.


That is correct; also to megger.



John Valdes said:


> So look at this on page 10. And maybe page 11.
> http://www.goevans.com/filesSite/EHB_pgs0803.pdf


Thanks! I've seen a similar drawing in my motor control book but I really like these diagrams; good quick reference. 

Too bad all the current EASA documentation is behind a $1k paywall!


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## John Valdes (May 17, 2007)

BrettC said:


> Thanks! I've seen a similar drawing in my motor control book but I really like these diagrams; good quick reference.
> Too bad all the current EASA documentation is behind a $1k paywall!


If you use an EASA motor repair facility (motor shop) they can provide you with most any document you need.
Ask them. They may let you make copies or even send you the doc's electronically.
After all, the educated customers are most likley your best customers.
I would have never had a problem sharing motor data with customers.


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## BrettC (Apr 10, 2016)

For those of you interested, I took the test this morning! (I passed.) Four hour bench test at 7am prior to the regular workday is just the thing for a Tuesday. 



telsa said:


> You haven't spelled out how big this test motor is.


They never told us! The nameplate had been replaced/covered with a plate with some sort of serial number on it, presumably some sort of SKU. 



John Valdes said:


> I have never seen 9 leads coming out of a DC motor.


Well, 2 were series field, 2 were shunt field, 2 were armature, one was Ground and 2 were...protection field? They were labelled P1 and P2. I wound up just asking one of the proctors and he said they would be linked to an OL relay somehow. It was news to me; I never saw anything like that in school. Glad I asked though! The motor is just used for this megger practical and it is never run. Insulation from being constantly over-volted is pretty shot.


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