Simple test using current values to prove polarity

[David Lowther]

I work in a large electrical machine shop. We have a test that no one can explain to me. We energise a dc field frame (in this case 6 pole) with ac and record the current draw, then reverse the polarity of connection, energise and compare values. If a pole is reversed polarity then a difference in these values appears. Why is that? My gut says it must be related to inductive reactance but I can't find an answer or quite visualise it. Any help would be greatly appreciated.

 

[Baluncore]

Welcome to PF.

What are you testing, a generator, DC motor or alternator?
Are the field windings in parallel or in series?
Do you cross the winding connections to minimise the AC field current during the test?
Is the armature present during the test?

You can usually use a small magnetic compass to identify the polarity of DC field windings without needing AC.

 

[David Lowther]

Thanks! It is a DC motor, shunt field circuit connected in series with each other. No armature present at time of test. I should clarify that I am aware of other suitable tests and indeed there is no polarity issue with the unit, I just can not figure out how this test works in regards to the theory.

 

[Baluncore]

Consider what would happen if one pole was wound backwards.
Instead of NSNSNS you would have NSSSNS.
The reversed pole does not support it's two adjacent poles but opposes them.
The reversed pole appears as a poorly coupled shorted turn.

Consider a 1:1 ratio transformer with the secondary connected in parallel with the primary.
Then imagine reversing the secondary connections.

 

[CWatters]

How do you reverse the polarity of an AC connection?

 

[davenn]

How do you reverse the polarity of an AC connection?

hahaha indeed, that's what I was thinking when I read that original comment

 

[David Lowther]

Sorry, sloppy terminology from a workshop floor. By this I mean we have two leads, F1 and F2. Test 1 is conducted F1 Active F2 Neutral, test 2 conducted F1 Neutral F2 Active. AC doesn't have polarity per se which is why I am confused as to why swapping the leads would make a difference but anecdotally it does.

 

[Baluncore]

AC doesn't have polarity per se which is why I am confused as to why swapping the leads would make a difference but anecdotally it does.

If the test is being done to identify which of the pole windings may be reversed relative to the others, then the lead swap must be done only to the pole that may have been incorrectly connected in reverse, while all others maintain the same phase.

 

[jim hardy]

Hmmmm that's very curious... it triggers my Ozarks "Show Me" reflex.

... What changes when you swap line and neutral connections to the machine?
Only change i can see is distributed capacitances between field coils and frame get driven with slightly different voltages, depending on where in the series string is the reversed coil.

If the reversed coil acts like a shading pole might a rotating field result ?

What kind of volts and amps do you apply with this test ?

Are there interpoles or compensating windings? Do you connect them to anything ?
http://www.studyelectrical.com/2015/01/compensating-windings-interpoles-dc-generator-motor.html