Groschopp Tech Tips | How to Check for a Damaged Armature

Here are three quick checks you can perform with a volt/ohm meter to test a DC motor armature winding to determine if a motor armature is functioning properly.

Groschopp Tech Tips is a series of How to & How it Works videos presented by our Engineers and Technical Sales Staff to help answer your fractional horsepower motor questions.

[anthonylemay]

Thanks for making the video. You saved me some money. I almost bought another amature for a KitchenAid mixer when the old one was fine.

[titanpolisher]

Holy Moly Groschopp. Y'all as great as you were twenty years back. Good to see you

[nackesww]

Great video. Almost ordered new brushes for my motor. After doing the test I realized the armature was faulty. you saved me $20 .Thanks

[russellmaguire]

Great explanation. What would you consider the variance of resistance allowable and that its ok?

[stansearing]

If you properly get continuity bar-to-bar going around the whole commutator in the second test, then the third test should just need to check one bar to the shaft ("ground").  It would be prudent to make sure the meter probe tips are not leaving indentations in the commutator bars (particularly where the brushes ride).  Note also that the first test can usually be performed with the motor in the housing by just measuring across the brushes.  This test is good for most motor configurations but there are exceptions (e.g. it won't work for a repulsion start motor, but they are rare these days).
Another good test for small motors is power the motor at a voltage just above where it starts turning (using a variable autotransformer for ac motors or a bench power supply for dc motors).  Then load the motor with your fingers and check for even torque throughout a complete turn.  The torque may change a little as each commutator bar crosses a brush, but should do so from bar to bar pretty evenly.  The current should behave pretty consistently from bar to bar.
A final test is to run the motor at full voltage and no load.  If the current is higher than expected and there are lots of sparks flying around the commutator, there is a good chance that some turns of the motor armature are shorted.  Shorted turns behave like shorted turns of a transformer.  That is, they may be hard to detect from dc measurements, but cause a considerable increase in ac current.

[Groschopp]

Hi Peter, from what you described there is a possibility the armature is not centered within the motor resulting in interference with the field. To confirm this you could check the field to see if there are score marks on the inside of the field. It might be possible to center the armature when putting the motor together by taking care when tightening the bolts. If this doesn't solve the issue you may need to contact a motor repair service. Hope this helps.

[chemic]

Thanks for the clear, concise description, nice video!

[uriel-heavensguardian]

Do you have a video on how to rebuild one? Or is it best just to buy a new one?

[cosmicdust]

even though my DC motor's winding is disconnected, motor is running fine and making pressured air.
it is a compressor motr with 18 commutator bars. one bar to bar test fail. the motor power consumtion has decresed little bit ~ 20%.

[pcfletch]

Hi, Many thx for the tips on testing, I Have a bosch gof 1300 ce router, did all your tests and all seems fine, notices that parts of the stack teeth are scored and not shiney any more, fitted a new field, and brushes, motor still running at half speed and still smoking, any ideas would be much appreciated.

[azukark]

Hi, my blentech displays "overload" within five second. it does run but stops shortly... people tell me change the catbon brushes. after opening and examining these, i noticed one is much worn. but i cant find where to buy a new one. can you advice?

[JOEZEP]

This is a very clear instructional video. Well done. A big 👍!! This will help me with one of my projects.
I am not being critical & was considering not adding this to the comment due to you not knowing me & thinking that I am one of those trolls. But the music could be lower, mind you after years of work in loud environments ( & age 😞 ) my hearing is not what it was. Stay safe, Joe Z

[MegaRiffraff]

I know this is an old video, but is this test done the same on 3 phase ?

[donaldpowers]

I subbed. Actually I was looking for this information for testing a armature. I just took apart. Pretty cruddy inside, though not surprised its a drive motor from a Tennant floor scrubber. It jumped it just sparked a little. Had to break the motor down the bolts from the face of the moter were Phillips. Slmost stripped one but like I mentioned. The armature looks bad. I'll find out soon enough. Thanks for your posting this video. Don.

[peterowens]

Need info about degree of slack for brushes in their holder & how to decide when brushes are too worn. I have a tool where the brushes are mounted 90 deg apart not the 180 deg you normally see, how does this influence things?

[isakwilkinson]

Thank you so much for the tech tip I just took this job as a janitorial equipment repair and maintenance technician. I've got some idea but finding out with the ohm test will see if the company a lot of money. And it will also tell me maybe possibly how much longer the motor's got or doesn't got

[pcfletch]

Hi Its me again, fitted new armature, checked the centering of the armature, motor ran at full speed for a few seconds then started to slow down again, on closer inspection i found that the bottom bearing is spinning in its housing, and there is a slight vibration on the machine, no smoke anymore, so iv'e just ordered bottom housing and bearing and top housing, wish i had have just bought a new machine now lol.

[mathsnetic]

Having problem with test 2, two segments in test 2 are giving 8.4 ohms rest of all are 0.9 what does it means plz tell

[alaskanken]

Hi I have a dewalt grinder and I did the 3 test and I also had continuity in my field .I put the grinder together and it turns slowly and gets hot enough to smoke. I am not sure where I am going wrong. Question on the bars where the point touch on the 180 test . If I am off a bar or two I have the same reading what dose this mean thank you for your time.

[jackfarrell]

Great information, thank you for the video