DC motors need preventative maintenance, they are not fit-and-forget electric motors. Unlike their AC motor counterparts, DC motors have wearing components that need to be checked regularly.
By carrying out regular simple preventative maintenance tasks you can reduce the chance of an in-service failure massively.
First of all, REMEMBER SAFETY, all equipment should be isolated and a safe working method written up before you do any of these checks.
Some of the figures stated are my person opinion, of course when it comes to Insulation Resistance (IR) the higher the better, yet it gives a good place to start. There is a standard for minimum IR levels IEEE 43 and can be found here – EASA – IEEE 43
Electrical tests should be corrected for 40 degrees C. However, in the field I know this is not always done, so for consistency of your results I’d suggest doing the tests when the motor is at ambient temperature. The reason for this is that IR and winding resistance changes with temperature.
Creating a simple form for each motor, even creating an Excel sheet will give a good record of the tests.
1 – Measure the IR of the field circuit. (min 20MOhms @ 500V)
2 – Measure the IR of the armature circuit. (min 5MOhms @ 500V)
3 – Measure the field & armature circuit resistance. (Use a low-ohm scale meter for the armature, a standard multimeter may not be suitable for this)
4 – Measure the brush length. (as a general rule brushes should be replaced when they are 1/3 of new)
5 – Check the brush surface for chips or burning. (this can give an indication of running condition, record or photograph the edges if there are any chips or burning marks evident)
This is an example of an excessively worn brush
6 – Photograph the commutator surface condition. (Ideally there should be a uniform patina with no streaking or lines on the surface. A photo can be useful to compare change against the previous photo. Here is a good guide to brush and commutator condition – Brush & Commutator condition guide
7 – Check the wear depth of the commutator. (max 2mm wear. For larger commutators over 250mm diameter this may be ok)
This commutator has seen better days
8 – Vacuum (don’t use compressed air) to clean carbon deposits. (Compressed air may force carbon deposits deeper into the armature or field windings causing the IR value to worsen)
9 – Measure the brush spring tension. (Brush springs can weaken in service, if insufficient pressure is on the brush it can ‘bounce’ on the commutator causing problems
10 – Check the filters and replace if blocked. (this is one of the most commonly overlooked problems along with missing filters)
How often should these preventative maintenance tasks be done?
I would suggest that these preventative maintenance tips are done every month, but at a minimum every three months.
Record and compare all the results. Hopefully by taking action and removing a DC motor from service when it’s needed will help to prevent an in-service failure.
We are on hand 24 hours a day to repair and service DC motors if you need – Electric Motor Repair Page