Tech Tip: DC Motor Armature MegOhm Testing

OK, AC equipment gets an awful lot of attention. However, in our Motor Diagnostics and Motor Health Study we were able to determine that more than 20% of facilities, of all types, still use a noticeable number of DC machines. Well, yes, we were aware of that from field experience, but a little different than having hard numbers.

In the IEEE Std. 43-2000 and later editions, the tables show that the temperature corrected value for DC armatures should be 100 MegOhms. When I have presented these numbers in classrooms and presentations, I normally get at least a ‘snort’ if not full blown laughter. The reason? Most facilities do not properly maintain their DC machines and allow enough carbon dust to build up in the armature and commutator for the insulation resistance to drop well below that value.

I mean, after all, the DC machine keeps running to well under a MegOhm, so why should we care that it is above 100 MegOhms.

There are several reasons including:

1. Low insulation resistance represents leakage current. That means that the point of issue will have current flow generating heat and a potential failure in the future in the windings or bearings;
2. It is a loss, meaning a reduced efficiency within the motor; and,
3. Unstable speed control.

Temperature corrected insulation resistance readings can be used to trend and determine when a winding should be blown out with low pressure air or cleaned. A change to a lower IR reading will represent that such things are occurring and once they drop below 100 MegOhms corrective action is required.