DC motor theory

[Eric Nelson]

I STILL question why the motor operates with the CL set at zero...

beerchug

-Eric

 

[DickDV]

Eric, I checked the instruction manual and the current limit pot goes down to 10% at the minimum point. With no load on the shaft, It will probably spin up ok.

Interesting that a new motor fixed the problem. Must be that the current limit pot controls A1 to A2 current but not armature circuit to ground current.

Must be that the bad insulation spot was flashing over periodically. 930 ohms at 240 or 120 volts wouldn't open a 30amp breaker.

Thanks for the brain teaser, russmartin

 

[Eric Nelson]

Must be that the current limit pot controls A1 to A2 current but not armature circuit to ground current.

That makes sense... :nodi:

beerchug

-Eric

 

[rsdoran]

I know I aint the sharpest knife in the drawer BUT

Must be that the bad insulation spot was flashing over periodically. 930 ohms at 240 or 120 volts wouldn't open a 30amp breaker.

Cant DC motors provide an inductance or low impedance factor when a connection to ground exists? I hope I am stating this correctly...using a continuity meter with a low dc voltage may offer a reading that is actually larger than the resistance/impedance when connected to the larger power source?

I stated in the beginning if the motor had that low of a resistance to ground it was suspect...btdt...would not have hesitated to change that motor nor would in the future. I have found in my meager period of dealing with this situation that if the motor isnt bad it is going bad, which means that problems happen intermittently.

 

[russrmartin]

More theory

Questioning RSDoran's last post, does this mean that almost any reading on a typical ohmeter would probably indicate a bad or going bad motor? If not, what would be the threshold where one could say, X resistance is adequate? I do know that the best way is to use a megger, lets just pretend that one is not available. I do not have a lot of experience in this particular area, and some general rules of thumb would be helpful.

Russ

 

[DickDV]

Ron, I'm trying to picture a situation where the AC impedance would end up being less than the measured DC resistance and am not coming up with any. In every case that I can think of, the AC impedance would be higher making the current less on AC than on DC.

I think the bigger problem is that the resistance as measured at only a couple of volts may be a lot higher than the resistance or impedance at several hundred volts due to flashing and arcing effects. I've seen a few situations where carbon dust, for example, measures high resistance at low voltage and conducts like crazy at several hundred volts. (I know this can make life complicated but, if it was simple, they'd pay us less! ha ha ha!)

russmartin, the instruction manual for your drive states that resistance from motor windings to ground should be higher than 100K ohms. You might want to measure this resistance on your new motor. I think you will find that good DRY insulation will measure several megohms at least. As any good motor shop mechanic will tell you, the only accurate way to get these resistance readings is after a "steam and bake" cleaning cycle. Especially on DC motors, carbon brush dust can cause low resistance readings that will go away after a good cleaning. Motors operating in coal mines and wet areas like paper mills can also give misleading readings if not cleaned up first.

But, as stated by many of us early in this thread, a motor measuring 930 ohms armature to ground is highly suspect and not to be trusted even if it runs ok. In retrospect, you did the right thing by changing out the motor. Good thing you didn't follow my advice! You'd still have your problem and a spare drive!