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Re: (ET) Triage - ideas from an unexpected result

Hi, Walter, hi all,

Thanks for the note, and the sympathy on the dryer incident. I was the test subject... like a chimpanzee in the space program.

So, yes, right with you about the limitations of the DMM. But let me be clear about the basis of my confusion: I measured an almost-zero resistance for the FIELD windings. And I measured the ARMATURE resistance to be *NOT* near zero.

Bench test: I have made this motor run by applying voltage to the armature and field in parallel, and it spun. (Yes, normally one would have given the field full voltage and ramped up the armature, but it was just a test.) It was when I was getting ready to wire it up conventionally (full field, ramp up the armature) that I took these measurements and got the reverse of what I am used to seeing.

AFAIK, this motor is from a big ol' Caterpillar forklift.

There are no "S" terminals on this thing, so I don't think I've got a compound motor here... but maybe it's some unusual variation in the compound motor family.

I think I should take measurements between A and F terminals to see if there's buried compound series-motor connections within.

-M

On Thu, Feb 17, 2011 at 9:58 AM, Konstanty, Walter (GE Energy Services) <Walter Konstanty ge com> wrote:
Max,
  Sorry about the clothes dryer incident.
 
Couple of thoughts;
- DVM's don't measure resistance accurately below 40 ohms or so.
- The field ohms is direct Ohm's Law...
- Armature resistance practically determines "inrush current".... once it rotates, a DC armature generates a back EMF or internal voltage and will keep drawing amps based on RPM per armature volts input until the back EMF almost equals this applied volts.  So, it really doesn't mean too much as brush contact drop influences the resistance and a bunch of other stuff.  
 
Try it on the bench and see if it runs.
...Walt

From: Max Hall [mailto:mhall maxmatic com]
Sent: Wednesday, February 16, 2011 6:42 PM
To: Elec-trak
Subject: (ET) Triage - ideas from an unexpected result

You guys, I came upon a little mystery. At least, it's a mystery to me!

I have an e-motor that I thought was just a run-of-the-mill shunt-wound (run-of-the-mill except for being really large)... but when I measured the FIELD windings' resistance, I got about .6 ohms, and when I measured the ARMATURE resistance, I got about 30 ohms. Is it because my brothers put me in the clothes dryer (and turned it on) when I was little, or is that just perfectly backwards? (And yes, I'm sure the armature measurement is the armature... not just because of the giant "A"s stamped into the metal, but also because you can see the physical connection between the A terminals and the brushes.)

Thanks for any light you can shed, ETers. Maybe I'm gonna wind up doing armature weakening to make this one go fast...

And since this is tangent to ElecTraks themselves, I guess I should suggest responses off-line to my regular e-ddress, mhall maxmatic com... but I also guess that, unless I'm just backward anyway, the story might be interesting.

Best to everyone,




--
www.maxmatic.com