[TravisR100]

This is one of those purchases I probably shouldn't have made but just couldn't pass up. It's an old Dumore grinder/buffer. Picture 4 below is a good overall shot of it. It just looks neat. My questions are on the rebuild of this thing. In picture 1 you'll see the overall shot of the bottom with some of the parts loosened. Picture 2 appears to be a resistor. On the end is written 140 and what appeared to be an Ohm symbol. I put a multimeter on it and indeed it measures 140 Ohms. Picture 3 seems to be a variable resistor. The speed switch had a wiper that contacted each of the copper pads and a wire was hardwired to the stud. Measuring the resistance from the stud to each of the pads I came up with (in Ohms): 0, 50, 80, 102, 128 and 10. Obviously there is a short somewhere on the last pad. Picture 6 and 7 show an "arm" that threads over the shaft and into the housing. Picture 8 seems to be some kind of oiler I'm assuming. You can also see it just below the shaft on the housing in picture 4. Sticking out of the threaded end is what appears to be a wick of some sort. Picture 9 is a shot of the screw that you take out to get to the brushes. The brushes seem to be in good shape and look to have a lot of life in them. Picture 10 is the motor with the cover removed. Oddly, there are no bearings anywhere. The shaft seems to ride in a velvet or cotton seal held on by the arm in picture 7. The oiler (???) in picture 8 contacts the bottom of the shaft.

On to the questions!

1. Am I wasting my time here? The motor seems to spin freely by hand with no noise.

2. What type of speed control should I use to replace the big resistor and the variable resistor? Not sure what power pot or resistors I'd need. The name plate reads 115 volts, 250 watts, null load speed of 5000 rpm. Under "rating" it's hard to make out but I believe it says "1/6 30 MIN." Serial number is 2671 and type is D3.

3. The motor has 3 wires going to it however one of them was not hooked up to anything. Not sure if it should be.

I plan on doing what I can to take the rust off of the external parts and plan to strip and repaint the housing and base. Any help or advice is appreciated.

Upon further inspection I noticed the end of each housing has a brass bushing in it. I guess that's the reason for no bearings.

[pfred2]

It looks OK to me. Just so long as the motor is not shorted to its own case that is.

Any speed controller that can handle the motor will work. The motor only appears to draw 2 amps (I=W/E 2=250/115). You want a router speed controller style not an induction VFD.

Motor should be series connected so 2 wires will run it. Maybe the disconnected wire is a safety ground?

Only other advise I can give is don't go crazy oiling the motor. That is how I fried my last one here, over oiling it! Cleaning the commutator up on old motors can make them run nicer too. I use 400 grit wet or dry to just clean the copper.

[TravisR100]

I'm familiar with a VFD but not a "router speed controller." The original wiring just used plain old wire wound resistors. Could I not replace it with a pot appropriately sized for power?

[pfred2]

VFDs are for squirrel cage induction motors. Universal motors aren't that. Universal motors are in items like corded drills, or vacuum cleaners, or Dremel moto tools. And use the same sorts of speed control. Oh yes, and routers too.

You could use some kind of a huge rheostat, but a router style speed controller would be cheaper and easier today. Better too. A 250 watt rheostat would look like prop out of a Frankenstein film!

Not an endorsement but here is one as an example:
http://www.harborfreight.com/cpi/cta...emnumber=43060

[TravisR100]

I'm a bit confused on the wiring here as well if anyone can help. The motor has 3 leads. I'll call them A, B, and C. Lead A goes to one of the brushes. Lead B goes to one of the brushes. From the brush with lead B another wire is soldered internally that goes to one end of the stator wiring. Lead C goes to the other end of the stator wiring. Seems odd that lead C wouldn't just be internally soldered to the first brush doesn't it?

[Geof]

We really need Al-the-Man to come in here for an expert opinion, but I think the wiring has been altered. From your description of the low wattage variable resistor bank I suspect the speed control was obtained by varying the voltage drop across the field. For this I think it would be wired parallel with the resistor bank in series with the field.

I think you may be better off to clean it up and turn it into a lamp standard or something. 250 watts is very low powered for a grinder/buffer and I think the 1/6 30min indicates a low duty cycle.

[TravisR100]

Yes, I was hoping Al might step in here. Actually I'm only planning on using it for very light aluminum deburring so I'm going to go ahead and rebuild it anyhow. I'm a sucker for antique machine tools with some style to them.

[TravisR100]

The restoration is coming along. I blasted everything with aluminum oxide to remove the old paint. Pictures 1 and 2 show the main motor after blasting. After that 2 coats of primer (pictures 3 and 4). Picture 5 shows the before and after on the oilers. Amazing what a little scotch brite and a lathe will do. Pictures 5, 6, and 7 show most of the major parts after being painted and cleaned up. I chucked the rotor in the lathe to clean and polish the shaft. I also cleaned up the copper pads where the brushes make contact. I was looking for some black satin paint but couldn't find any so went with the black "hammered" paint. I think it's looking good. I just need to wait for the paint to fully cure and then start reassembling it. Still stumped on the wiring. Al???

[TravisR100]

And here is the (nearly) finished product. Still needing some wiring help here. Anyone?

[TOTALLYRC]

Hi Travis,
Looking good.
If nothing else it will be great for those delicate buffing jobs.

My cousin was buffing some radiator brackets with a 3 hp buffer and the bracket caught in the wheel and tore it from his hands. He still has the scar.

Mike

[ger21]

As Geof said, PM Al_The_Man and direct him here.

[TravisR100]

I think I understand the reason for the 3 wires now. 1 wire is common and goes to the commutator and the field. 1 wire goes to the field and the other wire goes to the commutator. I think it was done this way because so that the field (or commutator) could always get a constant voltage while the voltage was varied to the commutator (or field) by the pot. I'm just not sure whether to supply the constant voltage to the field or commutator or vice versa.

[Al_The_Man]

According to the rpm, it would certainly appear to be a series motor, in that case the common connection of armature and field would not be connected externally.
Usually the only reason to bring the field out in a series motor is if you want to reverse it.
It is possible someone jury rigged it for speed control, or even played around as using it as a DC shunt motor with the variable res. in the field.
If run as a series motor, you can vary the rpm with a triac style controller, these are available or easy to make.
The bearing were often bronze oilite (porous bronze, oil impregnated).
Took a closer look and see it was a built in speed control? the size of the resistor, I would expect that to be a crude form of rpm control by putting the resistance in series with the motor itself.
Al.

[TravisR100]

It definitely doesn't appear to have been modified in any way. There is a common lead to the field and commutator. There is another lead to just the field and another to just the commutator. So to translate what you're saying into something I can understand, you're suggesting that I wire this such that the field and commutator are in series (not parallel by tying the the other two wires together) and then use a simple triac style (router) speed controller?

[Geof]

I think this is correct, maybe not, it is a long time since I did anything much with motors.

If it is wired parallel and a variable resistance is put in series with the field that will give crude speed control; when you increase the field voltage you increase the back emf in the armature and the rpm at which the free running motor stabilizes is reduced. Conversely if you reduce the field voltage you reduce the back emf and the armature spins faster; actually if you take the field voltage too low you can badly overspeed the motor.

[Al_The_Man]

Actually that is the way it runs in series, overspeed! it is actually running away and the only thing that restricts the rpm is load or windage etc.
This is the way all AC hand tools and vacuum cleaners operate.
You can actually hear a vacuum increase in rpm if you take the load off by blocking the inlet tube.
So you need the armature and field in series to run as Universal motor (two connections).
Al.

[TravisR100]

Thanks for the help with this. Indeed all the wires seem to be original however with the help you all provided I was able to determine which wire to ignore. For testing purposes I hooked it up to a variable benchtop DC power supply. Works great. After determining nothing was going to blow or short I plugged into an AC outlet. Again, works great.

For anyone interested, I thought it was odd that it had such thin/narrow shafts and no apparent means of attaching any kind of wheel. I started doing some searching on the Internet. I think that this thing is actually what is known as a dental polishing lathe. Reading up on these I found dental polishing lathes are made to take different chucks that fit a tapered shaft. I found that indeed it does have a tapered shaft on it. It goes from .359 at the end to .373 at the base. Not sure if this is what's known as a "standard dental taper" because I haven't been able to find a definition of the standard. But it doesn't seem like an accident as both ends have the same taper.

[TOTALLYRC]

Well at least when you get it running you will have a bright smile.

Mike

[TravisR100]

I actually found it will work perfectly with the wheel I wanted to use by using a "stone chuck" specifically made for these lathes/polishers/grinders. Overall I'm pretty happy with how it turned out. Only took a days worth of work.

[Ehren]

Gotta ask, how much asbestos is in this thing?