Novel Method Of Converting A Universal Motor To Pm

My $20 Harbor Freight angle grinder burnt out one of the field coils and melted the other, so I replaced them with magnets.



I cut the gap in the core with a cut off disk by hand as shown in the photo.

I've never seen this method of construction before. I like it because you can change the air gap without effort, though you will not be able to push the flux much higher than original design without adding more iron. The magnets are 6mm square, N48.

When I burned out the motor, the rotor was hot enough to boil water instantly and the field coils shorted out against the core because they melted through the .5mm thick plastic sleeve.

from the looks of the commutator segments there was not many hours on that motor before it let go .

I've only had it 2 years. It was probably pulling 10 amps for 15 minutes before it failed, the field coils were 20 awg.

The flux could be 50% higher if there wasn't such a big bolt hole in the stack.

What are you going to use to power it now? I assume it'll draw about 10 amps at 110 VDC at full load and quite a bit higher inrush at startup. That'll take some hefty diodes.

hmmm ...
i wonder how this would work (about a third of the way down the page) :
http://www.bristolwatch.com/ele/triacs.htm

the circuit would have to be improved upon a bit .
a foot controller off of an old sewing machine would be great .

the armature resistance is at least 5 ohms, not worried about inrush current.

speed regulation would be cool, for the moment i've been using a variac for all my fixed speed power tools.

I have a feeling it will need 300 volts to get to the same speed it used to run on ac.

How are you gonna clean out all the filings? Not even try?

Ok, it's a Horrible Freight tool, it's not like the bearings were going to last another 10 hours of run time or whatever anyway. But I can't see justifying the cost of those poor magnets ($5?) on a $10 tool plus labor (small cost even if it's free time) either.

Tim

Welding magnets? how much?

getting the filings out ?
a very high velocity air stream and some goopumpucky (aka : blu-tak , etc .) .

300V ...
the flux density must be nowhere near the original (before it crapped out that is) .
maybe cutting the center of the pole faces and inserting a set of magnets there would help out .

that would be backwards, more flux = more voltage at same rpm.

hmm, was it worth it? perhaps if i had waited another week or so to find a coupon to buy the angle grinder at $20 then its up for debate, but the price varies from 39 to 29 depending on where you live.

And from the looks of it, I practically doubled its hp "rating" if indeed i have to run it from 300vdc.
The bearings are solid, the gears are not, they only mesh properly when torque is sent though in the normal direction. not worried about the fillings, though if it fills up with them it will run faster because they would help short out the magnets...

I was wondering that myself. The case vents aren't usually big enough to permit serious air stream cleaning & that stator is going to be a ball of iron dust before too many operating hours have passed.
Interesting concept, though. I am waiting to see how it performs.


BTW: Those bolts aren't the only reason for less-than-solid lamination stack. The spaces are also required for cooling. Those motors run so close to the edge that if the cooling air velocity drops below 100kph or so, they'll fry, quickly.

hah, how it performs?

One of the rotor coils shorted out.

I've got another motor i intend to convert and some actual measurement data on, but i can't find it right now.