field powered motor to pm motor

[hiker]

had this motor laying around--had it on my old moped for awhile--it was a real juice hog..it had field winding in it--i checked the field coil stringth by hooking a 12 volt batt to them--not to impressed week magnetic stringth--so i tossed in 8 ceramic mags from lowes..put the motor back togeather--hooked it to a 12v auto batt--it spun up even more with the mags then the fiel coils--puts out some great amps but low volts--just by testing with my hand drill..should make a good gas powered gen..

heres a picture of it frying a couple of wires--powered by a hand drill..

any way it was a real easy conversion...most power ive seen from 8 ceramic mags!!

[RP]

Hey, that's pretty neat.  I wonder how much the armature and brushes can handle current wise.

Have you considered trying it the other way with a permanent magnet armature and using the field pole shoes to extract power?

BTW:  You might be able to increase the voltage by playing with the phasing between the magnets and brushes.  Try rotating the brush holder end of the motor in relation to the magnets.

Cool.

[hiker]

it has- bosh-300watt-on the side of it..might be an old VW gen..it had a bent shaft on the back side--which i sawed off..also it has 4 power out bruches on it-2for plus and 2 for negative output--real heavydudy type....

i tried putting mags on the armuture[using a diffrent motor]-and using the field coils for output--not  much power at all...

i have some old 1950s auto gens--might try replacing the field coils on those with mags--and see what kind of motor or gen -i get out of them................

[DanG]

If you're going to hassle a conversion and retain the brushes, here's what I know about brushed commutators. Brushes ride on layer of ionized air plasma w/ a little bit of copper and carbon/graphite thrown in. The only time when brush actually touches copper is when not energized or self-destructing. After the commutator surface gets seasoned very little copper or brush material is spent as plasma redeposits it's substance to be re-used next revolution.

Mess up a seasoned commutator and you will eat up the first set of brushes installed & with the oddball units we salvage thats eating up the only set of brushes had to be installed. Unless you are looking at a high commutator bar that is starting to chip brushes or some flash over slag do not burnish, polish, sand, or otherwise mess with it. No scratches, no dings, don't mess with it.

Oils be bad, use high-dielectric electronics cleaner or other safe oil-free solvent to chase off fingerprints, bearing lube fuzz, coil weevils etc. & use nothing more abrasive than a soft paper towel & be sure to blot dry the lowermost bars since the solvent carries junk there & evaporates enough so alot of residue isn't dripping clear, just being condensed at the lowest point. Then repeat.

A noticeable wear groove is something to be watched, but since the curved surface offers more contact area then a true flat surface it is usually self-limiting since the current and heat is disapated over a larger surface. Threading, where surface actually looks like a bolt's thread, can be burnished lightly to take the sharp edges off but I don't think its curable, maybe imbalanced motor rotor or arcing occurred on stall and changed hardnesses, maybe grease contamination when new - if you find an otherwise great motor w/ threading on commutator I've never seen one 'field' fixed w/o being warrantied back to factory.

Measure brush holder - commutator clearance & note any variables before you dissemble item, brush reinstalled a true 90° to shaft blah blah. Thin plexiglass can make a good go-no go gauge. Easy on spring tensionors, light setting unless brush worn to stub and no new brush available but know you're on borrowed time. Lube springs if neccesary from corrosion etc. w/ dry powder lube like graphite or teflon. K, well, lecture mode off.

[hiker]

thanks for the info..

hooked it up to my briggs and stratt.--nice to have electric start--just bypass the rectifier-to start..works great for that-as of now i just need a bigger pully on the engine--other wise i have to have it throttled up kind of high to reach past 12volts.

i guess i kind of lucked out on those mags --they were just the right size to go in there...wasn"t any real hassle at  all to change it over..just take out the field coils and glue in some mags..and you end up with a pm motor or gen...............later.

[RP]

Hiker,

Can those two brush pairs be connected in series?

[Ungrounded Lightning Rod]

so i tossed in 8 ceramic mags from lowes..put the motor back togeather--hooked it to a 12v auto batt--it spun up even more with the mags then the fiel coils

Which indicates that the magnets you put in were weaker than the field-coil system.

DC motors speed up until their spin generates a voltage that almost matches the voltage applied across the commutator.  (The amount they fall short controls the current.  The current controls the force pushing the rotor around.  Load it, it slows, the current goes up, the force goes up, and it stablilize at a lower speed, drawing more power.)

Lower the field voltage and you speed up the motor.

This is/was a big problem with large wound-field DC motors.  If they lost field excitation they'd speed up as the field collapsed, sometimes reaching hysterical speeds that would tear up themselves or the equipment they drove.

Of course as the field goes down the maximum force they can put out also goes down.  But on the big ones even the residual magnetism could keep them delivering enough horsepower to keep accellerating after all the field current was gone.  If they were hooked to something that presented an intermittent load - like a stamping mill - which wasn't presenting a load just now, they could spin up the flywheel and tear it apart, throwing the massive chunks all around the plant.  Very hazardous.

[hiker]

no.. the negative bruches are grounded--tried series -no go....

still puts out more power than before --no dou"t..

as a gen it works great--haven"t checked the amps yet--waiting until  i put a bigger pully on the engine--right now their the same size--as a electric start it spins my engine right up--real torkey...

[hiker]

i hooked the field coils directly to a 12volt auto batt..the mags still had WAY more pulling power.......it has no field coils now.

all i know is that it has more tork--and more rpms..its going to be used as a gen..and electric start for the engine..

so no worry about over speed--unless of course i crank up the throttle on the engine..

works for me :}.......................................................................

[Drives]

ULR  I had the experience of witnessing the aftermath of an 800HP DC motor in Buffalo, NY that went into "fly apart" speed.  GE calculated the shaft hit 4000RPM (on a motor rated at 1200RPM)  before the centrifugal force caused the windings to "spread".  It looked like someone threw a hand grenade into the armature.  The motor drove a 2000 ton stamping press, and shot flames 30 feet into the air catching the ceiling on fire.  The flywheel attached to the shaft had so much inertia that when the windings spread, it twisted the motor's 18 inch hardened steel shaft 180 degrees from end to end.  If it wasn't for the key shearing, the motor probably would have come off the mounting and killed someone.  The guy who hit the start button said it took seconds to accelerate from a stop to sounding like a Dremel tool before it exploded.  

I was called in to rebuild the DC drive which had malfunctioned, causing the failure, and blew up during the catastrophic failure.

Because most members of this board deal with PM motors, it is not likely anyone will experience this, but if you play with shunt wound DC motors...ALWAYS check the field current!  Just checking field voltage will fool you if you have an open winding.  A shunt wound DC motor's torque is equal to the field flux times the armature flux.  Flux is derived from current in a electrically powered winding.  Weak field flux equals high speed, no field flux, just residual magnetism equals a BOMB!

[ghurd]

How well do you think it would have worked as a gen if you put the magnets on the rotor, and used the field coils?

G-

[hiker]

the coils were on the small side -in this motor-so i dout if it would work out to be a better way to get power from this motor...like i said the mags have WAY more pulling power than the field coils ever did...heres a pict. of a 2 pole conversion using the field coils as output --it still needed lots of rpms do get any power from  it..

most likley would have worked better if i replaced the field coils with mags..

well im outta here--bright and sunny time to hit the trail..later...............

[nothing to lose]

Ok, I got "some kind of motor", not shure what yet. Says 90V fields I think and something else for the armature. Just something I picked up from a salvage yard cheap.

So if I supply less currant or volts to the fields to create weaker flux and this happens to be a shunt wound motor are you saying I can beef up the torque (HP) and shaft speed?? Does not sound correct to me the way I am thinking, but if it works then maybe I can make an electric vehicle motor from it somehow?? I forget the HP rating off hand.

Now if I can up the HP and speed by using lower field strenght, then how would I shut it off? Just kill the armature volts?

[hiker]

if you want more speed[rpms]you will have to rewire the armature with less turns of wire--if you want more tork you will have to rewire the armature with more turns of wire..what dean is talking about most likly only applys to the big motors[real big]..

i dont think you need to worry about a runaway hand drill motor..i just took the field coils out of a vacume motor and replaced the field coils with mags--makes for a good

pm motor--not  much of a gen thou..need to rewire the armature with smaller wire..