On Jul 30, 2009, at 5:57 PM, Kyle Mcallister wrote:
--- On Thu, 7/30/09, Horace Heffner <[email protected]> wrote:
Some of the youtube experiment videos show motors that seem
to run well with low current. I would not have expected the
contact to be so poor. Maybe it *is* indeed poor and
that is key to getting enough torque. The fact
flywheels are required on some experiments is a sign that
contact is poor. Torque is proportional to i^2 in a
good operating range, so putting all the current through one
bearing at a time will maximize torque for the total current
draw.
The one I have runs with two ball bearing race assemblies, has a
piece of 5/8" threaded steel shaft through the center, and a ~5"
diameter aluminum pulley as a flywheel. You have to turn the thing
first before applying current, then it takes off. As far as I can
tell, this prevents welding.
Any suggestions for improving steel-steel electrical
contact in the bearings would be appreciated.
Not sure how good you want it, exactly. I suppose you could clean
the thing in a few solutions to get rid of all the oils/grease in
the thing. Maybe stand the entire assembly on its end, remove the
top side grease seals from the bearings,
Yep, did all that. Running without grease seals. The bearings run
very smooth and a long time with a manual spin.
and drip in some melted Wood's metal while heating the bearings.
You'd have to preheat the bearings before each run, but Wood's
metal is cheaper than galinstan. Or buy some real mercury from
United Nuclear.
Yes, I have the mercury, but I of course wouldn't want to use that
except as a last resort!
But you might not want to go that route.
The power supply I used was a 12V 1200 peak ampere car booster
pack. Never had problems with bearings attempting to seize. I don't
think it is really a problem, especially since things are in motion
to start with (the initial spin.)
Don't know if I can be of any more help, but if I can, let me know.
--Kyle
Say, thanks for the response! There's nothing like robust equipment,
eh!
If I can get my dual motors to work in series, which I doubt, I
expect the stainless steel bearings to simply wind down to a stop. I
expect the stainless ones to wind down to a stop also when stand
alone, but if they run alone it will be easier to just turn off the
power right before it freezes (hopefully). The stainless ones may
run down before the ordinary ones get up to speed, especially if the
current makes them "sticky". I suppose I could make a run using one
stainless and one regular bearing on one motor to demonstrate that
both bearings conduct enough.
Did you ever take a look at your actual current? The lowest
resistance I've been able to measure is 0.2 ohms per bearing, which
is 0.4 ohms for the system. The resistance of the rest is about 0.1
ohms for me, but I don't know what the resistance of your shafts and
power leads are. My shafts are 1/2 diameter aluminum rod. In any
case, using 0.4 ohms for the system, just the bearing resistance,
that is a max of 30 amps for continuous conduction, and way less I
expect due to the highly intermittent nature of the conduction.
I added a 0.3 ohm nichrome resistor to my circuit as a current sense
resistor for my scope, and as a means to try to keep the current down
initially. That gives me a 0.8 ohm resistance overall, or a max
continuous current of 15 amps to start out. I can probably increase
the effective amps greatly, if necessary, by placing a large
capacitor around the motor, in parallel to it.
It is probably the case that I don't have any problem at all, except
the desire to demonstrate the two motors running in series, which
looks to me impractical at this point. I could run them in parallel,
but that would not prove much visually. I wanted to post a single
continuous short video on youtube demonstrating that magnetic
material is required to get the effect. I'll just have to do it in
parts.
Best regards,
Horace Heffner
http://www.mtaonline.net/~hheffner/
