which is quicker

[quakebo]

which is quicker a motor that draws more amps or one that draws less amps.

I tied a deathstar runing at 4v with shuted brushes and it drew about 1000 ma on my power surply.
I also tried some normal but slopeing brushes and got about 1000 ma
but when I tried normal unmodified brushes it ran at 500 ma

[Mike R]

I'm not Monty, no where near it, but there is a thread that kinda answers what you want to know

http://www.slotcartalk.com/slotcartalk/showthread.php?t=22857

The thread was label brush phenomenon and explains a lot regarding types of
brushes, what Monty sees as far as armature draw and such.

Me personally, on a SpeedFX 16D with a chinese arm, after doing a water break with Gold Dust Pro motor brushes, I'm looking for an almost 2.0 amp draw on the power supply at 2.0 volts after it runs for a few minutes. That's with the endbell set at max timing allowed and good brush spring tension. Anything between 1.8 and 2.1 I consider good, and at more volts the amperage will go up. On the track I run on, to me the higher amp drawing motors perform better than one that only draws 1.0 amps @ 2 volts.

Read that thread I listed above, and read Monty's replies, then test on the track you run on. That is the only surefire way to know for sure.

Mike R

[[email protected].]

Quakebo,

Somehow I feel a bit 'baited' by your question, like you are just begging to find the exception to generalities which have worked for me, and which have been clearly noted as being generalities.

What I have stated, many times, is that ALL OTHER THINGS BEING EQUAL, a motor drawing more power on the bench is most likely the faster one.

Place great emphasis on the capitalized phrase, and on the 'most likely' clause.

If you are changing the brush geometry, all other things are NOT equal!

Shunted brushes almost always exhibit a raised current draw. This is because the shunt conducts beter than the spring, and because the spring is coiled a good .020 furthur to get over the thickness of the shunt.

Brushes, being made from a sintered mixture of metal powder, carbon, and adhesive, are rather high resistance compared to a metallic conductor. This is why they run hotter than the rest of the motor. This is also why your motor runs with less current draw when you cut down the brush: the resistance is increased as the cross section of the brush is reduced.

After plenty of experimentation, my findings are that cut brushes are NEVER faster, even if the cut is arranged properly to increase timing. Reduction of friction is not a factor. The amount of increased conductivity from a larger brush more than offsets any gains from reduced friction. Actually, the reduced unit pressure on the full size brush track may also be the condition of least friction as long as the spring tension remains the same.

[Martyn]

Monty:

If you don't mind. I think a point you've eluded too need be made more clear.

IF the increase in amp draw is soley from an increase in spring force, e.g. more preload due to shunting on insulation the net power is less as the frictional losses consume a greater percentage of the total power. Too much spring pressure is akin to a gear to tight or a tire rubbing the body, right?

Cutting a brush to a smaller foot print at the same preload has a zero net "total force" effect. Unit loading is higher, ounces per square inch, of course but total force is the same. Higher unit pressure will increase com and brush wear rates.

Normal brush ampacity is about 50 amps per square inch, as such almost every motor brush in our world is too small to begin with. From this stand point, and I've heard you say it many times, it's impossible to over spring the brush. However true this is, and it is true from this view, using more spring than is required to get good commutation is a power waste.

Marty

[formerracer]

In several books on DC electrical equipment, they state that, in use, brushes really never touch the commutator: there exists a plasma barrier which is microscopic. That plasma conducts the electricity from the brushes to the commutator. Motor brushes are about 1/8x1/8 square, meaning they are .0156 square inches.... if you are "pulling" 10 amps that means you're putting an amp density of 640 amps/square inch across the brushes. 10 amps is a very conservative number, 50 amps cited before yields 3200 amps/square inch!

When properly tuned, there is no friction between the brush and comm under power. Brush tension is used to control the brush "backoff" resulting from the plasma and gas generation under power. This is why lower power invites low brush tension and high power vice-versa.

Some things to think about.

[Martyn]

Formerracer:

I didn't mean they carry, or can't carry 3200 amps per unit area, I mean brushes (graphite/copper) are designed to carry but 50 amps per square inch pointing out there already over taxed state. These numbers are like bearing 10X life values. It doesn't mean they won't carry stupid excess above this number it just means if you wish "normal" brush life these limits are what testing says give acceptable life. Most of this type of data is based on industrial use. 20,000 hour service life. A healthy strap motor is drag service can draw in excess of 225 amps for a brief period on the launch.

Even though the plasma does carry the current, don't mistake this to mean that by some magic it prevents the "pressure" from being transmitted to the com. Remember a fella named Newton? Equal and opposite reaction?

So, I will beg to differ with you that there is zero friction "if properly tuned", reference this link.

http://books.google.com/books?id=TsXEHPofiAYC&pg=PT484&lpg=PT484&dq=motor+brush+friction+versus+spring+pressure&source=web&ots=QNIJdbfgb_&sig=FZ7R9mwvSJJip6zX4dZrWNibPF4&hl=en&sa=X&oi=book_result&resnum=9&ct=result

If this link doesn't work, cut and paste into your browser.

Yes, more powerful motors need stronger springs, don't think I said otherwise.

If your still unconvinced try drag racing a 16D. Get the car dialed in with a light spring then change springs to something like a Camen extra heavy you will have added several tenths to your ET. Maybe a second :)

Marty