Assuming one has a fairly straight approx 4.5" OD x 10" long round motor with smooth surface, what would be a good way to get some heat sink type of deal ?
I am thinking about
A) getting say 3/8 or 1/2 alum channel, cutting it into pieces the length of the motor and attaching the "strips" flat bottom toward the motor around the circumference. 3-4 zip-ties holding the Us against the motor
B) doing something with copper or alum sticky foil, where you create as many folds as possible in a "pleaded" fashion - infinitely messier affair, but would it be more efficient in dissipating the heat ?
Either one will work, either one won't get a terribly good thermal interface to the motor case. To get a good thermal interface to the case you'd need to machine your heat sink surfaces to the diameter of the case, and possibly strip any paint off the motor.
If this is a brushed motor, or otherwise has windings on the inside, you're wasting your time. The heat is generated in the windings, so unless they have a good thermal path to the case you'll probably gain more by blowing air through the motor than by trying to conduct heat away from the part that's not getting hot.
This being a brushed DC motor, most heating is prolly coming from the rotor coils. Or is it ? Could there be eddie/induction currents that are heating up the exterior ?
When I do heavy milling that this motor/mill was never designed for :) , the exterior gets hot to the touch.
If the heat is coming squarely from the rotor, via the 2 bearing and air gap, the rotor itself must be mighty hot ! :(
I am thinking about getting some compressed air to enter the motor's interior - somewhere near the brushes, go up through the gap stator- rotor gap and then out (will prolly require a hole or two on top).
Rather than a lot of separate channels, perhaps get some heatsinks like ebay # 220255259730. (I have some heatsinks about twice that big, from junked motor controllers.) Bend it to be a reasonably close fit to the diameter, or cut a shallow 2.25"-radius channel in some
3"x1" aluminum bar and mount the heatsink on the flat side. If you made two assemblies you could bolt them together with 7" bolts across the motor. Make the motor casing as smooth as you can and use a minimal amount of heatsink grease between the motor and heatsink assembly.
If you go with your first plan, use large stainless steel ring clamps (like for round ducts) rather than zip-ties to hold your channels against the motor.
Ebay # 170237692452 will cut wide heatsink channel to size, but is somewhat more expensive than big surplus heatsinks.
Folded fins of foil won't do much because the tiny cross-sections won't conduct enough heat away from the motor.
If you have a good chilled water source or a small radiator, you could wrap about 30' of 1/4" copper tubing around the motor and circulate water or antifreeze through the line. But as noted in other posts, cooling the outside of the motor might not be as good as blowing air through the case.
Look at the chart on this page before you get too concerned. "Hot to the touch" is not necessarily anything to worry about. Good DC motors are usually class F or H.
The motor body has about 141 sq in of surface area. Each 1/2" high al U-channel 10" long adds 20 sq in (four exposed fin sides)so, assuming intimate thermal contact (not bloody likely!) you'd need 7 such channels to double the effective area or halve the thermal resistance to ambient. In fact, the "fins" won't be anywhere near that efficient due to thermal drop across the interfaces. Some good thermal compound between the channels and the motor could help considerably.
Increasing air velocity over the motor will very likely work a lot better.
I would assume so. The armature winding resistance will rise with temperature, which will increase I^2R losses. If the speed control utilizes the usual IR compensation, then the RPM setpoint will drift as the motor gets hot. But I don't think either was a concern here.
I've experienced this firsthand. I built a device that tested the leader to hub attachment of video cassettes as they were assembled. A pair of DC motors engaged the hubs and applied a torque that was controlled by limiting motor current. The torque would drift as the motors warmed, not enough to be a problem, but measurable.
1/4 inch copper pipe, brazed to the outer case of the motor with chilled water will keep the case cool - of course the armature will still overheat - perhaps you could consider forced air cooling through the motor rather than heat sinking the case?
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One some smaller motors (2.5" OD, 4" long), I made some by boring out some 3.5" OD 6061 Aluminum billet and using a 1/8" parting tool to make fins. The heatsinks were bored about a thou undersize, slit, and glued on with some thermal adhesive.
Ended up attaching a muffin fan at the bottom of the motor to suck the hot air out. For now it is a rather weak CPU fan, will replace later with a 24V/80CFM fan. Used some sticky alum foil to make sure it is air tight.
The problem is they already have an impeller fan on the shaft that "tries" to drive the air out of the bottom exhaust. Now that one is so inefficient of design that I can barely feel the air moving even at the top RPMs .
Sounds like a ideal application for the 120V. muffin fans I salvaged from outmoded Dot Matrix printer soundproofing enclosures. Gerry :-)} London, Canada
You might try to find a small centrifugal blower to establish some positive pressure in airflow, which can be directed into one end of the motor (with some flexible tubing).
Fans don't push much air when a restriction is encountered (the interior of the motor case, or being piped to another location).
A small AC motor blower that doesn't have the blower motor in the air intake stream, so as to not add heat to the cooling air, will potentially provide years of service with only some routine cleaning to remove dust.
Dayton, among other manufacturers, make small blowers that will be well-suited for the purpose of cooling small motors.
The only additional feature that one of these small blowers might need, would be to add a screened cover for the air intake to reduce the chances that big pieces of debris from entering the intake.
Second hand range hoods are often replaced rather than cleaned up, and can be picked up at the curb or bought for a couple bucks at yard sales. Gerry :-)} London, Canada
The kinds of small blowers I use are on the scale of palm-sized, AC induction motor (no brushes to wear out) with outlet ports of about 1.25 to
2", speed approximately 3,000 RPM, wheel size 2-3" diameter and 10-20 CFM or thereabout.
They're quiet, compact (so they can be mounted almost anywhere) and are essentially trouble-free if cleaned occassionally and oiled with about 4 drops of oil per year.
From my experience, an external heat sink is counterintuitive. Blast lots of cool air into the brush holder end of the motor instead. just my 2cents worth ben
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