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Cool down a mini-mill motor.

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Johan Crous12/01/2013 17:21:50
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I have a mini-mill with the classic motor (smooth, un-ribbed sides).

When I mill a big cut it eventually get hot. I have to stop perodically to let the motor cool down. Same with my lathe.

I have been thinking to add a fast running (computer type) fan on top to force suck air through the motor at high speed. This may assist the motor to take more time before I have to take a break.

The ordinary 100mm (4 inch) fan is too big as the fans blades are extruding to past the holes in the top and the dead, solid area of the fan blocking the motor's holes. A 75/80mm (3 inch) will have a smaller inner area, causing the blades to be exactly over the breathing holes of the motor.

I have read an article on Andyf's Warco WM180 lathe page about adding a computer fan to the motor to cool it down and this inspired me in this direction of thinking.

Any ideas or references?

Stub Mandrel12/01/2013 17:36:30
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4311 forum posts
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Hi Johan,

I have had similar thoughts in the past - the built in fan is ineffective at low speeds.. My thoughts are:

  • You need to make sure the air flows the same way as the built in fan sd they don't fight against each other.
  • If it needs to 'suck' rather than blow agood seal will be more important.
  • Make sure it can't suck swarf into the motor.

Neil

martin perman12/01/2013 18:23:09
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Johan,

Have you considered putting a tube on top of the motor to adapt to your 4" fan, have you also considered putting filter media across the air intake at the other end to stop muck going into the motor.

Martin P

 

 

Edited By martin perman on 12/01/2013 18:24:29

Johan Crous12/01/2013 19:36:44
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Posted by martin perman on 12/01/2013 18:23:09:

Johan,

Have you considered putting a tube on top of the motor to adapt to your 4" fan, have you also considered putting filter media across the air intake at the other end to stop muck going into the motor.

Martin P

Edited By martin perman on 12/01/2013 18:24:29

Martin,

No I have not, but it is a very good idea. If the air goes from bottom to top I am going to do a filter, but it is not necessary if the air goes from top to bottom.

I don't know if the motor has a fan at all. I will test tomorrow witha feather. I will then also see the air flow direction. The suggestion of a tube/funnel/shrowd is a very good idea, as I can then forece more air through the motor.

Anyway, I doubt if the air will completely cool the motor down so that it does not go hotter than a certain amount to be able to run continious. If I can only slow the warming effect down by at least 50%, then I will be able to work longer. Maybe I must slow down the rate of feed to even more slow, as it is only a mini mill. I am now doing a big cut: a tool holder for a QCTP. Lots of milling through steel for a mini-mill, but it does just that.

The next step will be for my lathe as well.

Stub Mandrel12/01/2013 21:25:36
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Johan,

The motors have a pressed steel fan inside. The efficiency of the fan is related to something like the square of the motor speed, so when running slow it hardly functions.

I have done the task you are planning (milling QCTP blanks) and I found I could work for quite a long time without getting worried about a warm motor. How fast are you running the mill?

Neil

Andrew Johnston12/01/2013 21:56:56
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5636 forum posts
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The motor getting hot may not be a problem. Certainly induction motors are designed to run hot when operating at full load. Hot means you will burn yourself if you touch the motor casing.

Regards,

Andrew

FMES12/01/2013 21:59:54
603 forum posts
2 photos

I have a portable magnetic drilling machine that suffers from slow speed lack of cooling, I made up a small adaptor to take a vacuum cleaner hose over the fan shroud, - start up the vac, no more cooling problems at slow speed.

Lofty

Stub Mandrel13/01/2013 09:09:35
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Andrew.

These are permanent magnet DC brush motors. Hot casing means VERY HOT coils.

Neil

Andrew Johnston13/01/2013 09:27:32
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5636 forum posts
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Neil,

Thanks for the information. In electric motors (DC and AC) it is common to have winding temperature ratings in the order of 100-200ºC. Granted in a DC brushed motor, with rotating coils, the cooling is less effective than an induction motor where the coils are static, but the case can still get pretty hot without a problem.

I suppose what I was really trying to say is there actually a problem, or is the OP just being understandable cautious?

Regards,

Andrew

Ian S C13/01/2013 09:59:58
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7468 forum posts
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Even with a motor designed to run at higher temperatures, an extra fan to drop the temperature a little can't hurt. I believe that some high performance electric motors used in model boats are actually water cooled. Ian S C

Andyf13/01/2013 10:10:55
392 forum posts

I know from other forums that there have been occasions when small (up to around 600W) Chinese DC permanent magnet motors have overheated and burned out. The problem arises in high torque, low speed situations such as turning large diameters.

My lathe has such a motor, and even when running at high speed its internal fan doesn't shift much air. I fitted a computer fan to the end, blowing the same way as the internal one when running forward; Of course, when the motor is reversed the two fans fight one another, but the internal one is so weak that the external one wins, and in any case the motor is only reversed to reposition the carriage, rather than to do any real work. I'm not sure if Johan's mini-mill has a reverse switch for its motor, but if it does it will be used very rarely.

Andy

Johan Crous13/01/2013 14:55:14
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I have no idea at what power I am running the current job (I don't want to remove the job item I am working on before I am not done).

I am cautious (naturally). The motor is not getting as hot that I cannot touch it. It is still low enough that I can touch it without burning or without pulling away your hand when yoy touch it by accident. I guess still lower as bathing or shower water temperature.

I must also mention that the ambient temperature here in the summer is between 28 and 36 degrees of C (82 - 97 degrees of F). My swimming pool pump is equiped with cooling fins and it get very hot. Too hot to touch.

I just don't want to burn the motor of the mill. I have no previous experience with induction motors. At the low speeds I could not find the direction of air flow. I am now thinking of adding a plastin agricultural water pipe reducer over the motor that I can attach to the motor with a hose clamp. On the other side it is a little bigger, wher I would like to add a small extracter fan that can deliver 90 cfm air. I think this should delay the getting hot some time. I can even keep the fan running when the mill motor is swithed off. I guess it will assist a bit.

But again, I may be over cautious, but I have no experience of these type of motors.

Johan

Michael Gilligan13/01/2013 15:04:45
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Johan,

As a convenient point of reference [remembered from using laboratory glassware]

If you can just hold the palm of your hand on the motor, without pulling away ... it's probably around 70°C.

MichaelG.

Stub Mandrel13/01/2013 15:31:08
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I have burnt out two mini lathe motors. You can keep them staggering on by shorting the blown coils - naughty but effective, but the slow speed perfomance suffers.

I bought a spare from a genial chap out of the ME classifieds. When I cam to try it two coils were blown!

Because it is an armature motor, the heat on the outside needs to get through the magnets and the casing - theres a lot of thermal inertia to overcome and the windings can blow with an impressive amount of smoke with the casing quite cool enough to touch comfortably. I think a fan has to be the best option.

That's why my mini lathe is now powered by an ancient hoover fractional motor, as I have a tendency to abuse it work it quite hard. I tried an experiment an hour or two ago using my box tool to reduce 7mm stainless to 0.090" - a cut depth of over 90 thou which is going some for such a light lathe! I couldnt understand where teh swarf was going, until I realised it was widnin under the tool tip.

Neil

Ian S C14/01/2013 12:10:50
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Neil,?? widnin under the tool tip. Ian S C

KWIL14/01/2013 12:40:40
3308 forum posts
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widnin=windingdevil

David Jupp14/01/2013 13:14:30
750 forum posts
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Posted by Michael Gilligan on 13/01/2013 15:04:45:

Johan,

As a convenient point of reference [remembered from using laboratory glassware]

If you can just hold the palm of your hand on the motor, without pulling away ... it's probably around 70°C.

MichaelG.

I'd suggest that the surface temperature probably isn't as high as that if you can hold your hand in place. Maybe with glassware you 'get away with it' because glass is not a fantastic conductor of heat, with metal object I certainly would not want to put my hand on anything very much above 50 C.

ChrisH14/01/2013 15:20:00
878 forum posts
29 photos

I have a welding machine which used to overheat and cut out when doing any serious prolonged welding. I also fitted a computer fan - the first one that came available, no research or calcs done there - and it transformed the welder, you can weld all day with no problems now.

Gordon W14/01/2013 16:01:24
2011 forum posts

My lathe motor is a modern "tin" cased job, I had thought about drilling some extra holes for cooling. The welder was greatly improved by drilling extra holes, in the bottom and along the top edge, with feet to allow air flow. This mod. moved the machine up a class.

Clive Hartland14/01/2013 16:11:52
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2604 forum posts
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Have you considered wrapping some small bore copper tubing around the casing and pump some water around to cool it? Needs a tight wrap though to make good contact.

Quite feasable using a small pump and a header tank away from the machine. Ypu could even stick a thermometer in the tank to see the temp. rise.

Clive

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