Thread: Fan Modification.

Originally Posted by Hornetb

Just another thought.

When you burnt your motor up what speed did you have it running and how big was the cut you were taking and for how long? High or low gear?

With these variable speed controllers it can be tempting to just dial in whatever speed you want and think it's going to be ok but generally these motors like to run on the faster side. If you were running it slow you might have been overloading the motor and drawing a high current, coupled with the reduced rpm decreasing air flow and you get your over heating issue.

Just saying it might be how you are using it that is causing the problem?

At the time I was using a 3/8" slot drill to square up a small 2"x3"x0.25" alloy plate. Running in low gear at about 1000rpm. So no great load on the motor.

In actual fact the original motor failed because two of the comutator segments had shorted together, more acurately two windings ! I did hear the motor noise suddenly change at the time but didn't realise what it was until I saw the smoke.

Rob (Nearnexus) and i put 180v DC treadmill motors on our lathes. We both ended up burning them out as we just could not keep them cool. I got mine hot enough to start to melt the lacquer on the windings and then of course it shorted. What really gets them hot is when they brake, does your mill have a brake on it? We both had computer fans pulling air through the motors but it was not enough.
https://www.woodworkforums.com/f65/ma...ersion-159263/ It's a long thread but may help you out....

Cheers,
Ewan

Originally Posted by BaronJ

A comment about the vent holes.
I monitored the air temperature coming out and added extra holes to increase the through flow of air. The idea is to restrict the airflow enough for it to pick up the maximum amout of heat and carry it out of the vent. If the airflow is too high then you don't get all the heat you can out. The other consideration is dirt and dust falling into the motor housing.

I don't know where you get the idea that slowing down air flow will carry away more heat. Rate of heat transfer between materials depends on the nature of the material and the temperature difference between the coolant and the object being cooled. As soon as the coolant heats up it's ability to draw heat away from the object being cooled is rapidly reduced.

If it were me I would have just made ~20 mm wide full length louvered slots on 3 sides near the top. Now that you have put holes in the top I would cut out all bar about 10 mm near the sides of the whole top and cover the hole with a sheet metal cover ( to stop gunk falling in) stood off about 20 mm from the top.

as bobl says...to remove more heat you need more airflow

Axial fans do not I repeat DO NOT like restriction. best to use a backward curve centrifugal fan if restrictions are present..and even then air flow volume is still dependent on static pressure...the higher the static the less volume. ...air is like water ..it likes the easiest path

If you look at most motors incorporating a fan on the back of the motor the fan is a paddle type which also has the ability to handle higher static pressures than axial.
paddle fans can also deliver more air for comparative size of axial diameter


and that was a big part of your problem...you let the smoke out

Originally Posted by Ueee

Rob (Nearnexus) and i put 180v DC treadmill motors on our lathes. We both ended up burning them out as we just could not keep them cool. I got mine hot enough to start to melt the lacquer on the windings and then of course it shorted. What really gets them hot is when they brake, does your mill have a brake on it? We both had computer fans pulling air through the motors but it was not enough.
https://www.woodworkforums.com/f65/ma...ersion-159263/ It's a long thread but may help you out....

Cheers,
Ewan

Hi Ewan,
Very useful information about the treadmill motors. I've been looking out for one or two of those without any luck. I'm not sure about the brake. It does stop very rapidly when the stop button or the shield switch is actuated. Though I did discover that the spindle has a fair amount of drag which reduces when it gets warm.
Thanks for the link. I'll peruse that with interest.

Originally Posted by BobL

I don't know where you get the idea that slowing down air flow will carry away more heat. Rate of heat transfer between materials depends on the nature of the material and the temperature difference between the coolant and the object being cooled. As soon as the coolant heats up it's ability to draw heat away from the object being cooled is rapidly reduced.

If it were me I would have just made ~20 mm wide full length louvered slots on 3 sides near the top. Now that you have put holes in the top I would cut out all bar about 10 mm near the sides of the whole top and cover the hole with a sheet metal cover ( to stop gunk falling in) stood off about 20 mm from the top.

Thankyou for that info. It seemed somehow right to allow the air to absorb some heat as it flowed over the hotter surface. I can see where as the temperature difference reduces the cooling effect becomes less. I like the idea of a big hole and a flat cover over the top. That would give roughly the same apature as the fan.

Originally Posted by eskimo

as bobl says...to remove more heat you need more airflow

Axial fans do not I repeat DO NOT like restriction. best to use a backward curve centrifugal fan if restrictions are present..and even then air flow volume is still dependent on static pressure...the higher the static the less volume. ...air is like water ..it likes the easiest path

If you look at most motors incorporating a fan on the back of the motor the fan is a paddle type which also has the ability to handle higher static pressures than axial.
paddle fans can also deliver more air for comparative size of axial diameter


and that was a big part of your problem...you let the smoke out

Yes I did ! The fan project I hope will reduce the risk of doing it again.

Thanks guys. The wealth of knowlege and experience here is very much apreciated.

BaronJ

how big is the diameter of that hole

I may have a fan that will certainly deliver much much more air that that peeezly little axial


One of the ones I am thinking of needs a hole diameter of around 90mm or 125mm...but they are 240v

Originally Posted by eskimo

BaronJ

how big is the diameter of that hole

I may have a fan that will certainly deliver much much more air that that peeezly little axial


One of the ones I am thinking of needs a hole diameter of around 90mm or 125mm...but they are 240v

Hi Eskimo,
I'm not sure 80mm I think. Its a standard computer PSU fan salvaged from a scrap one. The chrome finger guards came from a scrap HP server that had a dual redundant power supply. I have several fans of varying sizes and voltages, some 250mm 240V ones out of instrument cabinets. They have alloy blades that will take a finger right off if you don't take care. I'm tempted to scrap them and salvage the ball races.

The fan I used has a high airflow, much higher than is normally found in fans of this size, which is why I picked it from the ones I have.