Power Pro headstock cooling

[rat1932]

120 degrees is called 'the threshold of pain'! But I never found that much heat from the motor. The heat came from the upper poly V-belt, caused by too much belt tension. After the belt tension was reduced, the belt temp on the left quill shaft dropped from above 180 to 122 degrees. This test was made when the headstock only had itself running, no load on the motor, not even a saw blade.


Bob

[shopsmithpaul]

I found this thread while looking for info on heat problems with the powerpro when it was mentioned in the recent competitors thread. I did not know this was a problem as I am thinking about buying one.

Since I am rebuilding a headstock I decided to look at that. The original motors have a fan that blows out through the bottom by design of a shroud around the motor sheave. The air is pulled in though the various small openings in the headstock and then through the motor.

Looking an the powerpro it looks like there is no directed air flow to the outside from the end bell of the motor. It looks to me that the air coul just swirl around inside with no direct path.

For the powerpro I was thinking one might cut a hole in the end of the motor pan by the fan bell and then put some foam sealing tape between the fan bell and the inside of the motor pan. This would only allow cooler ouside air in around the motor and the heat sink above, creating a positive preasure inside and force air out the various other openings. Obviously a filter of sorts would be needed over the hole. I was thinking a foam type often found on electronic cabinetsin industrial settings.

I'm not sure how clear that is but any thoughts on this

[JPG]

WHAT is creating the 'positive pressure'?

[shopsmithpaul]

Not sure if I'm explaining properly. By sealing the intake part of the fan shroud to the inside of the motor pan it can only pull air in from the outside through the new hole made in the end of the motor pan. So the outside air coming in through the fan and blowing over the motor should create a slight positive pressure in the headstock as it seeks places to escape such as the vented end plate on the opposite end around the SPT drive hub. This might create more of a direct flow of air over the components like a standard headstock. It also may be necessary to cover the bottom air vent.

[WileyCoyote]

You would have convection but I doubt enough to move much air. The addition of a small fan would add positive air flow. Calculating the size needed should be a simple task for an engineering student or a SS user with some fine Googling skills.

[dusty]

While insufficient air flow could certainly cause a heating problem, I seriously doubt that this is the issue with the PowerPro. Look at how the motor and power supply are packaged. The entire encasement is one big set of cooling fins.

If I understand the overheating issue correctly, the problem arises only when at the higher speeds. What varies as the speed changes? Current drain on the power supply thus heat variations! If I had to troubleshoot this, I would start with a power supply performance analysis varying load demands.

Having been an environmental test engineer, I would be prepared to overheat until total destruction to find the problem.

If I owned a PowerPro, I certainly would not be operating at the higher speeds.

[BuckeyeDennis]

While insufficient air flow could certainly cause a heating problem, I seriously doubt that this is the issue with the PowerPro. Look at how the motor and power supply are packaged. The entire encasement is one big set of cooling fins.

If I understand the overheating issue correctly, the problem arises only when at the higher speeds. What varies as the speed changes? Current drain on the power supply thus heat variations! If I had to troubleshoot this, I would start with a power supply performance analysis varying load demands.

Having been an environmental test engineer, I would be prepared to overheat until total destruction to find the problem.

If I owned a PowerPro, I certainly would not be operating at the higher speeds.

As the motor spins faster, the winding-commutation frequency increases proportionally. As that frequency increases, the corresponding eddy-current loop diameter gets smaller. As the eddy-current loop diameter approach the thickness of the laminations in the motor iron, the laminations become much less effective at blocking the eddy currents. So you get little eddy-current loops circulating in the iron, doing nothing useful, but heating the iron.

This is the exact same principle as an induction heater, except that it is an undesirable artifact in a motor. It’s also why 60 Hz transformers will overheat if operated at 400 Hz - the iron laminations aren’t thin enough.

[Mike907]

Thanks, Dennis, for the explanation of transformer heating. As a former airline pilot, I was always unsure why the company had to provide different iPad chargers for use in the cockpit while plugged into the aircraft's 400hz power (almost every airline pilot in America carries an iPad instead of 40 pounds of charts nowadays). However, I never saw a case of someone's personal Apple charger getting hot.

Mike

[DLB]

As the motor spins faster, the winding-commutation frequency increases proportionally. As that frequency increases, the corresponding eddy-current loop diameter gets smaller. As the eddy-current loop diameter approach the thickness of the laminations in the motor iron, the laminations become much less effective at blocking the eddy currents. So you get little eddy-current loops circulating in the iron, doing nothing useful, but heating the iron.

I'm bringing this thread back because, as a more recent PowerPro convert, I'm trying to decide what, if anything, I should do to improve cooling. There are some great ideas here, and we see some of those ideas implemented in another recent post: https://www.shopsmith.com/ss_forum/view ... 88#p273588 My first comment is to expand on Dennis' explanation. I believe that higher speed operations of the headstock also contribute a burden to the motor as the bearings and belts are turning faster. I'd have to find my notes or redo the experiment, but by measuring AC input current while varying speed on a conventional headstock I found input power to have minimal change up until, IIRC, around Saw speed. From that point on current increased rapidly as speed was increased. Assuming I'm correct in attributing this to belts and/or bearings, the impact would be much greater at the higher speeds of the PP.

Second comment - I'd like to know more, if possible, about the impact of belt tightness discussed in an earlier post on this thread: https://www.shopsmith.com/ss_forum/view ... 38#p251538 While I agree that 1/4 inch under light pressure was once in the manual, it has been changed and that seems way too loose for the PP. Speed was not specified, unless I missed it, but I've noted that Idler shaft temp elevates quickly at all speeds I use. (Normal, per Jim McCann, but I didn't measure.) Are the temps cited a concern for others? I can't make any obvious correlation with Power Supply over-temperature, but Idler Shaft heating is a secondary concern.

And a question: I went the DIY route, so my motor pan has a number of holes with no obvious utility, other than improved convection cooling. I'm wondering if those holes are present on the 'new headstock' version PPs. Specifically, I'm talking about the motor vent from the AC motor, the motor mounting holes, and the original power cord hole. If some, but not all, are present, please specify. Thanks.

- David

[BuckeyeDennis]

I believe that higher speed operations of the headstock also contribute a burden to the motor as the bearings and belts are turning faster. I'd have to find my notes or redo the experiment, but by measuring AC input current while varying speed on a conventional headstock I found input power to have minimal change up until, IIRC, around Saw speed. From that point on current increased rapidly as speed was increased. Assuming I'm correct in attributing this to belts and/or bearings, the impact would be much greater at the higher speeds of the PP.

- David

I may be able to shed a little light on the belt-drive issues. In general, power losses do increase with increasing speed. If the bearings have a constant drag torque, then their power loss (i.e. heating) will be proportional to RPM. Similarly, the belt has to flex in order to wrap around the pulleys, and the mechanical hysteresis in the belt consumes a certain amount of energy as it flexes. So again, the higher the RPM, the more frequent the flexing (at a certain spot on the belt), and thus the more power is lost to heat. For V-belts, there is also a significant amount of loss due to friction between the belt and the sheaves. These effects are all generally lumped into the belt-drive efficiency ratings. Most that I've seen are in the ballpark of 85% - 95% at rated load.

If you really want to dig in, Gates publishes volumes of belt-drive design data. Better yet, they provide their Design Flex Pro software for free. You can select from a wide variety of Gates belts and pulleys, input the speed and power requirements and center-center distances, and the software will provice a rather complete design evaluation, including operating efficiency and recommended belt tension.