Shopsmith Forums

Ed in Tampa wrote:A motor is stressed the most when it is turned on. A continuously ran motor if properly cooled should last the longest. Also most motors draw their max amperage on startup so any cooling problems should be accentuated by an off on cycle.

I believe we aren't talking about motor heat. The heat issue in the PowerPro is related directly to the electronics.

Some 'basic' physics (etc.).

Higher speed with a pulse controlled motor requires faster switching.

Higher speed will also likely require higher current.

Both add to heat generated by the motor and control electronics.

The confined space of the SS headstock does not allow much air flow for heat transfer.

IMHO getting all that 'stuff' to fit was a notable undertaking.

IMHO the heat buildup is a 'detail' not sufficiently considered nor revealed.

AIUI all that surface planing to test it was done with considerable 'rest' times.


I agree it is a good improvement, but has some limitations that IMHO are not mentioned enough in the marketing etc.




I still want one!:rolleyes:

nuhobby wrote:This is true generally, but an electronic Soft Start, as the PowerPro offers, seems like it would greatly reduce that stress.

I turn my PowerPro on and off all the time. It's a finessed experience, no hard switch-clacks, and no dimmed lights.

Actually I think it a natural law, starting anything requires more effort than just maintaining a constant state. I agree soft start lessens the effect on the motor in that it transfers some of that stress to the speed controller but it still eventually translates into heat.

Heat is a killer to electronics. That is why most PC's have at least two fans, one on box and one on the cpu chip itself.

Having worked in huge computer rooms most of my life, I saw all sorts of ways to deal with the heat. Early on most computer rooms had under floor air conditioning that blew very cool air up into the computer. When you had to work on the machine you almost froze to death.

Later many big installations went with chillers that actually had coolant lines and cooling liquid running through the computer circuit boards carrying away heat.

Every effort was made to keep the component temperatures low.

Yet many problems turned out to be heat related. A standard diagnositic tool was can of spray gas that would drop the temperature almost to frost point instantly.

I worked on many bugs that would shut down the computer and then allow it to power back up only to shut it down again a little later. We shot those types of problems by blocking the air flow to the circuit and or freezing it.

Just the heating cooling cycle can adversely effect electronics and even break down solder joints.

How much heat? I can't tell you but the engineers can. I suspect when the SS shuts down due to heat it has reached the point the engineers feel has the potential to cause damage.

Again the Power Pro is a different animal from a typical induction motor.

An induction motor draws many times its normal running current at slow speeds and that creates the heat at startup. It only takes a few seconds to reach oxidation temperature(burning!) of the insulating varnish if stalled.

The PP also has this potential, but it is more severe with a pulsed stepping motor. The windings have much lower resistance and the pulse currents are much greater(albeit shorter duration), but the slow rotation creates an even greater risk of overheating than the induction motor. So much effort is expended to 'ramp up the speed'(to keep the armature motion in sync with the controller as well!!!).

A zebra is not a striped horse. Oh and 'zebas:rolleyes:' come striped two different ways!:D

So the comparison is:

A) Heat generated by the motor, spindle, bearings, etc. operating at 10,000rpm

vs.

B) Heat generated by the motor starting and powering up to 10,000rpm MINUS the heat lost during the time the motor was off.



I think B generates more total heat if the time off between use is short. When the time off between use reaches a certain point, it becomes better to cycle it off.

I'd be curious to know if the engineers know how long the off time has to be to make it worth cycling off between uses...

I've been following this thread with interest, and virtually all the posts show excellent instincts for electromechanical and thermal issues. But I figured it was time to jump in and clarify a few things. Designing motor-drive circuits and integrating them with motors on CNC machines is one of the things that I do for a living in my day job. Most of my experience is with permanent-magnet brushless motors, but switched-reluctance motors like in the PP are similar.

1. Yes, high heat does indeed degrade electrical and electronic components. A modern system will shut itself down before the temperature is high enough to cause a sudden catastrophic failure. However, long-term exposure to sub-catastrophic heat levels will accelerate the aging and eventual failure of the insulation in both the drive and the motor -- most notably the magnet-wire insulation in the motor windings, and the dielectric insulation in the drive's electrolytic bus capacitors. The rule of thumb is that every 10C (18F) rise in temperature cuts the insulation life in half. Or conversely, every 10C reduction in temperature (below the max rated) doubles the life of the components. To put this in perspective, standard electrolytic capacitors are typically rated for a 2000 hour operating life at max rated temperature and current.

2. Yes, mechanical stresses from thermal cycling can indeed lead to premature connector, solder-joint, and more rarely silicon failure. And those failure are a major PITA to diagnose, as Ed in Tampa has already attested.

3. Modern motor drives employ motor-current feedback, and completely eliminate excessive peak currents. The peak current is reliably limited to whatever the designer wants it to be. Pulse-width modulation (PWM) does apply large square-wave voltages across the windings, but the motor-winding inductance is used as an integral part of the drive circuit, and it limits the current ripple to a level that adds almost nothing to the thermal losses.

4. Unless you are in a huge hurry, accelerating a motor to full speed doesn't require much current. For example, if I apply the rated continuous current to a servomotor on a CNC machine, it will go from zero to full speed (or vice versa) in less than 50 milliseconds. Attaching a load with lots of inertia to the motor will increase the acceleration time, in direct proportion to the motor inertia. (If you double the total inertia, for example, the constant-current acceleration time will double).

5. Running at high speed does, by itself, increase the heat generation in a motor. Transformer iron is laminated in order to limit the heat-generating eddy currents in the iron, which are caused by the alternating magnetic fields. The higher the frequency, the thinner the laminations must be. For example, if you tried to substitute an ordinary 60Hz transformer for a 400Hz aircraft transformer, the eddy currents would cause excessive heating of the iron. Motors have the same issue, and the magnetic-field frequency is directly proportional to motor speed. At their top rated speed, high-speed motors generally have very substantial eddy-current heating of the iron. Because of this, the continuous torque rating is reduced substantially at top speed.

6. This part is pretty obvious: when you put heat-generating components in an enclosure (such as the SS headstock), the temperature inside the enclosure becomes the "ambient" operating temperature for the components, not the outside air temperature. The air-temperature rise inside the enclosure directly reduces the effective high-temperature operating range of the components.

7. The best way to manage the heat generated by the motor and drive depends on the operating environment. Ranging from "cheap but vulnerable" to "bullet-proof but expensive", options include A) natural convection, B) open forced-air fan cooling, C) forced-air cooling with filtration, D) sealed enclosure with passive heat exchanger, E) sealed enclosure with active heat exchanger, and F) sealed enclosure with active cooling. Whew! (And I won't even go into the various enclosure environmental ratings.) The effectiveness of these options can all be calculated fairly accurately at design time. A metal enclosure itself serves as a passive air-to-air heat exchanger, and for many applications is a fine solution.

Shop temperature 79 or 80 degrees. The A/C is set at 79.

Hat been running at 1500 RPM for about an hour (sanding), followed be running at 6150 RPM for another hour (bandsawing through the speed reducer). The headstock was warm and I was watching for the High Inverter Heat message. When it came on, I shut down and went to blowing out the DC motor. Came back to the headstock in 1/2 hour and the display showed normal info. (500 RPM when powered up)

No harm, no foul!

My many thanks to this forum and those that will take the time to post things like High Inverter Heat. Never would have been watching for that warning without this forum. Woul only have had an option to eference the manual, but only as an after-thought.

charlese wrote: ... followed be running at 6150 RPM for another hour (bandsawing through the speed reducer).

I'm off topic here, but why were you band sawing at that rpm through the speed reducer with the PowerPro? At 7/1 reduction that's a speed of roughly 878 rpm which you could just set the PowerPro to (for that matter a regular headstock will be at that rpm with the speed control set a little past the "B" speed setting).

Has anyone who has experienced these heat related shut downs had a conversation with one of the engineers at Shopsmith? I certainly would have.

However, if I had one, being here in this desert climate would just compound my problems. My Mark V sits in the open doorway with the Tucson sun beating down on it until about noon. There are times that I cannot lay a hand on the headstock - even without running it.

Not off the subject a bit!:)

Sorry to say my PowerPro still, after three re-builds, still presents me with bangs and jumps. Not all the time, but after two or three of them it's time to bring on the speed reducer. The jumps and bangs bother me worse than the high heat warning. The Speed Reducer completely solves the banging/jumping, but cuts down the time I can use the bandsaw in that mode.

Got the heat message today at about 1 hour's sawing time. Same 79 degrees in the shop. Guess like Shopsmith answered another forum member - about 45 minutes is the maximum running time at higher speeds.

Tomorrow only 7 more cars to saw out. Should only take 15 minutes to 1/2 hour.

Problem was/is I have 100 little cars and trucks to saw out. Am cutting 5/4 poplar.

P.S. - - I used to bandsaw with the older headstock at B speed.