PM 4224 speed variation

[Jon Spelbring]

I've got a used 4224, and I love it. However, recently it's been acting up. Specifically, when turing at low speeds for finishing/sanding, the variable speed has gone on auto pilot. Using the slowest belt setting, I can get my 50rpms (I haven't changed any of the program settings). Last night, I was finishing an oak bowl - maybe 5lbs, and the speed would vary between 50 and 150, seemingly at random. No excess pressure, just finish sanding with 220grit and walnut oil.

Any ideas what might be causing this?

Also, (maybe related?), how can you tell when bearings are going bad? (I've heard less than smooth noises a couple of times).

Thanks!


-Jon

 

[Bill Boehme]

... recently it's been acting up.

Tell it that you're going to send it to the Principal's office if it doesn't behave. 😀

.... Using the slowest belt setting, I can get my 50rpms (I haven't changed any of the program settings). Last night, I was finishing an oak bowl - maybe 5lbs, and the speed would vary between 50 and 150, seemingly at random. No excess pressure, just finish sanding with 220grit and walnut oil.

Any ideas what might be causing this?

Yes.*

Also, (maybe related?), how can you tell when bearings are going bad? (I've heard less than smooth noises a couple of times).

Remove the belt from the motor pulley and position it so that it does not touch the spindle pulley. Make certain that it is quiet enough to be able to hear faint sounds and then slowly turn the handwheel while listening for ticking or clicking or other sounds from the bearings. If you do not hear anything, put your largest chuck on the spindle and give it a spin and listen for any sounds. (Have something that is balanced mounted in the chuck or otherwise you might hear the chuck jaws rattling.

* You didn't ask, but I thought that I would give you my ideas anyway. I'll forgo giving my opinion about sanding with an oil/dust sludge and, instead, suggest that the lathe might be trying to tell you something about friction. First, verify that the drive belt was set to the low speed position last night when you were sanding it. Next, you may not have been applying much downward pressure, but perhaps the oil/dust sludge was creating more drag than you assumed. Mount the bowl on the lathe again and do not sand it -- instead just watch the speed as it turns to see if it is stable. If the speed remains fairly constant when not touching the bowl, repeat the sanding that you were doing yesterday to see if the problem reappears.

If the problem came back, that is normal behavior at very low speeds when using a sensorless vector inverter. The output frequency to the motor at 50 RPM is somewhere between 1.5 and 2.0 Hz. As a rule of thumb, most low cost sensorless vector inverters are not stable below about 5 Hz for any significant torque load.
​

 

[Jon Spelbring]

Thanks

Bill,

Thus far, I've only threatened it with a ruler smack, but I'll keep the principles office in mind.

I'll do the tests you recommend tonight. Here's to hoping for a quiet night.

FWIW, I did notice the speed variations even when I was not sanding. Strange thing is that I did the exact same steps a couple of days ago, and had no problem.

I have noticed that I can only get down to the 50rpm with the belt set to its slowest setting. In the mid-range (where I usually keep it), the slowest I can get is 100rpm - in that normal?

 

[Bill Boehme]

.... I have noticed that I can only get down to the 50rpm with the belt set to its slowest setting. In the mid-range (where I usually keep it), the slowest I can get is 100rpm - in that normal?

Yes, perfectly normal. Often, manufacturers set the minimum speed to avoid the really low speed regime where the inverter's sensorless vector speed control algorithm can't maintain reasonably constant speed.

 

[Jon Spelbring]

Seems OK

I did the tests you mentioned (though I don't own a stethoscope, so had to do the screwdriver trick). Everything sound OK, I think. No loud ticking. I also switched the belt to all three different settings to see what speeds I was getting:
Lo: 47-970
Mid: 101-2020
Hi: 177-3550

I did have it spazz out a little at the lowest speed, so I cranked the speed to max, then slowed it back down. I was able to get a steady 47 rpm both with and without a chuck/blank attached.

For now, I guess I'll chalk it up to gremlins.

Thanks for the help!

 

[robo hippy]

The converters can be programmed to go to almost 0 rpm which I prefer for sanding my warped bowls. The first 3520A's would do this. Then they changed it because "you will overheat your motor and fry the electronics".

"Well, gee, I have sanded out thousands of bowls at these low speeds and never had a problem."

"That is impossible".

Anyway, I have my Robust programmed that way, and no problems. I have found with my lathes when I am at the lowest rpm, just fiddling with the knob, there is a point where the rpm will jump up and down a tiny bit. This is right before I turn the knob far enough for it to turn off. I never got that much variation in speed. I do love the low rpm for sanding only.

robo hippy

 

[Bill Boehme]

...... "you will overheat your motor and fry the electronics"......

I think that they are mistaken about frying the electronics unless you add cooking oil and batter the inverter. When it comes to the motor, it is a much more complex question to answer so they give the easy and safe response. I am sure that they said that because --

1. they do not know anything about the sort of environment in your shop and how you are using it at that low speed
2. no amount of notes or warnings in the owners manual would prevent some people from doing things outside of things that would be allowed at very low speeds
3. the number of warranty repairs would increase as a result of #1 and #2
4. the particular motor used may not be rated for the environment in which to use it
5. they are not the design engineers so they really do not know what could happen
6. they just like to give you in particular a hard time (varies depending on company)
7. with a named like robo, they do not think that you are human

Sanding puts very little load on the motor except for the unlikely case of "turning" using the 40-grit gouge as the only tool. There are, however, other things inherent in the motor that are unavoidable sources of heat. How much heating is the sticky wicket that makes the manufacturers reluctant or unwilling to allow operation down to zero speed.

An "unloaded" three-phase motor running at normal speed uses roughly 25% of the full load current. Much of that lost energy is due to aerodynamic drag that heats the motor. Other causes of heating are from magnetic hysteresis in the iron, eddy currents (electrical current flowing in the iron), resistance in the copper windings, and bearing losses. At very low speeds, aerodynamic drag goes away, but overall efficiency is lower because energy lost as heat to magnetic losses goes up as the iron reaches saturation and copper losses also increase. The energy lost as heat in the motor has to somehow be dissipated. At full speed the external shrouded fan on a TEFC motor provides the mechanism for removing the heat. At very low speeds, the fan no longer provides any cooling.

How about the real world? It takes several minutes for the motor to reach a steady-state thermal gradient where the temperature that you feel on its exterior is cooler than the internal motor temperature. How much different depends on a lot of things including the ambient temperature. My "studio" is the great outdoors where the temperature might be 110° F or more in the summer -- that is about 40° over normal room temperature. The motor's internal core temperature will go up more than that amount. From experience I know that the motor on my lathe could run continuously at minimum speed without overheating if the only load is sanding. The inverter has also held up, but I am more concerned about it because electronics generally do not like high ambient temperature, especially if battered and fried.

 

[robo hippy]

Probably some of 'all of the above' except they never knew me as robo hippy. My guess it was due to lawyers and warranty issues. The motors can idle at that speed no problem, just about forever. Main reason, and I am no electrical engineer, is that there is almost no load unless you hand sand. Since hearing their side of the story, and after conversations with Brent English the owner of Robust, I put my hand on the motor to feel for warmth. It ALWAYS runs cooler when sanding than it does when turning.

robo hippy