G0752 speed control problem

[John281]

I have a Grizzly G0752 lathe which is the same as the G0602 but with variable speed that I bought new about three years ago. Last year, the speed control started acting up in that the speed would change erratically as the speed knob was turned. I replaced the potentiometer and it was OK for about a year but started doing it again recently.

Has anyone else had a similar issue. I'm not sure, but it looks like the manufacturer might have changed the part number of the potentiometer on the newer machines. I haven't called Grizzly yet but that's the next step.

 

[roadie33]

I'd call Grizzly and explain that you replaced it once already and now it's acting up again and you noticed they use a different PN now.
Might be able to get a free one out of them if they were defective to begin with.

 

[mksj]

I have had the same thing happen with the Chinese speed pots on other machines with factory VFDs, the tension on the wiper seems to go and possibly the film resistance wears out. If you need to buy a replacement, you are better off sourcing something better. I typically replace them with higher quality mill spec. 1 or 2W pots either conductive plastic or wire wound with a rating of 1M+ turns MTBF. I would check the resistance of the current pot, based on the Delta-E VFD specs used for this machine they indicate a speed pot of 5K ohm. I have been using the precision 3 turn ETI wire wound, or 1 turn film pots, they are military grade and should last a long time as they are rated for 10 million turns.
3 turn 5K wire wound
https://www.ebay.com/itm/2-ETI-MW22B-3-5K-5-1W-3-Turn-Wirewound-Potentiometers/201057249723
1 turn 5K conductive plastic
https://www.ebay.com/itm/ETI-System...SP22E-SB-5K-1W-SP22ESB5K-Nib-New/122837307285

 

[markba633csi]

You might try spraying some contact cleaner into the pot if you can find an opening in which to spray it- sometimes they get gunked up and cleaning
can work wonders. If you don't have contact cleaner you could try dunking it in alcohol and then drying it out thoroughly- I've revived a fair share of pots that way
Mark

 

[tlrascal]

I believe there are a number of things that might cause the failure. One might be the back EMF that the motor generates. On some machines you are cautioned to slowly reduce the speed on a VFD because it might sense an over voltage condition and issue a fault. I use a ten turn pot to get a more adjustable speed and to help reduce this back generated voltage spike. I think the potentiometer might be absorbing some of the back EMF and could lead to failure.

 

[RJSakowski]

I wouldn't expect back EMF to be affecting the potentiometer. The control circuitry and output circuitry are usually fairly well isolated from each other. More likely, it is just wear of the resistive element.

I would second the advice to get a better grade potentiometer. Allen Bradley Type J pots are good. They are well sealed. I have used one for control of a trolling motor in a marine environment for over twenty years before it failed. Allen Bradley was acquired by Rockwell Automation and the pots aren't made any more but they can be found on eBay. Bourns is another trusted name for potentiometers. If you buy a pot from DigiKey, you can download the data sheet which will give you a lot more information than the catalog description.

Pots come with different resistive materials. Wire wound are arguably the most robust but resistance changes in finite steps. Second is probably Cermet for durability. Carbon would be last. Another consideration is raper. A linear taper pot varies linearly with rotation so at half the rotation, the resistance is half of full scale. A logarithmic pot will have a mid scale resistance value of 10 to 20% of full scale. This expands the lower range and compresses the upper range. A reverse log taper pot will have a mid scale resistance value of 80 to 90% of full scale.

 

[markba633csi]

I could see on some machines that are too "smart" that the logic might fault if the pot is moved too fast. The back emf wouldn't normally affect the pot itself though, unless the circuit is poorly designed/protected/filtered
mark