Another minor VFD puzzle

[bulgie]

I have a buffer with a polishing wheel (cotton buff). Just a generic 3/4 HP 3-phase TEFC motor bolted to the bench, driven by a cheap KB (USA) VFD, model KBMA-24d, running on nominal 120V input power. That drive is spec'd for a 1 HP (max) motor, and worked fine with that motor for maybe a few hours of cumulative use, just a few minutes at a time usually. Until one day it didn't. I never heard a zap or saw magic smoke escaping the box, it just seemed bricked, not even the power-on light came on. Maybe squeezing three-quarters of a pony out of that drive while on 120 V was unwise? Maybe because of large start-up load with that big 10" buff on there? Or when I really leaned into it while buffing — but again, it didn't stop working while I was using it. It worked one day, then didn't work the next time I switched it on.

Actually I didn't know if it was the VFD or the motor that was bad, but 3-phase motors seem super reliable in my experience, so I was betting the problem was in the drive.

On a hunch, I changed the input power to 240V (just a jumper inside the case), changed the plug, and plugged it in to a 240 V socket. Works like new (for now anyway!)

I will probably not delve any deeper, I don't need to know what went wrong, just a little curious. My working hypothesis is I fried some component (transformer?) that's only used when input is 120 V. For the same horsepower, I was using twice the current at 120 V, right? Now with half the amps on 240 V, maybe I'll be fine now (touch wood), hope I didn't just jinx it.

This is my primitive level of understanding of electrical power. It's all voodoo magic as far as I'm concerned.

I have another identical KBMA-24d driving a 1 HP motor on my drill press, also running on 120 V input. That one gets a lot more use than the buffer and hasn't failed yet, so maybe the drive on the buffer was just bad somehow. Or my drill press motor never gets to deliver a whole horsepower, because the belt always slips before that happens? This cheap POS drill press is the bane of my existence, the way the belt slipping limits the power it can deliver, but I live with it because I'm poor, and I can always drill on the milling machine. I have made some decent holes with it though, like pushing a 2" holesaw through 3/4" thick 6061. It's not totally useless.

 

[markba633csi]

My wag is that there was a bad connection and somehow in the process of changing input voltages you fixed it- lucky save

 

[Eddyde]

Not sure what happened with the VFD. I assume it didn't show an error message. Did you try resetting the parameters to default then reprograming it?

As far as the belt slipping on the DP. isn't there some sort of tension adjustment? If it's all used up maybe a slightly smaller belt or swap out a slightly larger pulley. Or try a spray can of "Belt Dressing or Conditioner" this will reduce slip.

 

[bulgie]

My wag is that there was a bad connection and somehow in the process of changing input voltages you fixed it- lucky save

I agree that's the first thing to check, but I'm pretty sure that wasn't it. The only changes I made were (1) removing a jumper — to go from 120 to 240V, you simply remove a jumper, and the jumper was very tightly installed, no chance of a discontinuity there (I think). And (2) changing the plug that goes in the receptacle (power outlet). I inspected the "bad" plug and it looks 100%, passes continuity test, and it was definitely screwed to the 3 wires fully with no visible damage. I will re-use that plug on anothe rproject for sure.

Of course I can't fully rule out your hypothesis, I just think it's unlikely.

 

[bulgie]

Not sure what happened with the VFD. I assume it didn't show an error message. Did you try resetting the parameters to default then reprograming it?

As far as the belt slipping on the DP. isn't there some sort of tension adjustment? If it's all used up maybe a slightly smaller belt or swap out a slightly larger pulley. Or try a spray can of "Belt Dressing or Conditioner" this will reduce slip.

This KBMA model VFD is very crude and basic. There is no display, not even an LED other than the "power on" light — no way for it to give an error message. Programming is done with jumpers and trim-pots inside the case, which I never changed from default, so they were still at default settings. I didn't change any of those except the one that switches it from 120 to 240 V. You can't even input the "nameplate amps" for your motor, they just have a row of pins for 1/4, 1/2, 3/4 and 1 HP. I guess it assumes all 3/4 HP motors use the same amount of current, or within a narrow enough range.

I just thought of something, I think one of the trim-pots is for deceleration time, and the default may be too short a time, with this heavy 10" buffing wheel on it. There's no external braking resistor and probably no way to add one, for this uber-basic drive. Maybe I should try to set that to no braking, just let it coast to a stop. Gotta dig into the manual to see what the options and defaults are.

The problem with the DP belts is I can only apply so much tension by hand (swinging motor-mount method). I doesn't run out of travel, I just run out of strength. I could try adding a long lever to the motor mount for more leverage, but at some point the side-load on the bearings has to become a factor, no? I did replace the Chinese belts with new ones from Gates, maybe was a little better but they still slip when trying to use all the power the motor can muster. I think the problem is they're just too small. Sorry I don't remember the belt size designation, but they're much smaller than the ones on my RF-30 mill-drill. This DP was made for a 3/4 HP motor and I upgraded it to 1 horse, so I'm not too surprised the motor can over-power the belts.

Anyone in Seattle got a more skookum DP they want to sell me really cheap?