Need help from the motor gurus...

[daved20319]

Looks like the start cap on my 30 year old Grizzly lathe has finally given up the ghost. The motor will start if there's no load on it, but as soon as I put the belt back on, it just buzzes. So here's where I need some help.

The motor is 1 1/2 hp, dual voltage and currently wired for 220 volts. The cap is 125 volt, 300 microfarad. I've never been able to run this thing at top speed, which leads me to believe the cap may have been breaking down for awhile. My question is, would there be any advantage to going with a higher rated cap? Or is that just asking for trouble? Some of the research I've done says start caps should be sized at 500 microfarad per HP, this one obviously doesn't come close. Something else leads me to believe that a higher voltage cap could also be a good idea. Don't know if it makes a difference, but I do tend to start and stop the lathe frequently.

One other question, are start caps temperature sensitive? As it's finally been warming up a little here, I've let my shop furnace go out, so the temp in there is probably in the 40's instead of the 60's I have when the furnace is going. I have an older Craftsman table saw that gets temperamental when it's cold, I need to give the blade a spin sometimes to get it going. Could the same thing be going on with my lathe? Thanks, gents.

Dave

[Iraiam]

I have adjusted the size of start capacitors up as much as 20% without problems for a higher starting torque, however, a start capacitor that is too big will cause problems, the motor windings will overheat and shorten the life of the motor.

Since your motor has ran for 30 years I would say that the capacitor size is pretty close, I would only increase the mfd rating of the cap very slightly if at all.

How about a Grainger 6FLK5

My Grizzly lathe is newer than yours, I have problems running it at top speed also, I am convinced it is just under powered and I am in the process of converting it to 3 phase with a VFD.

[daved20319]

Thanks, Ira...

Of course, your answer brings up another question :-). This motor IS 30 years old, and although everything else seems to be okay, I'm no motor expert. Would I be smarting thinking of a new motor, and if so, should I consider bumping the HP? It's a 12x24 belt drive, which I bought used. Although it's showing its age visually, I don't think it saw a lot of use, and I don't think that was heavy use. Not much wear on anything except the drive belts, and it runs nice a smooth (when it runs!). I work primarily in plastic and aluminum, so big power isn't needed, but it would be nice if I could get it to it's max rated 1400 rpm.

I've considered bumping it to a 2 hp. I know nothing about 3 ph. motors and VFD's, other than what I've read here. Seems that might be expensive over kill for my use. What say you? Thanks.

Dave

[paulx]

Are you sure that the motor is being supplied with 220 volts ? Might want to double check your lines with a volt meter.

[MaxHeadRoom]

If this is a ROC motor, I am surprised that it lasted 30yrs?
IF they did the engineering right, you should stick with the same value, as the cap is designed to give as close as possible to 90° phase angle for the start winding.
If it is a start capacitor, then your low speeds are not normally capacitor related as the cap is only in for start?
If you are looking at a replacement, a 4 pole 3 phase and VFD may be the best solution.
Variable speeds and a possibility of running the motor up to 120hz!.
Max.

[RenoF250]

Electrolytic capacitors are very temp sensitive as the temp drops so does their capacitance. Not sure how you got away with a 125V cap on 240VAC, definitely replace with 240VAC or better.

[RenoF250]

Electrolytic capacitors are very temp sensitive as the temp drops so does their capacitance. They also dry out with age and loose capacitance. Not sure how you got away with a 125V cap on 240VAC, definitely replace with 240VAC or better.

Maybe this guy:

http://www.grainger.com/Grainger/capacitors/motor-supplies/motors/ecatalog/N-9ypZ1z13mnlZ1z0o4gaZ1z0maxo

[J Tiers]

The 125V cap is fine, don't worry about it. the way it works is that it is applied across only 1/2 the windings, so it never sees the 230VAC.

It is not a factor in the high speed issue.

[daved20319]

Thank you all for your responses...

After some more research, I have a new cap on order, hopefully it'll arrive by the middle of the week. I actually ordered two, one with approx. the same MFD rating as the original, and one about 20% higher. I'll try the OEM rated one first, if that doesn't cut it, I'll try the higher rated cap. If I burn out my motor, well, it IS 30 years old. Sounds like a good excuse for a new 3 ph. motor and VFD :-). Later.

Dave

[rock_breaker]

PAULX has a good point espesially when you say you have slow start-up on your saw.

[garagemark]

The motor is getting 220 volts. If it weren't, it would have fried long ago from undervoltage heating. It probably never would have started in the first place. You would very well know it if that were the case.