Lathe Motor Run Capacitor

[briankenyn]

Hi All,
I have the 8x12 HF lathe and have had the original 35 Mfd 250VAC Run Capacitor fail after 2 yrs. The replacement [which I may have improperly spec'd] lasted one week. Failure seems have to occurred with a fairly heavy load at the lowest spindle RPM.

I'm hoping that someone has a source for the Cap [I have looked all over], and some notion of whether increasing the rated VAC from 250 to 300 or so, might improve its life. Incidently, both caps shorted out.

Many thanks,
Brian

 

[stevehuckss396]

Grainger has 9 caps 35mfd and all above 250 volt.

 

[briankenyn]

Thanks Steve,
I spoke to them and physical size was an issue since the original has a tight plastic cover. I'm coming around to the idea fabricating something in its place to shield the terminals.

 

[Ken I]

Brian, Watch out for the voltage rating - start caps are typically rated AC but off the shelf caps are often DC rated.

AC peak is root 2 bigger than DC so 250VAC = 350VDC

Electronic caps generally don't bother to specify a VA rating but this is important on a starter cap.

So if the existing cap is proving unreliable a higher V & VA value is indicated. The actual capacitance value is not that critical (but if 35UF was original I would presume it to be optimal - lower values will diminish starting torque, higher will improve starting torque, higher or lower will increace parasitic currents resulting from out of phase from optimum - depending on motor loading.) I would Guess anything from 22UF to 47UF would work fine.

Regards,
Ken

Ken

 

[Admiral_dk]

WARNING - Starter capacitors are AC or bipolar capacitors, NOT unipolar capacitors as in most (if not all) electronics !!!!

Unipolar capacitors with a plus or minus sign WILL blow up if you use them in this application.

 

[n4zou]

Grab the phone book and look in the yellow pages for a motor rewinding shop in your area. They will have the capacitor you need. You might think about taking the motor with you and let them check it out. They have the ability to check for shorted or open windings in the stator without taking the motor apart. Both shops in my area check motors for defects without charge as long as you bring it to them.

 

[Chitownmachine]

Depending on where you are in the world, Dreisilker motor repair in the midwest of the US has all that stuff. Super slow speeds with high torque draw the most current and cause extreme heating, so if you are running into this situation quite a bit, maybe size up the motor. Now does the cap pop on motor start, or during operation? If its happening on start, your windings could be drawing too much initial current causing the cap failure(winding short, or winding burnt out all together causing phase heating) , or if its during running operation, the centrifugal switch that cuts the cap out after startup may have crapped out and been left in the closed position keeping the cap in the circuit the whole time. Good luck and safety first!

 

[Ken I]

Good point Admiral - when I mentioned the DC rating I should have added that caution - I was obviously refering to the differences in voltage rating - not suggesting a DC cap should be used.

Electronics still use lots of bipolar caps (ceramic, Mylar) for filters etc - the bigger values are of course electrolytic or tantalum etc and as you said will blow up if reversed.

Chitownmachine - most capacitor start motors do not have a cut out switch (although some do) - a centrifugal switch is normally for inductive start.

Normally the start and run windings for a cap start are identical and at 90° for reversibility - since the actual phase angle will not be 90° depending on load / impeadance this leads to parasitic currents.

On constant (or near constant load) applications the windings are sometimes "skewed" to bring the windings into line with the resultant phase angle (improved efficiency) - such motors should not be reversed (well they can but can easilly be converted to smoke).

A capacitor "run" motor is not ideal for a lathe motor because of wildly varying load - however they are commonly used. The start cut out switch (if used) eliminates the problem of phase angle error between start and run.

Inductive start motors usually employ a lighter "start" winding. Again these can be reversed if you can reverse either the start or the run winding. With inductive start the phase angle (which gets the motor tuning) is much smaller and you can hear the motor labour until the switch cuts out - failure to disengage the start winding on an inductive start motor will result in smoke.

Ken

 

[briankenyn]

Hi Guys,
Many thanks to you all for your kind advice. The motor uses two capacitors, for Start and for Run. In both instances, the Run capacitor has failed. The Run Capacitor is of 'film' construction and not polarized. The replacement which failed was ordered on-line, and although I knew better in other years, I neglected to check to see if it was VAC rated. I believe in retrospect that it was a DC Cap and popped for that reason.
Failure in both instances has occured under slow spindle speed conditions and high load. The original cap melted, causing my terminal board to melt, and then opened up. Its quite a sight!
Grainger has replacement Run Capacitors but they are all physically oversize. I have had the motor inspected by a rewinding shop, but they did not have a suitable Cap replacement. I am thinking or mounting a replacement Cap outboard of the motor in a fabricated PVC plumbing pipe container. It will be my last attempt to salvage this motor before I go to a 3Ph VFD arrangement.

Thanks again!
Brian

 

[n4zou]

I am thinking or mounting a replacement Cap outboard of the motor in a fabricated PVC plumbing pipe container.

DO NOT USE PLASTIC PIPE!

It will melt from heat. Remote mounting will work just fine except it will not be cooled by the motors fan.
If you remote mount it put it in a box with a small fan to provide cooling. You could also mount the capacitor to a heat sink.
Here are links to surplus outlets where you can find that stuff.
http://amasci.com/surplus/surpls.html

You could also put the capacitor mounted in a larger box attached to the motor where the smaller capacitor was mounted so it can be cooled by the motors fan. Usually the motors case is cooled by the air flow inside the case. This cools both the rotor, stator windings, and the outside of the case where the capacitors are mounted.

 

[briankenyn]

Thanks N4, that is just exactly the info I was looking for. The caps have No exposure to the ambient, and are encased in plastic containers right on the motor case, as is customary. The fan is in a cowling which directs a laminar flow over the case, and I suppose that is enough to control the caps internal temperatures. Since the replacement will be somewhat larger than the original plastic container, can I get way with strapping the cap to the motor case without the container?

brian

 

[briankenyn]

Thanks to all, and to wrap this matter up, I puchased a 35mfd 370VAC Run cap at Graingers. Its biggish, didn't fit in the original container, so I strapped it in place in the slipstream of the fan, against the motor casing. Its a little too soon to tell , but it seems happy there.

Thanks again.
Brian

 

[bob ward]

Hi Guys,
Many thanks to you all for your kind advice. The motor uses two capacitors, for Start and for Run. In both instances, the Run capacitor has failed.

I've had similar problems, and in my case FWIW, the capacitor failures were caused by the centrifugal switch operating unreliably.

 

[n4zou]

Since the replacement will be somewhat larger than the original plastic container, can I get way with strapping the cap to the motor case without the container?

brian

That will work as long as you make sure it doesn’t become a shock hazard. Motor capacitors need a little air flow or conductive heat flow into the case to prevent excessive heat buildup.