Replacing Start Capacitor

[Mt. Gomer]

Greetings,

I'm finally getting around to cleaning up the HF lathe I got with the large tool purchase last Fall. I was told when I got it the start capacitor needed to be replaced. Which makes sense becuase when you flip the switch it just hums but if you give the wheel a good spin it starts right up. So, I took the old capacitor out and it's labled 200MFD, 125 V AC. I drove around to a few places here in Durham until I found one I thought would work at an HVAC supplyier. It's labeled 189-227MFD, 110/125 VAC. The guy said as long as the MFD ratings lined up it should be fine. So, I got it installed and it does the exact same thing, just hums until I spin the wheel.

Any ideas? Is it the wrong or possibly bad new cap? are there polarity issues I should be aware of (there weren't any +/- markings so I figured it didn't matter). Now that it's there (and possibly charged up) I'm a bit leery of messing with it as I know it can give me quite a shock. Any advice as to how to proceed would be much appreciated.

Travis

 

[Bryan S]

Travis there is a centrifugal switch in the motor that is closed when the motor is idle and opens during startup as the speed increases taking the start capacitor out of the circuit. It is possible that the switch is bad or not closing when the motor slows.

 

[Mt. Gomer]

Travis there is a centrifugal switch in the motor that is closed when the motor is idle and opens during startup as the speed increases taking the start capacitor out of the circuit. It is possible that the switch is bad or not closing when the motor slows.

Is that something I can fix? Or is it time to take the motor in?

Travis

 

[FredP]

Is that something I can fix? Or is it time to take the motor in?

Travis

is the motor open or sealed [tefc]? I dont know HF stuff so. if it is an open motor try high pressure air to blow all the dust out. the contacts may just need cleaning on the centrifugal switch.

 

[aplpickr]

The first # can be changed within 10% only in the larger direction. The vac rating is an insulation rating. You can go up with it as far as needed. The larger physical size will stop you because the plastic between the layers of aluminum foil is thicker. The problem is when sucessive replacements keep adding more 10% errors.

Your motor has two windings: start and run. The start winding is long and small diameter ( lots of magnetic force, but little capacity for heat from load). The run has less magnetism, but can take more heat. The centrifugal start switch (in series with start cap and start winding drops them out when the motor reaches ± 75% of full speed). The switch contacts slowly burn, get dirty, or get mechanically stuck from sawdust. Clean(sand with 320 grit sandpaper) the contacts or the mechanism so that they make in the off cycle.

A charged cap can really eat your lunch! Not enough to kill you, but you might bleed to death after you jerk your hand back. Short across the cap's contacts with an insulated screw driver. Repeat until it does not "pop". However this will shorten the life of the cap. Better is to use ±15K ohm bleed resistor, available from Radio Shack. This will take fifteen seconds of contact. Make reference marks on both end bells and motor body before taking it apart to get to the switch in the non-shaft end. When putting back together, lightly tap with small hammer WHILE tightening the thru bolts. Any normal mechanic can do this operation, just be sure that you do not pull any wires loose!

 

[zapdafish]

You can try googling for an online pdf manual of that particular lathe. If you need a part, the online manual should have a list of parts and a telephone number for ordering. Not sure which is cheaper for an HF machine, fixing the motor or just ordering a replacement

 

[bob vaughan]

A very common problem with chaiwan motors is that the centrifugal switch mechanism comes loose and slides back away from the spring contacts. Take the fan cover off and fan to see if this is what happened. The dent of the setscrew's original position should be visible on the motor's shaft once the centrifugal throw-out mechanism is removed.

Note that what looked like a conventional enclosed motor is anything but.

 

[Mt. Gomer]

Thanks for all the great replies. Looks like I'll be trying to take the motor apart to see if I can figure this one out. It seems like I can't make it any worse and a HF motor is a lot better to experiment on than others.

A few more bits of information: I've tried the compressed air, no go yet but I'll try it again just to be sure. Would it be a good or bad idea to do it while it's running? What PSI would you suggest (something gentle like 40-60 or stronger 90-125)?

I did notice a solid "click" sound after I shut turned the motor off the last time which I didn't remember from before (could have been there though). Could that be a sign that the centrifugal switch is starting to break loose?

Travis

 

[bob vaughan]

As a cap start single phase motor slows down that 'click' is as it should be. Its the centrifugal throw out falling back into place and allowing the capacitor points to re-connect. If the centrifugal throw-out mechanism has slid back on the shaft, it will still click upen and shut.

 

[Mt. Gomer]

Thanks Bob. Not sure when I'll have the time to dissasemble but I'll post back when I've made some progress.

Travis