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Old 11-01-2010, 09:58 PM
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Default ac motor trouble

I was using my lathe today and when I went to turn it on it just hummed I grabbed the chuck and spun it by hand and it fired up after stopping the motor and firing it up again it would come on by itself once in awhile but most of the time I was using it today I had to spin it to start it.I took the belts off to make sure there was no problems in the gear head and it still does the same thing .I called grizzly and they said its either the capacitor or the point/contacts on the centrifugal switch but when i told it it would start once in awhile he said it was probably the points because when a capacitor goes it just goes .So I took it apart and cleaned the points and made sure springs on switch are working put it back on and it started the first time then the same thing again .so my question is is it true that when a capacitor goes bad it won't ever work or can it work intermittently and why is there two capacitors ?can I test these types of capacitors with a meter and does anyone have any other things that might be causing this? Thanks Phil
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Old 11-01-2010, 10:38 PM
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Phil, I had to replace the capacitor on Mildred because her motor would just barely start.
New cap fixed it right up.
A couple years ago my house air conditioner was just barely cooling. I called in my reliable neighbor who's a former ac tech; he replace the capacitor on the compressor.
Said the old one was "weak".

So the Grizzly guy is full of crap, at least in this one area.
You most likely have a start capacitor and a run capacitor.
Maybe the Grizzly guy can tell you which is which if you can't figure it out from tracing the wiring.
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Old 11-01-2010, 11:20 PM
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Quote:
Originally Posted by cutter View Post
Phil, I had to replace the capacitor on Mildred because her motor would just barely start.
New cap fixed it right up.
A couple years ago my house air conditioner was just barely cooling. I called in my reliable neighbor who's a former ac tech; he replace the capacitor on the compressor.
Said the old one was "weak".

So the Grizzly guy is full of crap, at least in this one area.
You most likely have a start capacitor and a run capacitor.
Maybe the Grizzly guy can tell you which is which if you can't figure it out from tracing the wiring.
Cutter could one of these caps be for reverse ?I'm not up on ac motors and how they reverse.
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Old 11-01-2010, 11:45 PM
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Quote:
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Cutter could one of these caps be for reverse ?I'm not up on ac motors and how they reverse.
No, just start & run.
Take the covers off both and post the values here: mfd and voltage. The numbers should be stamped on the capacitors.
That information will give the real motor guys enough information to help you.

Now, a few words of caution: capacitors act like momentary storage batteries and they do carry a jolt of high voltage ac.
1. unplug the motor.
2. Ideally, they will have bleeder resistors across the capacitor terminals but if they don't then you want to short across the poles to discharge them or you can get a nasty shock.
3. You might want to wait for more professional help than I am if you're too skittish about handling them.
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Old 11-01-2010, 11:56 PM
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Quote:
Originally Posted by cutter View Post
No, just start & run.
Take the covers off both and post the values here: mfd and voltage. The numbers should be stamped on the capacitors.
That information will give the real motor guys enough information to help you.

Now, a few words of caution: capacitors act like momentary storage batteries and they do carry a jolt of high voltage ac.
1. unplug the motor.
2. Ideally, they will have bleeder resistors across the capacitor terminals but if they don't then you want to short across the poles to discharge them or you can get a nasty shock.
3. You might want to wait for more professional help than I am if you're too skittish about handling them.
I will post the values tomorrow but i don't remember seeing anything on them as they are Chinese I had them out today to inspect them for any signs of problem and I have worked with capacitor on dc circuits and know about discharging them they can pack a punch even small ones
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Old 11-02-2010, 10:01 AM
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You can test a capacitor with a analog meter using the ohm setting if you have one. First discharge the capacitor using a screw driver across the terminals keeping hand on insulated handle then using the probes one on each terminal the meter should show a high resistance then slowly go back to zero. If open (no reading at all) it is bad.
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Old 11-02-2010, 10:30 AM
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A good quality digital meter (such as the Fluke 179 I have), will measure capacitance, so you could check with such a meter, if you have access to one. I used mine a while ago to check a no start situation on my radial arm saw, and it worked to eliminate the capacitors as the problem.
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Old 11-02-2010, 10:32 AM
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Adding to what digr said.
http://www.shopfloortalk.com/forums/...5&postcount=46

More then likely the larger valued cap will be the starting cap.

One problem will be, most Chinese caps are physically smaller then what replacement you may find.
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Old 11-02-2010, 10:15 PM
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I took out the capacitors one was 150 uf 230volt ac the other was 20 uf 430 volt ac . I tested them with a digital multimeter on ohms 1k setting the 20uf cap would charge and discharge when I would alternate the leads the 150 uf cap would not take a charge. So I'm guessing thats my problem and I believe the higher uf is the starting cap? I ordered one from Msc it will be here in the morning .I am also going to replace the 20 uf cap because they were both soldered connections and Chinese and I prefer the spade terminal type. I was surprised to see the 20 uf said 430 volt when this is only a 220 machine is this normal?
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Old 11-02-2010, 11:28 PM
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The peak voltage of 240 volts AC is about 337 volts so the cap will need to be rated above that.

Some of the run caps tap off the center of the run windings which is 120 volts, 169 peak.

http://www.shopfloortalk.com/forums/...&postcount=332
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