# Startup capacitor keeps blowing



## snowluck2345 (May 24, 2013)

I have a harbor-freight 8x12 metal lathe I bought used around 6 months ago and a month or two ago the startup capacitor blew. I finally got around to replacing it today, but guess what, it blew again. I used it for around 4 hours and then it blew. Most of the four hours was relatively high spindle speed, 1000 rpm or so and it blew when I had the spindle speed turned down for threading. 

I wouldn't think the speed would matter because the centrifugal switch relies on the speed of the motor, not the spindle correct? They wouldn't put a centrifugal switch on the spindle right?

It seems that it would blow twice in the same circumstances. It blew when I was turning at low speed for threading last time aswell.

The capacitor was smoking and leaking oil. It was too hot to touch. The motor was aswell. 

I had also been turning the lathe on and off alot in a short time period, could that have overworked the capacitor?


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## tripletap3 (May 24, 2013)

It is possible to overheat a "start cap." by rapidly turning the motor on and off, but I doubt that is it. I would think you would have to "jog" the lathe quite a bit thus not allowing the start circuit to cutout for before this would happen. The start circuit should cut out at about 3/4 of the rated motor rpm taking the cap. out of the circuit. I think that more than likely you have a shorted start winding or a stuck / lazy centrifugal switch in the motor. It is also possible that the motor is weak or under voltage not allowing it to reach the required speed to disengage the start circuit. Low voltage could be caused by using a too long or too light of a ext. cord or high resistance in a switch. Check the windings for bad or burnt areas , check the connections, check the centrifugal switch then spring for another new cap. (much cheaper than a motor) then check voltage at the motor if it still blows it is new motor time.


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## snowluck2345 (May 24, 2013)

The odd thing is it worked fine for several hours amd it blew doing the exact same thing as last time.


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## 4GSR (May 24, 2013)

When you start the motor, does it make a very erratic hum for several seconds after the motor reaches full speed?  If so, the centrifugal switch is not kicking out properly to disconnect the start capacitor.   The next thing I would do is determine if the start capacitor is connected properly to the correct wires in the motor.  Last, if this is a 115V motor, check to be sure you don't have the hot wire placed on the neutral and the neutral attached to the hot side of the circuit. Don't think it would matter, but you never know!
Ken


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## snowluck2345 (May 24, 2013)

I'm not sure bout the humming. I'll check if I replace the capacitor again. To check if the centrifugal switch trips properly should I just measure the voltage over the capacitor once it has come up to speed?

The thing that seems really odd to me is that it worked for a few hours without problems. It blew doing the exact same thing as last time, but I can't hink of any of reason that threading at low speed would put an especially large load on the motor?


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## snowluck2345 (May 24, 2013)

I just turned on the lathe and it seems that the centrifugal switch isn't cutting out and I measured 210v between the terminals on the capacitor. It is buzzing


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## snowluck2345 (May 24, 2013)

Sorry for the triple post. 

I just tried turning it on. 

When I turn it on it hums. However once it is up to speed it doesn't hum. There is stilll voltage across the capacitor so I assume the centripetal switch is not tripping.

It is odd though, it is reading 208v across the capacitor? I' powering the lathe off of a 120 outlet, so it isn't three phase power. 

How difficult is a centripital switch to fix or replace? 

Would it be easier to just get a second switch and disconnect the capacitor once it is up to speed?

If it isn't easy to replace the switch, is there anything wrong with what I'm about to proprose:

Add a relay in  series with the capacitor, put a black spot or white spot, on the output pulley of the motor, use a light dependent resistor and a microcontroller to measure the rpm and when it is above a certain number, turn off the relay?


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## Alan Douglas (May 24, 2013)

Anything that would disconnect the capacitor will work, even a time-delay relay.  But then you're assuming the motor is up to speed in a certain time.  With no load, it probably will be.  But the centrifugal switch is overall the simplest and safest solution.


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## snowluck2345 (May 24, 2013)

I'm pretty sure. Its significantly larger capacitance and the lathe worked with it removed.


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## CluelessNewB (May 25, 2013)

marinusdees said:
			
		

> Start caps are paper and lighter and not typically round.




Other than some old Craftsman motors which have flat metal can caps in the base,  I have never seen a flat starting cap, most are round.  Then again I don't do much with "Made In China" motors but the few that I have worked on also had round starting capacitors.


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## strantor (May 25, 2013)

I agree it's starting to sound like a run cap. I have seen mostly oblong or round run caps. These run caps are super stout; assuming you are using the right voltage, you should be able to put (L) on one side of the cap and (N) on the other side and plug it into a wall and it should go all day like that - a single series impedance. So being in a circuit with a motor could only be less impedance than by itself, so less likelihood of blowing up. As I said, that is assuming the proper voltage; but you said something about 208V. I would investigate that. I'm not aware of any method a motor could employ to boost voltage.


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## CluelessNewB (May 25, 2013)

A good video about start and run caps:

[video=youtube_share;OMd9QkinXz4]http://youtu.be/OMd9QkinXz4[/video]


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## Alan Douglas (May 25, 2013)

> I'm not aware of any method a motor could employ to boost voltage.


The winding inductance and the capacitance could be series-resonant at 60Hz, so more than line voltage could appear across the capacitor.

The video could be better. He didn't explain the difference between starting capacitors, which are nonpolar electrolytics, and run capacitors which use mylar or polypropylene film and metal foil or metal deposited on the film.  Also the oil is not necessarily there for cooling, but is to prevent voids at the foil edges which would allow corona to form at high voltages. Corona creates ozone which attacks the film.

Start capacitors have significant impedance, which creates heat when they carry current.  They need time to cool off between uses.  Jogging a motor could, it seems to me, overheat them pretty quickly.


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## markba633csi (Jun 29, 2016)

Agree with Alan- also sounds like a good excuse to install a variable speed motor! Best thing I ever did for 
my lathe..
Mark S


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