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| Motor Question - Regarding Capacitor http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10101&t=15705 |
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| Author: | SimonF [ Sun Feb 03, 2008 7:21 pm ] |
| Post subject: | Motor Question - Regarding Capacitor |
I thought someone might have some insight into this situation. I have had a 3hp Grizzly Cyclone for well over a year now. I had the magnetic switch and a start capacitor blow on me within the first couple months of owning the machine and they were replaced. It has run fine ever since. About 4 months ago, I rewired and reorganized my shop. I installed a dedicated 30 amp circuit with 8 gauge wire and it has worked just fine. Last week the starting capacitor blew up. I replaced it with a starting capacitor of the same microfarad rating. I got the capacitor locally and it was a smaller cylinder than the ones from Grizzly but the specs were the same. It just blew up again today. At $15 a capacitor - this isn't something I want to be happening all the time. Before I talk to the Grizzly tech support - I thought I might see if anybody has some suggestions. I did check the centrifugal switch and it seems fine (i.e. no sticking). Any information you think might be relevant - I would love to hear. Thanks, Simon |
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| Author: | Rick Turner [ Sun Feb 03, 2008 7:28 pm ] |
| Post subject: | Re: Motor Question - Regarding Capacitor |
Marginal capacitor on a marginal motor? Is that a starting cap or a run cap? |
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| Author: | Rick Turner [ Sun Feb 03, 2008 7:33 pm ] |
| Post subject: | Re: Motor Question - Regarding Capacitor |
Check this out: http://www.allaboutcircuits.com/vol_2/chpt_13/9.html A good starter on AC motors... |
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| Author: | SimonF [ Sun Feb 03, 2008 8:05 pm ] |
| Post subject: | Re: Motor Question - Regarding Capacitor |
Interesting link, thanks. The capacitor that blew was the starting capacitor - the run capacitor is fine. Perhaps the capacitor I just purchased was defective - but I don't know how likely that would be. I was just thinking that there might be a deeper problem/solution than the start capacitor itself. In other words, if you have a motor in relatively new condition with a proper power supply but the start capacitor keeps blowing - does that point to a specific issue with the motor or do I just need to buy a new capacitor? But if it blows again, what do you all think might be the problem? I am very limited in my understanding of electric motors - I thought somewhere here might have encountered a similar situation. -- Simon |
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| Author: | Judge W [ Sun Feb 03, 2008 10:19 pm ] |
| Post subject: | Re: Motor Question - Regarding Capacitor |
Hey finally something I know something about... your starter capacitor relay (or centrifugal switch) is probably not opening up and that is what is blowing your cap. The voltage to the starter capacitor must be removed after the motor starts turning or it will blow. That is the switch's job. It lets the motor turn and then removes the voltage from the capacitor. It is the voltage rating that is critical on the cap for it not to blow- not the microfarad rating. Hope this helps Judge |
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| Author: | SimonF [ Sun Feb 03, 2008 10:29 pm ] |
| Post subject: | Re: Motor Question - Regarding Capacitor |
How do I know if the centrifugal switch isn't opening? I checked it and it seems to be working just fine without any trouble. I have noticed that you can raise the centrifugal "part" of the switch up or down along the motor shaft (as it is held in place with a set screw). Would moving it up the shaft allow the switch to break the circuit sooner - or does that not really matter? In other words, should I set it so that as soon as the "wings" begin to fold outward from centrifugal force the circuit is broken or is there a little bit of wiggle room in regards to the placement. Thanks for the help, Simon |
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| Author: | Judge W [ Sun Feb 03, 2008 10:48 pm ] |
| Post subject: | Re: Motor Question - Regarding Capacitor |
The ones I have dealt with you can hear a noticable "click" when the switch clicks in and out. If youve ever lived in a house with a well pump you know what this sounds like... If you havent ever moved it out of its original position it should either work or not work. Check the contacts for arc marks, kinda like points in an old vehicle - if they are worn enough they wont work properly (they could stick together). I would verify the centrifigul switch is clean and opens/closes when you work it by hand (power disconnected). Check the contacts on the switch for arc or burn marks, clean off with a small file/piece sandpaper or replace. I would verify the capacitor voltage is rated at least as high as the original. (what is the voltage rating on the cap?) Then I would replace the cap. Power connected test: Place a digital voltmeter across the capacitor (set on AC). Connect to the capacitor leads. Turn the motor on. You should see a voltage reading on the meter - then somewhere just before full rpm the centrifugal switch should click and then the voltage across the capacitor as read on the voltmeter should fall to zero. The voltage reading should not exceed the voltage rating on the capacitor. if the cap says 100V then that is the maximum the capacitor can see and explosions occur above that voltage... If the switch does not operate - turn off power immediately so you dont damage another capacitor... Good luck! |
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| Author: | Rick Turner [ Sun Feb 03, 2008 10:53 pm ] |
| Post subject: | Re: Motor Question - Regarding Capacitor |
I love capacitor explosions! Poof and a nice cloud of nasty smoke... Capacitors are not forever, and I would suggest one at the same farad rating but for at least 50% higher voltage than this thing will run at. |
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| Author: | Rick Turner [ Sun Feb 03, 2008 10:56 pm ] |
| Post subject: | Re: Motor Question - Regarding Capacitor |
BTW, the capacitor is there to provide a phase lag to fool the motor into thinking there's a multi phase current coming in. It provides a second phase that is less than 180 degrees out of phase with the line current, but once the motor gets going it's creating its own phase lag because of the motion of the shaft, etc. |
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| Author: | SimonF [ Sun Feb 03, 2008 11:18 pm ] |
| Post subject: | Re: Motor Question - Regarding Capacitor |
The manual for the machine lists the start capacitor as being rated for 400 microfarad at 125 volts. So you all are suggesting that I get a capacitor rated for a higher voltage - if so what do you suggest. The motor is a 3hp 240 volt motor for a cyclone dust collector. You all have been very helpful - thank you. -- Simon |
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| Author: | KenH [ Sun Feb 03, 2008 11:51 pm ] |
| Post subject: | Re: Motor Question - Regarding Capacitor |
you can go 10% higher on the Mfd rating with no problems. I would suggest a 277v start cap if it is running on 240v of power. By the way, start caps are round... usually, but not always, they are black (sometimes metal) Run caps are oval shaped or square. Find someone who has an account with WW Grainger or Johnstone supply to get the caps at a cheaper rate. |
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