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| Electric motor question ? http://luthiersforum.com/forum/viewtopic.php?f=10101&t=37125 |
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| Author: | ernie [ Tue Jul 17, 2012 1:24 pm ] |
| Post subject: | Electric motor question ? |
I tested the 2 capacitors on my delta 3400 rpm 1.5hp tefc motor one is a startup capacitor, the other is a large capacitor, they both read o on the ohms scale. Before I go rushing off to grainger I/m guessing that both of them are not working??? The motor is 15 yrs old and was used on my delta stationary sander. Are these motors worth repairing by replacing the capacitors? Thanks |
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| Author: | B. Howard [ Tue Jul 17, 2012 3:11 pm ] |
| Post subject: | Re: Electric motor question ? |
I believe that capacitors should be measured for capacity in farads, not ohms. A good cap will read 0 ohms, a cap that does not read 0 ohms is bad. That does not mean that reading 0 ohms means it is still good. There should be someone in your area like Industrial Motors that re-builds and re-winds motors, take it to them and have them check it out. they will tell you if you should repair or replace. |
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| Author: | charliewood [ Tue Jul 17, 2012 5:06 pm ] |
| Post subject: | Re: Electric motor question ? |
Agreed capacitance is measured in farads Most newer multimeters measure capacitance as well as ohms.... Mabye check for the capacitance symbol on your selector dial on your multimeter - if you dont know what that looks like its sort of like a reverse small cap u with a long stem on it if I remember correctly...(isnt it?) Nope its not just looked it up - its mF uF pf nF milli micro pico nano farads Cheers Charliewood |
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| Author: | klooker [ Tue Jul 17, 2012 6:55 pm ] |
| Post subject: | Re: Electric motor question ? |
The meter would have a setting that looks like μF for micro Farads or it would have a capacitor symbol - Google capacitor symbol. Kevin Looker |
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| Author: | ernie [ Tue Jul 17, 2012 7:24 pm ] |
| Post subject: | Re: Electric motor question ? |
My meter does not read out in farads ,it/s old school.Will take it to the motor shop |
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| Author: | John Coloccia [ Wed Jul 18, 2012 5:43 am ] |
| Post subject: | Re: Electric motor question ? |
So what you typically get when you measure resistance across a capacitor is: 1) first you measure a resistance as the cap charges up, because current is "flowing" through the cap. The resistance will move from low to high. 2) once the cap charges up, the resistance should basically go to infinity, because the cap is no longer able to take any charge Many meters will display the symbol OL for this, which stands for "overload", not 0-something. You can't really test the capacitor while it's in the circuit. It's attached to large coils of wire. Has your motor stopped working? |
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| Author: | ernie [ Wed Jul 18, 2012 10:02 am ] |
| Post subject: | Re: Electric motor question ? |
Yup abt 3 mo ago .Replaced it with sears used 1/2 hp 1725 rpm bsaw motor.The replacement motor for the delta is $385.When I discharged the capacitors the other day saw no sparks at all and 0 ohms across both capacitors I used an old style multimeter. |
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| Author: | Fred Tellier [ Wed Jul 18, 2012 11:00 am ] |
| Post subject: | Re: Electric motor question ? |
Quote: 2) once the cap charges up, the resistance should basically go to infinity, because the cap is no longer able to take any charge Disconnect the caps and check with the meter, they should be as stated above. |
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| Author: | TonyKarol [ Wed Jul 18, 2012 12:19 pm ] |
| Post subject: | Re: Electric motor question ? |
yep .. 0 ohms means a short, as the meter uses DC current to measure ... caps are frequency based and pass high freq, and shouldnt pass or short DC (freq is 0), they block it. So if they still read 0 out of the circuit, they are pooched. |
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| Author: | ernie [ Wed Jul 18, 2012 1:25 pm ] |
| Post subject: | Re: Electric motor question ? |
I did not take off the connectors from the capacitors when checking for resistance. Grainger is abt 15 min from my home . Does someone know of a reliable inexpensive source for capacitors ?? |
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| Author: | John Coloccia [ Wed Jul 18, 2012 2:50 pm ] |
| Post subject: | Re: Electric motor question ? |
I bothers me greatly that you're reading 0 ohms. Even if you touch the leads together, you will read some small resistance (because nothing has 0 resistance). I've never in my life seen a reading of 0. |
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| Author: | Fred Tellier [ Wed Jul 18, 2012 3:04 pm ] |
| Post subject: | Re: Electric motor question ? |
Quote: I bothers me greatly that you're reading 0 ohms. Even if you touch the leads together, you will read some small resistance (because nothing has 0 resistance). I've never in my life seen a reading of 0. Actually 0 ohms is what one should see when shorting the leads together, the resistance of the leads on a good meter is so low that it will not show up as anything but 0. |
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| Author: | EddieLee [ Wed Jul 18, 2012 6:04 pm ] |
| Post subject: | Re: Electric motor question ? |
Quote: yep .. 0 ohms means a short, as the meter uses DC current to measure ... caps are frequency based and pass high freq, and shouldnt pass or short DC (freq is 0), they block it. So if they still read 0 out of the circuit, they are pooched. I agree with this statement for the most part, as an EE. The OHM meter applies a voltage across the resistor and reads the current. R = V/I. If you place a resistance meter on an uncharged cap the meter will at first read close to 0 and then raise in value as the current from the meter charges up the cap. For a small value caps this can happen in the sub-second range. For a large value stater caps, it can take 10s of seconds depending on the value of the cap and how much current the meter sources. The bottom line is, if you put a meter across a cap and the reading stays a 0, the cap is shorted and not good. If reading starts low and then goes up, the cap maybe OK. If a large value cap immediately reads a high value and stays there, it is probably no longer a cap and has become a resistor. |
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| Author: | ernie [ Thu Jul 19, 2012 1:29 pm ] |
| Post subject: | Re: Electric motor question ? |
Pulled the startup capacitor out of the motor an measured again this time I got some resistance, but the meter did not climb as it was being charged by the battery in the ohmmeter.Took the motor to graingers and swapped out for a new startup capacitor, even with new capacitor .it would not run. Service tech checked capacitor ,does not matter now. the motor is kaput.Thanks to all who tried to help. |
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| Author: | John Coloccia [ Thu Jul 19, 2012 4:39 pm ] |
| Post subject: | Re: Electric motor question ? |
Oh, come on. Even my Fluke 85 reads .2, or so, across the leads. |
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| Author: | ernie [ Fri Jul 20, 2012 12:31 pm ] |
| Post subject: | Re: Electric motor question ? |
John I did get some resistance from the startup capacitor, but it doesn/t matter as the motor does not work. |
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