My Ingersoll Rand compressor would not start today Motor starter clicks in, motor does not move. It came to me as a 10 HP 3 phase, I have no 3 phase. power. I used a large single phase motor, either 7 1/2 or 10 HP based on amp draw but no name plate or tag anywhere. Bought it at an auction for $60, so who knows, but it worked for years on my 5 HP compressor and fine on this one with a motor starter. I am guessing the start capacitors are bad. It has 2 - 700 MFD start capacitors, and 1 run capacitor which I have not checked yet. The 2 start capacitors smell burnt, one has a melted spot near the connectors, other one has rust at base of connectors. Capacitors failed the multimeter tests you can do without a capacitance function. My question is are capacitors wired on series or parallel? I thought series wired capacitor sets only gave the capacitance of the highest value capacitor. It looks like me like they wired series.
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Capacitors Parallel or Series Wired?
- Thread starter CJ5Dave
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Parallel caps add, but series caps divide. For example, two 100 uF caps in parallel would be 200 uF, but in series they would measure 50 uF
For two caps in series the value equals the product of the values over the sum of the values:
(100x100 = 10000 divided by 200 = 50)
How are they failing the multimeter tests? Are they showing shorted, or open? Or something else?
(start caps are usually in parallel)
For two caps in series the value equals the product of the values over the sum of the values:
(100x100 = 10000 divided by 200 = 50)
How are they failing the multimeter tests? Are they showing shorted, or open? Or something else?
(start caps are usually in parallel)
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How where they wired when you removed them? Typically in series is to achieve higher voltages but 1/2 the capacitance parallel is additive. Start capacitance is usually 50-100uf/Hp. Size of compartment would limit a single larger capacitor. When they are bulging or spilling their internals, need to be replaced.
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This is how they should behave.
This is NOT how they should behave. Probably a shorted cap. Is it bulging, or leaking fluid?
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Assuming "one has a melted spot near the connectors" is the capacitor you are talking about, to me that indicates an internal short/rupture of the capacitor with excessive heating. If in series then if either one fails then you have no start capacitor. They have a finite lifespan, so I would replace both. Check the physical size of the replacement to make sure it fits, if you can find ones with the same uF and a higher AC voltage rating it may last a bit longer. Example below are 330VAC, electrolytic start capacitors, they typically are something like +/-10% or even 20% of a rated value depending on the application. Run capacitors are either solid wound or oil filled with tighter tolerances.
www.northamericaphaseconverters.com
630-750 MFD Start Capacitor: 330V - 2-9/16" D & 4-3/8" H
Are you looking for a 630-750 MFD start capacitor? Order this 330V unit from NAPCco. This start capacitor is rated at 20 starts per hour and is UL-recognized.
www.northamericaphaseconverters.com
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One way to determine the capacitance value is to set yourself up a voltage divider using a potentiometer and capacitor in series. Since the motor runs at 60 Hz this should give a good representation of what you have.
1. Apply a 60 Hz AC signal across the series combination.
2. Adjust the potentiometer until the voltage across the capacitor equals the voltage across the potentiometer.
3. Kill power and measure the resistance of the potentiometer at this point.
Xc (Capacitive Reactance or AC resistance)=1/2πfC. So rearranging the formula to determine the Capacitance Value gives C=2π*60*Rpot.
One problem here is 120V unless you have a step down transformer. At low resistances the current will be quite high (maybe smoking your POT). So I'm not advocating you run and grab a lamp cord but in a pinch this method could be useful in some situations. Of course a new multimeter would be nice.
1. Apply a 60 Hz AC signal across the series combination.
2. Adjust the potentiometer until the voltage across the capacitor equals the voltage across the potentiometer.
3. Kill power and measure the resistance of the potentiometer at this point.
Xc (Capacitive Reactance or AC resistance)=1/2πfC. So rearranging the formula to determine the Capacitance Value gives C=2π*60*Rpot.
One problem here is 120V unless you have a step down transformer. At low resistances the current will be quite high (maybe smoking your POT). So I'm not advocating you run and grab a lamp cord but in a pinch this method could be useful in some situations. Of course a new multimeter would be nice.
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If run capacitor has a bleeder resistor attached then you will read a steady resistance- otherwise it's bad. Caps must be disconnected to test.
Normally, a capacitor (without bleeder resistor) can be checked with an ohmmeter- you touch the leads to the cap and the reading should quickly go low and then start to climb to infinity. If you reverse the test leads it should do it again, but more strongly, and each time you reverse the leads.
This won't tell the cap value, only if the cap is open or shorted. Works best with a needle type analog meter (easier to interpret)
Everyone should have a cheapie Radio Shack analog meter laying around just for things like this- Shameless plug for the Shack
Normally, a capacitor (without bleeder resistor) can be checked with an ohmmeter- you touch the leads to the cap and the reading should quickly go low and then start to climb to infinity. If you reverse the test leads it should do it again, but more strongly, and each time you reverse the leads.
This won't tell the cap value, only if the cap is open or shorted. Works best with a needle type analog meter (easier to interpret)
Everyone should have a cheapie Radio Shack analog meter laying around just for things like this- Shameless plug for the Shack
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The resultant capacitance of series capacitors is C = (C1 x C2)/(C1 +C2). The voltage rating of series capacitors will be the sum of the ratings of the individual capacitors with one caveat. If unequal value capacitors are wired in series, the voltage across each will be determined by the capacitance value (actually the reactance value). For more than two capacitors in series, the resultant capacitance is C =- 1/(1/C1 +1/C2 + ....1/Cn).