Capacitor Help needed

The recommended capacitance will be specific for every motor. It is an electrical equivelent to the inductive (winding) part of the motor. I had commented that 250 MFD was a starting point for a 3/4 HP motor. For a 1.0 HP motor, the capacitance will be a little higher. How much higer depends on the inductance involved. Someone had specified a ~500 MFD for that motor. To me, a bit high, but definately doable.

As far as voltage goes, if the motor is used on 120 volts, a 120 volt capacitor would do the job. But having spent many years in the field, I have found that spurious electical noise can damage equipment rated just above line voltage. In this case, where the motor is dual voltage, there is all the more reason to use a higher voltage device. A 240 volt cap would work fine, until some noise comes along and blows a hole in it. Far better would be one rated for 480 volts. It may never see 480 in normal service, but if it did it would survive. Call it cheap insurance. . .

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The motors coils are in parallel for 120VAC operation and in series for 240VAC, the start capacitor is attached to the midpoint of the two run coils so would always see's 120VAC. Although I agree that a higher voltage capacitor is desirable, in many cases the capacitor compartment will only fit the designated replacement and not anything larger. So a higher voltage capacitor will probably not fit, as they are typically larger as the voltage is increased. In addition there is temperature ratings and in some cases the manufacture will provide MTBF at rated conditions. Suggested capacitor size for a 1 Hp motor start only capacitor is 500-600uF from the tables I looked at. Motors with a run capacitor I usually see the start capacitor smaller. Not a motor guru like others here, but I have worked on my share of single phase motors replacing the capacitors. The pool pumps were often the most challenging because the capacitor compartment was always to small for off the shelf units and were forced to buy it from the OEM manufacturer at a premium.
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NTE has a line of bi-polar electrolytic motor start capacitors. I found a 540uf 250V unit on Digikey for $36.53. You'd need to compare package dimensions to see if it would fit. Digikey just drop-ships it from NTE so you might be able to save some money by going directly to them....that is, if they have an online store. Be sure to compare prices if you go to ebay. I often find stuff there that costs more than what the manufacturer charges for the same item. Sometimes it's a hefty difference.

I suspect that 540uF in the right package size will have to be a bi-polar electrolytic. That's about the only capacitor technology with a high enough capacitance/volume ratio @ the working voltage you need.
 
Capacitors are basically two sheets of metal with some form of insulation between the sheets normally paper, the size of the sheets generate the micro-farad rating as the larger sheets can hold more electrons. the Voltage rating describes how much the insulation (paper) can handle without burning a hole through it. they are basically a mechanical battery the larger they are the longer time it takes to charge and discharge it. They are normally used with a coil such as the start winding in a motor. the time factor for forming the magnetic field in the winding and the time factor for charging the capacitor are tuned to give a frequency that would equal 35 to 50% of 60 cycles per second or 60 hertz. the magnetic field of the start winding is strong enough to mask the run winding field and because it is a slower speed it has less slip and will deliver the torque of a 1 HP motor at half the speed which is double the torgue of the same motor at full speed. The start capacitor must be removed from the circuit before or as close to the tuned speed or it will fight the run winding and burn up the motor. The start capacitor MFD is added to the run MFD / the run MFD IS matched with the start winding for 100% of the 60 hertz and provides a more efficient motor.

I hope this helps with some of the confusion about capacitors
Art B
 
What ever? Bill's response worked for me. The motor started like a new one:grin:
Thanks again
 
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