# Thread: purpose of capacitor

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Capacitors make use of certain laws of electricity to decrease the amount of power consumed.

Here's a good website for calculations and formulas:
http://www.nepsi.com/formulas.htm

The pertinent term to lock in on is "power factor correction." This term means you are trying to mathematically offset some of the electrical inefficiencies of the system by adding capacitors.

Once you understand the losses associated with induction loads, you'll begin to see how the capacitor offsets some of those losses. This "offset of loss" equals a reduction in usage, and therefore a lower bill for large consumers (factories, hospitals, etc..) but for the design engineer of the HVAC system, it means lower amp draw, allowing smaller wires, both in the motors, and feeding the motors.

Here is a website that explains it much better than I can, and (combined with the formulas you get from the first website I posted) allows you to calculate power correction factors as if you had taken the class in college.

http://www.lmphotonics.com/pwrfact.htm

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## Start Cap

The Start cap is used as a switch. To get the compressor motor moving you need a large set of windings .There are two sets in the motor. At start both winding are working to get the motor spinning .Once it's up to speed running both would pull too much of a load then what it needs, so the start winding have to drop out. A cap will allow electricity through it at discharge but as it builds up a charge, resistance increases through it preventing electricity to pass . The cap is used here as a switch....

The run cap as I understand aligns the phase to make the motor run more efficant making more motion then heat.....

[Edited by Big Ed on 02-26-2005 at 08:30 AM]

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Thoughts on this are like a**holes, everybody has one, so heres mine... On a single phase motor, the electric is in phase, so instead of the poles repeling, they match up, and they match up in sequence, in essence, not moving the rotor... When you use a capacitor, you shift the sine wave, taking the electric out of phase, giving you poles that repel, giving you rotor rotation.... A start capacitor is used when a motor will start under a load, it'll shift the sine waves further apart, giving the motor much more starting capability... The start capacitor is designed to fall out of the circuit when the motor is reaching full speed by use of a PTC or a potential relay...

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Frame,

Run cap cools motor ?

I'm just goona let that one go, go back to basics read them and reread them at least once a year. There is no replacement for basic sound fundamentals,fundamentals are the key to the field. Without them you are lost.They will never let you down.If you can't apply fundamentals and work out a problem ,then you are applying them wrong.A run cap is virtually the same as a start cap it just has less capacitance.

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The capacitors do need to cool. Thats why they are strapped to the side of the unit by a metal band. Don't you just hate to see em dropped in the unit hanging with black tape all over em?????

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Schmuck,

That is as bad as his original post. Possibly even worse. At least he had some Idea that there was an electrical purpose for the thing.

7. Oh god, everyone that doesn't understand the basics of electric motors, leading and laging currents, induction and rotating magnetic fields, please take an electrical class that covers AC motors!

The capacitor isn't there to switch anything, or to dissipate heat, and the start windings of a compressor doesn't drop out when the compressor gets up to speed. A start capacitor will, but the start winding and run capacitor stay energized.

The run capacitor, in conjunction with the start windings having more turns of smaller wire, causes the current through the start windings to shift out of phase with the current through the run windings. In most cases, the current through the start windings will lead the current through the run winding. This causes the magnetic fields generated by the windings to rotate. The rotation of the magnetic fields in the stator act on the magnetic fields induced in the rotor to cause it to spin.

There are whole books on this subject, read one.

Reading some of these responses makes me wana run around in circles like the mupet Beaker screaming EEEEEEP! EEEEEEEP!

[Edited by mark beiser on 02-26-2005 at 10:59 AM]

8. wb
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For your test, just wright down.........

The run capacitor increases starting torqe and increases run efficiency of a PSC motor.....

9. Mark is right here !!

a start-capacitor has higher capacitance than a run capacitor. Is there to produce a strong starting torque, but is not manufactured to dissipate heat and causes an increase in current draw, therefore it cannot stay in the circuit & therefore must be removed. ( at app.75% speed )

A run capacitor is of smaller capacitance & is oil filled to dissipate heat, which allows it to remain in the circuit, mainly for operating torgue.

The torque needed for a motor to move from a still position and run is much more than the torque needed to keep it running.

And just to see if ya'll are paying attention:
A motor is an inductive device. That means that because of the inductance, the voltage & current are out of phase with each other. This reduces the power that is capable
from the device. The capacitor brings the phase between the voltage & the current closer together which causes an increase in power for the device.( purely resistive circuit provides maximum power )
The run capacitor moves the Voltage & current phase slightly closer together, to get a little more power (which is torque)
The start capacitor moves the phases even closer, and therefore allows the device to generate more power ( torque ). But the motors are not designed to handle the power from a start cap. for a long time, due to the size of wiring in the windings.
Remember power equals voltage times current. To get more power, with voltage remaining the same, current would have to increase.

[Edited by bornriding on 02-26-2005 at 12:17 PM]

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Well besides all of the technical stuff ( which I do know very very well ) The caps intention is to make the motor think it has three phase power which has more torque.

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A start capacitor assist in the starting of a motor. When motor reaches 75% speed start cap. is taken out of circuit with a centrifugal switch or some type of relay. If it isn't,the windings will fry. A run cap. stays in the circuit 100% of the time. A run cap. is also filled with an oil to keep it cool.

12. Originally posted by fat eddy
Well besides all of the technical stuff ( which I do know very very well ) The caps intention is to make the motor think it has three phase power which has more torque.

Eddy, step away from the pipe.

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Dave,

You don't think that installing an out of phase current in another winding resembles three phase, I'll let the heavies handle this one. ( any PE's out there disciplined in electrical? )

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