What is the purpose of the capacitor?


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Old 07-12-08, 07:02 AM
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What is the purpose of the capacitor?

I am somewhat familiar with what a capacitor is, but what is its purpose in the condensing unit? Is it used to kickstart the compressor and fan motor? Or just the compressor? Or is there a different purpose? My wiring appears to have a wire coming from the fan motor to the capacitor - which I am not sure what this does. I hope somebody can enlighten me.

Please note - I am not going to rewire my system - I merely want to know the why's of some things on the system.

Many thanks in advance!
 
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Old 07-12-08, 07:25 AM
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Not going into great depth, if you google it you'll find loads of info and formulas, about caps.

Basically, the cap moves the electrical sine wave forward a little. This keeps the motor running more efficiently and cooler.

This is why when you read the forum where someone's fan motor will not run. buzzes, runs then stops, the first thing we ask is "have you checked the cap?".

That's only the "Run" cap. Typically a dual cap, round or oval, with 3 terminals.

Some compressors use a "Start Cap". A start cap will give the compressor a kick to get it started and is then taken out of the circuit when the comp gets up to speed.
 
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Old 07-13-08, 11:04 AM
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The compressor and fan motors are 2 phase motors. 2 phase motors have more starting torque than single phase motors. Since we only have single phase current coming into a house we have to somehow convert it into 2 phase. That is what the cap does. The cap provides the current 90 degrees out of phase from the other leg to the leg that that cap is used on.

So, the cap does a couple of things, more torque for the motor to start under load and it gives better efficency resulting in less current being drawn.
 
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Old 07-13-08, 11:18 AM
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close but no cigar's

If you are not familar with electricity - this may be complicated, however I will try to explain as simply as I can with complete accuracy.

Home Air conditioners are single Phase 230 volts. ( Two Legs - 115V per leg)

All Alternating Current motors (home or commercial) have 3 windings. - or 3 legs (that is for single phase and 3 phase motors alike - 3 phase motors ( found on commercial building applications) dont have capacitors simply because 3 phase service from the power company provides the 3rd leg to the motor commercially.)

"Single Phase" service to your home from your power company can and do provide two legs of a motor with voltage. The 3rd leg for that same motor comes from the Capacitor (that is in parallel with one of the two other commercially powered legs)... in that, it stores the energy as the 2 single phase legs are building up on the sign wave.

As the single phase voltage decays going to zero volts (AC= alternating current), The capacitor discharges its stored voltage/current giving that 3rd leg voltage to your single phase 3 leg motor, just as the commercial power single phase switches direction (or zero volts) and begins to build up once again in the opposite direction. The capacitor once again store's the energy of the parallel leg.

When a cap is weak or bad - The motor is simply single phasing! or better yet, when 1/3 of the windings are not being supplied with power by the capacitor - it simply cant run and humms... Effiency?..well ok sure... the 3rd leg of voltage is basically free since it is stored energy and not provided from the power company in single phase applications such as your home. That is however misleading, because single phase motor's are relatively speaking less effiecent than 3 phase motors...lol

This happens 60 times per second. Now you know the rest of the story!
 

Last edited by The Real Deal; 07-13-08 at 12:15 PM.
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Old 07-13-08, 03:15 PM
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I stand corrected

Deal is right, the cap is on the 3rd (start) leg and the wave lags the other 2.

Sorry for the backward info. I'll try not to do that again.
 
 

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