Ask Me Help Desk - Understanding air conditioner capacitor wiring

I would like help understanding the wiring diagram for a window air conitioner.

From what I've read, a capacitor gives an electrical "boost" to the start windings in the compressor motor and the fan (this particular capacitor is a compressor/fan type.)
I've also read that the capacitor is wired "in series" with the line voltage to boost the starter.
Here's my confusion:
1. In the schematic, the Capacitor seems to be "downstream" from the Compressor if I trace current flow. Here's the flow--

Line voltage====>thermal cut out terminal #1
black wire to====thermal cut out terminal #2
black wire to======>Compresssor C terminal.
Leaving Compressor S terminalheavy white wire to=====>Capacitor H terminal and Leaving Compressor R terminal heavy red wire to=====> Unmarked Capacitor terminal.
This Capacitor Unmarked terminal also receives a White wire from fan,
and also connects back to the other line voltage wire.
This unmarked termainal strikes me like a neutral junction, but I'm stymied by the appearance the
Compressor S to Capacitor H wire.
If the flow is from Compressor C to Compressosr S and R, how can the Capacitor contribute being "downstream" from the current flow?
And if I've got it reversed, and that Capacitor Unmarked terminal is NOT the neutral junction, then where are the Compressor and Fan return lines?

And, if that's not enough, one quickie... does a Capacitor need a neutral wire?

The unmarked terminal you referred to is the common and it feeds the "Run" winding on the compressor. The other wire coming off capacitor goes to the "Start" winding on the compressor. Capacitors always go between "Start" and "Run" on any motor like a compressor motor. The third terminal on the compressor is "Common." The capacitor causes the wave in the Start winding to lead the wave going to Run so that it provides more torque. Three phase motors have no capacitors because each wave is 120° apart and a 3-phase motor has 600% more torque to start than a single-phase motor. The way you know which winding is which is you measure the resistance between the three terminals. Start to run is the highest, start to common is second highest, and run to common is the lowest. So, if you measure A to B and get 10 ohms, B to C = 6 ohms, and A to C=4 ohms you know that C=common, A= run, B=start (R+S=(C+R)+(C+S)). Always check a compressor this way before wiring it up. Also, remember that the wire that feeds the run winding feeds the capacitor and the other side of the capacitor goes to start. Check the capacitor with a meter because a shorted capacitor will burn up a motor whether compressor or fan, even a little water or a bug (as I have had happen) will do this. I spray the top with WD40 to prevent this as much as possible. Here is a book that will explain a lot: https://dl.dropbox.com/u/8010163/Com...ata_Manual.pdf

Hi Dr. Tom,
Many thanks for your clear explanation. It seems I was tracing currnet flow in reverse. Now, it all makes sense, EXCEPT for one little detail. If that Unmarked terminal on the Cap is actually a "hot" junction rather than a "neutral/return" junction as I first supposed, how is that the white wire from the Fan (I've assumed it's the neutral/return) can be connected there with the hots?

An AC capacitor has no + or - nor is there a hot and neutral. Both wires are treated the same on motors. Usually capacitors that are wired that way are two capacitors in one. One side is marked "Motor" or "Comp" and the other is marked "Fan." The middle terminal is a common, not to be confused with "Common" on the compressor. When you have such a capacitor NEVER switch the leads. The common on such a capacitor feeds the "Run" winding on the compressor and the common (Actually Run) on the fan motor. The speed windings go through the switch and are really different windings all tied to the common terminal which feeds the fan capacitor, too. On a capacitor that has only two terminals you will have one terminal marked which means that it is the one connected to foil closer to the outside and should be wired to neutral, or on a 240 volt system to Run on a compressor, or Common on a fan motor.

Quote:

Originally Posted by drtom4444

An AC capacitor has no + or - nor is there a hot and neutral. Both wires are treated the same on motors. Usually capacitors that are wired that way are two capacitors in one. One side is marked "Motor" or "Comp" and the other is marked "Fan." The middle terminal is a common, not to be confused with "Common" on the compressor. When you have such a capacitor NEVER switch the leads. The common on such a capacitor feeds the "Run" winding on the compressor and the common (Actually Run) on the fan motor. The speed windings go through the switch and are really different windings all tied to the common terminal which feeds the fan capacitor, too. On a capacitor that has only two terminals you will have one terminal marked which means that it is the one connected to foil closer to the outside and should be wired to neutral, or on a 240 volt system to Run on a compressor, or Common on a fan motor.

Hi Dr Tom,
So it sounds as if that white wire running from the Cap Unmarked to the Fan is NOT a neutral/return leg from the fan at all but actually a hot feed to the Fan Run Windings.

Why the white color on a hot wire?? Very confusing for this novice.

If all this is so, the fan now needs a Neutral/Return leg, and the only wires remaining all link back to the fan speed control switches. So... given all that, those three wires leaving the Fan and runiing to HI, MED, and LO must be returns and the Fan speed switches are actually interrupting the Neutral leg? I am getting it?? Scott

You seem to be understanding it. Just remember that electricity flows in a loop. On a 120 volt system either leg could be neutral; it's not important. White usually indicates common in a motor circuit and could be hot or neutral. It's not the same as wiring a receptacle or something in the house wiring. AC current flows both ways so with a motor both wires are considered power supply wires.

Quote:

Originally Posted by drtom4444

You seem to be understanding it. Just remember that electricity flows in a loop. On a 120 volt system either leg could be neutral; it's not important. White usually indicates common in a motor circuit and could be hot or neutral. It's not the same as wiring a receptacle or something in the house wiring. AC current flows both ways so with a motor both wires are considered power supply wires.

HI Dr. Tom,
With a couple of brief replies, you've explained more to me than I've found in over a week of hit and miss Google searching to try to resolve these questions. Thank you very much for your knowledge, patience, and knack for clarity. Best regards, scott