I have a capacitor in my AC condensing unit outside. The fan motor was not spinning but the compressor was on or making noise. The AC man changed the capacitor and it worked again. The question is: What does the capacitor do in the unit? Do most AC condensing units come with a capacitor? What about heat pumps?

All ac units, heat pumps, and furnaces come with capacitors for the fan and compressor unless you have a three pahse system. Three phase systems are generally commercial.

A capacitor helps the motor run or start depending on what type of capacitor it is. If one is to weak it will cause the motor not to run. This is not an uncommon thing. I would say more than half of the posts on here about fans end up being the same thing. Replaced capacitor.

Sorry I was going to give you more detailed and technical info but it was way to long and complicated.

It adds a brief boost of current to allow the motor to overcome inertia.

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Originally Posted by siberianair

all ac units, heat pumps, and furnaces come with capacitors for the fan and compressor unless you have a three pahse system. three phase systems are generally commercial.

a capacitor helps the motor run or start depending on what type of capacitor it is. if one is to weak it will cause the motor not to run. this is not an uncommon thing. i would say more than half of the posts on here about fans end up being the same thing. replaced capacitor.

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That's Ok. I am actually looking for a technical answer from a professional in the HVAC industry. I am a master electrician. I was curious as to how efficient Condensing units were as opposed to heat pumps for Air conditioning.

See your question said nothing about one vs the other. Neither one is really more efficant than the other. Seer ratings for both are the same. 13 is 13. 16 is 16. 19.5 is 19.5.
I find it funny that master electricians seem to know little about capacitors. I have found 5 on here all that seem to ask the same question. And just so you know I am a professional in the hvac industry. I even own my own company. I tend not to get technical with people on here because most of them are just normal people with little knowledge about systems, electricity, freon, or anything other that what they have read or been told. Getting to tech on them confuses them and can cause long winded conversations they will never understand anyway.

Let me attempt this for a bit. In motors they typically are designated srart capacitors and run capacitors.

A start capacitor roughly preforms the function of a centrifugal switch. There is a brief pulse to the start winding that causes the motor to rotate. The is because the cvoltage across a cap can't change instantaneously, so it initially acts as a short.

The run capacitors are designed to provide some sort of phase shift for the motor. One example is the PSC motor used in ceiling fans.

Condenser fans don't have to have much HP attached to them and neither do ceuiling fans. They are not doing a lot of work like moving water. The ceiling fan motor is vary lossy. You can grab a blade and hold it.

AC motor - Wikipedia, the free encyclopedia

Introduction : AC MOTORS

When capicitor don`t do its job,the motor don`t work anymore

So to make certain I understand the above answers, typically the "fan motor failure" issue is really that a capacitor is failing and not that the motor is overheating, stuck, old, etc. When the start capacitor can not provide the UMPH to get the motor started, the motor heats up but does not turn causing problems for both the fan motor and other items? Right?

Usually if the motor fails, either it's because the capacitor has failed or the bearings have become dry/gummed/stuck. The windings themselves usually withstand pretty much anything, they're usually impedance protected to protect against overheating in the case of stalling.

There are usually two capacitors. One for the compressor and one for the fan in the condenser. They can be classified as motor run or motor start caps and they have different functions. There are usually two capacitors in one "device" with 3 terminals. They can be replaced with two separate 2 terminal devices.

The fan cap is usually a motor run cap where it provides a phase shift for the motor to rotate.

If the compressor uses a start cap. It energises the start winding, to start the movement of the compressor.

Sorry, to be clear, I was only talking about the capacitor(s) pertaining to the motor. I know little of compressors.

What causes a Capacitor to fail ? I have replaced it at least 2 x a year, I did notice that my house has the AC 30 amp breaker sharing with some of the outlets in the house, would this add to the rapid capacitor failure rate ? The capacitors swell up and then no heat or Air is pushed through the ceiling, its an outside Trane, the repair guys still have no clue and I keep paying for the capacitor to be replaced.

Can the capacitator cause the AC not to cool?

Can the capacitator cause the AC not to cool?

YES

MAK Commu,
They actually stop the motors or compressor from working, which are all needed for cooling. If they are working, Capacitor is likely good.

Also, In the Newer units with control boards, The Capacitors can be used for Filter, Timers,.

The capacitor in our unit went out. Land lord just had someone come replace it. I was told what part needed to be replaced but not why it does or how it works so I looked on here. I understand all the answers that were given to his question just fine but I noticed a lot of them saying that it would cause the unit not to run if going out. In our case our compressor would run but it wasn't cooling and it ended up freezing up because come to find out when we cut our thermostat to off our compressor wasn't cutting off. Luckily I went out back to feed the dog and noticed that even though we cut it off it was still on so we had to turn the A/C breaker switch to off so it wouldn't run. So basically what I'm getting at is how does the capacitor going out cause the compressor to not shut off at all even when you cut the thermostat completely off?

"capacitor going out cause the compressor to not shut off at all even when you cut the thermostat completely off?"

There are a lot of conflicting things here and there could be other problems.

1) Some causes of "the compressor not cutting off with the thermostat" would be a stuck contactor in the outside unit. This is a common failure.

2) A faulty capacitor can cause a few problems. The compressor won't run, the outside fan won't run or runs backwards.

3) There are so many causes of not cooling, so I won't go there.

There are two major parts of the outside unit and they are the compressor and the condenser fan. High and low pressure switches are normally employed to protect the compressor. Usually there is also a high temperature switch on the compressor as well.

A standard condenser unit for a non-heat pump system is pretty simple. A two speed condenser fan adds complications in trade for energy efficiency.

Okay looks like all these questions were answered several years ago so I don't know if anyone is even reading this thread anymore. But I have another question. My outside motor stopped running yesterday and it was Humming. So of course I went to Mr. Google and it said it could be the capacitor and to try spinning the blades to see if it would run. I did that and it worked and my blades are currently spinning. But it still does not cool. I know I have to replace the capacitor but would the bad capacitor be affecting the unit not cooling as well?