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-   -   Run capacitor - how accurate should it be? (http://www.diychatroom.com/f17/run-capacitor-how-accurate-should-146740/)

jeremyp969 06-11-2012 04:35 PM

Run capacitor - how accurate should it be?
 
I recently had an A/C performance check done by a professional. I have 2 units for my house, a 2 ton and a 3.5 ton. One thing the tech told me is that the current draw of my larger unit at startup is huge and that is caused by a bad run capacitor. I'm looking at the writeup he did and for the "bad" system he noted a compressor amperage of 51A at startup and 13A when running. I agree that seems like a very large spread. Then I looked at what he noted for the run capacitor and it's spec'd at 35uf and he measured it to be 32.1uf. Is that enough of a difference to cause a problem?

In contrast on my "good" system the startup amperage of the compressor and the steady state amperage of the compressor are the same at 9.2A. The run cap is spec'd to be 30uf and he measured 28.3.

I can replace the cap, but neither one seems that good and yet the compressor on the smaller unit startup current is the same as the steady state current while there is a huge difference in the larger unit.

These units are now 11 years old and haven't had any issues yet. Recommendations?

Thanks!

Doc Holliday 06-11-2012 05:28 PM

Google "in rush current" and "5-2-1 compressor saver/hard start kit." The hard start kit for your 3.5 ton will be bigger than the hard start for the 2 ton so get one according to condenser size.



Your system is normal.

Doc Holliday 06-11-2012 05:32 PM

http://en.wikipedia.org/wiki/Inrush_current

Motors
When an electric motor is first energised, the rotor is not moving, and a current eqivalent to the stalled current will flow, reducing as the motor picks up speed and develops a back EMF to oppose the supply. AC induction motors behave as transformers with a shorted secondary, until the rotor begins to move, while brushed motors present essentially the winding resistance. The duration of the starting transient is less if the mechanical load on the motor is relieved until it has picked up speed.
For high power motors, the winding configuration may be switched during start-up to reduce the current drawn.

http://www.diychatroom.com//upload.w...rush-100Va.png

jeremyp969 06-11-2012 05:38 PM

Thanks for the info. So the in-rush current draw of one unit vs. the steady state vs the in-rush of the other unit vs the steady state(51-13 vs 9.2-9.2) isn't a concern? Is the capacitance value a problem?

Doc Holliday 06-11-2012 05:44 PM

Quote:

Originally Posted by jeremyp969 (Post 941127)
Thanks for the info. So the in-rush current draw of one unit vs. the steady state vs the in-rush of the other unit vs the steady state(51-13 vs 9.2-9.2) isn't a concern? Is the capacitance value a problem?

Only if the capacitor is either the wrong size for the unit or weak.

The hard start kit is a super capacitor and a relay which will assist the regular capacitor for the compressor on start up. It also helps reduce wear on the compressor's windings due to the fact that the motor slams right into action, less time in rush equalling less heat on the windings, why it's called a compressor saver. It is designed specifically to get the motor moving, fast. It can and will lengthen the life of a compressor for many years.

---------------------------------------

The capacitor is an invention that was used to store up an electrical charge, and then discharge it into a circuit. This can be used to smooth out electrical impulses, or turn a constant electrical flow into a series of impulses.

Simple process of capacitance
The average capacitor is made up of a few things. There are two lead wires, the case, two metal plates of a specific size, and the dielectric.
The way capacitance works is by having two metallic plates separated by a material called the dielectric. The dielectric must be a nonconductive (insulating, opposes flow of electricity) material. There are several types of dielectrics such as paper, plastics, ceramic, glass, and even air. When the capacitor is place in a circuit with an active current, the electrons from the negative side build up on the closest plate(the negative flows to the positive, that is why the negative is the active lead, although many capacitors are not polarized). Once the plate can no longer hold them, they are forced past the dielectric and onto the other plate, thus displacing the electrons back into the circuit. This is called the discharge.

jeremyp969 06-11-2012 05:46 PM

Quote:

Originally Posted by Doc Holliday (Post 941115)
Google "in rush current" and "5-2-1 compressor saver/hard start kit." The hard start kit for your 3.5 ton will be bigger than the hard start for the 2 ton so get one according to condenser size.



Your system is normal.

Haha, so I went to their site and they won't sell to a homeowner.....d'oh. Guess they don't trust me to install.

On the capcitance, how much do I let it differ from the spec value before I replace? Currently it's spec'd to be 6% and I'm looking at more like 8.3%

Doc Holliday 06-11-2012 05:51 PM

It's past time already. 6% of 35 is 2.1. If the capacitor is under 32.9 mfd than it is out of range, or higher than 37.1.

Change it. Get you a hard start kit regardless.

Doc Holliday 06-11-2012 05:52 PM

Quote:

Originally Posted by jeremyp969 (Post 941135)
Haha, so I went to their site and they won't sell to a homeowner.....d'oh. Guess they don't trust me to install.

On the capcitance, how much do I let it differ from the spec value before I replace? Currently it's spec'd to be 6% and I'm looking at more like 8.3%

Go to Grainger, they sell to anyone.

jeremyp969 06-11-2012 08:58 PM

Quote:

Originally Posted by Doc Holliday (Post 941141)
Go to Grainger, they sell to anyone.


Thanks for the info. I was looking at the Grainger website and they rate the hard start kits by torque. Any recommendation for a 3.5 ton unit?

Thanks!

Doc Holliday 06-11-2012 09:56 PM

The middle one.

http://www.compressorsavers.info/images/units_pic.png

REP 06-12-2012 03:04 AM

I agree that a hard start kit can bwe very helopful in helping a unit run at it top effiency.
The important thing is that your cap is wearing out.If I see anything below the listing I change it out.There is nothing as agrervating as a callback on one of the cheapest parts on the whole unitMost service tech will say 5% is the limit on a cap.
When I was working ,I always used a factory hard start kit as its designed to match perfectly with the unit.I only had so so luck with off the self third party subsitute start kits.


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