I don't know how many times a technician has said that they installed a part based on what was on their service truck.  I have heard of technicians wasting money over-sizing contactors, cutting down air filters, and even using controlled substances to clear condensate drains!  Of course, these scenarios all get the job done, but I would argue the many reasons why not to do these.  The one thing that gets to me is when a technician doesn't verify they are installing the correct size dual run capacitor.  Believe it or not, there is a simple method to figuring the correct size capacitor, without waiting on hold for the distributor's guru.  Of course, you could use a multimeter that reads microfarads (uf), but this will only tell you if the existing capacitor is weak - not the correct size!

 

Testing Volts / Amps

With the condensing unit operating under a load, you will need to measure the total volts between the HERM and COMMON terminals on the run capacitor (i.e. 345 VAC).  Next, measure the amperage on the wire leading from HERM to START on the Compressor (i.e. 4 amps). 

  Use the equation below to verify the size of the capacitor. The resulting microfarad (uf) should match the size of the installed capacitor.

 

  An over or under-sized capacitor will cause an imbalance in the magnetic field of the motor.  This hesitation when operating will cause noisy operation, an increase in power consumption, a drop in motor performance and eventually overheating or overloading motors like compressors.  The run capacitor should have the exact microfarad (uf) that the motor is rated for.  Capacitors rated above 70uf are considered Start Capacitors and are generally removed from the circuit electrically during operation.  This is where the rule of +/- 10% of the rating came from, for Start Capacitors ONLY!  The voltage rating should be no less than the listed amount for the motor, for central heat pumps and air-conditioners this is usually a minimum of 370VAC.  Most new condensing units are specified for 440VAC capacitors, and are more durable during fluctuations in power supply.  I have seen some universal type dual run capacitors rated for as much as 700VAC, as this voltage rating does not impact uf performance.  The changes in uf will impact amperage draw and reflect on kilowatt hour usage though.

  When completing your early season maintenance this year for your customers, do them a service by testing capacitors and verifying their size.  You could just increase your service ticket total and save some all important power.  You may even prevent a call-back during the next heat wave!

http://excessair.blogspot.com/2012/05/correctly-sizing-capacitor.html

Views: 34240

Comment

You need to be a member of Home Energy Pros to add comments!

Join Home Energy Pros

Comment by Bob Blanchette on May 25, 2012 at 7:54pm

I suppose "trial and error" could be used to determine resonance. Using several capacitors that are close to what you think you need determine which one has the highest voltage across it. That will be the capacitor closest to resonance which will give a 90 degree phase shift to the start windings.

As far as thermostats go, any digital stat that doesn't have the ability to be common wire powered (must have batteries) are mostly junk. There are a few exceptions, but not many. About 80% of the digital thermostats I see in the field are NOT common connected, go figure...

Comment by Bob Blanchette on May 25, 2012 at 7:46pm

Your test works great to figure the actual working value of the installed capacitor, but I'm not seeing it as a way to verify if it's correct value for the unit. The only way to verify correct capacitor without having manufacturers data is to test the phase of the power going into the start winding vs the run winding. They should be 90 degrees apart under "typical load", phase will vary based on actual compressor load. Unfortunately this requires a dual channel oscilloscope, something few techs are likely have on their truck, much less know how to use properly. I've thought of trying to get the voltages equal across the start winding and capacitor to verify resonance. However there is no easy way to separate the inductive, resistive, and load transfer components of the start winding. It's best to look it up on the manufacturer's website if you aren't lucky enough to find a readable label on the unit.

Home Energy Pros

Home Energy Pros was founded by the developers of Home Energy Saver Pro (sponsored by the U.S. Department of Energy,) and brought to you in partnership with Home Energy magazine.

Latest Activity

j. west shared Christopher Morin's blog post on Facebook
5 hours ago
j. west commented on Christopher Morin's blog post UPDATED: Big Changes for Duct Testing in MA
"You're right Christopher- the trick to saving the profit margin is being in several places at…"
5 hours ago
tedkidd replied to Christopher Talarico's discussion Heating with Tankless Water Heater & Hydronic Air Handler vs. Gas Furnace
"Nice posts Steven and Eric.   Scale on my combi is a recurring problem.  Attaching…"
6 hours ago
Eric Kjelshus replied to Christopher Talarico's discussion Heating with Tankless Water Heater & Hydronic Air Handler vs. Gas Furnace
"I find the tank less hot water lime up.   I have a condensate pump that I pump white vinegar…"
6 hours ago
Trip Smith joined James Sayers's group
Thumbnail

Marketing Energy Efficiency

Sharing ideas, tools and examples of promoting energy efficiency to consumersSee More
7 hours ago
Trip Smith joined Hal Skinner's group
Thumbnail

The RCC Classroom (Radiant Control Coatings)

A group where energy professionals can learn about all the different energy saving aspects of…See More
8 hours ago
Trip Smith commented on Diane Chojnowski's group Facebook Pages
"Just went through the first few pages liking the active Facebook pages. I've found that…"
8 hours ago
Trip Smith joined Diane Chojnowski's group
Thumbnail

Facebook Pages

Does your company or organization have a Facebook Page?This group is for pros who have facebook…See More
8 hours ago

© 2014   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service