NORCALer
Sacramento, CA
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Why is it that the manufacturers don't just install the upgraded capacitor? There has got to be some draw backs to this "upgrade" capacitor. Can we guarantee that going from the 43-53uF to the 88-106uF will not harm the unit?
Read below for information that generated these questions.
Capacitor Size:
The proliferation of potential relay type devices has resulted in the notion that one capacitor can be employed to start all compressors. That is, use the biggest capacitor and give the compressor a "big kick" to get it started. The sensing characteristic will drop the capacitor out of the start circuit when necessary and thus the compressor will not be harmed. This idea, however, is flawed. The use of a capacitor that is too large for the impedance characteristics of the windings in some compressors can actually result in significant compressor damage. Recent investigations indicate that this situation is particularly evident in voltage sensing devices.
If the start capacitor is too large for the application, the capacitor can actually mask the developed voltage in the start windings and keep the start capacitor in the circuit continuously. Figure 5. illustrates a compressor start with a capacitor that is too large. The motor is actually running, but the run-start voltage is suppressed below the trigger voltage of the start device. As a result, the start capacitor remains in the circuit as the motor runs. A secondary, fail-safe method is necessary to ensure that the start device is ultimately removed from the circuit. This event can be seen at the end of the time duration of the run-start current highlighted in Figure 5. A start device that fails to remove the start capacitor from the circuit has the potential to cause premature failure of the start windings in the compressor. Figure 6 shows the same compressor start using and oversized capacitor without a safety timing circuit. The run - start voltage is suppressed by the combined characteristics of the motor windings and the extra large capacitor. It never reaches the prescribed threshold voltage defined by the potential relay for removing the start capacitor from the circuit. The total supply current remains near the locked rotor value even after the motor has started (as highlighted in Figure 6). If the capacitor is never removed from the start windings, premature winding failure could occur. As such, care should be taken when selecting capacitor sizes for an application. Care should also be taken regarding products that tout a "bigger capacitor is better" approach to compressor starting. SUPCO E-Class devices provide a secondary timing safety device to ensure that the start capacitor is dropped from the circuit in a fail-safe mode. Figure 5. also shows that the start winding voltage drops appropriately after the start capacitor has been removed in a SUPCO E-Class device.
Info above taken from this source:
http://www.supco.com/eclassadvantage.htm
I do currently have the 43-53 uF capacitor installed and just want to make sure there are no negative effects of installing the 88-106 or the 130-156 uF capacitor. The last two sentences in the above paragraph may be enough to answer my question, but I just wanted to make sure and understand why these units dont already come with the better capacitor.
Thanks for the help! 
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MI Director
Michigan
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Most of us in the Appliance Repair field have know for years that putting a hard start kit on a compressor usually only will give it a short life span. I will only use a kit as a last resort and then tell the customer to start looking for a new unit cause in most cases the compressor fails within a year.
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garry1p
Oklahoma
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Interesting its been a long time since I was in electronics school but a capacitor is not "removed" from the circuit. What happens is the capacitor gives a 90 deg phase shift and once the motor starts spinning the voltage and current go back to a 0 phase shift.
The larger the capacitor in mfd the longer the voltage/current take to get back in phase and the higher the charge across the capacitor plates.
All of this happens in fractions of a second. The engineers design the circuit for optimum performance to small and the motor won't start or overheats trying to start. To large to high of a voltage across the windings and you can get arcing shorting out the windings.
A small increase in mfd can be tolerated and will provide a little extra boost but there is a limit and that is based on the motor windings.
I sure would no go with a 130/156 capacitor but that is just me.
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garry1p
Oklahoma
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I stand corrected some are removed from the circuit see below.
Motor capacitors include 2 common types, run capacitors and start capacitors :
• Run capacitors: - Run capacitors are designed for continuous duty, and they are energized the entire time the motor is running.[1] Run capacitors are rated in a range of 3-70 microfarads (mfd), with voltage classifications of 370V or 440V.[1] Single phase electric motors need a capacitor to energize a second-phase winding. If the wrong run capacitor is installed, the motor will not have an even magnetic field, and this will cause the rotor to hesitate at those spots that are uneven. This hesitation can cause the motor to become noisy, increase energy consumption, cause performance to drop, and cause the motor to overheat. However, a motor will not be ruined just because a run capacitor is faulty.
• Start capacitors: - Start capacitors briefly increase motor starting torque and allow a motor to be cycled on and off rapidly.[1] Start capacitors have ratings above 70 microfarads (mfd), with three major voltage classifications: 125V, 250V, and 330V.[1] A start capacitor stays energized long enough to rapidly bring the motor to 3/4 of full speed and is then taken out of the circuit, such as by a centrifugal switch that releases when rotating at or around that speed.
Examples of motor capacitors are: a 35 mfd, at 370V, run capacitor, or an 88-108 mfd at 250V start capacitor.
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southernkilowatt
North Carolina
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MI Director wrote:Most of us in the Appliance Repair field have know for years that putting a hard start kit on a compressor usually only will give it a short life span. I will only use a kit as a last resort and then tell the customer to start looking for a new unit cause in most cases the compressor fails within a year.
I agree totally with this point, and to further prove it without actually damaging a compressor, try it on a 250 watt Metal Halide bulb setup. Most of these come with a 15uf cap. Remove it and replace with a 20uf. The bulb still fires, but will be much brighter. And the life span is cut ten fold.
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BillyW
North ID
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I will argue the point to the other direction.
I believe that the manufacturers avoid the hard start caps simply to save money. It is mentioned as an available option on my Coleman model. Would they offer an optional part that would prematurely kill their product? Who knows? Maybe.
Pardon my ignorance, but can you really compare a Metal Halide bulb to the windings in a motor? They don't seem the least bit electrically similar.
I visited MANY websites in my quest for truth in regards to the hard start caps. Some manufacturers don't need them, others do. In fact it's most often touted that the hard start kits LENGTHEN the compressor's life. I did find reference to hard start caps shortening compressor life specifically because of their operational characteristics, meaning wrong tool for the job.
I put a Supco SPP6E on my A/C last year. It has significantly reduced the initial voltage drop (from shore power or generator) at start-up. The A/C also spends much LESS time producing that initial start-up humming noise, which I assume is when the compressor is experiencing the most current draw. Though I'm not 100% certain, I "think" it's the right tool for the job.
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Golden_HVAC
Fulltime, CA, USA
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Hi,
A higher MFD capacitor will cause more amperage in the start winding.
I put a hard start kit on my rooftop air conditioner many years ago, on a truck camper that I had. The compressor never went out. I wanted the hard start capacitor to lower the time the compressor was in the start mode.
There are two capacitors used with freon compressors. Normally all air conditioning compressors have a run capacitor. Some air conditioners have a start compressor. Most commercial refrigerators do not have a run capacitor, some do not even have a start capacitor, but use line voltage on the start winding. Most domestic refrigerators don't have a capacitor, and use line voltage to start the compressor.
The capacitor limits the amperage going through the start winding. Lets take the average 13,500 Btu rooftop A/C unit. It might use a 20 - 25 Mfd capacitor for run capacitor. This will cause about 11 amps on the common terminal, 7 on run and 4 on start. Installing a start kit with 90 Mfd capacitor will cause the start winding to draw about 25 amps for a fraction of a second until the potential relay takes the start capacitor out of the circuit.
Anyway a larger capacitor will probably not help, but might damage the start winding with to many amps. Not the first or second time you start it, but perhaps within a year or two.
It is just that the start and main winding is only rated for so many amps, before the windings start to overheat and the insulation melts.
Good Luck,
Fred.
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cougar88nw
Prescott Valley, AZ
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I was considering this mod (even ordered the supco capacitor) but after doing a little more research...I think I'll pass. I have a 13,500 Dometic/Duo Therm unit and I went to the manufacturers website to find out what size generator is recommended. Dometic recommends a 3500 (minimum) watt generator to run the 13,500. My ac does have the factory start capacitor .
I am not willing to take a chance damaging or reducing the lifespan of my compressor, just to run a small generator. Not to mention the damage or premature wear to the generator. Oh well, at least I got my coils, fan,etc cleaned while I had the cover off.
I guess I will be listing the Supco SPP6 on Ebay when snail mail delivers it. lol
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ryanallie1
Magalia, Calif
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cougar88nw wrote:I was considering this mod (even ordered the supco capacitor) but after doing a little more research...I think I'll pass. I have a 13,500 Dometic/Duo Therm unit and I went to the manufacturers website to find out what size generator is recommended. Dometic recommends a 3500 (minimum) watt generator to run the 13,500. My ac does have the factory start capacitor .
I am not willing to take a chance damaging or reducing the lifespan of my compressor, just to run a small generator. Not to mention the damage or premature wear to the generator. Oh well, at least I got my coils, fan,etc cleaned while I had the cover off.
I guess I will be listing the Supco SPP6 on Ebay when snail mail delivers it. lol
Hi All.
Our 13,500 Dometic Duo-Therm, Brisk Air, clearly stated in our owners manual that it would take a Minumn of a 3.5 generator to even start our A/C. Our Twin Honda EU2000i's would start our A/C, but it had to work at it. I installed the Supco SPP6 on our A/C. Now its as differant as night and day. The Twin Honda's don't even puff anymore trying to start our A/C. It starts it as easily as flipping a light switch. So in fact, It is making the A/C work much less to start, which is better. The Honda's are working a whole lot less now to start the A/C. I would re-think your idea of it shorting the life span of your A/C and your Generator. The original Capasitor on your A/C is very undersized at best. But its your RV, so you do what you think is best. I know what is best from actual proof. Good Luck. Happy Camping. Dan & Jill
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cougar88nw
Prescott Valley, AZ
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Most articles I have read on the start/boost capacitors state " for tight or worn compressors, low voltage conditions, etc." I guess having a small generator falls into the low voltage/wattage category.
If the larger capacitors would make the ac unit compatible with smaller generators they would use them IMO (better selling feature). I think they used a capacitor that was compatible with the compressor/wiring. We know the rv industry uses the cheapest parts available and the compressor could be damaged by the higher amp/voltage surge from the boost cap. I'm just not willing to take the risk and I hope all that have opted for the spp6 have many years of trouble free service.
I found another dometic spec sheet that has different specs for the 13,500 Brisk. Shows minimum 2500 watt generator so I guess my Yamaha 3000 will work fine.
http://www2.dometicusa.com/pdf/os2132BriskAirRooftopLO.pdf
* This post was
edited 11/29/09 09:57pm by cougar88nw *
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