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  1. #1
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    Sep 2014
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    Is a start capacitor required??

    Ok fellow techs I had a situation that threw me for a loop in the lab the other day. I had to rewire a simple ac from scratch and I noticed it had a run capacitor for the fan and one for the compressor. My run and common was connected to the contactor with no bus bar. I got confused because I thought every compressor had to have a start capacitor. Can someone explain this to me? Thanks in advance.

  2. #2
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    Mar 2015
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    Medford, N.Y.
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    That is something the comp mfrg decides based on the starting requirements . There are not too many comps that YOU can just add a start cap, unless it is in the "Comp electrical handbook".Some ice makers start "unloaded" so they don't need a start cap, but the comp can have a start cap AND relay added.A start cap needs some kind of relay to knock the start cap out of the circuit after a second or so.

  3. #3
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    Sep 2014
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    Portage, MI
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    At least what I have learned. A fixed bore system doesn't generally need a start cap because the system equalizes on shutdown. So the compressor doesn't have to work against high head. A TXV system will generally need a start cap due to the discharge and suction not being equal and it needing to work against the high head.

    Some TXV systems have a solenoid valve that opens on shutdown allowing the sides to equalize. I'm drawing a blank what that setup is called at the moment.

  4. #4
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    Jul 2007
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    Quote Originally Posted by Hauptmann6 View Post
    Some TXV systems have a solenoid valve that opens on shutdown allowing the sides to equalize. I'm drawing a blank what that setup is called at the moment.
    Are you talking about a Liquid Line solenoid valve?

    If so, the liquid line solenoid valve closes when the thermostat demand is satisfied to prevent liquid refrigerant migration. These are usually seen in long line applications.

    The liquid line solenoid valve opens any time there is a demand signal from the thermostat.

    An accessory start capacitor and relay are normally required when using a liquid line solenoid valve with single phase, single speed compressors.
    Instead of learning the tricks of the trade, learn the trade.

  5. #5
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    Sep 2014
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    The one I'm talking about is piped much like a hot gas defrost but on the liquid line and not the hot gas line. Allows both sides to equalize when the compressor shuts off.

  6. #6
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    Jul 2012
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    most compressors can be wired with a potential zstart relay and seperate run and start caps. The relay uses back voltage feed back off the start winding to energize the relay coil and "hold it open" after the compressor gets to speed. The potential relay is normally closed.

  7. #7
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  8. #8
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    Nov 2006
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    Quote Originally Posted by Deezo View Post
    Ok fellow techs I had a situation that threw me for a loop in the lab the other day. I had to rewire a simple ac from scratch and I noticed it had a run capacitor for the fan and one for the compressor. My run and common was connected to the contactor with no bus bar. I got confused because I thought every compressor had to have a start capacitor. Can someone explain this to me? Thanks in advance.
    You can only have a start capacitor if you have a device that ensures it is removed from the circuit once the compressor gets running. This is essential because a start cap has no oil in it and will pop if left in the circuit due to overheating. Run caps have oil in them for cooling, and they are designed to stay in the circuit all of the time. The best choice for controlling the start cap is a potential relay. Other small compressors use a current relay, and some cheap units use a PTCR.
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  9. #9
    Join Date
    Jan 2012
    Location
    San Gabriel Valley
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    Some AC compressors are designed to work with start caps, but most don't need it, however as they get older parts start to wear out, compressors may start to have a hard time starting, if you see it pulling close to LRA at start up then I would say add a startcap and relay

  10. #10
    Join Date
    Mar 2015
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    ARK.
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    some cheap units use a PTCR.
    I call them "fire starters". Most I've seen have usually burnt up itself along with anything close by.

    Back in the "old day's" start components were a necessity due to the use of TXV's and no "electronic" time delay gizmos like we have now to provide equalization "time" and if the system was "bounced" had to have the oomph to get going along with being low torque,1725 rpm,pumps.Carrier had their Time Guard (pita) mech delay unit which did the job along with others I don't recall the names of.
    Fast forward some years and most res pumps are 3450 rpm and "sorta" high torque but they were on fixed bore systems which bleed down along with improved Internal Overloads and Comp Manuf's "could give a damn" attitude and "eventually" started within a short amount of time,usually 1 IO cycle.We had at this time "electronic" time delay's to add on when needed and these were a hot item then for sure.
    Then came the Scroll with it's internal discharge check valve and no load start up.BUT,They required a Time Delay in order to keep the Comp from literally starting in Reverse on a cool call bump but no start kit required now.
    Scroll's have improved over the years and the time delay is gone as most boards & Stats have a delay in them anyway.
    Fast forward to Today and 410A along with hard shutoff TXV's and some R22 systems as well along with some Manuf's still using a piston type Comp.It IS a necessity to have a start kit on these as there is no bleed down of the system along with not enough torque for the comp to start on it's own in this condition.
    Yes,Start kit's do extend the life on an aging (abused) system and I use them now.I like some of the newer electronic designs such as the 3-2-1,5-2-1 Series but "some" of the Fit's All electronic standalone relays along with the famous SuperBoost are just junk and end up blowing up rather quickly.
    I'll throw one on a DODGE unit Dead Or Dying Going Eventually (and I'm a MOPAR guy) but it "fit's" the unit's disposition and customers price range.
    JMO.

  11. #11
    Join Date
    Sep 2002
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    Although many PSC compressors do not have a start capacitor - all PSC compressors would be better served by having a start capacitor. They are not included on many units because of manufacturing and warranty costs - but all compressors should really have a start capacitor. And of course; the relay which operates it.

    You wrote: "run and common was connected to the contactor with no bus bar"

    What does that mean? What is a "bus bar" in this situation?

    PHM
    -------



    Quote Originally Posted by Deezo View Post
    Ok fellow techs I had a situation that threw me for a loop in the lab the other day. I had to rewire a simple ac from scratch and I noticed it had a run capacitor for the fan and one for the compressor. My run and common was connected to the contactor with no bus bar. I got confused because I thought every compressor had to have a start capacitor. Can someone explain this to me? Thanks in advance.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  12. #12
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    Jul 2012
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    I use a potential relay and appropriate start cap with a bleed resistor. Best way.

  13. #13
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    Mar 2015
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    I agree and actually installed a "real" start kit on my H/P Scroll comp just for some extra torque as she is a '97 Bryant and all is comfortable.

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