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  1. #1
    Join Date
    May 2007
    Location
    San Francisco
    Posts
    258

    weak transformer?

    it's a small water source heat pump. it worked great for years with the mercury bulb tstat. I upgraded to the digital programmable today, but whenever the compressor contactor is energized, I get a voltage sag, the display drops for a second, and I lose the call for cooling. Compressor is only on for a second.

    found the primary tap of the control transformer on the factory setting (230 v) but my mains are 208. switched that. same problem, only stays on maybe a second longer.

    when 'jumped out' at stat location unit works fine, and in the control box as well.

    no unexpectedly high resistance through my control circuits (but I would expect the transformer to overcurrent and fail if I did anyways)

    I'm suspecting that years of running at the wrong voltage (and hence higher currents then rated for) have caused this transformer to 'weaken' and it can't handle the inrush current of energizing the compressor contactor coil. Does this line up with anybody's experience?

  2. #2
    Join Date
    Aug 2006
    Location
    Vancouver , Canada
    Posts
    297

    Weak Transformer

    I have seen a small transformer ' weaken ' on a bunch of old York Sunline units several years ago when hooked up to the wrong tap, and cause havoc on digital stats.

    Check your va ( volt / amp ) rating of the present transformer and ensure you are not drawing excessive current for some other reason. A 40 va transformer should not draw more than about 1.5 amps on the control side.

    Easy enough to hook up a auxilliary transformer and see what happens - also, what does the voltage drop to upon startup of the compressor? Is there a bad connection somewhere that is letting the voltage drop below 190 or so on initial inrush?

    Keep us posted.
    Superheat and subcooling tell it all !

  3. #3
    Join Date
    Dec 2005
    Location
    Spokane Wa
    Posts
    169
    If it works by jumping from stat and unit, sounds like a problem with the stat. It sounds like the problem arrived when stat was put in circuit

  4. #4
    Join Date
    May 2007
    Location
    San Francisco
    Posts
    258
    thanks.
    actually (and sorry I didn't mention this), I ruled out the stat as the problem by trying other stats (I keep several in truck stock) and had identical problem with all of them. I think it's either the transformer or a damaged common wire. I have other conductors available so will rule that possibility out tomorrow before changing the transformer.

    I'll measure my volt drop at compressor startup as well to see what's going on there.

  5. #5
    Join Date
    Dec 2005
    Location
    Spokane Wa
    Posts
    169
    Check for a faulty contactor coil, I have had that happen before.

  6. #6
    Join Date
    May 2007
    Location
    San Francisco
    Posts
    258
    would that not cause the transformer to simply overcurrent and fail, if it was a faulty contactor coil? I'll test it as well though. thanks

  7. #7
    Join Date
    Dec 2003
    Location
    Northern VA
    Posts
    622
    Any kind of problem with the contactor mechanism such as binding or a foreign object in the plunger mechanism can cause a high amp draw that will not always be fatal to the LV transformer.

    I think if the new t-stat was the cause there would be a slight ala Barbecue odor waft in the bargain.

    Maybe something got into the contactor when you removed a panel.
    "Iron sharpeneth iron..."

  8. #8
    Join Date
    Feb 2007
    Location
    Dallas, Tx.
    Posts
    572
    make certian that your common is really common at the xformer. if the stat is acting as "power robbing" it could be the issue.
    mike

  9. #9
    Join Date
    Aug 2006
    Location
    Vancouver , Canada
    Posts
    297

    Transformer / stat woes

    I was in a rush last night when I posted my reply but should add a couple of points -

    - when I say the transformers were weakened, I actually measured a difference in the winding resistance on both primary and secondary sides compared to a '' good '' identical transformer, and upon closer inspection under the winding wrap, discoloration was present - they still put out over 23 volts when wired up to line however. I believe the stats the owners were trying to use were Enerstats, which, especially in the early years, had their share of problems.

    - the incoming line voltage to this facility was a paltry 198 volts - 3 phase, but other than the transformers not being wired up on commissioning, we had few if any voltage related problems ( ie: compressor or fan motor failures ) except for shortened compressor contact life and control relays burning out.

    - the other posts to this thread have been right on!
    Superheat and subcooling tell it all !

  10. #10
    Join Date
    Oct 2002
    Location
    Lady Lake, Florida
    Posts
    799
    I've had found on occasion a weak contactor coil that closed the contacts but had a voltage drop across the contacts. When I visually inspected them after replacing you'd swear they looked ok.

  11. #11
    Join Date
    Apr 2007
    Location
    Kaufman county, Texas
    Posts
    10,011
    Quote Originally Posted by jcarlding View Post
    it's a small water source heat pump. it worked great for years with the mercury bulb tstat. I upgraded to the digital programmable today, but whenever the compressor contactor is energized, I get a voltage sag, the display drops for a second, and I lose the call for cooling. Compressor is only on for a second.

    found the primary tap of the control transformer on the factory setting (230 v) but my mains are 208. switched that. same problem, only stays on maybe a second longer.

    when 'jumped out' at stat location unit works fine, and in the control box as well.

    no unexpectedly high resistance through my control circuits (but I would expect the transformer to overcurrent and fail if I did anyways)

    I'm suspecting that years of running at the wrong voltage (and hence higher currents then rated for) have caused this transformer to 'weaken' and it can't handle the inrush current of energizing the compressor contactor coil. Does this line up with anybody's experience?

    Where are you mesuring this 'voltage sag'? If measured at the transformer it sounds like a VA problem. If it good at the transformer, but drops at the thermostat, then it sounds like you are dropping voltage between the two. i.e. a wiring problem or bad connection somewhere.

    I assume you have common wired in at the thermostat, if not and relying on a trickle from "W" to power stat then that is another problem altogether perhaps.

  12. #12
    Join Date
    Feb 2004
    Location
    Brooklyn, NY Occupation: HVAC Service Contractor
    Posts
    258
    My old partner had a problem trying to use an electronic thermostat with a McQuay Water Source Heat Pump unit that he installed in his apartment years ago. It seems that the McQuay unit uses a DC volt relay board to control the loads and it has compatibilty problems with newer electronic stats. He put back the old Honeywell electro-mechanical stat and all the conflicts went away. I don't know if this info will help you. Good Luck.

  13. #13
    Join Date
    Oct 2001
    Posts
    6,954
    make sure you have a ....TRUE and read it off subbase 24v.... to the new stat...no feedbacks(old styles R only with Y,G,W other side of TR) you have to have the C side at the digital to run it.with that primary 208V and your tapped for 230V could give you a lower secondary when it draws as your seeing.do you have 24 or better coming out ...final solution get a new 208V/24V and more VA if changed out
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

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