HVAC transformer odd problem

Discussion in 'Hangar Talk' started by Chip Sylverne, May 10, 2019.

  1. Chip Sylverne

    Chip Sylverne En-Route PoA Supporter

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    Fear is the poison of our lives.
    I have an A/C air handler with a 120vac whole house electrostatic air cleaner. The air handler uses a multi-pole 40va transformer for control voltage, input is 240v, as the blower fan is also 240v. Other legs on the transformer are 208v, which is capped off, and 120v which is supposed to run the air cleaner. I have 240 volts at the transformer, I have 24v out of the secondary side. I only have 2.4vac out of the 120v leg, and 80vac out of the 208v leg!
    All legs ohm good removed from the unit. If I plug the 120v input and common into a 120v ac outlet, I get 208 and 240 volts at the other legs. I have traced the nuetral from the air handler back to the panel, cleaned the lugs and all is good there. I get 240 at the breaker, 240 at the service switch, and as I said 240 to the control box. The fan works fine. I just cant get 120v to the electrostat. Tried it with 2 transformers, no joy.

    Any thoughts?
     
  2. Chip Sylverne

    Chip Sylverne En-Route PoA Supporter

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    Before you mention it, the orange tranformer lead is 240, the blk is common. Red is 208 and wht is 120. I know wht is usually the common, but not in this case. L1 of the 240 supply is common, l2 is the orng. The bare copper in the 10/2 240 line runs to the panel common lug, and is used as the common for the 120v electrostat.
    Not the way I would have done it, but that's the way it was installed.
     
  3. PaulS

    PaulS Final Approach

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    Sooooo, a 40 VA transformer running an air cleaner sounds like you are asking a lot from that transformer. What are the power requirements of the air cleaner? Is there a neutral wire?
     
  4. Chip Sylverne

    Chip Sylverne En-Route PoA Supporter

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    The tranformer doesn't run the air cleaner. The secondary 40va runs the a/c controller. The primary side of the transformer has 4 input taps; common, 120vac, 208vac and 240vac. If you use 240vac, the common and orange are L1 and L2. Whichever input voltage tap you choose leaves the other two taps hot as outputs. The 120v output is more than enough to run the electrostat, provided it would give me the juice. Yes, there is a neutral wire to the main panel.
    I don't understand how I can be putting 240vac in, getting 40va out, yet the other two taps are so low voltage.
     
  5. PaulS

    PaulS Final Approach

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    I still don't understand. My understanding of a 24vac transformer is you have the hot side, from which you can INPUT 120 or 240, then it will transform that to 24vac. You do not connect 240v to the 240 terminals, then expect to get 120 out of the 120 inputs. I'm not an electrician, but if you have a neutral and a ground, you should be able to use one hot and the neutral to run the air cleaner. If you have only a ground, then you shouldn't do that. You can buy multi voltage transformers that would give you 24 and 120 from a 240 input, but they aren't cheap, and you need to make sure you get one that can run the cleaner. You may want to get an electrician.

    A picture of the transformer would really help.
     
  6. Chip Sylverne

    Chip Sylverne En-Route PoA Supporter

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    Last edited: May 10, 2019
  7. PaulS

    PaulS Final Approach

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    Maybe you do get stepped down voltage on the other input legs, but that transformer is not designed to be used that way which is why it's not working. And pulling 120v from an input is a worse idea than unbalancing a breaker. I would run another wire with 120 if you don't want to use one leg from the 240 line, or you can buy a bigger transformer with dual OUTPUTS. You need to figure out the load that the air cleaner uses.

    Something like this, but even this is probably too small to run the air cleaner. https://www.amazon.com/X4075PSF1-Pr...4&qid=1557533271&s=gateway&sr=8-9-spons&psc=1
     
  8. Chip Sylverne

    Chip Sylverne En-Route PoA Supporter

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    Not sure why pulling the voltage from another tap on the primary side would be such a bad thing, especially when it's less than the input, as in a step-down. I don't believe the load of the electrostat is all that much. Once it's not an input, it's an output. Electronics engineers use it all the time. But , it ain't working for some reason so....Pulling the 120v for the cleaner from another circuit in the attic is probably the way to go, but it'd also mean another service switch. The wiring diagram in the air handler shows the way it's set up as being correct, but that may be with something other than a universal hvac transformer. The one that was in there was a 40va 240 in with a 208 tap out running the electrostat, but when I measured the 208 tap, it only showed 80volts, so I figured it was the transformer. The whole installation kinda looks like a DIY from a previous owner. As I said, not the way I would have done it.

    Thanks for the ideas.
     
  9. PaulS

    PaulS Final Approach

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    Maybe it should work, not the way I would do it though. Would this work? https://www.supplyhouse.com/Honeywell-203365A-240V-Conversion-Kit
     
  10. Chip Sylverne

    Chip Sylverne En-Route PoA Supporter

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    The load of the electrostat is 30 watts, so at 120v that's what, .25 amps? Not a lot.
     
  11. PaulS

    PaulS Final Approach

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    Yes, I thought it was a lot more. Check out the conversion kit, just a step down transformer, $40 ….. cheaper than running a new line.
     
  12. Art Rose

    Art Rose Pre-takeoff checklist

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    volts x amps = watts (VA rating)

    The primary side of your multi tap step down transformer is designed to accept either 120, 208, or 240 AC volts of input by connecting the proper terminals shown on the label (the unused taps are not to be used and are capped) Granted, you will see an induced voltage in the unused taps, but the voltage you see is not designed to be used, as you are thinking. The transformer will output on the secondary side (yellow wires) 24 volts AC at 40 VA. (It will handle a load of about 1.7 Amps at 24Vac) Suggest you view a couple YouTube videos on transformers. Once you let the smoke out, it's real hard to put it back in.
     
  13. 3393RP

    3393RP Pattern Altitude

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    You are wrong! The unused primary inputs are not to be used for outputs, they are to be capped. You have a multivoltage input/24VAC output transformer, which runs your AHU control board. That's all it is to be used for. Period.


    It's not the correct transformer for the filter. You need a separate 2 wire 240V input and a line and neutral 120V output. This is a 50VA model.

    https://www.amazon.com/Siemens-MT0050A-Industrial-Transformer-Secondary/dp/B0052N1TVY


    [​IMG]

    If you reuse the same kluged connection to an unused primary winding that someone else installed or one of the hot inputs to get 120V, you cannot resolve the issues of a separately derived source bonding and grounding to comply with code. The secondary is also required to have an over current protection device, and you won't have that either. It won't be safe, and it'll violate the NEC. Note the XFMR pictured above has clips for an AGC glass fuse to protect the secondary.

    Do it right or call an electrician. Don't jeopardize your family's safety over a $43.89 part.
     
    Last edited: May 11, 2019
  14. 3393RP

    3393RP Pattern Altitude

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    You are measuring odd voltages on the unused input taps because they are installed at other points of the primary winding than the 240 volt taps, to allow the proper length of the primary windings to correlate with 208 and 120 volt inputs. The lower input voltages on the other primary winding taps induce the correct magnetic field to excite the 24 volt secondary winding. If electronic engineers are using them as secondary outputs "all the time", they're wrong all the time.
     
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  15. Chip Sylverne

    Chip Sylverne En-Route PoA Supporter

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    The secondary is protected by an in line fuse. If I use the tap off the transformer, how does that create a source bonding issue? The load is only .25 amps.
     
  16. Chip Sylverne

    Chip Sylverne En-Route PoA Supporter

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    When I was a kid I worked in a tv and stereo repair shop. Part of basic troubleshooting was measuring voltages at multiple taps on transformers.
     
  17. Checkout_my_Six

    Checkout_my_Six Final Approach

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    interesting....didn't know that those 24VAC transformers were used for power. I thought that was the controls side of the system ....for relays.
     
  18. Chip Sylverne

    Chip Sylverne En-Route PoA Supporter

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    The 24v secondary side is just that. The open question is whether the primary side can be used as a step-down. Since this air handler is probably 40 years old, it may have been legal at the time to create a "floating" 120vac circuit inside the appliance off the primary. Probably not so much anymore, but, as you know, I hate to spend money on anything not aviation related.
    If I had any sense, I'd chuck the whole deal and replace it all with mini-split.
     
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  19. Art Rose

    Art Rose Pre-takeoff checklist

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    There is no open question. You're just not listening, or comprehending the information you've been provided here. Go watch those videos jr Sparky. Alternating current transformers still work the same way they did since the days of Tesla.
     
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  20. Chip Sylverne

    Chip Sylverne En-Route PoA Supporter

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    Tell me Art, why an inductor taken off half the primary winding in-line with the power source doesn't work. As long as the load is small, there is no reason taking power off the other primary taps should be problematic.
     
  21. EppyGA

    EppyGA Touchdown! Greaser!

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    The primary windings are separate entities along the core and are not intertwined while the secondary is intertwined. What you are seeing is some induced voltage that is of no use to you. Good bet is, if you throw a load across one of those unused taps the voltage goes away.
     
  22. 3393RP

    3393RP Pattern Altitude

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    That doesn't mean you understood what you were doing. Let's go back your original post.

    Other legs on the transformer are 208v, which is capped off, and 120v which is supposed to run the air cleaner. I have 240 volts at the transformer, I have 24v out of the secondary side. I only have 2.4vac out of the 120v leg, and 80vac out of the 208v leg!

    Your expectation there should be 120 volts on that primary tap and 208 volts at its primary tap shows you don't know what you're doing.

    It is improper to use primary windings for the purpose of a secondary voltage. Besides that, they will never produce the voltage you are expecting. As you mentioned earlier, there's something "wrong" because you have the wrong transformer for what you're attempting to power.

    Using the primary tap for secondary load will also alter the magnetic field of the transformer and change the 24 VAC secondary voltage, which could cause the control board in your AHU to fail.

    Run a 120 VAC circuit to the filter, buy the right transformer, or do it incorrectly. I don't care.
     
    Last edited: May 11, 2019
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  23. Chip Sylverne

    Chip Sylverne En-Route PoA Supporter

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    AHA! FINALLY A GOOD ANSWER!
    That's exactly what's happening! I didn't realize those primary taps had their own discrete windings. I thought the design of the primary coil was a connection to specific turns on the primary to get the desired voltage out of the secondary. That's what yhe schematics present. That is probably how the old transformer and new transformer differ in design, and the old was able to run the electrostat from a tap off the primary side. Discrete windings for the different inputs inducing voltage. That makes sense.
    Thanks!
     
    Last edited: May 11, 2019
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  24. Chip Sylverne

    Chip Sylverne En-Route PoA Supporter

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    I suggest you do more research on multitap transformers.
     
  25. 3393RP

    3393RP Pattern Altitude

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    I suggest you concentrate on the transformer you have.

    The inductive forces and number of turns from the 240 volt primary winding will not produce a 120 volt circuit capable of providing power and current from the 208 or 120 volt primary tap.

    It's rather obvious you are wrong about all this and don't understand it. Your own writing is proof.

    You:

    I have 240 volts at the transformer, I have 24v out of the secondary side. I only have 2.4vac out of the 120v leg, and 80vac out of the 208v leg!

    You again:

    I don't understand how I can be putting 240vac in, getting 40va out, yet the other two taps are so low voltage.
     
    Last edited: May 11, 2019
  26. Art Rose

    Art Rose Pre-takeoff checklist

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    Hilarious! Is it possible you have the wrong schematic, or are you misinterpreting it?
    Oh well. The air cleaner is probably taking 120V AC supply through a full wave rectifier circuit that powers the grids. The only way I know to check proper function for something like that is to stick my finger in it.
     
  27. flyingbrit

    flyingbrit Pre-takeoff checklist

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    Apparently many of the posters here have never heard of an auto transformer. This is a single winding device which can be used for stepping voltage up or down. A well known example is the Variac which has a sliding output tap controlled by a knob. Typically they are 110V in, and 0-130V out. The disadvantage of the auto transformer is that it provides no isolation. For example:

    https://www.circuitspecialists.com/variac-tdgc2-2.html

    A transformer with a primary that is tapped for multiple input voltages could be used as an auto transformer, even though that is not its design intention. You would need to be careful about how much current is drawn.
     
  28. Chip Sylverne

    Chip Sylverne En-Route PoA Supporter

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    EXACTLY! This thing should be acting like an auto transformer on the primary side. The draw for the electrostat is about .25 amp. Should be well within reason to pull off the 120v tap. I just need to figure out why I get such a low read on the vom when testing one lead on the 120v tap and the other to neutral wire from the house panel. Since there's no isolation, that means a floating ground in the air handler. One leg of the 220 supply goes to the common of the transformer, so that can't be the return for the 120 volt tap.
    Maybe this is a problem where I'm taking the reading and completing the circuit and not with the transformer.
    That is, of course, assuming the multiple windings theory is incorrect.
     
    Last edited: May 11, 2019
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  29. Checkout_my_Six

    Checkout_my_Six Final Approach

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    do you have another device to use as a load? sometimes you'll get voltage but no load.
     
  30. Stephen Poole

    Stephen Poole Pre-takeoff checklist PoA Supporter

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    Exactly. To the OP: I once had the 5,000 V primary in a transmitter die. We got it back on air with three "pole pigs" (i.e., big power company transformers). I fed them "backwards," 240V 3 phase Delta on the secondary taps and pulled high voltage from the primary. I temporarily erected a screen around it with a one of those, "Don't touch this or it'll kill you, and you'll hurt the whole time you're dying" signs. :)

    I agree that the type of windings can have some effect, but that's usually for special purposes and/or at higher frequencies. At 60Hz, it's not critical at all. I've used tapped and split primaries many times in my career. At the end of the day, a transformer is simply an inductor. The turns ratio is equal to the square root of the inductance ratio, i.e., the inductance looking into each winding.

    (Do the math sometime on, say, the transformer in a Fender Twin amplifier and don't be surprised if you come up with 80 or 160 Henries -- or more -- for the primary!)

    OP, I suspect you've already figured it out. The coils were isolated. I guess the low sides were tied together at one time, and then possibly corrosion or something else caused a loose connection. Edit: or as someone else here suggested, if you've replaced it, maybe the original had an auto-former for the primary, but the new one has separate windings.
     
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  31. Chip Sylverne

    Chip Sylverne En-Route PoA Supporter

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    I need to figure out the return for the 120v transformer lead. If it's really the common wire being used for L1 of the 240v input, not comfortable using it, even inside the appliance. If that's the case, plan B will be taking the 120v from another circuit in the attic, and putting a 24v dp relay in parallel off the secondary side so when the service switch is thrown, the electrostat comes off line too.
    Geez, I wish the darn handler and stuff were in the basement!
     
    Last edited: May 11, 2019
  32. flyingbrit

    flyingbrit Pre-takeoff checklist

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    Without a schematic showing exactly where you are measuring I can only guess. But if the transformer is connected to 240V on its primary, I would expect the 120V tap to read close to 0V relative to neutral. Most likely the 120V Electrostat is connected to the 120V tap on one side and either 240V line on the other side.

    BTW, if the Electrostat uses 35 VA of the 40VA transformer that does not leave much for the HVAC contactors/relays.
     
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  33. Stephen Poole

    Stephen Poole Pre-takeoff checklist PoA Supporter

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    Got this from the manufacturer's Website. This is the schematic for the part for which the OP posted a Grainger link. It is indeed an autotransformer on the primary side. You'd pull 120V from the Black and White, with 240V connected to the Black and Orange.

    By the way, in spite of what I said above, you really shouldn't pull a lot of current from the 120 V tap. As others have pointed out, you only have 40VA to play with. In this particular case, given that the transformer is capable of operating at 120V to provide the 24V output, there shouldn't be any problem with using the 120V tap with 240V input. BUT ... watch the current. :)

    [​IMG]
     
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  34. Checkout_my_Six

    Checkout_my_Six Final Approach

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    If it's anything like my system....it's all hosed. Mine was wired backwards. It worked fine for +30 years. As soon as I replaced the compressor....the relays and defrost wouldn't work. I then realized the neutral and power was reversed in the control circuits. That one was a real head scratcher....