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  1. #1
    Join Date
    Nov 2004
    Location
    Auburn, AL, USA
    Posts
    597
    I'm new to this forum and prefer to watch and read before speaking. I tried several searches on my question and found nothing. Perhaps I didn't search the archives correctly.

    Does using only two legs of 3 phase 220v for a 220/24 volt transformer prouduce poor and erratic 24v controls system power? Over the years I've seen this done I thought these systems went through a lot more contactor, relay, defrost conrol, and various other control problems. I've thought these systems had a lot more debris (left behind boxes, paper work and old components) and scars (re-mounting and rig mounting of components) left over by previous service inside these units after several transformer, relay, pressure switches, defrost control board replacements, etc. The service call would be poor or no heating/cooling. These units often had marginal control voltage; 18v - 24v at best and some days the main contactor would hum but not close with a control voltage of 11v - 18v. The units often have been serviced for the same kind of problem for years. Many others have replaced that "weak" contactor, "weak" transformer, or even added a small relay to power up the regular contactor solinoid. Every time I ran a neutral wire for a regular 110v/24v transformer and used one leg of the 3 phase 220v, I got good control voltage (27v - 29v) to the controls while under full load. Usually no more control system erratic problems and no more frequent control component failures. I will say more after a few replies come back. Thanks for the forum and allowing me access to it. james
    jt

  2. #2
    Join Date
    Jul 2000
    Location
    Dallas,Texas
    Posts
    4,948
    Most three phase supplied units will not have a nuetral supplied with the phase conductors. The contol voltage will be 240 volts to derive from two phases of the supply voltage.
    A lot of problems arise from not retapping the control transformer for 200\208\240 supply voltage at start up.

    In your personal solution, the question begs to be asked. Where did you get a neutral?

  3. #3
    Join Date
    Aug 2002
    Location
    manitowoc wisconsin
    Posts
    4,943
    Think about where the three phase power is.The power in most of the factories I have worked in is anything but stable.The bigger the factory the more power surges etc.You have all these big machines with big motors turning on & off etc.I will say it seems the factories with 480v seem to go through alot less controls than the 208-240 factories.Wolfdog is %100 correct on proper trans tapping.I have seen alot of controls damaged from improper transformer tapping too.
    Take your time & do it right!

  4. #4
    Join Date
    Oct 2004
    Posts
    3,400
    I would venture a guess that the primary voltage is "assumed" rather than verified.

    A transformer changes voltage at a fixed ratio. It does not have a built-in voltage regulator. If primary voltage is 208, you only get 20.8 volts, before load, unless you choose the right tap. If the wrong tap (or part) is selected, the VA rating for the new transformer may be overstretched, shortening the life of the new part. This could conceivably cause problems with contactors, relays, and other control loads, because they don't get their "rated" voltage. Then they get changed (more scars).

    Also, many times (BUT NOT ALWAYS), the poor guy that is responsible for doing HVAC work in a factory or large building is "in house" with minimal training. He might be forced to learn through on-the-job-training, by changing parts until it runs again.
    (Not a good method, but still employed.)

    I could see how a "dirty" supply power could shorten the life of controls, but I don't think just being polyphase is a death sentence. Rural areas have their fair share of blinks, too.

    PS (Watch out for the "wild" leg.)

  5. #5
    Join Date
    Nov 2004
    Location
    Auburn, AL, USA
    Posts
    597
    Wolfdog
    "Most three phase supplied units will not have a nuetral supplied with the phase conductors. The contol voltage will be 240 volts to derive from two phases of the supply voltage.
    A lot of problems arise from not retapping the control transformer for 200\208\240 supply voltage at start up.

    In your personal solution, the question begs to be asked. Where did you get a neutral?"

    Thanks for the reply, Wolfdog.
    These “weak” 24 volt transformers seemed to match or exceed the origonal factor installed ones. They are the two wire 220/208 AC primary and a two wire 24 volt secondary type. I replaced them with standard 110/120 volt to 24 volt tarnsformers with an equal VA rating. The primaries were re-wired to one leg of the three phase supply side and to a neutral wire which I had ran back to the supply breaker box that already had neutrals for other 110v service sharing the same 208/220 three phase supply. (It is a lot more difficult to find a true neutral in some instances, such as roof tops with only three phase equipment.)
    The non-factory 110/120v transformers produced 5 to 10 more volts at the secondaries under load. (on site)
    The replaced “weak” 208/220v transformers also produced 5 to 10 more volts (under a replicated load) when hooked up to a true single phase 220 VAC source.

    My guess is that single phase electrical induction devices do not do well using two legs of a three phase power source, even when the voltage is adequate.

    Wouldn’t using only two legs of a three phase power supply for work load made to use 180 degree phase shifts create a lot of induction losses (extra heat + poor power)? It would be tring to work with phase shifts of 120 degrees, then 240, then 120, then 240, etc.
    james
    jt

  6. #6
    Join Date
    Jul 2000
    Location
    Dallas,Texas
    Posts
    4,948
    Things are a little different when discussing utility company power supply.

    But on site, single phase power is simply 2 phases of the provided 3 phase service.

    On 240 delta, the single phase is A & C and B phase is the high leg and makes ABC.

    On 208 Wye, single phase is any 2 phases or 120 is any 1 phase to neutral.

    I have been in this business a long time as HVAC as well as Master electrician and have not experienced a big problem with transformers on 3 phase units.



    My concern was that you might be using cabinet ground for a neutral. An NEC violation and a safety hazard.

    Your description says you are not.

  7. #7
    Join Date
    Nov 2004
    Location
    Auburn, AL, USA
    Posts
    597
    Wolfdog, again thanks for the reply.
    I'll watch for those cabinet neutrals.
    Isn't regular household 208/220v single phase service a 180 degree differential in polarity at any given moment? (except at zero)
    I thought "Y" three phase was 120 degrees from A to B to C phases.

    On site, two of the 220v to 24v transformers produced almost 24v with no load and sporadicly down to 12v under load (70% VA rating). These same two transformers tested much stronger (28v+ at 70% VA rating) with no sporadic weakness (so far and I only tested two of them) when loaded while powered with 220v (my) household single phase. Perhaps I have an on-site power supply harmonics problem or some other three phase supply problem. Several transformer/contactor replacements did not work with these units (five heat pump units) until I used the 110v transformers, all five now with 28v+ control voltage under load and no control problems (for three years on the first one).

    Thanks, bwal2, also for your reply. So far, all the "weak" transformers I replaced only had a two wire primary (not multi-tap) and was used within its supply voltage rating. The commercial facility where I serviced these particular heat pumps have only "Y" three phase and no wild leg. My first encounter with a "wild leg" in 1968. It was felt, remembered, and almost my last experience (damp wooden screw driver handle and 480 delta).

    james
    jt

  8. #8
    Join Date
    Sep 2004
    Posts
    51
    Sounds like you are trying to reinvent the wheel for a secondary voltage drop on an improperly wired transformer.Everytime I see a drop in 24V and no problems with any contactor coils or shorts it is from the primary voltage does not match the proper voltage taps.I rewire it and my control voltage is back up to 27V or so and problem solved-no more chattering,etc. I would've bought a transformer with multi tap and hooked it back to 208, 230, or whatever.I can see low control voltage would cause problems. Nothing works right under 24V.

  9. #9
    Join Date
    Nov 2004
    Location
    Auburn, AL, USA
    Posts
    597
    To all:
    Sorry about my typo in this thread's title.
    It should be "24 volt problems /w 220 three phase".
    Also this situation has "Y" 220v three phase supply.

    james
    jt

  10. #10
    Join Date
    Dec 2003
    Posts
    81
    Three phase Y is usually 208 L to L needs a 208 transformer, three phase delta is 240 L to L

  11. #11
    Join Date
    Nov 2004
    Location
    Auburn, AL, USA
    Posts
    597
    "Three phase Y is usually 208 L to L needs a 208 transformer, three phase delta is 240 L to L
    kingfish"

    Thanks for the reply, kingfish

    Some power company workers changing out an underground a set of 4k supply lines at the site told me that all the facility's three phase is "Y" type and without a wild leg. There are at least two other high volt feeds to on-site step down transformers, so I could get the electrical info mixed. I thought the panel service in the new part of the facility was 480/220 three phase and 277/120 single phase. And older side's basic panel service is 208 three phase. Thanks for the voltage info. My meters (2 cheapos and one good RMS meter) read 210v~235v L to L if I remember correctly. I'll look more closely next time. I think the suspect factory control voltage transformers have 220v primaries. I'll try to check tomorrow. This could explain the weak control voltage if the supply available is actually 208 (RMS).
    jt

  12. #12
    Join Date
    Jul 2000
    Location
    Dallas,Texas
    Posts
    4,948
    Standard low voltage feeds will be 480\277 with the flourescent fixtures fed from the 277v to neutral. They feed a 480 X 208\120 transformer(s) and pick up all single phase and receptacles loads off the transformer(s).

    208\120 wye is more common is some areas and all the power company will provide in some areas.

    240\120 delta is still common in this area and usually available as well as 208.


    4160,7200 and higher voltages are available and if you want to provide your own substation the power company is very negotiable.


  13. #13
    Join Date
    Nov 2004
    Location
    Auburn, AL, USA
    Posts
    597
    Thanks again, wolfdog, for the information. The three different transformer rooms/structures that have 4k lines that come on the property are locked down and maitained by the local power company. Would these places be called "substations"?
    Sorry if I bother you by pressing on you (and others on this forum) but I enjoy learning and improving.
    I could also use some guidance and correction in how I use this forum. Is using the "search" toward the top each page the same as searching the archives?
    jt

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