Three Phase Heater on Single Phase?

Discussion in 'General Electronics Chat' started by sparks325, Nov 5, 2009.

  1. sparks325

    Thread Starter New Member

    Mar 3, 2009
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    0
    Hello All,
    I have an old Dayton electric heater that is rated for 208 volts 3 phase.
    After studying the wiring diagram I noticed that the fan motor is single phase running off of L1 and L2 only.
    One of the three elements is also across L1 and L2 as well. After very carefully wiring with the proper conductor size I applied power and received heat.:) I was drawing 40amps which is what each leg is rated at. The unit is set up so that each element is separate. I used a separate contactor to parallel a second element with the first (the unit has three). When I brought in the second element and checked the current draw it was 52 amps. I disconnected the second element and now draw 26 amps. :eek: I’m not sure why the current didn’t double and why now it is only at 26 amps with a single element instead of 40amps. I figured that a heater element is basically a resistive load and phase shouldn’t make a difference. Any ideas?
     
  2. sparks325

    Thread Starter New Member

    Mar 3, 2009
    5
    0
    I thought this diagram might help
     
  3. Duane P Wetick

    Active Member

    Apr 23, 2009
    408
    19
    It looks like each phase of the 3 ph. 208 line has 2 elements connected to it. But the other legs of the elements are split between the other two lines, so the current is 40 x .666 or 26.6 amps. Since the system is balanced, each phase should show 26.6 amps.

    Regards, Duane W.
     
  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    just out of curiosity, what conductor size did you use, and what breaker size did you place on this branch circuit?

    at 40 amps per leg, your demand is 208 x 40 x sqrt(3) = 14.4Kw. Divide this by 3 (assuming loads balanced) and each heater is 4.8Kw. Divide this by your voltage and find that each phase draws 23 amp. Divide this into your voltage to find a heater resistance of 9 ohm.

    Paralleling two 9 ohm heaters on one phase will give you twice the current on that phase, but this must be vectorially added to the other phase. So 46 amp at some angle plus 23 amps 120 degrees apart, gives you a line current of 51.4 amps.

    Is your breaker 3 pole tied?
     
    Last edited: Nov 6, 2009
  5. sparks325

    Thread Starter New Member

    Mar 3, 2009
    5
    0
    Thanks Duane and GDI!
    It kind of makes sense now...But I'm still wondering why I read 40 amps the first time I fired up with a single element and since then a single element gives me 26 amps. I'm using an Amprobe model ac/dc 1000.
    The math makes sense to me and I feel better knowing that the 26 amp figure is correct. I thought I might have smoked something when I threw the second element on the line.
    I'm using a number 4 copper conductor directly to each contactor. I have a 100 amp breaker (2 pole) on the circuit. GDI could you tell me what a 3 pole tied breaker is? I really appreciate your input;). Iv'e worked in electroncs for close to 30 years now. Avionics Tech. I wired my machine shop at home with a rotary phase generator and that was pretty basic. I realize that there is no substitute for experience, that is why I'm greatful for you sharing your expertise.
     
  6. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    Because you are using 3 hots, you'll want a 3 pole breaker, that has a connecting bar(tied) that causes all positions to switch together.

    Your choice of a #4 conductor is overkill, and the 100amp breaker does not properly protect your circuit. If your using a 2 pole, that means one leg is not protected. You have some code issues here.

    In regards to current draw, sounds like a phase has opened. But I'd get that other stuff fixed first.
     
  7. sparks325

    Thread Starter New Member

    Mar 3, 2009
    5
    0
    I guess I left some information out that might be helpful. This was a test setup just to see if the heater would work on single phase (two pole) power. As I was going into unchartered territory I wasn’t sure what to expect. I have included the diagram as it is currently wired. I have also included the spec. sheet. The model is the 3E347A. That is where I got the number 4 wire size and the 40 amps per element. As it stands now, it does draw 26 amps with one element and 52 amps with the second one in parallel. Thanks again. It will come in handy as winter is just around the corner.
     
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