Announcement

Collapse
No announcement yet.

Electric experts.. Question

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Electric experts.. Question

    I need to pick the electric guru's brains.
    Hypothetical question: If I were to build a new shop, with only single phase service, but wanted to put six 3-phase machines in it...
    could I hire an electrician to wire a breaker box, with several 240v circuts, directly from a phase converter in order to make all circuts 3-phase?
    (Please reference my sketch to see what I'm talking about.)
    It seems alot less expensive than buying several VFD's.
    If it's possible, how big of a phase converter would be needed to power 6 circuts? (For example)

  • #2
    Yes, what you are proposing is done commonly. As to the size of the phase converter needed, well... that depends on the total current requirements of all the machines you have connected into that breaker box. If you are not looking to run several machines at once then that requirement is only for the max current draw anticipated.

    Comment


    • #3
      I would think if you are looking at that many machines you should look for a small commercial area with 3ph supply.
      Otherwise I would still think the VFD would be the way to go, it is mainly for 3ph spindles anyway and the cost of running a large inefficient RPC over as-needed VFD use would recoup your expense in short order.
      Also, if your RPC goes out, all your machines are out of commission.
      Max.
      Last edited by MaxHeadRoom; 02-21-2012, 10:59 AM.

      Comment


      • #4
        What you propose is feasible. As mentioned, you will need to know the sum current draw of the machines that could be running at any given time (and then never go over that draw). This will size the PC required. Sizing the PC for the sum of all machines is not really necessary if you are doing a one man hobby type shop.

        You will also need a primary disconnect between your service and the phase converter to protect the PC.

        Comment


        • #5
          Originally posted by KiddZimaHater
          I need to pick the electric guru's brains.
          Hypothetical question: If I were to build a new shop, with only single phase service, but wanted to put six 3-phase machines in it...
          could I hire an electrician to wire a breaker box, with several 240v circuts, directly from a phase converter in order to make all circuts 3-phase?
          (Please reference my sketch to see what I'm talking about.)
          It seems alot less expensive than buying several VFD's.
          If it's possible, how big of a phase converter would be needed to power 6 circuts?
          Yes, it could be wired up that way but if you put a single phase electrical panel in after the meter, then you have single phase 120/240v circuits for those needs and run the phase converter as a circuit off that box.

          We can't answer the question of how big a phase converter since you didn't provide info on how many amps are needed for each machine.

          If you are running multiple machines that draw a fair load you might want to look into one of these:
          http://www.phaseperfect.com/

          Steve

          Comment


          • #6
            Buy the number of VFD for the number of 3 phase motors that will be running simultaneously and have switch gear to switch the 3 phase to each motor interlocked with the speed controls.

            it works for me 5 motors 3 vfds.

            Comment


            • #7
              KZH that would be a nice tidy installation, not sure how your electrical inspector will view it........you might want to consider a couple different idler motors small and large depending on your machine loads......then you could choose which one to run........the RPC's seem to start best at about 40% larger but once one is running they seem to purr along adding loads almost equalling thiers.......
              Opportunity knocks once, temptation leans on the doorbell.....

              Comment


              • #8
                FWIW, that is how mine is, and I have four circuits coming off a 3HP RPC. Here is what is connected:

                -floor standing drill press 1/2hp
                -lathe 1hp
                -mill 1.5hp
                -small bench lathe with integral motor 1/3hp

                Each of above has a fused disconnect box between the RPC 3-phase and the machine. I'm the only person in the shop so I don't ever need to start two machines at a time. That said, I can start one motor and let it feed back into the RPC for a larger capacity if I ever, say, needed to start a 5HP motor or something like that. I only run the RPC when I'm working on a machine that needs it. I put a high amp, enclosed disconnect switch to turn on/off the RPC: McMaster No. 6759K35 It is so much easier to be able to run practically any three phase equipment I might take in without any fuss. And before I get blasted for something I'm unaware of, this is how my licensed electrician decided to install it.

                Comment


                • #9
                  Unless you have access to a surplus 240 (listed and labled as 240) 3 phase panel with suitable main (optional if the RPC is fused via a disconnect) and branch breakers, it's going to get expensive fast.

                  Rather than a 3 phase panel, often they best way to do distribution is by fused disconnects - one at each machine. You can buy these on ebay for $10-25 all day long. I do this from a Phase Perfect in my shop, but had planned it for an rpc.

                  Your rpc will need to be fed from a single phase breaker (your existing panel) and feed a 3 phase fused disconnect prior to distribution. Your distibution wire will be sized for the RPC and its protection devices. If you have a large RPC utilize motor starter overloads as part of a contactor to protect the rpc.

                  Comment


                  • #10
                    Disconnect question

                    Agree with lakeside about having a breaker between the meter and the RPC. Not a code expert, but that won't pass.

                    I have a question about disconnects at the machines: Do mag starters with thermal overloads count as fused disconnects? What about VFDs with thermal overload protection?

                    Comment


                    • #11
                      That is the way I did it, except as lakeside said, you will still need a single phase panel in there somewhere, if not to feed the RPC , at least to handle your single phase loads.

                      I say this as it can be problematic feeding an rpc from a single phase panel as you may be limited by the largest breaker available/advisable for your panel. You didn't say what size RPC you were considering, I suppose anything under 30 hp wouldn't have this problem.

                      I was able to power my 100 hp converter with a 400 A, 3 pole breaker fed directly off the fused 400 A service disconnect. It wasn't too expensive as there seems to be a good supply of electrical "wrecking yards" here in the Spokane area, and I did most of the work myself.

                      Feeding a breaker panel with the RPC certainly does eliminate the rats nest that inevitably results from feeding "just one more unit/machine" from an existing lashup of cords and wires.

                      Dave

                      Comment


                      • #12
                        After buying a RPC, 3-phase panel, all the breakers, disconnects, ect. and then paying someone to wire it all you may find that a VFD at each machine is cheaper-plus you get reversing and variable speed.

                        Comment


                        • #13
                          Originally posted by kennyd4110
                          After buying a RPC, 3-phase panel, all the breakers, disconnects, ect. and then paying someone to wire it all you may find that a VFD at each machine is cheaper-plus you get reversing and variable speed.
                          There may be the deciding factor, how much putzing around you want to do yourself.

                          In other words, you can build an RPC yourself at nominal cost, dependent on your scrounging abilities. Vfd(s) will likely cost more depending on required capacities and number of units necessary, also taking into account whether you need/desire the greater flexibility that comes with a vfd.

                          Dave

                          Comment


                          • #14
                            Thats the way we did it over here when we wired up the shop, well almost, there was a single phase panel after the meter. 2 100amp single phase breakers to some flex conduit, into a dog house, 2 phase converters, some more flex back to a box, around the corner and into the shop, into 2 3 phase panels.

                            It worked fine and dandy, we mainly did it that way because we planned on getting some 3 phase off the poles. Made it easy, electrician just had to plumb the wires from our 2 3 phase boxes into the new bigger 3 phase box on the side of the building, we were only down for about 2 hours while they turned on the 3 phase and pulled the single phase wires off the building.

                            Comment


                            • #15
                              Why do you need a panel? The VFD should have some kind of internal protection so additional breakers should not be needed.

                              I would go for a minimal installation along these lines:

                              1. Carefully analyze how many machines you will BE ABLE to operate at one time. In my shop, that would be one or perhaps two at most.
                              2. Add the current draw of the two machines that would have the greatest current draw and buy a VFD to handle that amount of current.
                              3. Install the VFD in a central location where it is easily reached from all the machines.
                              4. On the output of the VFD install just enough outlet sockets for the number of machines that can be operated at one time (step 1 above).
                              5. Extend the power cords of each machine to the location of these power outlets, label them, and let them hang near the outlets.

                              Now, just plug in the machines you need to use as you need them.

                              Advantages: No cost for a box or breakers. Plugs and sockets are relatively cheap. Very easy to add an additional machine.

                              If you want redundancy, get two or several VFDs, each sized for the largest machine you have. Each one has a single outlet socket in that same central area. Now if one VFD fails, you can still operate any of the machines on any of the other VFDs.

                              With several VFDs of different sizes you would still have partial redundancy, but if the largest one went out you would not be able to use the largest machine.
                              Paul A.

                              Make it fit.
                              You can't win and there is a penalty for trying!

                              Comment

                              Working...
                              X