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Oddball Phase Convertor - will it work? Could it work?

Discussion in 'ELECTRICAL ISSUES - POWER YOUR MACHINES & SHOP' started by British Steel, May 13, 2017.

  1. British Steel

    British Steel United Kingdom Active User Active Member

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    With the New-To-Me Proper Mill likely to come to my "shop" in the next week or so, I really need to think about powering it: I saw an interesting rotary convertor http://www.mig-welding.co.uk/forum/threads/rotary-phase-converter-plans.32135/#post-350783 which is switchable between 240 and 415 volts from a 240v source - only downside is some needed complication and, for us Europeans, the lack of available 240/415 dual-voltage motors in the 7-10 HP (5 - 7.5kw) range (they seem to stop at 3kw)...

    I've seen a few rotary convertors where a second idler can be switched in which would make 8HP/6kw feasible, assuming I can find a box big enough for all the required contactors and capacitors (could be 6 or more, particularly with the way I overdo things!) can anyone see a reason why this would be a Very Bad Idea, other than needing some interlocking of the contactors so both motors would switch voltages together etc.? It could be possible to have a "dual-range" convertor where the second motor could be switched in for heavy loads* but it would save a little energy with a single motor if only a couple of HP were required, is my guess.

    Although I'm quite good with electronics, rotating fields etc. puzzle me !

    Dave H. (the other one)

    * a current-sensing relay feeding the contactor(s)? Manual would do, but why not over-complicate it if I can?
     
  2. markba633csi

    markba633csi United States Active Member Active Member

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    I see a problem keeping the two idler motors in phase with each other, unless the shafts are connected together. Is that what you were thinking? Seems like a workable scheme. You would need to switch the "tuning" capacitors in and out.
    Mark S.
     
  3. British Steel

    British Steel United Kingdom Active User Active Member

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    Thanks Mark.

    I don't think it's essential to couple the two motors, once an RPC's running other motors (e.g. an idling lathe) add to its capacity, e.g. a 3HP RPC powering a 2HP lathe motor gives a 5HP RPC to power other tools.
    My thinking is that the second idler motor could be switched in complete with its balancing capacitors using a couple of 3 or 4-pole contactors big enough for the individual motor current/voltage when extra HP was needed? Balancing should be possible per-motor: start with idler 1,, switch to 415 (star) and measure voltages, balance; run idler 1 and switch to 415, start idler 2 and disconnect idler 1, measure voltages and balance, add PF cap; switch idler 1 back in, checks, add idler 1 PF cap - Robert's one's parent's sibling?
     
  4. markba633csi

    markba633csi United States Active Member Active Member

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    If your first statement is true then I've learned something new today. I had no idea one could essentially "extend" the power rating with a second motor which would normally be a load for the first RPC. Very interesting. A couple of relays and capacitors for the PF adjustment. Let us know how it works.
    Mark S.
     
  5. 4gsr

    4gsr Global Moderator Staff Member H-M Supporter-Premium

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    Once you get one 3-phase motor running, you can start as many motors as you want as long they are not bigger that the largest motor in the circuit. Of course, there is a limit on how many motors to start. That limit is the available amperage. Used to do this in my shop back in the 1970's. Back then, the biggest motor I had was 5 HP. It was started under no load. All the motors thereafter were 3 HP and smaller.
     
  6. mksj

    mksj Active User H-M Supporter-Premium

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    In most RPCs of different types/voltages, the neutral cannot be used because of the phase different between the legs. The output would need to be isolated from the input to use a floating neutral, and I am even less familiar with UK wiring. I also do not know how the 240/415V motors are wound/connected to determine if the generated 415VAC is isolated from the incoming power. What seems to be the defacto configuration for generating 415VAC three phase from 240VAC single phase is a step up transformer followed by a 415VAC RPC. Finding a 240/415VAC motor over 5Hp seems to be a rare bird, and probably have to pay a significant penny for one. If just 1 or 2 machines there are VFDs that do single phase to 3 phase 240 to 240, or 240 to 415VAC.
    http://www.phasetechnologies.com/phaseconverterinfo/phaseconverter_deltawye.htm
    http://www.practicalmachinist.com/v...verters-and-vfd/uk-230vac-415vac-10hp-102334/
    http://www.drivesdirect.co.uk/Products/Digital_Inverters240_415.htm
    rpc 415.jpg
     
    Last edited: May 18, 2017
  7. Keith Foor

    Keith Foor Active Member Active Member

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    As far as adding 'idler motors' to increase the capacity of a RPC. Yes, that is indeed the case. In fact I have designed several that were high horsepower rated that were combinations of multiple smaller idlers in parallel. The way it worked is the first idler had a start capacitor circuit to get it running. Once it was running the second idler motor would be connected and it would start off the first idler that was already running. This staged startup would continue until all motors were running at which point an output contactor would close and the power was connected to the 3 phase panel. Each motor had it's own set of run capacitors to balance the created leg. The one I did was 100 horsepower and was created with 5 20 HP motors and it worked well. You can also do this with a 20, a 30, and a 50 making it smaller. The things to remember with doing this is everything needs wired for 100 HP. so even though a 20 HP motor requires a number 8 wire, a 100 HP motor requires 1/0 copper if I remember correctly. So anything that is in the buss that runs from the single phase input to the output needs to be 1/0 and the contactors need to be rated for 100 HP.
     
    British Steel and Ulma Doctor like this.

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