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  1. #76
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    Ok first let me say I am swimming out of my depth here.
    I don't believe you need a transformer. True 415V 3 phase is still only 240v to neutral. You only get 415V when you go phase to phase. So there is no need to "up" the voltage as such, just shift the timing.

    "Here is a question... With the people who have built 240V phase converters, what voltage do you get from a phase to neutral? I guess it is 133V. (240 cosine 60)"
    A rotary convertor?
    1 of the phases the still 240V to neutral, I think the cap and winding size effects the other two but they should be 240V.

    As I understand it, running a dual voltage motor in delta, 240V to neutral will not give full power. Of course many people seem to disagree with me.

    Stuart

  2. #77
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    Quote Originally Posted by .RC. View Post
    Here is a question... With the people who have built 240V phase converters, what voltage do you get from a phase to neutral? I guess it is 133V. (240 cosine 60)
    I can't directly answer your question as I run a split phase (480V) phase converter, but the voltages I get phase to neutral are (from memory) all at, or above 480V ... so I assume a 240V converter would produce similar results.
    Cheers.

    Vernon.
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  3. #78
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    Quote Originally Posted by Stustoys View Post
    As I understand it, running a dual voltage motor in delta, 240V to neutral will not give full power. Of course many people seem to disagree with me.
    Maybe I'm not understanding your question (most likely ), but a dual voltage motor either runs off 415V three phase (415 phase to phase, motor in star) or 240V three phase (240V phase to phase, motor in delta).
    Can it even be run off only one 240V phase (i.e. 240V phase to neutral) as you are suggesting?
    Cheers.

    Vernon.
    __________________________________________________
    Bite off more than you can chew and then chew like crazy.

  4. #79
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    Quote Originally Posted by Vernonv View Post
    Maybe I'm not understanding your question (most likely ), but a dual voltage motor either runs off 415V three phase (415 phase to phase, motor in star) or 240V three phase (240V phase to phase, motor in delta).
    Can it even be run off only one 240V phase (i.e. 240V phase to neutral) as you are suggesting?
    Sorry I didn't mean so suggest single phase.
    As I understand it a dual voltage motor runs in star(240V 3 phase connected phase to phase to give 415V across the windings) or delta (240V 3 phase connected phase to NEUTRAL to give 240V across the windings). But you dont get full power running in delta.
    Of course I could be wrong
    Stuart

  5. #80
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    Quote Originally Posted by Stustoys View Post
    Sorry I didn't mean so suggest single phase.
    As I understand it a dual voltage motor runs in star(240V 3 phase connected phase to phase to give 415V across the windings) or delta (240V 3 phase connected phase to NEUTRAL to give 240V across the windings). But you dont get full power running in delta.
    Of course I could be wrong
    Stuart
    Hi Stuart,
    I'll ask the Son-In-Law who is a motor rewinder, (4th year apprentice) what the power difference is, if nothing else it will be a good question for him to ask the lecturers.

    My take on it is that with 415 star you have two windings in series between each of the phases, and with 240v delta there is only one winding between each of the phases.
    So... my simplistic analysis is the 240 volt delta would draw more current, (because of less resistance and inductance) that the 415v, so the VA rating (power consumption) might well end up being similar, ignoring all the complexities of 3h phase, inductance, phase shifts, ie power factors and the rest of it....

    I think that higher voltages are always going to be more efficient in general because with lower currents (to acheive the same power) the I^2R resistive losses will be less, so my money would be on the 415v star being higher output power, than 240v delta, but probably not by much. Certainly not double, maybe 10% or so perhaps..

    Regards
    Ray

  6. #81
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    Quote Originally Posted by RayG View Post
    Hi Stuart,
    I'll ask the Son-In-Law who is a motor rewinder, (4th year apprentice) what the power difference is, if nothing else it will be a good question for him to ask the lecturers.

    My take on it is that with 415 star you have two windings in series between each of the phases, and with 240v delta there is only one winding between each of the phases.
    "and with 240v delta there is only one winding between each of the phase (and neutral)"
    Quote Originally Posted by RayG View Post
    So... my simplistic analysis is the 240 volt delta would draw more current, (because of less resistance and inductance) that the 415v, so the VA rating (power consumption) might well end up being similar, ignoring all the complexities of 3h phase, inductance, phase shifts, ie power factors and the rest of it....
    You half the resistance but you half the voltage(almost), so the current goes up a little. But you also half the number of windings doing work.

    Quote Originally Posted by RayG View Post

    I think that higher voltages are always going to be more efficient in general because with lower currents (to acheive the same power) the I^2R resistive losses will be less, so my money would be on the 415v star being higher output power, than 240v delta, but probably not by much. Certainly not double, maybe 10% or so perhaps..

    Regards
    Ray
    Ok the way I look at it. Large motors are started in delta and switched to star once running because it helps them start slower because they have less torque in delta.
    Stuart

  7. #82
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    Quote Originally Posted by Stustoys View Post
    "You half the resistance but you half the voltage(almost), so the current goes up a little. But you also half the number of windings doing work.
    Hi Stuart,

    Yep, I see your point, I'll see what the electric motor experts think. Might be a day or two to get an answer.

    Regards
    Ray

  8. #83
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    Apart from the power side I am fascinated that when you change from delta to star what the issues may be with switching the capacitors. WIth 240v delta RPC you have caps from neutral and active to the one unpowered phase. When you switch to star how are the caps fed to the 2 unpowered phases and are their values still going to be suitable for balance. Also most run caps from memory are around 400V rating.

    I would be interested in seeing more info on the circuit diagram and values and on actual load handling. From my perspective the only real advantage in going to a 415V rather than a 240V RPC is to save on changing wiring and starter contactors in every bit of machinery I buy (an on one occasion rewiring an old motor to enable delta). I am still fascinated by the concept though.
    Bill

  9. #84
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    Default Three phase converters

    Hi,

    I'm fairly new to this forum, and just happened upon this thread.
    I got hold of a three phase mill a few years ago, and decided to investigate this converter stuff. The thought of changing the motor to single phase wasn't too bad, but then the coolant pump, contactor coils, the reversing switch, the lo-vo lite, etc. etc was rather daunting. So somewhat sceptical, I started research... there's just so much out there!
    The first thing I tried was to connect one winding of an old three-phase motor I had to 240 mains, via a robust switch, spin the motor up with an electric drill, flick the switch, and bingo, it worked. Then I did the same thing with the mill connected to all 3 phases, started the idler motor, switched the mill on and again, it worked! and the coolant motor too, and the lo-vo lite.... rather dim because of the 240v instead of 415v... the contactors also tended to flutter occasionally... So I took a few cuts, that worked too! All with no transformer and no capacitors. I knew this wasn't the final answer, but began to realise just how easy it was.
    I then came across Douglas Arnt at unique 3 phase, an eccentric chap some might say, but watching him made me appreciate just how straightforward it is to build or modify a transformer. So I got an old welder, added some more windings made from domestic wiring rescued from a skip, reconfigured it as an autotransformer. and again it worked, this of course was meant to be an experimental prototype as I feared it would probably go up in smoke in short order, but at least I would have tested the principle. It worked perfectly, didn't get hot, no strange smells. So I connected it to the idler motor... that worked, the mill worked... everything worked. The voltages between the phases varied somewhat, so I built what is essentially a capacitor substitution box and switched them in and out until voltages were close enough (2 microfarads can make a big difference). You don't have to continually fiddle with it with changing loads, maybe massive changes in load.
    It still works perfectly today.
    Anyone who thinks I'm casual about electrical safety is plain wrong, I'm acutely aware of the dangers, especially with 415v.
    There's a lot more stuff concerning discharging capacitors, potential relays etc.
    I am now in the process of building a 12KVA converter, using a transformer from an X-ray machine (ebay, 40quid).
    One other thing, I don't know if its been covered before but you could consider investigating the possibility the Ronk 'add-a-phase' system, as it uses no idler motor, the noise of which, over time can be extremely wearing. Oh, and its Star-Delta... Star, start, Delta, Run.

    Cheers, Chas

  10. #85
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    Quote Originally Posted by RayG View Post
    Hi Stuart,

    Yep, I see your point, I'll see what the electric motor experts think. Might be a day or two to get an answer.

    Regards
    Ray
    I think I have it back to front and upside down..........and my head is starting to hurt lol

    Will be intersting to hear what you find out.

    Stuart

  11. #86
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    Quote Originally Posted by Stustoys View Post
    "and with 240v delta there is only one winding between each of the phase (and neutral)"


    You half the resistance but you half the voltage(almost), so the current goes up a little. But you also half the number of windings doing work.



    Ok the way I look at it. Large motors are started in delta and switched to star once running because it helps them start slower because they have less torque in delta.
    Stuart
    Not sure of reduced torque in Delta, but my understanding is that large motors designed to run in delta at mains voltage are sometimes started in Star, at that mains voltage, then switched to Delta when they are up to speed. Because the start voltage has been reduced by a factor of 1.7 (square root of 3), the power during the start has been reduced to 1/3 of what it would have been if it had been started in Delta
    (Power=Volts X Current. =v X i, but substituting i = v/r, Power = v X v/r =v^2 divided by r.)
    Sorry about the poor illustration of my example, but I cant get the hang of the equation editor!!

    The voltage sag caused by the high inrush current of the stationary motor, could be problematical, if the motor was started in Delta, but will be greatly reduced when started in Star. I'm not sure but I imagine that this might not be so good in the case where a motor is started under load, as the motor may struggle to get up to speed.

    On a slightly different point, I wonder if instead of considering a dual voltage motor to be one which has all the ends of all the windings bought out to the connections box, so that it can easily be connected in either Star or Delta, but instead having a motor with each of the windings having a tapping at an equivalent point. Each of the 3 windings could be considered as two windings in series, so 12 separate wires would be brought out to the connection box.
    I am wondering if the tappings were organised such that when 240v was put across the first part of the winding, a voltage of say 415v was then developed across the full winding. If I am correct, I don't think it would matter if the design was delta or star, but it should be possible to build a dual voltage motor, to be able to run on 240 or 415 volts, and if such a motor was used as an idler in a R.P.C. we should be able to get 415v 3 phase output with only 240v, single phase input, with the motor basically providing an auto-transformer action. Perhaps that is what was done in the youtube video earlier in this thread.
    If I need putting straight on this, hopefully someone will say, but if I'm on the right track, who has a dual voltage motor of about 10 H.P. with 240 and 415 volt inputs, they have no further use for?? Cheers boys,
    Ropetangler

  12. #87
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    Quote Originally Posted by ChasWolf View Post
    The first thing I tried was to connect one winding of an old three-phase motor I had to 240 mains, via a robust switch, spin the motor up with an electric drill, flick the switch, and bingo, it worked. Then I did the same thing with the mill connected to all 3 phases, started the idler motor, switched the mill on and again, it worked! and the coolant motor too, and the lo-vo lite.... rather dim because of the 240v instead of 415v...
    That's pretty much my experience. I tried 2 similar 3/4hp 415V 3ph motors. There was no info on the nameplates of the GMF Cadets as to whether star or delta - they are not dual voltage. It just needed a little flick with the fingers to start. Anyway, to show they run on 240V single phase:
    [ame="http://www.youtube.com/watch?v=pDNLkdyFiL0"]YouTube - 1 2 3phase[/ame]

    Jordan

  13. #88
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    Hi Jordan,
    That's brilliant, makes the point so clearly,
    I originally used a 4hp 2-pole motor as idler which needed a bit more help to start. I read somewhere of of chap recounting how, as an apprentice, his first job every morning was to kick the idler motor into life!

    Star-Delta or Delta-Star?
    For example, on a 415v supply
    The two ends of each stator winding are brought out to the starter. When set in ‘start’ position they are connected in star. The voltage per phase at start is 1/√3 of the supply voltage ie. 240v, (each winding has 240v across it) this must mean that less torque is developed, in fact, one third of that obtained if the motor was started delta connected at 415v per winding.
    As the motor accelerates the starter is moved over to run, changing connection to delta, increasing the voltage to 415v per winding.
    The whole point of a Star-Delta starter is to allow a larger motor to run more gently up to speed, reducing the massive, possibly damaging, inrush current and consequent voltage drop which would certainly endear you to your neighbours!

    As regards dual voltage polyphase motors, its fairly well known that you can build quite a reasonable generator with balanced phases from a squirrel-cage induction motor, some require a small amount of excitation to get them started, I know I do. Nadroj has the perfect set-up to demonstrate this.

    Looking at it this way, you might imagine that at least the ‘empty’ legs should work as a generator if the motor was spun up, and that the energized leg could work as an auto transformer, or you could see it as an RPC with the successively lowered voltages provided by the empty legs (as is normal) being autotransformed up to numerically even lower voltages than the 415v required, which it then may be possible to raise using capacitors to balance out the phases.
    At least with a traditional RPC you have one phase fixed and independent of the RPC, you only have 2 extra phases to create from the idler.
    I’m finding it hard to visualise the magnetic circuit in such a beast at the moment, but it still sounds almost plausible.
    One stumbling block is the old ‘no one else has done it so it can’t be possible’ argument.
    In the US, where dual voltage motors are commonplace, it might seem reasonable to expect that someone may have used this method to power a non dual-voltage machine where motor change was not feasible. I’ve researched phase converters at some length, and I’ve never come across this method… “just because you’ve never come across it”…. I know, I know.

    Why not beg, borrow or buy on easy credit terms, an old dual voltage motor, ˝ horse, anything at all, and test out your theory? I’m going to have a go, how long can it take just to see if there are any legs on it!

    Chas

  14. #89
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    Default Dual voltage motor

    Hi,
    Looks like you might have something with the dual voltage idea!

    [ame=http://www.youtube.com/watch?v=BBzW1hc11Ps&feature=related]YouTube - 220v-440v Rotary Phase Converter[/ame]

    Not sure that its a totally convincing performance, but well worth a look.

    Chas

  15. #90
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    Hi All,
    I think that it is worth checking out, even if in a really basic form as in the Youtube videos above. If it works, we could build a more permanent version. I don't think that safety items should be left out however, so earth wires, and fuses need to be there from the start IMO. I will have to see if suitable motors can be found to get the right voltages. Using a VFD to clean up the output has its good and bad points I think though. The good is that a good clean output is generated, which can be of varying frequency, but we cant use the one converter for multiple motors as easily as a RPC can. In any case I am out of the loop for the next 3 weeks or so, so if anyone wants to give it a go in the meantime, go for it and I look forward to hearing the results upon my return. Regards,
    Ropetangler. - a non sparkie!

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