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    Default 3 Phase inverter,

    Hi,
    I know that this subject will have been done to death,so apologies in advance.

    I am thinking of building my own 3 phase rotary inverter, but need to know what is the difference between 3 phase 415v and 3 phase 240v.

    Just bought a lathe with a 2hp 415/240v motor, I am looking build a plug and play inverter.

    How do I know if the input is 415v or 240v 3 phase..

    regards
    N

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    Your motor can be run on either 415 or 240v, if there is not a wiring diagram on the data tag, open cover for electrical connections and look in there. An inverter is a VFD, you cannot reasonably build one of those, a 3 phase rotary converter can be easily built.

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    For you, the two voltages are essentially the same thing, but are presented differently from US "240V".

    You have 240V from line to neutral. When dealing with 3 phase, that is termed a "wye" voltage, since the motor wiring can be drawn to look like a letter "Y". But that very same voltage system has 415V from line-to-line, termed "delta" since the diagram can look like a triangle.

    US 3 phase 240V is used with motors wired "delta". The 240V is a "delta" voltage. I do not think you have an equivalent in common use..

    A motor that can use a 240V line-to-line voltage (common in US, probably not in UK) when wired "delta", can be used with 415 V 3 phase when wired "wye". The latter would presumably be what you would choose, as it is compatible with your power.

    P,S, An "inverter" is the term for a device which created AC from a DC source. True of a VFD, not true of an "RPC" or "Rotary Phase Converter"

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    hi, thankyou for both your replies.
    I knew I would get the converter/inverter the wrong way round.
    so basically 415v and 240 3 phase are both the same, is that correct..
    I have seen a few youtube clips of rotary converters been built but they do not specify the size of the 3 phase pony motor, should this be the same size as the lathe motor or does it not matter?

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    I would size the idler/pony motor at 2x the largest motor you intend to run, you might get by with less, but it would suck to build one then find out its not enough.

    In the USA, motors with 240/480v on data tag can be run on either voltage by changing the connections on leads, I'm not familiar with european motors, so JST's explanation may be correct.

    Open the connection box on motor, how many leads are there? Please provide a pic of data tag and a pic of the leads.

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    Quote Originally Posted by griff25-06 View Post
    hi, thankyou for both your replies.
    I knew I would get the converter/inverter the wrong way round.
    so basically 415v and 240 3 phase are both the same, is that correct..
    Those can both be 3-Phase, yes. Was common for MIL-SPEC generator sets to offer 208/416 V "Wye" OR 120/240 Single_phase, OR 220/440 (240/480..) VAC 3-Phase "Delta". The winding taps were already built-in to the "generator head".

    Even so, a bit of labour was wanted to make the selection - it was neither "automagical" nor "don't care" interchangeable. Voltages look different? That's because they ARE different.

    I have seen a few youtube clips of rotary converters been built but they do not specify the size of the 3 phase pony motor, should this be the same size as the lathe motor or does it not matter?
    (At least) one of the makers of commercial RPC has published tables. The oversize is required by starting needs more than any other factor. Those can differ enough that some loads want a significant multiple of their running load. Ten times, sometimes more-yet. Multiples beyond ten times are published, but of questionable use. Situation that dire, an RPC may not be a good idea anyway.

    Where the load-motor is clutched and/or otherwise starts "unloaded" (kinetically, not electromagnetically) a fifty-percent up-step is a reasonable minimum. Eg; 5 HP load, 7.5 HP idler, 10 HP load, 15 HP idler.

    While there is no theoretical lower-bound, it seldom makes labour and materials sense to build an RPC of much LESS than 5 HP idler, even so. Small stuff, it is often cheaper to just swap to a single-phase load motor and not NEED conversion to 3-P at all.

    At the other end, go TOO large, and the idler itself presents a nasty "starting" load. That's where one or both of "pony start" and "supplementary" idlers can help.

    Ex:

    - Start a 10HP idler. Once "up", add a 5 HP idler. THEN start a 10 or 12 HP load-motor of the net-net 15 HP idler.

    - Start a 30 HP or larger idler with a 1-P "pony" motor. Or a Dee Cee pony, or ...., etc.

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    Quote Originally Posted by griff25-06 View Post
    Hi,
    I know that this subject will have been done to death,so apologies in advance.

    I am thinking of building my own 3 phase rotary inverter, but need to know what is the difference between 3 phase 415v and 3 phase 240v.

    Just bought a lathe with a 2hp 415/240v motor, I am looking build a plug and play inverter.

    How do I know if the input is 415v or 240v 3 phase..

    regards
    N
    Open up the connection box on the motor, normally on proper British/European motors you'll find six terminals attached to the windings, if there are three linked together it's in star (higher voltage)and you connect to the unlinked terminals, if there are three parallel links pairing terminals it's delta (lower voltage) and you connect to the three linked pairs.

    The 240 / 415 comes from the difference in wiring the same coils in different ways - picture a triangle with sides 240 long, that's delta, now a 3-pointed star with arms 240 long and measure between the points - 415, that's star or "wye" connection.

    If it's a Proper Lathe there'll be a hatful of contactors (huge relays) to do all the switching, they'll often have 24v ac coils fed from a smallish transformer - you can look see whether incoming power goes to the 240 or 415v terminal on it, to help determine the voltage it's set up for - if it came from anywhere other than a home workshop it's likely to be 415v as that's all the electricity board provide as 3-phase in Blighty unless you're a big industrial user

    Dave H. (the other one)

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    Hi N, If you want to build a Rotary Phase Converter that you can plug and play, I think it would be better to build a 240v single phase input with 415v 3phase output, because most 3phase things you would play with will be 415v. If you would be kind enough to put a photo of the rating plate of your 2 HP lathe motor and a photo of the inside of it's terminal box.
    Usually an idler motor HP is about two times the size of the motor you are running. In this case a 5 HP would be fine.
    If a motor rating plate stated that it could run from two voltages 240v and 415v that means when this motor was designed and manufactured it's 3 coil voltages are 240v each and when connected is Star you would require a supply voltage of 415v for normal continuous operation. But if you had a supply voltage of 240v this means you would connect this motor in Delta.
    So if you wanted to run your lathe motor on 240v 3 phase you will need an idler motor, say, 5 HP that can be run from 240v (in Australia they are rare). But if you run your lathe motor on 415v you will need a 415v Idler motor and you will need a single phase 240v to 415v transformer. The transformer can be overcome by using an old welding machine's primary winding connected as an Autotransformer or if you could not find a suitable duel input voltage welder 240v-415v it's not hard to convert an ordinary 140amp AC welder to get 415v.
    P.S. You can not Supply a 415v motor with 240v and output 415v in the kind of RPC I am talking about here.

    Jim

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    Rather than building a rotary converter, a VFD is a far simpler approach to converting single phase power to three phase power, and it's far more efficient. Rotary phase converters require a motor twice the size of the motor that you are powering. Several advantages of the VFD include ability to control the speed of the motor, soft start the motor, and electrically brake the motor with the use of a resistor. If you do go the VFD route, take caution of buying cheep Chinese junk. Here are a list of several reputable brands for VFds that I have personally worked with and can recommend: ABB, Allen Bradley, SEW Eurodrive, Hitachi, Toshiba, Yaskawa, Mitsubishi. Since you are converting from single phase, you will need a VFD that can accept single phase, and you will also need to oversize the hp of the VFD to to motor by approximately 30 percent. So if you have a 3hp motor that you need to power, a 5hp VFD should be plenty sufficient.


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