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  1. #1
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    Default VFD questions from an electrical idiot

    Hi folks,

    Thinking about buying a 3 phase disc sander - I do have 3 phase in the workshop but as it shouldn't need a heap of power and a sander is something where variable speed would be nice, I'm thinking about going down the VFD route, with single phase feeding into the VFD, sending 3 phase to the sander.

    Couple of questions I'm hoping someone can help with:

    There seems to be no shortage of sellers on ebay - can anyone recommend a reputable one?

    Do all VFDs need to be installed by a sparky, or are there units which can plug into a 10A or 15A GPO (ideally with a 3 phase output which has a plug but I realise that's unlikely) ? Would be nice to be able to move it around the shop rather than have it hardwired

    Cheers,

    Danny

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  3. #2
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    Quote Originally Posted by rtyuiop View Post
    Thinking about buying a 3 phase disc sander - I do have 3 phase in the workshop but as it shouldn't need a heap of power and a sander is something where variable speed would be nice, I'm thinking about going down the VFD route, with single phase feeding into the VFD, sending 3 phase to the sander.
    STOP!
    NOT all 415V Three Phase (3P) motors can be driven at full power by a 240V VFD.
    BEFORE you buy you need to ensure the motor is able to be made compatible with a 240V VFD.
    You need to firstly know, is the motor wired as a 415V 3P ∆ (delta) OR Y (Star).
    Some 415V motors can be readily converted from Y to ∆ in the motor mains junction box, - this automatically converts them into a 240V 3P - if you is like that - great.
    If the motor is "hard wired" as a 415V ∆ - you are plumb put of luck and the entire motor has to be changed - it will run but it will generate <half power and is not recommended you do this
    If the motor is "hard wired" as 415V Y it is usually possible for a qualified person to convert is to ∆ this making it 240V 3P compliant. This requires internal surgery to the motor - ie definitely not recommended unless you know what you are doing.
    If you have access to the mains motor junction box with the power disconnected remove the cover and take as clear a phot as you can and post it here and we may be able to work out what it has.

    There seems to be no shortage of sellers on ebay - can anyone recommend a reputable one?
    Before you even think about buying sort out those issues above.

    Do all VFDs need to be installed by a sparky, or are there units which can plug into a 10A or 15A GPO (ideally with a 3 phase output which has a plug but I realise that's unlikely) ?
    Sparky is recommended - also don't buy any VFDs without consulting the sparky - they after all have to guarantee the thing and machine will keep working - if they are not comfy about using the VFD that you bought they won't want to install it.
    That's not how they work - The VFD has to be connected direct to the motor by a suitably experienced and preferably qualified person.

    Would be nice to be able to move it around the shop rather than have it hardwired
    That's not how they work and its not good for them to do that either. VFDs have to be programmed for the specifics of a machine so unless you are using them on an identical machine the thing will have to be reprogrammed every time it's moved.

    Suggest taking the time to read through this thread as it will shows you most of the practicalities involved.
    VFD install summaries

  4. #3
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    Default

    Thanks Bob - that's exactly the kind of info I need. Going through your summary thread now.

    The sander is interstate, but the seller has posted this:



    Any red flags from that?

    Little sad I won't be able to roll the machine around the shop, but I can live with it!

    Thanks,

    Danny

  5. #4
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    Use a VFD that has 415V input and you can then control the speed and other parametrs such as start speed etc. Powtran can supply one at a nice price.
    CHRIS

  6. #5
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    Motor looks suitable for conversion to 230V ∆.

  7. #6
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    This info below the is present on the motor data plate indicates the motor can be easily converted in the mains/motor junction box but I have been caught out before - to be sure we need to see a photo of the inside this box.

    Screen Shot 2019-06-16 at 1.35.35 pm.jpg

  8. #7
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    As suggested by Chris, if you have access to 3 phase power in the workshop, and just want variable speed etc, go with a 415V in to 415V VFD. Then the only thing you need to be aware of is that you hard wire the VDF output to the motor terminals, rather than using the motor control systems in the system, and use the inbuilt controls in the VDF, or remote controls added to it, to control the motor. VFD's do not like having their load disconnected while operating, as would happen with a user operating the original stop switch on the machine, or having a 3 ph plug and socket between VDF and motor disconnect while the motor is running.

    That way, you have no issues with whether the motor can develop full rated power at 50Hz, and no issues with whether or not the motor is easily convertible for 240V operation.

    As Bob suggested in the previous post, the existence of a diagram showing links and connections for star and delta on the data plate does not guarantee that the motor is readily convertible, particularly if the data punched onto the plate shows parameters for only one configuration. The plates are prepared for use with the range of 3ph motors coming out of the factory, and the relevant data for a particular motor is punched onto a blank plate immediately before it is attached to the motor. Therefore the blank plate is designed to cope with all possible motor configurations.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  9. #8
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    There is actually a valid reason for using a 240V VFD on a 3HP or less machine in a shed even if you do have 3 Phase and that is chances are that you don't have a lot of 3 Phase sockets and that they are not always where you want them to be. The cost of a 3Phase socket alone can be quite eye watering. Using a 240V VFD allows you to plug into any 10A GPO provided you use slow start so the start up currents won't be too high.

  10. #9
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    Default VFD Answer from another electrical idiot

    Quote Originally Posted by rtyuiop View Post
    ...I won't be able to roll the machine around the shop, but I can live with it!
    You will still be able to move the combined sander and VFD around your shop. The plug and cord go directly into the VFD, which is hardwired to your machine.

    Cheers, Vann.
    Gatherer of rusty planes tools...
    Proud member of the Wadkin Blockhead Club .

  11. #10
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    Ah, I think the light has dawned - the VFDs I can find good photos of are hardwired on the input side, so I assumed that would be an 'anchor' keeping everything immobile... But if I wire that (more accurately, get a sparky to do it!) to a 3 pin plug for a GPO then I'm mobile again?

  12. #11
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    A lot of electricians have nil experience and knowledge of VFD's because they are generally used in industry and not residential situations.
    CHRIS

  13. #12
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    Quote Originally Posted by Chris Parks View Post
    A lot of electricians have nil experience and knowledge of VFD's because they are generally used in industry and not residential situations.
    I bought my used 3 Phase compressor from a sparky who said he was selling it because he had moved to a house that did not have 3 Phase. After the transaction was complete I asked him if he had heard of VFDs and he said no. After I told him he said he would have to look into them as they sounded useful. A more likely type of electrician who would know all about VFDs are electrical fitters.

    FWIW one of my Italian cousins has a motor manufacturing business and most of the motors he makes including those for single phase applications are internally 3 phase with a built in mini VFD. EU rules for maximum motor efficiency to save electrical power is a major driving force behind this. The motors themselves are made with much higher efficiency than in the past and adding a VFD saves further power in many applications.

  14. #13
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    Decided to buy the sander so will be figuring out a way to hook it up one way or another! Deciding factor might be if I can find a sparky who knows something about VFDs and will work for a reasonable rate.

  15. #14
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    Pics including the inside of the mains motor connection box would be interesting

  16. #15
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    Quote Originally Posted by rtyuiop View Post
    Decided to buy the sander so will be figuring out a way to hook it up one way or another! Deciding factor might be if I can find a sparky who knows something about VFDs and will work for a reasonable rate.
    If I may make a suggestion.: contact Jack English Machines. He has installed numerous VFDs on his old Wadkins etc. He now sells them (mods please remove if this is not permitted). He wil get the motor details off you and recommend a suitable VFD then provide assistance to enable you to wire it in (saves a lot of sparkie costs). Then you can get a sparkie to do a final check and wire in a plug and cord.

    Cheers, Vann.
    Gatherer of rusty planes tools...
    Proud member of the Wadkin Blockhead Club .

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