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  1. #16
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by BRADFORD View Post
    Hi Bob,
    If you connect the VFD output to the line side of a contactor on each grinder (connected in parallel) and then electrically interlock the contactors so only one can operate at a time, you are all sorted, no need for a changeover switch.
    When grinder one is running, grinder two cannot be started and vice visa.
    If you don't have contactors with the required spare contacts, you could probably add relays to achieve the desired result.
    Yep I understand that setup and have considered it but that only fixes half the problem , that of making sure that only one of the grinders are connected to the VFD.
    The other problem, which is just as importan,t is that of someone "breaking an ON contactor while the VFD/Motor are running".
    This is why it's always better to fit VFDs with no switchgear in between the motor and the VFD.
    We could put the contactors inside a box the door to which can only be opened when the VFD is off so now we are back to square one.

    When I do things like that I tend to use whatever I've got about the place, so sometimes it can end up a bit unnecessarily complicated.
    Yep I know what you mean, and that is why I'm looking at a simple change over switch and a micro switch on th e i.e. I have several of these at hand and ready to go.

  2. #17
    BobL is offline Member: Blue and white apron brigade
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    Something I have thought about is the 3P crossover switch I'm thinking of using has zero (0) position and then 2 (1 & 2) contact positions.

    This means there is a danger that the VFD could be set to run with the Cross over set at the 0 position i.e. VFD not connected to anything.

    Fortunately the Crossover has 4 contacts so one of these can be wired up in series with the VFD remote switch.
    So if that contact is not at 1 or 2 the VFD wont start.

    Any other ideas?

    Thanks

  3. #18
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    Quote Originally Posted by BobL View Post
    The other problem, which is just as importan,t is that of someone "breaking an ON contactor while the VFD/Motor are running".
    Part of the theory behind my 4 pole switch* is that the front pole(VSD control) will open first(I use coasting stop). If you change to three wire control and momentary switches the VSD wont restart until the start button is pressed again.**
    I think that will sort out your problem unless they hold their finger on the start button while changing the 4 pole switch.........If you think thats likely you could add at 5th pole at the back...... or hit them over the knuckles.

    Also if you're going to use some sort of interlock or parallel contactors you could likely make use of the VSDs internal relay which can be set to close(or open) when the VSD has output on(it can also be set to do many other things but I doubt they will be useful in this case)

    Stuart

    Quote Originally Posted by BobL View Post
    Ah yes I remember that thread. My requirements are almost trivial compared to yours
    Yes the price of VSD has past that setup by really... still it seemed like a good idea at the time

    *its two throw with center off.

    ** once you've done that, may as well have a start button for each grinder?
    Last edited by Stustoys; 24th Jun 2015 at 05:07 PM. Reason: **

  4. #19
    BobL is offline Member: Blue and white apron brigade
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    OK I have changed my mind about the locked door because the 4 pole 2 position 3P switch looks like it might so what I want without locking it behind a door.

    Heres the pseudo schematic.
    When the X-over switch is turned the NVS is series will drop out which in turn will drop out the 240V relay and turn off the VFD
    It will also stay off until the NVD is triggered.
    The big question is will the "turning off" be quick enough for the VFD?

    Attached Images Attached Images

  5. #20
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    Adding timers to the circuit?

  6. #21
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    Quote Originally Posted by BobL View Post
    The big question is will the "turning off" be quick enough for the VFD?
    My guess would be "No"

    Why not just run the VSD control through the 4 pole switch in series with the 240V relay? (is it so you can stay with two wire control?)
    Stuart

  7. #22
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    What about the trick that they do for large industrial cabinets - having the handle to open the box that turns off the power to the VFD (supply side) when turned. Once the box is open, have a change over switch (grinder A or grinder B), but nothing will run before the cabinet is closed. Still use your pendant to turn the VFD on and off.

    Michael

  8. #23
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by Michael G View Post
    What about the trick that they do for large industrial cabinets - having the handle to open the box that turns off the power to the VFD (supply side) when turned. Once the box is open, have a change over switch (grinder A or grinder B), but nothing will run before the cabinet is closed. Still use your pendant to turn the VFD on and off.

    Michael
    That was what I had in the first post

  9. #24
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by eskimo View Post
    Adding timers to the circuit?
    yeah but delaying the 3P is not gunna be easy, perhaps a negative timer to turn the VFD off before I need to.

  10. #25
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by Stustoys View Post
    My guess would be "No"

    Why not just run the VSD control through the 4 pole switch in series with the 240V relay? (is it so you can stay with two wire control?)
    Stuart
    Err that's what I thought I had?

  11. #26
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    I mean in series with the switched wires of the 240V relay, the VSD controls (but if you stay two wire the vsd will restart, so that would be no good)

    The reasons I say 'no' is that I think 240VAC will arch longer than 12VDC, then you have how ever long the nvs takes to cut power to the relay and how long before the relay contacts open? (I need a recording CRO so I can test some of my thoeries
    If you want to stay with two wire I think you're better off having a cover over a 3 pole switch with a micro switch wired as the 4th pole is now, that would give you the delay you're after.

    Stuart

  12. #27
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    Quote Originally Posted by BobL View Post
    That was what I had in the first post
    Sorry - the first post sounded like you were going to make something up. I was thinking of something off the shelf along the lines of

    http://zjbeny.en.alibaba.com/product...ng_Handle.html

    We have some at work where the rod on the handle physically keys into the switch so that the door can only be opened when the power is off.

    Michael

  13. #28
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    Also it doesn't have to be a cover. a spring loaded pin that has to be held back to allow the switch to be turned would do.....must be a 1000 different ways to do it. Anything so there are two actions.

    Stuart

  14. #29
    BobL is offline Member: Blue and white apron brigade
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    OK I did the microswitch on the covered box method.

    The 3P Xover switch extends thru into another box joined to the box with the transparent cover on it.

    The 3P XO switch (from the 2 speed llinisher I converted to 240V 3P +VFD) has a ZERO position (i.e. VFD NOT connected to either grinder) so the switch could in theory be moved to the zero position and the XO box door closed, BUT the 4th contact on the XO itself will be in series with the XO door micro switch (from an old x-ray machine) so the VFD wont start unless;
    - Xover switch is in position 1 or 2
    - door to switch is closed
    - NVS is finally pushed



    This is effectively what the circuit will be


    Still a fair bit of work to do on the rest of the circuit so there won't be much more to see for a few days.
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  15. #30
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    With my "take no prisoners" attitude to electrons. I would have done it like this -

    diagram.jpg

    A No Volt Switch could be added but the VFDs that I have can do that anyway I think. The door switch will just be a power interuption.

    Michael

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