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Thread: Two GMFs and one VFD
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24th Jun 2015, 04:12 PM #16Member: Blue and white apron brigade
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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.
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24th Jun 2015, 04:25 PM #17Member: 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
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24th Jun 2015, 04:33 PM #18Most Valued Member
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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
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: **
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25th Jun 2015, 10:05 PM #19Member: 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?
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25th Jun 2015, 10:19 PM #20Most Valued Member
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Adding timers to the circuit?
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25th Jun 2015, 10:36 PM #21Most Valued Member
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25th Jun 2015, 10:42 PM #22Philomath in training
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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
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25th Jun 2015, 10:57 PM #23Member: Blue and white apron brigade
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25th Jun 2015, 11:34 PM #24Member: Blue and white apron brigade
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25th Jun 2015, 11:36 PM #25Member: Blue and white apron brigade
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26th Jun 2015, 12:22 AM #26Most Valued Member
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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
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26th Jun 2015, 07:49 AM #27Philomath in training
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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
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26th Jun 2015, 12:46 PM #28Most Valued Member
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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
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27th Jun 2015, 09:18 PM #29Member: 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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27th Jun 2015, 10:33 PM #30Philomath in training
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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