Tablesaw On/Off Switch with VFD

[Malcolm McLeod]

I was thinking of eventually running two tablesaws with a single cheap VFD using a rotary switch,
I've obviously heard that switching it, will destroy the inverter while the machine is powered on, but I'm not in any kind of environment
where I would have to do this quickly, so can wait for the machines to fully stop (and some more) before switching .

I just wasn't sure if I would be able to, with the cheap drives ,as one machine is 3HP and the other has only 2HP .
I wasn't sure if I'd have to do a parameter change, or if it's possible to have another rotary switch for changing the allocated settings
for machine 1 and machine 2 ...
So I would have a panel nearby with two rotary switches, and only then switch over, when the VFD is powered off ?
Or does the VFD need to be powered on, to recognize the rotary switching with the interface/mobus/low voltage controls ?
Can someone clarify this please,
as I have been wanting to know this for a while .
Thanks kind folks
Tom

The industrial system I referenced used a PLC to manage the switching (multiplexing) to the various motors. The PLC talked to the VFDs and would not allow motor starters to switch the VFD's load from one motor to another, unless the VFD was stopped (the output buss was at 0volts & 0hz). I could have accomplished the same thing by training the operators to open the control enclosure, verify that the VFD in question has in fact stopped, and only then switching to another motor. (People being people, I changed the program.)

Assuming you can successfully manage the 'logic' part of this (don't switch the saws while VFD is active), then you CAN do what you describe. I'm also going to assume you'll run these at rated speed - - just using the VFD to get 1ph --> 3ph power conversion.

I'd put 2 starters in a electrical enclosure with the VFD and use a single 3-position switch on the door to select the respective saws: "3Hp - OFF - 2Hp". Two contact blocks on the switch would have XOO (on-off-off) (:: 3Hp saw starter ON in left position), and OOX logic (:: 2Hp saw starter ON in right position).

You can put a pilot light, fed from the VFD terminal strip, to indicate the VFD is running and/or stopped, and so inform you it's safe to switch. This would allow you to leave the VFD itself powered all the time, should you choose to do so.

That leaves the logic to change the VFD settings to appropriate values for the respective saws (most important would be the current limits for each). It can, I believe, be done on some VFDs, but I would have to know more about the specific make/model to offer more. You could add a third contact block on the selector switch to feed this signal to the VFD.

Or, you could operate the 3Hp on same settings as the 2Hp??

Edit: This scheme begs the question of how you start/stop each saw - - at the respective saws. The STOP function is relatively easy: use a STOP PB at each saw with maintained, normally closed contacts. Both switches would be wired in series. For START, you could wire a pair of switches in parallel - - but it means you could start the selected saw from the switch on the other saw...not a good thing. To prevent this means the START circuit has to run thru the selector switch as well, and that gets pretty complicated to design remotely ....on a woodworking forum. And, I suspect somewhere in here, it's cheaper to just get a second VFD!

[Dan Friedrichs]

Tom, I recently bought a cheap VFD on ebay that has parameter settings for 2 different motors. I'm not immediately sure how you specify which motor is being used, but it definitely allows programming of 2 motors and switching between them.

That said, it was only $100 for a 2HP VFD, so I'd probably just buy a separate one....

[Matt Mattingley]

The problem here is, there is a 3 hp in a 2 hp Motors that do not have the same characteristics. They can be run in the V/H mode with the 2 hp motor having overload fusing.

These can be run together if desired. The drive needs to be sized accordingly. You can start both motors using the same drive.
If you're going to hard start using a contactor the VFD has to be rated for the smallest motor + 450%.

So if the 2 hp is already online and you want to add a 3 hp, the VFD needs to be rated for 5 hp +450% of 3 hp. Average amps per horsepower is 3A. So to accomplish a hard start on the largest motor you would need
6 A output + 3hp X(3A x450%)= A 15 hp VFD. They are about $1000.

This is a phase perfect technology. They are pretty much running a 20 hp VFD. This creates three-phase with allowable hard starts up to 10 hp. Pretty neat kit if you want to spend that kind of money. Phase perfect cannot variable speed a selected machine. They are only a very well-balanced phase converter.

I run in my shop numerous VFD's and an 20 RPC and a 5 hp RPC. My RPCs are only used to run my machines that are in the 550 to 600 V range. I do have one plug for 240 V three-phase off the RPC. I am comparing apples to oranges here. A RPC or phrase perfect will convert and whole shop. A VFD is usually just one machine. Most of my southern friends don't need a Rotary phase converter (RPC). Last year I built my own portable for testing machines. It converts single phase to three phase 240 V/600v. These are not toys!

[Tom Trees]

Assuming you can successfully manage the 'logic' part of this (don't switch the saws while VFD is active), then you CAN do what you describe. I'm also going to assume you'll run these at rated speed - - just using the VFD to get 1ph --> 3ph power conversion.


I'd put 2 starters in a electrical enclosure with the VFD and use a single 3-position switch on the door to select the respective saws: "3Hp - OFF - 2Hp". Two contact blocks on the switch would have XOO (on-off-off) (:: 3Hp saw starter ON in left position), and OOX logic (:: 2Hp saw starter ON in right position).

You can put a pilot light, fed from the VFD terminal strip, to indicate the VFD is running and/or stopped, and so inform you it's safe to switch. This would allow you to leave the VFD itself powered all the time, should you choose to do so.

That leaves the logic to change the VFD settings to appropriate values for the respective saws (most important would be the current limits for each). It can, I believe, be done on some VFDs, but I would have to know more about the specific make/model to offer more. You could add a third contact block on the selector switch to feed this signal to the VFD.

Or, you could operate the 3Hp on same settings as the 2Hp??

Edit: This scheme begs the question of how you start/stop each saw - - at the respective saws. The STOP function is relatively easy: use a STOP PB at each saw with maintained, normally closed contacts. Both switches would be wired in series. For START, you could wire a pair of switches in parallel - - but it means you could start the selected saw from the switch on the other saw...not a good thing. To prevent this means the START circuit has to run thru the selector switch as well, and that gets pretty complicated to design remotely ....on a woodworking forum. And, I suspect somewhere in here, it's cheaper to just get a second VFD!

Thanks Malcom
Judging from what you've said ...
It does not sound possible, to have a rotary switch to change over, from machine 1 to machine 2 by using the low voltage panel ...
Even if the VFD was plugged out/power off no illumination on screen and then,only then, ... use the second rotary switch that would be inbetween
the motor and the VFD to change over motor to the other .
As you said, it seems that the only parameter difference between machines, would be the motor HP rating (for just using the VFD for phase conversion purposes)
I did not think it would be necessary to have a really cool light to tell me which machine is ready to be initiated, as I thought a rotary switch with an arrow
or similar would be enough. (thanks for the suggestion though)
I have not used a tablesaw yet so can't say for sure that 2HP would not be enough ..I'm patient though (not to mention very cautious about using a TS)
and the scheme you mentioned could be made feasible ...As I was thinking of making a latch on the start button/bar system I previously mentioned
anyway , ( something like Carl Holmgrens switch design, but with a hidden latch )
Since I am the only one who is in the workshop, the possibility of starting one TS with the other TS switch is not a concern .

Dan
I am interested in knowing what VFD you got, which has the option of having the ability to change between machines .
I'm struggling to save up for the one VFD so having two is not an option ...
I originally got this second TS for cheap,to use for the motor to make a cyclone with , as I thought it was 3HP and a farm style tablesaw in the advert pics .
As It turns out it is an OK machine, just needing a lot of work ,and I couldn't bare not souping this baby up .
Some might like it here because it's another Italian beast in the shop .

Matt
I think I confused things somewhere, by not stating that I would be only using one machine at a time .
Thanks though for your comments ,I'm sure their is some lurking folks gleaning from your expertise !

So if it all comes to it, and I can't figure it out ...
I know with a cheap 100 quid unit, that I can stick a rotary switch between the motors and the VFD and change
the max voltage parameter ...
I imagine you could get used to it
I might ask some folks on youtube, with the same VFD I have allready, can it be done ...
It might take some time to get a reply though and I worry that this thread might have lost momentum, so to speak .
Can't mention any Italian machine on this forum without some piccys though !
That's the rules
magic tablesaw 2.jpegmagic tablesaw.jpegMagc tablesaw.jpg

Magic tablesaw (Possibly Funken too, from the only other pic I've seen on the web )
Funken TS.jpg
Thanks again folks .
Tom

[jack forsberg]

Not all VFD's have internal break resistors or if they do they are usually pretty small. Not all VFD's have the internal hardware that external break resistors can be added. If high inertia stopping is required, this needs to be a prerequisite when selecting a VFD. I'll be testing this break resistor in the near future on an 18" Direct drive table saw. It is 1000 W 30R. VFD's cannot displace the heat generated (by themselves) in heavy inertia stopping. A external break resistor needs to be added. This thing weighs more than the VFD.

Attachment 351973

You know one thing not mentioned here is ithat the majority of VFD's in the world are actually being fed three phase power and as such in a lot of cases don't need an external break resister to dump the high DC bus voltage . This is your typical fault if the deacceleration is too fast for the internal break sector . As others have suggested the motor is the generator and regenerative braking can dump power back into the three phase mains makes the VFD more energy efficient . The reason for breaking machines faster than they would normally coast down in terms of safety is to do with rundown accidents . Many a finger is lopped at run down . Blades run silent and rundown accidents caused by reaching in to grab a piece of scrap And not realizing the blade is still spining . It's well-documented in Europe in accident reports being dramatically reduced by having machine stopped in less than 10 seconds .

[Matt Mattingley]

You know one thing not mentioned here is ithat the majority of VFD's in the world are actually being fed three phase power and as such in a lot of cases don't need an external break resister to dump the high DC bus voltage . This is your typical fault if the deacceleration is too fast for the internal break sector . As others have suggested the motor is the generator and regenerative braking can dump power back into the three phase mains makes the VFD more energy efficient . The reason for breaking machines faster than they would normally coast down in terms of safety is to do with rundown accidents . Many a finger is lopped at run down . Blades run silent and rundown accidents caused by reaching in to grab a piece of scrap And not realizing the blade is still spining . It's well-documented in Europe in accident reports being dramatically reduced by having machine stopped in less than 10 seconds .

Jack, even some three-phase equipment that is being fed three-phase using a VFD from three phase utility line, for hirer inertia stops, Break resistors are still needed. But, they are about a 1/20th the size, for reasons you just stated. For example this 5 hp motor is getting 1000 Watt break resistor. This same break resistor could be used on a three phase VFD that (with three phase utility) is 100 hp. A VFD can create reactive current in breaking state. Single phase shops electrical is not really set up with utility to be back feeding this kind of energy. Most of the time the energy that is back fed is usually consumed in the industrial plant and never makes it back on to the Power grid. I guess you can say it is kind of a on-site generator.
IMG_5913.jpg

[Matt Mattingley]

http://www.sawmillcreek.org/member.php?130924-Tom-Trees
You can actually use the same VFD running two different machines that are different horsepower. You need to put adequate fusing (after the VFD) for the smaller machine (and you can use a simple forward reverse contactor rewired and use the onboard relay in the VFD to switch at 0 V/hz). But you need to buy the VFD for the larger machine motor.

[Todd Willhoit]

You know one thing not mentioned here is ithat the majority of VFD's in the world are actually being fed three phase power and as such in a lot of cases don't need an external break resister to dump the high DC bus voltage . This is your typical fault if the deacceleration is too fast for the internal break sector . As others have suggested the motor is the generator and regenerative braking can dump power back into the three phase mains makes the VFD more energy efficient.

What you are describing is the exception rather than the rule. Very few VFDs have an active front end which permits regeneration back to the line. Aside from vehicles, for the vast majority of applications it 1) is not necessary, and 2) does not fit the price point.

If someone is going to use a braking resistor, please make sure it is kept free of dust. Resistors make nice fire starters. I know of one case in which the line voltage was elevated over night due to reduced loading on an upstream transformer. The increased line voltage caused a high DC bus voltage. The algorithm activated the braking circuit to dissipate the bus voltage. Since the line held the voltage high, the resistor was active for an extended period. Wood dust on the element ignited causing a large fire.

[Malcolm McLeod]

Another common sense reminder (like keeping them dust free): If you plan to use external brake resistors, make sure they are mounted appropriately. Don't put them where you can hit 'em with a bare leg, lean back on them, let a child touch them, etc....

It's easy to forget that VFDs and their accessories were developed for industrial environments, with UL/NEMA/NFPA/OSHA/MSHA codes typically requiring they be placed in a control enclosure or dedicated motor control center. As these devices migrate into our home shops, make sure you don't take their design roots for granted. Please.

I'd hate to think my advice got someone a spot on a GEICO commercial.

[jack forsberg]

Probably a good idea to keep the breaker resistor out of the VFD enclosure . Generally they don't get Warm Unless repeated breaking Over short periods of time . Should all be listed in your manual as to proper breaking intervals. It's also not the best thing for the motor to rapid stop. Some programs will lock the rotor with full torque at zero RPM . So the application should be reviewed as often a larger drive is required . Matt let me know how that big five horse power break unit works out and if you need one for your 3 hp