Tablesaw On/Off Switch with VFD

[Rod Sheridan]

For those of you who use a VFD on their tablesaw I would be interested in knowing how you turn you saw on/off.
I personally have a rotary phase converter and have a magnetic switch on the saw outfitted with a "paddle" that lets me shut the saw off with my knee or foot. However I understand you can't use a switch in between the VFD and the saw.

On tools like drill presses, lathes etc an emergency shutoff may not be as critical as a tablesaw so am wondering how others do it.

Thanks

Fred

I don't have one on the table saw however I would use the standard saw stop/start pushbuttons like I do for the band saw, band mill and lathe.

I also like to have a disconnect switch on the machine so I don't have to unplug it when changing tooling............Rod.

[fRED mCnEILL]

My understanding is that the VFD should be hardwired to the saw and any emergency type switch should use the terminals provided because just turning off the power between the VFD and saw could do damage. Does this only apply when the saw is running?

[Dan Friedrichs]

My understanding is that the VFD should be hardwired to the saw and any emergency type switch should use the terminals provided because just turning off the power between the VFD and saw could do damage. Does this only apply when the saw is running?

I think Rod is saying to put a disconnect between the supply and the VFD, in this case, to facilitate blade changes, etc.

Technically, you could put a switch between the VFD and the motor to use as a safety disconnect - only to be opened when the motor is stopped - but that's an awful idea, since somebody is likely to accidentally open the switch while the saw is running, someday. Much better to put the disconnect between the VFD and the supply.

[Eric Keller]

depending on the vfd, the manual may give a way to switch off the output of the vfd to the motor, but you're not going to like how much that will cost you. At least a couple of contactors. Been a while since I have seen a circuit for that.

The low voltage side is the way to go. Most vfds allow you to add external braking resistors, might be worth it on a tablesaw

[Van Huskey]

Most vfds allow you to add external braking resistors, might be worth it on a tablesaw

You have to be really careful with the deceleration rate dialed in on a TS, too much and it will loosen the arbor nut. It is are great on some machines like big disc sanders and bandsaws for example.

[Matt Mattingley]

You have to be really careful with the deceleration rate dialed in on a TS, too much and it will loosen the arbor nut. It is are great on some machines like big disc sanders and bandsaws for example.

I think tablesaws are the best for high breaking rates! What can be better than hitting your stop paddle and having your tablesaw come to a stop in two seconds( or less )? I would call this safe! Actually in Europe, it is code to have stationary equipment come to a stop in less than 10 seconds.

Now all arbours don't allow this. On most consumer grade saws you will spin the armour nut off with blade bouncing uncontrollably! Better quality saws have arbour nut/outer flange inertia stopping. This is not new, this is old technology. Tablesaws in the 50s and 60s started preventing this with shear pin for blade plates. I think most want to get a good deal on an Asian saw. So you might be right.

[Van Huskey]

I think tablesaws are the best for high breaking rates! What can be better than hitting your stop paddle and having your tablesaw come to a stop in two seconds( or less )? I would call this safe! Actually in Europe, it is code to have stationary equipment come to a stop in less than 10 seconds.

Now all arbours don't allow this. On most consumer grade saws you will spin the armour nut off with blade bouncing uncontrollably! Better quality saws have arbour nut/outer flange inertia stopping. This is not new, this is old technology. Tablesaws in the 50s and 60s started preventing this with shear pin for blade plates. I think most want to get a good deal on an Asian saw. So you might be right.

You are never going to get any real safety benefit from resistive or DC injection braking on woodworking machines, it could help if you find yourself in a precarious position AND can hit the stop button. Tablesaws have so little momentum they spin down fairly quickly, nothing like a big bandsaw that might take 3-5 minutes or a big disc sander that can take 10 or more minutes to stop. The vast majority of American and Asian saws in hobby shops and even in small cabinet shops don't have any arbor locking mechanisms.

If you want added safety in a table saw buy a Sawstop or a slider you really aren't going to add much safety with any form of braking except Sawstop (or Bosch) type braking. VFD braking as it applies to woodworking is mainly the convenience factor as long as you use the rule of waiting until the tooling stops to move your cut pieces etc.

BTW I don't want any machines with high breaking rates... sorry couldn't resist.

[Matt Mattingley]

So do you consider a TS stoping in 2s better than 45s??? Or it doesn't matter???? My 30" disk sander takes 14 minutes to stop. I hit the stop button and walk away. Maybe if it was in a Federal penitentiary, for safety, it might want it to stop like a SS.

OH,... By the way... I did buy it out of the old Kingston Pen and it was in their wood working shop for inmates. No stopping with 14 min spinning... I haven't found any bone, blood or skin yet. I guess I can say, even our less brilliant in society are not idiots. A SS is for the schools and those without a fearfull respect.

[Erik Manchester]

By the way... I did buy it out of the old Kingston Pen and it was in their wood working shop for inmates. No stopping with 14 min spinning... I haven't found any bone, blood or skin yet. I guess I can say, even our less brilliant in society are not idiots. A SS is for the schools and those without a fearfull respect.

LOL, Matt, I think that most inmates of the Kingston Pen were there because they preferred to spill others blood as opposed to their own.

Your JTA would certainly trim the fingernails of the careless in short order.

[Malcolm McLeod]

For those of you who use a VFD on their tablesaw I would be interested in knowing how you turn you saw on/off.
I personally have a rotary phase converter and have a magnetic switch on the saw outfitted with a "paddle" that lets me shut the saw off with my knee or foot. However I understand you can't use a switch in between the VFD and the saw.

On tools like drill presses, lathes etc an emergency shutoff may not be as critical as a tablesaw so am wondering how others do it.

Thanks

Fred

Already lots of good advice on how to execute the Start/Stop function, and I'm also reassured by the uniform recommendation not to insert a switch between the VFD and the motor.

If anyone is interested in 'why' no interposing switch, I can provide a case-in-point: I was called to a site where an industrial user had a production line with multiple stations that each used a single VFD multiplexed to 5 different small motors (1/4 to 2Hp). Each motor would be selected by rotary switch, then the VFD 'jogged' in Fwd or Rev to make adjustments. The selector switch would disconnect one motor (turn off it's starter), and connect another (turn on it's starter). Each starter connected the respective motor to the station's VFD. They had ongoing issues with the VFDs faulting (line would run for 5-6 hrs out of 24).

The problem was traced to the motor starters. The operator was changing the selector switch while the VFD was still in it's accelerating or decelerating phase. The starters would switch immediately. A fault is generated on the VFD when a motor load is suddenly connected to the VFD output bus that is in the middle of it's accel/decel ramp. We installed PLC code to prevent motor starters from switching while the VFD was 'active'. The next day they ran 22 of 24 hours.

The above system is probably well beyond what the average Creeker will use in their shop, but the lesson is still applicable - don't put any switch between motor<>VFD. The above were premium quality VFDs and were 'just' faulting. The imports may very well be damaged by such.

[Malcolm McLeod]

You are never going to get any real safety benefit from resistive or DC injection braking on woodworking machines, it could help if you find yourself in a precarious position AND can hit the stop button. Tablesaws have so little momentum they spin down fairly quickly, nothing like a big bandsaw that might take 3-5 minutes or a big disc sander that can take 10 or more minutes to stop. The vast majority of American and Asian saws in hobby shops and even in small cabinet shops don't have any arbor locking mechanisms.

If you want added safety in a table saw buy a Sawstop or a slider you really aren't going to add much safety with any form of braking except Sawstop (or Bosch) type braking. VFD braking as it applies to woodworking is mainly the convenience factor as long as you use the rule of waiting until the tooling stops to move your cut pieces etc.

BTW I don't want any machines with high breaking rates... sorry couldn't resist.

You are dead on that VFD's certainly aren't safety devices, but the braking function can be ....reassuring? Maybe?

If you do choose to use the braking function, make sure you follow the instructions for external 'braking resistors'. When a motor is spooling down it becomes a generator. In the typical On/Off switch-operated TS or BS, the generated voltage has no where to go, so no current flow, so no braking (simplistically). If a VFD is connected and configured for braking, it can apply a resistance to dissipate this power (as heat).

VFDs have small internal resistors that can stop relatively light loads (different VFD sizes have different internal resistors, so can stop different loads). Braking capacity is dependent on both time and load. You can brake a (relatively) big load over a long time, or a light load in a short time. If you try to stop a big load in a short time, the internal resistors can be overheated and damaged.
I've not tested this, but as an example you could probably stop a TS in 2-3 seconds with VFD braking (internal resistors). I would think a similar Hp 14" BS, should be set for no less than 12-15 seconds on a similar VFD.

Most VFDs offer external braking resistor packages. I'd think they are well worth the investment if you need or want a VFD to stop a 24" BS (or a disk sander with a 14 minute over run!!).

[Dan Friedrichs]

If you try to stop a big load in a short time, the internal resistors can be overheated and damaged.

I'm not certain this applies to EVERY VFD, but I would suspect that the vast majority of VFDs have protection to avoid damaging themselves - either an over-voltage limit or an over-temp limit. In the event that you set the decel time too fast, it will try to generate more power than the internal resistance can handle, and simply trigger an over-voltage fault and stop trying to brake the motor (instead, it will just allow it to coast down).

Most VFDs offer external braking resistor packages. I'd think they are well worth the investment if you need or want a VFD to stop a 24" BS (or a disk sander with a 14 minute over run!!).

The ones sold by the VFD manufacturers are way overpriced, though. These are simply power resistors that you can buy very cheaply - just size the value of resistance and wattage per the manual:
http://www.mouser.com/Search/Product...-HS100100R5%25

[Matt Mattingley]

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.

IMG_5904.JPG

[Dan Friedrichs]

Good points, Matt. And that's a sexy resistor ("phrases I thought I'd never say for $100, please, Alex...")

[Tom Trees]

The above system is probably well beyond what the average Creeker will use in their shop, but the lesson is still applicable - don't put any switch between motor<>VFD. The above were premium quality VFDs and were 'just' faulting. The imports may very well be damaged by such.

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 nearby panel with two rotary switches, and only twist these rotary switches , 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