Fast E-Stop for a Powerflex drive

I have a client who has a circle saw directly attached to a 3KW, 3x380 V, 50Hz, 3000RPM motor.
The application is a ply cutter where operators work with their hands very close to the saw. The safety measure is a life line that acts over the control circuit and disconnects the power to the contactor therefore the motor is disconnected, pretty much like an E-stop. When power is disconnected, by inertia the saw keeps running for about a minute and a half until completely stops. Obviously this is not very safe so my client wants to install a drive that can stop the saw in a shorter time (no more than 3 seconds).
I´m thinking about a Powerflex with driveguard and a braking resistor, my question is how can I calculate the braking resistor and what drive should I use for this application? Maybe there is an application where this already was implemented?
I hope you can help me.
Rodrigo

I do not know the setup on the saw BUT
in general the blade must be guarded. That is best practice and is required with very few exceptions.

Automatic work clamping works wonders to keep operators fingers away from the blade and attached to their hand as they are supposed to be.

I see no reason for people to get within six inches of a saw blade on well designed equipment - especially so on production lines. Job shop with continuously varying setup is another situation I know.

After that in normal shutdown I would brake with dynamic braking and braking resistor.

In E stop situation I would use DC injection braking. I would limit this to emergency only and not a normal shutdown

There is a setup on the market to immediately stop the blade upon contact with meat. A hot dog is frequently used in the demonstration. I have never seen one so cannot say much more about it. May be worthy of investigation.
Dan Bentler

http://www.sawstop.com/?gclid=CPX2_fCcpLACFQlN4AodxBuCXA

Not sure I would want to try this for real???

You will have to know what the inertia of the blade/rotor is to get an idea as the amount of energy you have when the saw is running. And you will have to select a VFD drive of a sufficient horsepower that can stop that load within the time your client has specified.

That said, you will have to come up with the inertial load based on the customer's setup. Up-size your drive as necessary (probably at least 10hp) to give it a chance to control the load while braking.

I have had problems in the past with drives that wouldn't stop the load (but should of been able to) when they were told to stop. The drive would get an overvoltage fault on the buss and fault out.

Fooling with DC injection solved the problem in that case, but it really wasn't the ideal solution.

You can't stop a motor as fast as you would like with a drive. You need a motor with a mechanical brake that is released when it is energized.

gmferg said:

You can't stop a motor as fast as you would like with a drive. You need a motor with a mechanical brake that is released when it is energized.

Click to expand...

If stopping is all that's required without special regard to how or where, then I think this is probably the safest option (and may require less control changes). I would use this in combination with the DriveGuard solution so you have redundant safety measures though, given this blade sounds pretty dangerous.

gmferg said:

You can't stop a motor as fast as you would like with a drive. You need a motor with a mechanical brake that is released when it is energized.

Click to expand...

Well, assuming that the existing motor can be fitted with one, then that would be the way to go. I had heard of the other system (the hot dog one), which would be ideal. But if the existing motor can't be fitted with a brake, what then?

rkukl said:

http://www.sawstop.com/?gclid=CPX2_fCcpLACFQlN4AodxBuCXA

Not sure I would want to try this for real???

Click to expand...

That is the one. Would not bother me to put hot dogs in it all day - provided someone else was paying for them.

Pretty amazing - do not know how they do it that fast. Supposedly it is a one time deal - if you activate it it has to go back to factory.

Dan Bentler

leitmotif said:

Pretty amazing - do not know how they do it that fast. Supposedly it is a one time deal - if you activate it it has to go back to factory.

Click to expand...

You only have to replace the brake assembly for ~$60 (plus whatever your newly destroyed blade cost you). Here's a more detailed video with slow-motion:

http://www.youtube.com/watch?v=OMD3agP5hv0



-Eric

Eric Nelson said:

You only have to replace the brake assembly for ~$60 (plus whatever your newly destroyed blade cost you). Here's a more detailed video with slow-motion:

http://www.youtube.com/watch?v=OMD3agP5hv0
-Eric

Click to expand...

Thank you Eric - that is the best demo of this I have seen yet. Most amazing and cheap at twice the price. I still wont stick my finger in a running saw blade no matter what bell n whistle is on there. My luck someone took out the clapper to clean it or whatever.

Dan Bentler

RodV said:

I have a client who has a circle saw directly attached to a 3KW, 3x380 V, 50Hz, 3000RPM motor.
The application is a ply cutter where operators work with their hands very close to the saw. The safety measure is a life line that acts over the control circuit and disconnects the power to the contactor therefore the motor is disconnected, pretty much like an E-stop. When power is disconnected, by inertia the saw keeps running for about a minute and a half until completely stops. Obviously this is not very safe so my client wants to install a drive that can stop the saw in a shorter time (no more than 3 seconds).
I´m thinking about a Powerflex with driveguard and a braking resistor, my question is how can I calculate the braking resistor and what drive should I use for this application? Maybe there is an application where this already was implemented?
I hope you can help me.
Rodrigo

Click to expand...

My advice would be to stay away from this one due to liability issues. If you must get involved then make sure you do so following all safety rules & regulations and fully document everything you do. (Make certain you charge appropriately)

The only way I would touch this one is to use a mechanical brake on the motor and connect it, a pair of redundant motor contactors and redundant pull cord switches to a Category 3 or 4 safety monitoring relay or control.

DC injection braking is not failsafe. If the injection fails and someone gets hurt you will get sued and will probably lose or be advised to settle out of court.

I touch safety systems only if the customer will allow me to do it right, otherwise I try to dodge them. Quote it doing things the right way.

How about somehting like this right on the saw blade. Granted, it will probably warp the blade. But if someone is hitting an emergency stop a warped blade is the least of your worries. The reason I like these is you can develop much more stopping power than a motor mounted brake will allow.

On a different note, I don't think a cable operated e-stop actuator is the best option for an e-stop actuator in this case. By the time a user gets around to pulling a cable switch they may not have the fingers left to pull the cable. E-stop actuators are by nature reactive devices. This is a case where you want to be proactive.

Keith

Eric Nelson said:

You only have to replace the brake assembly for ~$60 (plus whatever your newly destroyed blade cost you). Here's a more detailed video with slow-motion:

http://www.youtube.com/watch?v=OMD3agP5hv0



-Eric

Click to expand...

Great video! Did you see the post where it saved one guys finger(s)?

$60 is nothing compared to a lifetime without fingers.

I have a problem when I use RSLogix5000 to read value from PowerFlex 700s by datalink.

The returned value is not same with the value on PowerFlex 700s.

Any ideas to solve it?
But I can't understand this returned value. For example when I read "Speed ref 1" = 17, the returned value is 67633152, sometime the returned value is the internal value of PowerFlex 700s, I can't understand this value too.
Can you explain this value? And how to get true value?

Thank you so much!

We use thes redundant brakes http://www.mayr.com/en/products/safety-brakes/roba-duplostop/

But as mentioned in a previous post you need to see if clamping would be possible to keep operators hands away and maybe use a light curtain to detect if hands do get close.

DC injection braking is the fastest stopping method for a drive but it is not failsafe so do not rely on this alone.

You may want to consider the safe off system for many popular drive like rockwell powerflex and not break line or load power with a contactor as this can damage a drive if not done right and I say this because even though you train them better many operators still use the estop to just do a standard stop of the machine.