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Old August 29th, 2005, 06:19 PM   #1
kolyur
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Exclamation Safey issue with motors & VFDs

We are building a new piece of equipment and a significant safety issue has come to light. I'm hoping I can get some suggestions here.

We are using a number of rotary trimmers and wire brushes which are individually belt-driven by AC motors (VFD-controlled). All motors are fractional HP. Drives are Delta M-series. There are a lot of exposed belts in the machine, which was not seen as an issue initially since the entire system is guarded.

The problem is that some of the brushes are fairly heavy and take awhile to stop (up to 10 seconds). It became apparent that if an operator was in a hurry and yanked open a guard door and jumped inside, he/she could easily be injured by the equipment even though all power was off at that point.

Part of the issue is the way I have the VFDs wired. The 3-phase lines going from the VFD to the motor pass through a contactor, which is controlled by the safety circuit. So, if a guard is opened or an e-stop activated, this provides a mechanical interlock. For safety's sake, I really wasn't comfortable with just dropping the enable line on the VFD. The problem with this is that when the contactor drops out, any kind of slow-down ramp in the drive won't work; the motor just coasts to a stop.

Does anyone have any suggestions? I am concerned more about the exposed belts than anything else. Unfortunately it would be difficult to guard them individually at this point. I considered adding a braking resistor to the VFD but that would have no effect because of the contactor. I also thought of using brake motors but we don't have room for the additional length. I would like to keep the contactors between the VFDs and motors for safety, but I'm open to other wiring ideas if they still provide a safe system.

Thanks for any help!

-John

Last edited by kolyur; August 29th, 2005 at 06:22 PM.
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Old August 29th, 2005, 06:32 PM   #2
Eric Nelson
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You could use use solenoid-operated safety switches on the guards that will only unlock when it's safe to enter. Couple of choices here.

1.) Time-delayed unlocking using a preset time for the brushes to stop.
2.) Zero-motion detectors to ensure all brushes are stopped.
3.) A guard switch that requires time to unlock. Operator must unscrew the locking pin, which takes as long or longer than the brushes take to stop.

I'd give you direct links to itmes like this at STI, but they require registration now. Their loss...



-Eric
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Old August 29th, 2005, 06:40 PM   #3
Eric Nelson
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So you don't get lost at STI. Here are a few part numbers:

Time delayed unlocking: SMT01
Zero-speed unlocking: SMD02/SMD03
Slow unlocking switch: BL6009

These are on the Safety Interlock Switches page.



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Old August 29th, 2005, 06:55 PM   #4
jstolaruk
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Funny you mentioned this, I just shipped a wire brush deburr machine a couple of weeks ago and we had the same issue. One option we thought about was a solenoid operated safety gate switch as Eric described but the customer didn't want to have a "request to enter" procedure. Our process involved reversing the brush so that the opposite side of the part could be deburred so we used motors with brakes; a signal to the brake solenoid releases the brake on the motors. So, if the operator pulled open the door, the power to the motor and the signal to the brake disable dropped out via the safety relay and the brush stopped instantly.

Some of the VFDs (we used AB Powerflex) documented that the output to the motor be not run through contactors and that the drive enable signal run through a safety relay will be adequate for e-stop. We used the dynamic braking option too.

Hope that helps.
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Old August 29th, 2005, 07:37 PM   #5
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I'd get rid of the contact block between the VFD and the motor and use the ramp down or dynamic braking function on the VFD. We have mainly AB powerflex and 160C all of which use a relay on the enable line.

Is there any room for a mechanical brake on the motor?
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Old August 29th, 2005, 08:40 PM   #6
jimtech67
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I use SSD (Eurotherm) drives 690+ a lot they have a fast stop parameter. This does exactly what it says. When an input goes low the VFD stops fast ( stop time is a setting). Then after 3 seconds we open up a motor contactor.



GO JETS
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Old August 29th, 2005, 09:26 PM   #7
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You can probably combine both for added safety. "Request to Enter" button opens the drive enable circuits and starts the safety timer. When timer expires, unlock guards. Guard circuit opens the contactors.

Oddly enough, we also built a wire brush machine a few years ago to polish industrial door hinges. Four 1HP motors with the brushes mounted directly on the motor shafts. I used the drives to quickly stop the motors when a guard was opened, but the individual dust collection hoods encompassed nearly the entire brush, so there wasn't much chance of 'accidental' contact anyway.





-Eric
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Old August 29th, 2005, 09:52 PM   #8
Allan Barnes
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For printing presses we had all the guards on a safty relay, then off the safety relay there was a safety timer. The Main saftey relay kills the Run and jog signals. This puts the drive into regen/breaking, then after a set time the timer dropps out killing the enable. Part of the safety codes is disipating the mechanical energy, the safety timer alows the engery to "Drained" out of the system. If your equipment can not be stopped before the operator can touch it then you have to put the locking soliniods on, or move the gaurd acces out further so the operator takes longer to get to the danger.
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Old August 29th, 2005, 11:47 PM   #9
leitmotif
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HOLD THE PHONE HERE
Regarding your statement:
The problem is that some of the brushes are fairly heavy and take awhile to stop (up to 10 seconds). It became apparent that if an operator was in a hurry and yanked open a guard door and jumped inside, he/she could easily be injured by the equipment even though all power was off at that point.

What are operators doing "yanking open door and Jumping inside ?"
AFTER 20 years in safety I KNOW employees do goofy things and sometimes get hurt or killed.

I appreciate you can do training and lockout tagout have policies procedures etc etc etc

I appreciate you can make it fool proof
BUT some fool will prove you wrong.

To the basics
1. You said these are fractional HP units. Even if they get in there will they really get mangled? I think in 95% of cases you have to say yes.
2. DO operators really have a justifiable need to open the guard?
if NO then bolt the dang thing in place. If you have to use them tamper proof screws
IF YES then I guess you maybe should do one of two things
A train them to let the machine stop
B investigate interlocks brakes etc etc to protect fools from themselves.

Sorry but that is the way it is in industry. I have investigated some bloody accidents and more fatals than I ever want to see again. In most cases if the injured just had stopped to think,,,,,,,,,,
Not that I am smarter -- just lucky - there but for the grace of God have gone I.
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Old August 30th, 2005, 12:08 AM   #10
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Quote:
Originally Posted by leitmotif
I appreciate you can make it fool proof
BUT some fool will prove you wrong.
Unfortunately, a human by definition is smarter than any machine can ever be.

"Foolproof" does not exist, only "reasonably safe" does...
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Old August 30th, 2005, 01:11 AM   #11
Eric Nelson
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Quote:
Originally Posted by leitmotif
In most cases if the injured just had stopped to think,,,,,,,,,,
I posted a link to this picture over at Ron's a week or so ago. Dan and LadderLogic's statements remind me that it should be seen here as well...





-Eric
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Old August 30th, 2005, 01:32 AM   #12
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We have many machine with a similar problem. We also have a few different solutions.
1, use a captured key isolator. You isolate the three phase power, this releases a key that can then be used to unlock the gate.
2, Safety circuit with delay timer. Your safety circuit immediately drops the enable to the VFDs, causing them to ramp down with a braking resistor. An interposing timer then opens the contactor. In this case use only safety rated timing relays such as the Pilz PZE X4 VP.
3, Use VFDs with a built in safety function. At the moment, I think only the SEW Movidrive B has this function, although people on this board may know of other safety rated drives.

Hope this helps,

Doug
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Old August 30th, 2005, 02:18 AM   #13
Wino
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Quote:
1. You said these are fractional HP units. Even if they get in there will they really get mangled? I think in 95% of cases you have to say yes.
Humans are fractional horsepower devices, too.
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Old August 30th, 2005, 02:29 AM   #14
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The advice I have from my drives manufacturer is "DO NOT USE A CONTACTOR IN THE OUTPUT LINES FROM THE DRIVE!! IT IS NOT GOOD FOR THE DRIVE!!!"
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Old August 30th, 2005, 03:15 AM   #15
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We use a "captured key isolator" like Doug refers to. There is a version with a mechanical time delay (no electrics inside, so you can access the machine when the plant is down). If you want, I can get the make and type. But I think it is not cheap.

To use brakes on the motors is a simple and as far as I understand an approved way to achive fast and safe stop. The brakes must activate when power is removed.

To use VFDs to make an E-STOP is approved for certain makes that are specifically approved so. I know Siemens and AB have special types and special options to achieve this, others maybe too.
But this is not the same as using the VFDs to achive standstill by braking. I think Siemens has this possibility, and I know that AB is working on it. It will probably be more expensive than just adding mechanical brakes to the motors.
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