Emergency stop for agitator

kallileo · Oct 18, 2016

I have a small agitation motor controlled by an Atv12 vfd that gets a start command on one of it's DI from a programmable controller which also controls heating and cooling processes and I need to install an e-stop for the agitator. Normally it's not needed but in this particular case the agitator is equipped with a planetary gearbox and some sharp blades to cut curd.
I see 3 option:
1. Remove the power from the whole control circuit to turn everything off when the e-stop is pressed
2. Remove the power only from all DO
3. Remove the power only from the DO that controls the agitation motor

What would best the best approach?

OkiePC · Oct 18, 2016

My preference (and many will disagree) is to interrupt power between the VFD and the motor. Use a fourth contact on the safety contactor to break the enable signal to the VFD.

This fourth contact ideally would be early break, late make. The idea is to break the enable to the VFD before or simultaneously with breaking the power circuit between it and the motor. Then the VFD cannot be re-enabled until that safety circuit is satisfied closing the contactor.

Some VFD manuals will warn the installer not to break the circuit between a running VFD and its load. I have done it anyway. I'd rather violate the recommendations of a book than pick up severed appendages off the production floor. It is even more important not to close the load circuit while the VFD is already in run mode. Using a fourth contact makes it highly unlikely that the drive can go into run mode until the other three poles are closed.

Some folks like to kill power feeding into the VFD. This is even worse for wear for the VFD in my experience. I have not seen a drive fail because I used a contactor between the drive and motor using this method. I have seen many of them go up in smoke when being powered up. Many of those situations were caused by short cycling of the safety circuit which supplied power input to the drives. (Operator abusing the e-stop button for non-emergencies).

Breaking the digital input to a standard VFD is not safety. What if that VFD digital input decides to quit in the "true" state and the drive "thinks" the run command is still active? Unlikely? Yes, but that exact scenario does happen. I have seen it twice in 20 or so years of working with automation.

You need to break power to the motor when the safety circuit is not satisfied. How you go about doing that can be debated.

The overall approach to any safety design should begin with a safety analysis. Physical guarding that prevents a human from getting into the danger zone is always preferred to electrical circuits that attempt to save her or him. Even redundant and highly reliable designs are not as good as a big guard that must be unbolted and moved to gain access. I know this is not always possible or feasible.

I would never trust a digital input on a powered VFD to keep it from running. I would much more trust (but not 100%) a contactor to open and break to motor power circuit.

ASF · Oct 18, 2016

Your best approach is to remove all three phases through a proper safety relay and a safety contactor (or a pair of safety contactors, if your application requires Cat 3/4). You might also investigate using a drive with Safe Torque Off (or Safe Stop) instead - removing power from the drive too often is not really good for them, and will save you messing about with safety contactors.

(edit) Totally agree with OkiePC above. Excellent breakdown of the concept

kolyur · Oct 19, 2016

OkiePC said:
My preference (and many will disagree) is to interrupt power between the VFD and the motor. Use a fourth contact on the safety contactor to break the enable signal to the VFD.

This fourth contact ideally would be early break, late make. The idea is to break the enable to the VFD before or simultaneously with breaking the power circuit between it and the motor. Then the VFD cannot be re-enabled until that safety circuit is satisfied closing the contactor.

Some VFD manuals will warn the installer not to break the circuit between a running VFD and its load. I have done it anyway. I'd rather violate the recommendations of a book than pick up severed appendages off the production floor. It is even more important not to close the load circuit while the VFD is already in run mode. Using a fourth contact makes it highly unlikely that the drive can go into run mode until the other three poles are closed.

Some folks like to kill power feeding into the VFD. This is even worse for wear for the VFD in my experience. I have not seen a drive fail because I used a contactor between the drive and motor using this method. I have seen many of them go up in smoke when being powered up. Many of those situations were caused by short cycling of the safety circuit which supplied power input to the drives. (Operator abusing the e-stop button for non-emergencies).

Breaking the digital input to a standard VFD is not safety. What if that VFD digital input decides to quit in the "true" state and the drive "thinks" the run command is still active? Unlikely? Yes, but that exact scenario does happen. I have seen it twice in 20 or so years of working with automation.

You need to break power to the motor when the safety circuit is not satisfied. How you go about doing that can be debated.

The overall approach to any safety design should begin with a safety analysis. Physical guarding that prevents a human from getting into the danger zone is always preferred to electrical circuits that attempt to save her or him. Even redundant and highly reliable designs are not as good as a big guard that must be unbolted and moved to gain access. I know this is not always possible or feasible.

I would never trust a digital input on a powered VFD to keep it from running. I would much more trust (but not 100%) a contactor to open and break to motor power circuit.

+1

This has become my M.O. in virtually all VFD applications. I like to use a safety relay or controller with an adjustable off delay output to drive the contactor between the VFD and motor. This way, when the motor enable signal drops immediately at E-stop, the motor is allowed to ramp down before the contactor is opened (otherwise it would coast to a stop). It may be overkill in some cases but I sleep better at night and I suspect that Okie does also.

Gene Bond · Oct 19, 2016

Safe-Stop is a tricky situation. All of the drives with Safe-Decel or similar functions require closed loop (encoder FB) ($$$).

If an application requires a motor to stop fast, better use a mechanical brake or closed loop.

If an application requires a drive to be disabled, better use STO.

Just my opinion, and I WON'T be a safety expert for any amount of money

ianingram · Oct 19, 2016

Thinking outside the square a little bit..

What type of guarding is on the agitator?
Is it possible to fit a guard locking switch that will only allow access to moving parts once it is proved that the machine has come to rest?

janner_10 · Oct 20, 2016

I would get somebody in that is experienced with risk assessments and Safety Circuits.

That is your first port of call to establish what level you need to ensure your safety function meets what you are asking of it.

If the blade is big enough or fast enough to cause irreversible damage to someone, you need to be whiter than white.

'I posted on PLCtalk.net' won't be a defence if you are in the dock because some guy is minus a hand.

twocozz · Oct 20, 2016

My preference (and many will disagree) is to interrupt power between the VFD and the motor. Use a fourth contact on the safety contactor to break the enable signal to the VFD.

This fourth contact ideally would be early break, late make. The idea is to break the enable to the VFD before or simultaneously with breaking the power circuit between it and the motor. Then the VFD cannot be re-enabled until that safety circuit is satisfied closing the contactor.

Some VFD manuals will warn the installer not to break the circuit between a running VFD and its load. I have done it anyway. I'd rather violate the recommendations of a book than pick up severed appendages off the production floor. It is even more important not to close the load circuit while the VFD is already in run mode. Using a fourth contact makes it highly unlikely that the drive can go into run mode until the other three poles are closed.

Some folks like to kill power feeding into the VFD. This is even worse for wear for the VFD in my experience. I have not seen a drive fail because I used a contactor between the drive and motor using this method. I have seen many of them go up in smoke when being powered up. Many of those situations were caused by short cycling of the safety circuit which supplied power input to the drives. (Operator abusing the e-stop button for non-emergencies).

Breaking the digital input to a standard VFD is not safety. What if that VFD digital input decides to quit in the "true" state and the drive "thinks" the run command is still active? Unlikely? Yes, but that exact scenario does happen. I have seen it twice in 20 or so years of working with automation.

You need to break power to the motor when the safety circuit is not satisfied. How you go about doing that can be debated.

The overall approach to any safety design should begin with a safety analysis. Physical guarding that prevents a human from getting into the danger zone is always preferred to electrical circuits that attempt to save her or him. Even redundant and highly reliable designs are not as good as a big guard that must be unbolted and moved to gain access. I know this is not always possible or feasible.

I would never trust a digital input on a powered VFD to keep it from running. I would much more trust (but not 100%) a contactor to open and break to motor power circuit.

This is how we have done it for almost 20 years. We break the connection between the VFD and the motor using two separate contactors. Both contactors have feedback to the safety relay that monitors their correct operation. We have used several different VFD from Siemens, Mitsubishi and Omron over the years and never really had a problem doing this (despite this layout not always being recommended). As a guide the motors are usually interrupted about a dozen times an hour.

JesperMP · Oct 20, 2016

Kallileo, your own suggestions show that you do not have enough knowledge about machine safety.

janner_10 said:
I would get somebody in that is experienced with risk assessments and Safety Circuits.

This is the proper recommendation.

The other posters advices are correct too, but do not attempt to try and design the safety on your machine by these forum advice only.

In the end, the machine is not allowed to put into operation without an EC declaration of conformity. This is a document signed by someone responsible (the CEO or the owner of the company) that states that the machine is safe to operate and that it follows applicable standards (which must be mentioned in the document).

robertmee · Oct 20, 2016

Pretty much do the same as okie and others although lately some customers prefer just using the safe off feature of powerflex drives. If it were my hands at risk I still prefer killing the 480V...don't want to rely on some drives firmware taking a ****. The time off red safety relays are suited for this app....the immediate contact breaks the drive enable for a quick ramp stop and the timed relay opens the 480....that assumes you want a controlled stop like on a Web line. In the case of an agitator you might not want a torque ramped stop but a coast free wheeling stop...in that case kill the 480 period. You really need to do a safety audit and make sure all are in agreement.

John Morris · Oct 20, 2016

Planetary gearboxes don't backlash very well and with a load of curd the mixer probably will not move one more inch without power, even at full speed. The same can be said for an empty mixer. Mechanically it will play hell on the ring gear.

+1 to OKIE

OkiePC said:
My preference (and many will disagree) is to interrupt power between the VFD and the motor. Use a fourth contact on the safety contactor to break the enable signal to the VFD.

But be prepared to rebuild your gearbox after a dozen or so e-stops. (unless it's about fifty years old and made in Germany, in which case a stick of dynamite would probably make it run better)

Best bet is to just put a cage around it.

kallileo · Oct 22, 2016

In reality the danger of injury is low because the agitation speed is low (10-30rpm) and the vertical tank is covered by a stainless steel lid.
I will go with solution of using a contactor between the vfd and motor but I have seen similar devices from other manufacturers and all them where cutting power to the vfd when the e-stop was pressed and it shouldn't be pressed just for fun.

I will try to have deeper look in to it and maybe find some some European directive or standard regarding pasteurisers with curd cutters and safety.

Emergency stop for agitator

kallileo

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OkiePC

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ASF

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kolyur

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Gene Bond

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ianingram

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janner_10

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twocozz

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JesperMP

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robertmee

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John Morris

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kallileo

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