VFD Drive, Acceptable E-Stop options

[iqak]

Multiple AB PowerFlex 40 Drives, in a Packaging Line Control Enclosure (bottles, cans, cases, pallets).

E-Stop Connections
Historically, we have cut power to the line side of all drives during Emergency Stop Conditions using a Large Contactor.

Is it acceptable to code or safe to De-Enable the drive rather than cut power. By De-Enable I'm referring to the drive enable terminals with the drive.

Any Ideas?

 

[sawdust454]

Yes, if the drive has a dedeicated safety input listed for that purpose.

 

[petersonra]

Yes, if the drive has a dedeicated safety input listed for that purpose.

Just out of curiousity, why would it need to be a dedicated safety input?

Wouldn't the level of control required be determined by the risk assessment?

 

[iqak]

I would like to avoid contactors, but I'm not completely convinced it is safe or acceptable to do so.

The drive in question:

AB Powerflex-40, 340-528VAC, 3-P, 2HP/5.7A, Fixed HMI
Catlog number 22BD4P0N104

Our revised safety stop connection would be between terminals 1 and 4 (the regular stop terminal). This is contrary to the "Enable" I initially described.

What concerns me is this "Attention Note" from the PDF Manual as follows:

ATTENTION: The drive start/stop control circuitry includes
solid-state components. If hazards due to accidental contact with
moving machinery or unintentional flow of liquid, gas or solids exist,
an additional hardwired stop circuit may be required to remove the AC
line to the drive. When the AC line is removed, there will be a loss of
any inherent regenerative braking effect that might be present - the
motor will coast to a stop. An auxiliary braking method may be
required.


Our conveyance systems are tabletop conveyors, belt and chain driven roller
conveyors, chain driven pallet conveyors. All the conveyors are guarded, however, It does remain possible for new personnel to injure themselves if they stick their hands where they should not.


Any further suggestions / comments would be greatly appreciated!

 

[pfalcon]

I always suggest industrial sites adopt NFPA79.

9.2.2(1) Category 0 is an uncontrolled stop by immediately removing power to the machine actuators.
9.2.2(2) Category 1 is a controlled stop with power to the machine actuators available to achieve the stop then remove power when the stop is achieved.
9.2.2(3) Category 2 is a controlled stop with power left available to the machine actuators.

9.2.5.4.1.3 The emergency stop shall function as either a Category 0 or a Category 1 stop (see 9.2.2). The choice of the category of the emergency stop shall be determined by the risk assessment of the machine.

 

[mxslick]

I would like to avoid contactors, but I'm not completely convinced it is safe or acceptable to do so.


ATTENTION: The drive start/stop control circuitry includes
solid-state components. If hazards due to accidental contact with
moving machinery or unintentional flow of liquid, gas or solids exist,
an additional hardwired stop circuit may be required to remove the AC
line to the drive. When the AC line is removed, there will be a loss of
any inherent regenerative braking effect that might be present - the
motor will coast to a stop. An auxiliary braking method may be
required.


Our conveyance systems are tabletop conveyors, belt and chain driven roller
conveyors, chain driven pallet conveyors. All the conveyors are guarded, however, It does remain possible for new personnel to injure themselves if they stick their hands where they should not.


Any further suggestions / comments would be greatly appreciated!

Your answer is in the manual section you posted. You will need to provide a contactor to kill power to the drive. Chances are that your AHJ or OSHA regs may require it as well.

I installed some Danfoss drives on a set of film projectors that can be operated remotely, and I used their "QuickStop" input tied to E-Stop buttons at each projector. The E-Stops will stop the projector but it coasts to a stop. With the E-Stop engaged it will also inhibit the projector from starting..but...I did experience a glitch of some kind (never found out exactly what it was) which caused one of the machines to bump start with the E-stop engaged.

So as that note in your manual advises, in a critical application where an E-stop is required, it is best to cut power to the drive.

 

[StephenSDH]

With that drive the enable input is not a safty input, so you shouldn't use it to disable the drive for safty. The best you can do is kill power with a contactor.

The next drive up PF40P can be purchased with a safe off board. This saftey input will disable the drive internally in hardwire. It also provided relay welding feedback to prevent the safety circuit from resetting if the relay sticks.

Steve

 

[pfalcon]

PowerFlex 40 will need outside control.

Checkout these parameters (and their associated wiring)
P036=1 P037=9

You also need a safety device like the MSR57P or guarding

 

[richwaskowitz]

This is interesting. Our customers in the automotive industry and even the Ministry of "Labour" in our neighboring Canada have taken the stand that E-stops are not devices intended to provide protection of personnel like light screens, gate switches, laser scanners and other types of perimeter protection devices are since they only provide protection IF they are used. As such, they do not require safety relays in their circuits and they can simply drop master power like a conventional master stop circuit. Gates with interlocks, light screens and the such do not require someone to take any action. E-stops do not protect anybody from anything that a panel lock-out can't do much better. If someone is counting on stopping a machine with an e-stop and then intentionally putting themselves in harm's way without putting proper protective measures into place, that is a mistake. If someone is going to argue that hitting an e-stop AFTER someone has already gotten in the way and is being hurt makes it a safety device, that is like closing the barn door after the horse got out. That having been said, a standard master stop or an e-stop which drops control power to a drive (by interrupting the enable or run command) could very well be sufficient and no further measures would be required to open contactors or utilize "safe off" features on drives.

 

[Rockyd]

A call to the Manufacturer might be worth your effort. They live and breathe their brand, might as well capitalize on whatever they can give you.

 

[TxEngr]

An interesting discussion. In my industry, an EStop is a ?balls-to-the-wall? full regen/I don?t care if I blow up the drive or not ? just stop it as quickly as I can event. The assumption is that someone is caught in the equipment and you want it to stop moving. A Coast Stop is used to simply remove power from the equipment and just let it coast down ? an event that can take many minutes due to the inertia of the equipment. Removal of the power from the drive will let the load coast down so you might want to investigate what you really want to have happen in the event of an EStop ? Stop the Drive NOW or let it coast down.

 

[Besoeker]

I suppose the practice and implementation varies between industries and countries.
Here, in the UK, it is common practice to have a hard-wired emergency stop push button adjacent to the motor to drop the main contactor. It's safety that's of primary concern and if interrupting the output from the drive causes it to fail that's of secondary importance. That said, we put an early break auxiliary contact of the contactor in the drive inhibit circuit.

On large DC drives with a high inertia load, we have dynamic braking resistors that are connected across the motor armature on an emergency stop. As it happens, we had one fail (open circuit) just before Christmas and it's a specially made resistor and added to that, the manufacturer no longer exists. Six weeks delivery on a new unit. The customer decided to keep the machine in operation - production is king.

Our guy on site called me and asked what I thought about that. A bit of a morbid or macabre conversation followed. The drive would run just fine but stopping time would be much longer up from about 30 seconds to maybe five minutes. If someone was caught in the machinery for 30 seconds the prospects of survival would be minimal to zero. Would the longer stopping time pose a greater risk?
I'm glad it wasn't my decision to make but I still have concerns.

 

[dbuckley]

My take on E-stops is this: If the failure of an e-stop system could reasonably be assumed to cause or make worse immediate risk of injury or death than it needs to be an e-stop arrangement that can be relied upon, using either old school drop out contactors, or safety rated PLC and drive technology.

The other category is similar to EPO type systems, typically used in data centres, where you would (truth be told) actually would prefer the EPO sytem to fail altogether, and just keep the power running, as there is generally no safety of life issue. For these systems then something much less fail safe is required, typically shunt breakers.

It's not rocket science to figure out which category of arrangement one is dealing with, visual inspection is all it takes; imagine a realistic bad scenario, and that it was you that was about to experience it....

 

[petersonra]

I think you need to look at the overall system.

there is no code that says it is Ok to rely on an E-stop to shut down a machine in case someone is caught in it. You are supposed to eliminate the hazard of being caught in the machine.

The e-stop is there in case something happens that you did not anticipate. The vast majority of times it is not about protecting humans but about protecting the machine.

Many machines it is "safer" to have a delay off feature on your estop so that the drive can go into maximum braking mode before the power is dropped (a category 1 stop). Otherwise the machine will coast to a stop and take substantially longer to shut down.

I have seen machines that can take 5 minutes or more to stop motion if they coast to a stop. The gates are interlocked with motion switches that keep the gates locked until all motion stops. Even under maximum braking it can be 20 or 30 seconds before the thing stops.

 

[pfalcon]

<snip> that E-stops are not devices intended to provide protection of personnel <snip> since they only provide protection IF they are used. <snip> E-stops do not protect anybody from anything that a panel lock-out can't do much better. If someone is counting on stopping a machine with an e-stop and then intentionally putting themselves in harm's way <snip> If someone is going to argue that hitting an e-stop AFTER someone has already gotten in the way and is being hurt makes it a safety device, that is like closing the barn door after the horse got out. <snip>

E-stops are safety devices.

Case in point: Arm caught in conveyor.
Many conveyors will continue to drag the arm into the conveyor until motion is stopped. The limb does not create enough drag to trip the protective circuits. E-stop cords are provided along the conveyor. They are the only thing the trapped operator can reach and nothing will trip to save them. As a child my arm was caught in compression rollers. It was the E-stop I slapped that released my arm. As it was, the rollers bruised the muscles in my forearm. Had I been slower the rollers would have reached my armpit and started tearing tissues. Just because an accident has started doesn't mean it's finished.

Case in point: Improperly clamped part.
Most machinery does not include a tilt meter like many washing machines include. An improperly clamped part can spin eccentrically until it gets thrown. Attentive operators often notice the problem at cycle start and hit the E-stop. This prevents a thrown part that the machine cannot otherwise detect until after launch.

Case in point: Maintenance
Automated vehicles scan areas in front of them where obstructions are expected. Maintenance people are often in strange positions like pits in the floor or on booms that are not in the scanned area. I've seen two maintenance people hit by such vehicles. One was bumped, the other was trapped by the vehicle. No injuries. I've also seen at least twelve potential incidents stopped by alert people engaging the E-stops.

Case in point: Unknown issues.
Exactly how sure are you that you have discovered all the failure modes of your machine? It's amazing how many times I've often seen operators save people or equipment by hitting the E-stop before damage occurred. Usually the next question asked is "Who would have thought that could be a problem?". And just as often "Who'd have thought someone would be dumb enough to do that?".

Yes, I vote that E-stops are safety devices. NFPA79 says they are safety devices. And I want them as reliable as any other circuit on the machine.