Conveyor System E-Stops

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Thread Starter

Paul T.

Question for those of you who have dealt with material handling and conveyor systems using VFDs:

Do conveyor systems typically use contactors (either before or after the VFD) to allow isolating the motors and meeting NFPA 79 requirements for Category 0 or Category 1 stops? We have a newly installed conveyor system in our plant with 120 VFDs. Most of the VFDs interface to the E-Stop circuit only through their external run inputs, and the present installation is such that if a drive is being run from the VFD keypad, the conveyor E-Stop will not disable the drive and stop the motor. (As an aside, the electrical contractor wired all the drives as 2-wire
start/stop. Is 3-wire better or worse from a safety standpoint?)

Failure of the E-Stop to interlock all modes is a cause of some concern to us <g>. The contractor's response, however, is that use of the drive keypads (which are locked in the electrical panel) is negligence on the part of whomever may be so bold as to do so, and that NFPA 79 is not a
standard normally applied to industrial conveyor systems in any case. I'd appreciate any input from the list on your usual practice with conveyor
controls and conveyor safety systems.

Thanks... Paul T
 
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Roland LIlja

Hi Paul,
OoooNooo is my first responce to your question! If this would been a conveyor in Sweden it would be closed for ANY use.! Since the VFD in itself can be malfunktion this would be a SERIOUS DANGER. The normal (In Sweden that is) would be two contactors independently braking the powersource to the VFD's when the E-stop is pushed. And i think that's a god safety.

Regards,
Roland Lilja
 
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Arturo Arevalo

From my standpoint the selection between two wire control and three wire control depends on your application, both have a stop signal and an enable signal, removing any of this signal will stop the motor.

In the applications that I have seen the E-stop removes the enable signal to the VFD or deenergizes a contactor that goes before the VFD but I dont know about how convenient is this second option.

It is not recommendable to use a contactor between the VFD and the motor. Removing the connection between VFD and motor while the VFD output is energized could produce serious damage to the VFD. You can use a delayed contactor between the VFD and the motor so that it deenergizes after the output of the VFD is off.
 
M
Most drives do have an Enable type input or programmable "External Fault input" that can be connected to the e-stop contact. This input is not the same as the run input. If you have the type of the drive I may be able to check it out for you.
 
When installing VFD's I always install a contactor on the line side of the VFD. Typically the contactor would be energized by an output on the PLC. The power supply to the PLC output card is then hardwired through an E-stop relay. So when the e-stop is pressed, the power to the output card is removed through the hardwiring and thus the contactor for the VFD is de-energized, removing the supply power to the VFD. SO THERE IS NO DOUBT THAT THE MOTOR IS DE-ENERGIZED.

When the E-stop is reset the PLC would energize the appropriate control signals to the VFD ie 1st energize the contactor supplying power to the VFD, 2nd energize the enable signal to the VFD. This all depends on the type of VFD you are using and the setup parameters for the VFD.
 
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Jesper M. Pedersen

Hi Paul.
A typical conveyor belt is a very dangerous piece of equipment. Parts of a persons body can be trapped at a number of places with grave, even fatal, consequences.

So, you must consider everything carefully and then adopt a safety strategy for your conveyor belts.

The best is to avoid any danger during normal operation by means of protective covers. The covers should be in place EVERYWHERE a person can get to the belt (over, under, on the sides, and at loading and unloading points).

Also, there should be a local isolator switch for the drive motor(s) for use when the covers are off (for maintainance work). There should be warning signs, informing about the danger and informing to use the isolator when doing maintenance.

But what if maintenance work must be done while the belt is running ? A quick and reliable way of stopping the conveyor is needed. That's where the E-stop comes in.

When using E-stops, they MUST be utilised in the proper way.

That means:
1. The E-stop(s) must connect to a dedicated safety-relay.
2. With two independendent contacts in the safety relay two relays (or contactors) can be engaged.
3. With serially connected contacts in the two relays the conveyor belt can be engaged.

All of the above is two make sure that a single error in the circuit can't fail the safety.

Before you loose faith, read on:
An acceptable way to cut down on all this complexity, is to arrange the E-stops in groups. That means that one E-stop circuit can stop a complete group (or maybe all) of the conveyors. The safety relay and the two relays then engages either: a common pilot voltage or: common 3-phase power for the conveyors (preferred).

So you only have to do this once (or for every group). Hurra!

Hope this helps you,
Jesper M. Pedersen
DISA Industries A/S
 
If NFPA isn't the standard for you industrial application, I don't know what is. NFPA 79 9.5.2 requires at least one Cat 0 stop. I don't fully understand the architecture of your control system, but you must have the ability to "remove power from the machine actuators(motors).

We E-stop VFDs all the time by one of two methods-
1. We remove the ENABLE signal to the VFD which will turn off the output transistors and at the same time open a contactor between the drive and the motor (Cat 0). This will not fault or hurt the drive if it is done correctly.

2. Where there are high inertia loads, e wire the e-stop(s) to a pneumatic timer so that the RUN (not the enable) signal is dropped immediately and then after a brief time delay to decel, open the output contactor and the enable signal (Cat 1).

If we do the latter (number 2), we also have a Cat 0 method such as a master stop circuit that drops all control power in the control panel.

Hope this helps.
[email protected]
 
S
A contactor should be placed on the line side of the drive, definitely never on the load side of the drive, as in in regen and the load side contactor opens it can damage the drive. The best practice is to follow the european codes that provide the best safety, and always exceed U.S. requirements. This is expensive however. In the U.S you are per NFPA (which does apply!) required to have an electro-mechanical e-stop (ie. no logic) which prevents you from simply using the enable input of the drive, and requires that you place a contactor between the line power and the drive input. It is a good practice to take away the enable as well. This section of the code is often misunderstood, and may people only use the enable as there estop function, but if you want to meet codes then you need the contactor, no question about it. Even if you are not concerned about code, do you really want to live with the knowledge that someone got hurt and you could have prevented the accident!
 
J
Although, I've heard of damage done to drives by diconnecting the load side, I believe this is not an issue with quality drives with the acception of DC drives. We always E-stop control power to a load side contactor. (between the drive and the
motor) Your contractor is using very good sense in telling you that it's your problem.
What type and brand of drives are you using?
[email protected]
P.S. We do alot of this type of conveyor from engineering to installation.
 
Rich- thanks much for your reply. This is what I'm recommending to my plant manager. OK... so it looks like even if you have a drive enable line (which these drives don't) you use a contactor to meet the intent of NFPA 79? That agrees with my interpretation, but since I don't do a lot of VFDs I appreciate the input from others who use them.

Paul T
 
That is exactly what I'm recommending to my plant manager. Thanks for the comment on "typical practice;" we need some examples like yours to offset the contractor's "we don't usually have to do this" assertion.

Paul T
 
First thing I checked. 118 of the drives are Reliance SP120's, which have no enable input. In fact, they have five *programmable* external inputs. This raises a secondary question of whether the drive can ever meet the NFPA 79 EStop requirement in 9.6.3 "... operation shall not depend on electronic logic (hardware or software...". I spoke to one of the Reliance
product managers, who told me that the SP120 is the first drive of theirs he's seen that didn't have a dedicated enable and that he's not sure of
it's NFPA 79 status either given the fact that the inputs are programmable; the run input can be any of the five.

Paul T
 
B
> Question for those of you who have dealt with material handling and
> conveyor systems using VFDs:
>
> Do conveyor systems typically use contactors (either before or after the
> VFD) to allow isolating the motors and meeting NFPA 79 requirements for
> Category 0 or Category 1 stops? We have a newly installed conveyor system
> in our plant with 120 VFDs. Most of the VFDs interface to the E-Stop
> circuit only through their external run inputs, and the present
> installation is such that if a drive is being run from the VFD keypad,
> the conveyor E-Stop will not disable the drive and stop the motor. (As an
> aside, the electrical contractor wired all the drives as 2-wire
> start/stop. Is 3-wire better or worse from a safety standpoint?)

I prefer 2 wire. I think the safety aspects are not substantially different. I also like to put contactors on all VFDs. I am a suspicious cuss and want to make sure that when I trip the estop, the thing stops. However, a pretty good arguement can be made that if the e-stop opens up the enable inputs on the drive, its a safe bet the motor is not going anyway. The reliability
factor of such an arrangement probably approaches that of a hard wired contactor. The real issue is what is required by code and practice, and what can the lawyers twist into a liability 50 years from now.

> Failure of the E-Stop to interlock all modes is a cause of some concern
> to us <g>. The contractor's response, however, is that use of the drive
> keypads (which are locked in the electrical panel) is negligence on the
> part of whomever may be so bold as to do so, and that NFPA 79 is not a
> standard normally applied to industrial conveyor systems in any case. I'd
> appreciate any input from the list on your usual practice with conveyor
> controls and conveyor safety systems.

Its not unusual for people to have some modes of operating such equipment even if the main e-stop is tripped. I am not overjoyed by this practice, but its common. Often you will see MCCs with a hand-off-auto switch that has a start and a stop pushbutton that runs a motor in the hand position. Often, the stop only affects the circuit in the "auto" psotion. I do not like this as I think the thing should stop when the estop is pressed. I suspect the reason this is done is that they really need both an "estop" and just a regular "stop" pushbutton but don't realize it and they use the estop as a stop.

For this reason I usually suggest having both remote stop and e-stop pushbttons along the route of the conveyor instead of just estops. The
remote stop/estop buttons are of the pushpull variety. The conveyor automatically restarts after the stop is pulled back out. The estop requires someone pull out the estop button and then go push the master start button. Its probably a good idea to have a horn/siren/flashing lights energize for some period of time before the conveyor actually starts moving again.

Bob Peterson
 
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Michael Griffin

We use palletised conveyor systems (smaller than yours), but I won't claim widespread experience the business. I also don't know what risks are present in your system. However, I hope this reply is of some use.

From your description, it does not appear that you *have* an emergency stop system, regardless of what the push button may say or look like. You may have conveyor start/stop buttons, but no emergency stop circuit. An emergency stop system would stop the conveyor regardless of what the drive's opinion on the
matter may be. Perhaps the proper question should be - do you need an emergency stop system?

What caught my attention was the statement from your contractor that "use of the drive keypads (which are locked in the electrical panel) is negligence". Sticking your head into a stamping press and cycling it is also "negligence", but most presses seem to have a lot of extra hardware which is intended to
prevent this. If you didn't have to allow for people being occasionally careless or foolish, then most machines wouldn't need e-stop systems.

************************
Michael Griffin
London, Ont. Canada
************************
 
JLD, thanks for your response. You imply that you *do* consider a contactor to be a necessary part of the E-stop circuit... thanks.

The issue we have is not the location of the contactor... it is using a contactor. The contractor drops out the drive remote run circuit with the E-stop and supplied no contactors anywhere. The drives are Reliance SP120's, which have no enable input, only a remote run input. When the drive is operated from the keypad, there is no external input to the drive which will disable it. Hence, our request to the contractor for contactors in the motor power conductors to provide E-stop under any condition, not just when the drive is being operated via remote run. Our contractor was required to comply with national, state and local codes... NFPA 79 comes to mind... so his telling me that he does not follow NFPA 79 for industrial systems leads us to question his "good sense."

Paul T
 
The contractor is jerking you around. I would never buy anything from them again, if it were me.

The E-Stop circuit should drop either power in to the drive, or the motor leads out of the drive. The drive enable interlock on most drives is
implemented in firmware. Read: Software. Safety should _never_ be dependant on properly operating software. You have already identified one failure mode.

Using front panel controls is part of the troubleshooting process. This is not negligent use.

I do not know whether NFPA 79 applies or not, but conveyors are considered part of an industrial control system, and standard safety practices should apply.

As I said, if I were to identify this type of problem, and the contractor refused to address it, it would be the last time they stepped foot inside my plant.

--Joe Jansen
 
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Michael Lindstrom

Typically on an underground conveyors, there is a contactor, the disabling of the enable signal and the removal of the run signal. I am unsure of what NFPA 79 requirements are but AS3000 allows for power retention on a VFD for the purposes of electric braking. This is particularly important with trunk conveyors as there is usually a large
amount of current to dissipate. Also with the invention of regenerative drives allowance was made for stopping these drives with the power still retained and generally the contactor dropped out after a period of time, after the stop was completed (this is usually been done by a hard-wired timer).

Under AS 1755(again unsure of the IEC standard) the E-stop clause states that the conveyor shall stop in the shortest practical time. It does not
say anything about the removal of power !. This is particularly evident again with larger systems using regenerative/Dynamic braking options.

Again I am unsure of the IEC standard but AS 4024 P 1-7, machine guarding standard, does not allow for starting of drives through their keypad
unless the appropriate risk assessment has been undertaken.

If someone installed your current system in my plant I would be asking for an appropriate risk review. Again I may be right off my mark with my
comments, what do other people think ?

Michael Lindstrom
 
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Ralphsnyder, Grayg

<P>Here is what I have used for many years. The ESTOP circuit's MCR energizes a power CR contactor which applies line voltage to the motor
controller (Variable Speed Drive). A seperate stop/start circuit to start and stop the motor controller. This start/stop can be a signal directly from a PLC. The MCR is in the start/stop to prevent the VSD from restarting
when the control power is restored and the MCR turned on. This circuit is good for AC or DC applications. Putting a contactor between a VSD drive and the motor is a bad idea... damage to the VSD drive will occur. Removing the run or enable command from a VSD is no guarantee that the motor will stop... when the semiconductors short out and essentially connecting the motor
directly to the incoming line... run or enable have no effect. ESTOPs should not be confused with Process Stops (i.e. the motor start/stop or
enable circuit). Hopefully, the ESTOP will never get pushed. The process start/stop or enable circuit is what should be used for everything except for an ESTOP situation - threats to life or property.</P>

<P>thanks,
<BR>grayg ralphsnyder </P>

<PRE>
(Display using a non-proportional font.)



Control Power ON PB

ESTOP _|_ MCR
--o_|_o----o o--------()--
| |
| MCR |
--||-


MCR VSD Power CR
--||---------------()--



VSD Power CR
--------
--||----| |--\
3PH | VSD | \
--||----| |--(MOTOR)
Power | DRIVE | /
--||----| |--/
| |
RUN CR | |
--||----|RUN |
| |
--------



Drive Drive Drive
MCR STOP RUN RUN CR
_|_
---||--o_|_o----o o--------()--
| |
|Drive|
|RUNCR|
--||-
</PRE>
 
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