Remote start/stop of drives 1

[maypot]

Hi,

I`m quite new in my job and there some couple of things there which look amazing.
All the drives , including conveyors , are remotely started via the SCADA with a soft start and a soft stop via the screen. They only have an emergency stop for all drives , which surprisingly is also wired to the PLC . The only information being sent to the PLC is the run command from each drive.
I am requesting that position detectors , field start/stop, be hard wired for each drive . I am also looking to provide additional inputs for the drive ready state and faulted condition . In addition I am also requesting that the emergency stop be hard wired so as to trip main contator in case of trouble.
The management is not ready to accept these amendments as they will involve extra expenses . The former instrumentation engineer told them that this mode of start/stop were compliant with international standard WHICH I OBVIOUSLY DO NOT BELIEVE.
What are the current standards for remote start/stop of drives and how are they implemented ?

Bob

 

[Skogsgurra]

Most industries use some kind of field-bus system to start and stop motors today and also to transfer information from limit switches and so on.

But it is not considered good practice to wire the emergency stop via the bus or via the PLC. It shall work directly on the main contactor (or via a relay/auxiliary contactor with forced opening of the contacts).

The emergency stop can (and should) also be connected to the bus/PLC system for indication. But not for stopping.

If you are using one of the newer systems with a SIL level according to your application, then you may also wire the emergency stop directly to your PLC. But only then.

 

[dpc]

If the emergency stop involves personnel safety, I agree it should be hard-wired.

For a belt conveyor, it is customary to have an emergency stop pull cord running the length of the conveyor if it is accessible. This pull cord terminates in "pull cord switches" that must be hard-wired.

I'm not an OSHA expert, but I believe this is an OSHA requirement. If the PLC system is designed to be fail-safe, such that it will cause a conveyor to shut down if it fails, then maybe this is acceptable, but it wouldn't be acceptable to me.

 

[jraef]

IMHO, it depends upon the definition of "Emergency Stop" in your particular application. Drives often have braking abilities built-in and implemented, so if the emergency were related to rapid speed reduction, it may actually be "safer" to have the VFD stop the motors rather than allowing them to coast to a stop by opening the contactor. In many process industries, the concept of "Emergency Stop" is actually "Emergency Shutdown", because there is a specific safety proceedure that must be followed to prevent unintended consequences, as in petrochemical reactions. Cutting off power may actuall make it extremely dangerous!

Without knowing the particulars of all of the decisions that went into the design process of your system, I would be carefull about making a blanket statement. Do a key word search on "VFD emergency stop" in this forum and you will find several lively debates on this subject over the past few years.

"Venditori de oleum-vipera non vigere excordis populi"

 

[dpc]

I agree with jraef - the term "emergency stop" has many different meanings and interpretations. But if conveyors are involved, they should have some sort of safety stop.

If it is an enclosed pump system, the "emergency stop" may be a misnomer.

 

[jraef]

Agreed DPC, but even with conveyors, the point I was making is that there is no universal answer without knowing the specific application. I deal frequently with downhill material conveyors for instance, and cutting power to them means a runaway, exactly the opposite of what you want if someone falls onto the belt! E-Stop in that case means engaging the brake mode fully on in the VFD(s) because the mechanical brakes are often not capable of stopping a fully loaded belt, only holding it after the VFDs stop it.

I'm just being "meticulous" I guess (meticulous being a more socially acceptable term than the other "Freudian" version I would use in normal conversation).

"Venditori de oleum-vipera non vigere excordis populi"