large motor VFD local control

[HamidEle]

I had a discussion with our customer engineer regarding a 600V 500Hp VFD Control. He suggested not to add hand position for this size of VFD, But just field wire Off-Auto switch in the field. I can understand the hand position can be diabled for this size of VFD. But why do we need off/Auto position in the field. I selected 2 position switch, at Off position,Auto function ( PLC control) is not functional. At Auto position, VFD is controled by PLC run command. We also have a ESD( Emergency shutdown button at field).
Field ESD is very common in Canada. But what is the purpose of having Off-Auto Switch in the field.
Any inputs will be appreciated.

 

[itsmoked]

I'm having trouble understanding your question as it is so obvious I suspect I'm missing something...

OFF = OFF - motor won't run.
Auto = Motor can run - at anytime. Someone who walks up and looks at the machine that is not running can - at a glance - recognize that the motor could start at anytime.

If they see OFF/ON in the ON position and the motor isn't running, then something is possibly/likely wrong.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

If an automatically started motor is stopped with an emergency stop button, once the button is released the motor may again be started automatically. Local implies local to the motor. A control position may be in the field but nowhere near the motor. Local Hand/Off/Auto switches are common on automatically controlled equipment. They are used for servicing. If your customer wants to omit the "Hand" function, I don't see a problem. The "Off" position should prevent the motor from being started from any other control, whatever it takes to achieve that.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HamidEle]

Thanks for the reply. It is geting clear to me.
Waross,actually, the maintained push button by the motor or two position switch can function as Off-Auto switch, which one is more common in North America(I am relatively new to North America).ESD button is also located in the field.

 

[DickDV]

I would suspect that there are important safety or operational conditional signals being processed by the controlling automation system that would be bypassed if the drive could be switched to Hand Mode.

OFF simply means the drive is forced off, not Faulted. It just can't run.

AUTO means that the control of the drive and motor is in some other system communicating with the drive. At any particular time, the motor may or may not be running.

 

[LionelHutz]

Is the ESD connected to the PLC? In some plants the ESD is not used for normal operation but rather reserved for emergencies. The PLC monitoring system will flag this. So, the Off/Auto switch would be used for normal operation instead.

Controlling a large motor is not a valid reason for eliminating the "Hand" position. If the operators/mechanical group needs or wants local control then they need local controls installed.

 

[waross]

With a VFD driven motor the hand position, if used, may call up a preset speed for maintenance.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HamidEle]

lionelHutz,
My undersanding is, if ESD is connected to PLC, we don't really need to install a ESD buttoon in the field. Stop command will be issued by PLC output. If not, we would have to install one at the field. Is is correct?

Thanks

 

[ozmosis]

If you are going to use an ESD and PLC then the PLC needs to be a certified safety PLC. How the PLC will STOP the VFD will depend largely on the VFD. A number of VFD's will have "Safe-Stop" integrated into the drive and this can be used in conjunction with an ESD from your safety PLC. If the VFD does not have this capability, then you will have to resort to removing power to the incomer to the VFD as part of your safety circuit design.
However, you seem to be confusing the issue of safety with the 'normal' stop of a VFD. If in Auto the normal stop is via a PLC then this is one thing but for an ESD you should have 'local' devices to allow an emergency stop and not have to go to a external means to shut down the VFD.
Trust this makes sense.
Split out safety and your normal stop/start functions.
Hand(local) and Auto(Remote) are typically defined in the function of a VFD when commissioning. The VFD should give you the choice to define these settings as 'local' is not always a button on the VFD, it could be a remote switch on the front of a panel somewhere.

 

[HamidEle]

I talked to VFD vendor and I was told this VFD doesn't have "Safe-Stop" integrated into the drive. VFD input is fused disconnect without any contactors.
I was not aware we should have a safety stop on VFD. What is the purpose of this Safety stop?

 

[itsmoked]

Safety stop is what you do when you see someone falling into a machine, or their neck tie is catching on the drive shaft, or the machine is coming apart in front of you.

On large complicated machinery it is sometimes better to drive the system to a stop using a VFD's abilities than it is to just have the power disappear and the system coast to a stop taking seconds or minutes. A conveyor would be such an example.

A pump that needs valves shut might be another example.

Or a machine requires a human to reach into it occasionally to retrieve a piece normally not done. A safety stop can suspend the machine safely to achieve this. (In other cases it might be better to disconnect all power.)

That is the action of a safety stop.


Your VFD has a way to stop it. A "RUN" input for instance. All you need to do is interrupt this with a mechanical contact and you have an E-STOP. You can also program your VFD to stop the load not let it coast to a stop. You just have to set it that way. So YES you can E-STOP your VFD.

Now as was stated if your E-STOP switch doesn't directly act electrically on your VFD but goes to some other controller like a PLC then that PLC needs to be safety rated as a berserk PLC used as a safety device is not a good thing.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

In North America, it seems you have to determine what is safe enough by yourself. No-one wants to give free advice because that makes them liable so to get help you'd have to pay for it. If you decide that a digital input on the VFD is not safe enough then you add a contactor. Now, you have a contactor that could experience contact tip welding and not open so you go another step and add a shunt trip in the breaker. But, hey, that shunt trip is a coil of wire which could also fail.... I think you get the idea.

I've gone through this with a safety system using a PLC before. Sure that PLC was safe in it's operation but when it would tell the motor contactor to open there was no guarantee that contactor actually would open. I was also informed to use safety contactors. Well, these just have a pair of contacts that are forced to operate the opposite of each other. The safety part does not include the fail safe opening of the main contactor tips.

The minimum that you must follow is to wire the ESD button contacts directly to the equipment. They must not go through a PLC unless it is safety rated. You can feed a second contact to the PLC and use it so you know the ESD button was pressed, but there still must be a direct hard wired shut-down. This is about the first thing a health and safety inspector will check when looking at the electrical part of the safety program. Been there, done that with some test bench equipment - first thing asked and the only thing that changed.