# Off Delays



## dronai (Apr 11, 2011)

John, Here is the Off Delay Timers that have been bypassed. These were to slow down the on off switching of the VFD's.


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## Netree (Sep 3, 2011)

Why the delay timers? Was there necessary to do this without the PLC?


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## nolabama (Oct 3, 2007)

Off delay timers. I got lucky the last time I set one of those.


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## dronai (Apr 11, 2011)

Netree said:


> Why the delay timers? Was there necessary to do this without the PLC?


 
I have no idea !! You would think they would use the internal TD's ? They were only set for .5


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## JRaef (Mar 23, 2009)

dronai said:


> I have no idea !! You would think they would use the internal TD's ? They were only set for .5


Here's the likely application.

For whatever reason (probably safety protocol*) they were required to have contactors ahead of the motors. But using contactors on the output of VFDs runs significant risk of damaging the VFD transistors. It's OK for them to be there, it's just not OK for them to switch )open or close) while the VFD is running the motor.

So how you make that work is to have your control system, i.e. PLC, control the CONTACTOR. Then you use a contactor Aux. contact to signal signal the PLC that the contactor has closed before it gives the VFD the command to run. This makes sure the contactor is ALREADY closed before the VFD is operating the motor.

But that doesn't help when you want to shut down, i.e. a safety shutdown. So what you do to get around that is to attach an Off-Delay timer to the contactor coil circuit. In an Off Delay timer, the coil changes state immediately but when you remove control power, it stays on for the delay time. So in this case, your contactor coil circuit is actually going through the Off Delay timer first, THEN the contactor coil. So when the PLC tells the contactor to close, it closes immediately. But when the PLC tells the contactor to open, it opens 0.5 seconds later. But in logic, the PLC also told the VFD to stop at the same time it removed the contactor control signal. That way, the PLC turns off the drive and 0.5 seconds later the contactor opens, which prevents the VFD from being damaged by opening the circuit while it is operating the load.

* If you want to achieve SIL-3 Safety Integration or higher in a machine control system, you are often required to remove ALL power from a motor circuit during a safet related shutdown, which on VFD controlled motors, means a contactor downstream of the VFD. Newer drives now have what is called "Safe Power Off" features that allow you to get around that from a compliance standard, but even then it's not acceptable to some people. This scheme is universally acceptable. If your machine has a Siemens PLC controlling it, and any of the I/O cards and/or the CPU have yellow terminal markers, that means it is a Safety PLC (Siemens can mix Safety and Standard I/O in the same PLC rack or even in remote racks connected to a Safety Approved PLC).


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## John Valdes (May 17, 2007)

dronai said:


> John, Here is the Off Delay Timers that have been bypassed. These were to slow down the on off switching of the VFD's.


I need you to tell me where those wires were connected. I see what appears to be a row of fuse blocks on top, and the row below are the contactors. 
Point to the timers as I only see contactors and aux contacts added to them. Does the timer attach to the contactor like an Aux does? If so it seems its not an electronic timer, but maybe a pneumatic timer? Help me understand. :thumbsup:


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## nolabama (Oct 3, 2007)

Thank you thank you thank you for that explanation jraef


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## McClary’s Electrical (Feb 21, 2009)

John Valdes said:


> I need you to tell me where those wires were connected. I see what appears to be a row of fuse blocks on top, and the row below are the contactors.
> Point to the timers as I only see contactors and aux contacts added to them. Does the timer attach to the contactor like an Aux does? If so it seems its not an electronic timer, but maybe a pneumatic timer? Help me understand. :thumbsup:


 









John, I believe the auxillary contacts circled in red have a programmable off delay


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## JRaef (Mar 23, 2009)

mcclary's electrical said:


> <img src="http://www.electriciantalk.com/attachment.php?attachmentid=11544"/>
> 
> John, I believe the auxillary contacts circled in red have a programmable off delay


Yes, the taller units on top of the contractors are Off Delay timers that attach to the contactor directly.


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## Runion Energy (Aug 17, 2011)

Just right the timer in the logic of the plc get ride of the timers just adds to components to fail in the system.


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## dronai (Apr 11, 2011)

Thanks JRaef !!!! finally that makes sense.

I couldn't get a straight answer out of anyone at work, about why they are there, and why they are bypassing them.

One guy tonight told me, They are sometimes sticking ? Then why don't they want to replace them ? Another guy told me, they were causing cascading on the belts in certain areas.


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## John Valdes (May 17, 2007)

JRaef said:


> Yes, the taller units on top of the contractors are Off Delay timers that attach to the contactor directly.


Thanks. Are they pneumatic and how do they time out? I remember seeing them before. My memory is not great. I was expecting an electronic timer.


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## JRaef (Mar 23, 2009)

dronai said:


> Thanks JRaef !!!! finally that makes sense.
> 
> I couldn't get a straight answer out of anyone at work, about why they are there, and why they are bypassing them.
> 
> One guy tonight told me, They are sometimes sticking ? Then why don't they want to replace them ? Another guy told me, they were causing cascading on the belts in certain areas.


Most likely nobody ever explained why they were there and what they were doing, so other problems were blamed on them as the "unknown" factor. The timers should not have been able to cause casecading if the PLC logic was done right. However if someone changed the PLC logic without knowing why it was configured this way, that could cause issues. 

That sort of thing happens a lot, especially when an OEM is involved, Some OEMs are notorious for not documenting things like that BECAUSE they don't want users to be able to work on them. When I worked for Siemens we came across this a lot, especially with European equipment. The OEMs were very protective of their processes and engineering but to get anyone to help, you had to pay to fly a technician out from Europe. So they would come to us at Siemens because it was our components, yet we had no way to second guess their engineering either.


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## Netree (Sep 3, 2011)

JRaef said:


> you had to pay to fly a technician out from Europe.


Not true; I moved here. Only have to pay to fly me from Massachusetts now! :laughing:

But yes, you have it.


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## JRaef (Mar 23, 2009)

John Valdes said:


> Thanks. Are they pneumatic and how do they time out? I remember seeing them before. My memory is not great. I was expecting an electronic timer.


They offer both pneumatic and electronic, the ones in the picture are electronic (the pneumatic version has a big knob on it for adjusting the time).

Data sheet link


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