Relay Suppression

[nic00b]

I am looking for information regarding relay suppresion. I have read about various different types that can be used, I'm curious what situations you would use the different types in. For example MOV's, Snubbers, Diodes etc.... Also how do you size the suppression correctly? I seem to get different answers from my colleagues and as im only two years out of my time i see it best to learn the correct ways now than "Wild Guessing".

 

[PLCMentor]

Selecting the type of suppression depends on whether your system is AC or DC. I tend to like MOV's for AC; however you can also use the RC circuit type. For DC a diode is used (obviously not a good choice for AC).

Russell White, P.E.
Automation Technologies, Inc.

http://www.AutomationNC.com

Automation Help

http://www.PLCMentor.com

 

[cjhut]

If it's an ac system there are as mentioned above MOV and RC types. One is for arc suppression for contact wear, and the other is for noise suppression I believe. Allen Bradley makes terminals that incorporate both of these devices. It should also be noted that these should be placed as close to the load as possible.

 

[PLCMentor]

cjhut,
I assumed he was interested in protecting a source against inductive backlash from the coil. I should be clear that my comments are for MOVs/diodes installed in parallel with the coil for this purpose.

Russell White, P.E.
Automation Technologies, Inc.

http://www.AutomationNC.com

Automation Help

http://www.PLCMentor.com

 

[OperaHouse]

It should be noted that suppression can slow the drop out response of a relay. If that is a problem, some resistance can be added to the diode in DC applications. The automotive industry has be going to using just parallel resistors to reduce noise and reliability. We had one noise sensitive application and resistors sized at 10X coil resistance produced the lowest noise.

 

[jraef]

Here's my quick and dirty rule (for AC control systems).

MOVs protect the device from problems in the circuit.
RC Snubbers protect the circuit from problems in the device.

Not universally true of course, but it works.

By the way, I dislike MOVs in general because of their failure mode. I have spent a LOT of time and money cleaning up messes caused by vaporized MOVs.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[nic00b]

Hi jraef i recently come across MOV's on our syncdrive motor and it was literally blown to pieces...Primarily are plant is DC and we have had problems in certain areas with interface relays when the diodes have been used (Im guessing the diodes affect coil dropout)

 

[jraef]

I had some MOVs vaporize on Medium Voltage Soft Starters when an up-stream event sent a massive spike down the line. Then when the plant operator (ex operator) panicked and re-closed the plant feeder without checking anything, the vaporized MOV materials still in the boxes provided a flash over path inside the starters and took them all out with a somewhat spectacular, albeit brief, fireworks show. $250,000 mistake, not counting down time.

Yes, I know that's not the fault of the MOVs, they were just doing their job. But still, their mode of "sacrifice" left a little to be desired.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[PLCMentor]

Ok, I have to ask. What did you replace them with?

Russell White, P.E.
Automation Technologies, Inc.

http://www.AutomationNC.com

Automation Help

http://www.PLCMentor.com

 

[nic00b]

jraef, see the attached pictures of our "incident"