Surge suppressor across inductive load

kiran

Member
Join Date
Jul 2003
Posts
43
Hi all,

Do we need to put surge suppressor across an inductive load if it is switched on thro' a relay contact output from PLC?

Thanks.
 
We generally use some type of supressor when switching inductive loads from a relay output on the PLC. Varistors for AC loads and diodes for DC.
 
Yes, for at least 2 reasons

1) It helps protect the relay contacts. PLC relay outputs are sometimes rather small. Their ratings, typically 0.5A, are based on the expectation of a basically resistive load. Using a suppressor keeps the contacts from degrading. Most relay vendors have Tech Notes that illustrate what happens to contacts switching inductive loads when they are NOT protected.

2) In my opinion, an even MORE important reason - Suppressors greatly reduce the amount of electrical noise broadcast into the surrounding electronics. These transients are quite capable of taking out some very expensive PLC components.

One example from personal experience - An Allen-Bradley PLC3 system ran for years on a converting line. Then it was upgraded to a PLC-5 system on the same line (ie, same valves, motor starters, drives, etc.) All of a sudden, we started having random failures in the processor and the I/O scanner. After MUCH trial and error, someone suggested putting suppressors on all the valve and starter coils... that did the trick!
 
I bet if I have answered this question once, I have answered it a thousand times.

Foghorn22 is exactly right. Also you want to protect any other devices that are powered by the same power supply from the transients caused by the CEMF (Counter Electro Motive Force) produced when the coil de-energizes.
 
Surge suppressors are cheap compared to the potential costs and headaches of not using them.

You may or may not acually need them, but they will certainly extend the life of the contacts on the output card.

I recently was called in to troubleshoot some problems for a customer who had all of their controls designed and built by their japanese shop. They had a number of output relays that were used to directly switch solenoid valves. A number of the relay contacts had fused after just a couple of months of use. I had them install suppressors and replace a bunch of the relays (they were these neat mitsubishi blocks where you could pop out the relay and replace it).

I havn't had any more calls about that problem, so I consider it fixed.
 
I have heard this solution time and again and was just curious as how surge suppressors are sized. We currently do not use any but I feel that some of our large motor starters need them.

Can some simple rule of thumb be used or is it more involved?
 

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