which relay will always fail off when the relay is damaged or something happened to it

Thread Starter

nspr

Joined Jan 31, 2019
4
Hi,
i am a controls engineer at a industry where we use a lot of spot welding machines imported from japan,
here we got a problem when the relay fails the it fails ON leading to switching on Welding Gun, which in turn falls down,
because of this our operators are getting injured.
please can anyone suggest different relays when it fails it fails OFF.
i tried to used regular Terminal Relays and Transparent Relays(ice cube) i got no good output from them, input for welding gun is 24VDC
all the spot welders are automated and controlled by PLC
any suggestions appreciated.

TIA.
 

Yaakov

Joined Jan 27, 2019
1,612
How is it actually failing? That is, what is the defect that is causing it to fail on?

If the contacts are welding the solution might be some kind of snubber circuit, if it the design of the circuit requires the relay to be energized to turn welding OFF, that's a really bad design!

Can you provide more information about the failure mode, and the actual circuit (and the ratings of the parts)?
 

MaxHeadRoom

Joined Jul 18, 2013
20,697
I assume these are contactors, rather than 'relays'. Are they switching DC by any chance, if so you could look at arc blow out types, some posses a magnet close to the contacts that are probabally welding due to a plasma arc across them.
Ice cube relays generally switch simple control circuitry, they are not made to switch spot welding currents.
Sounds like the wrong device in the wrong application.
Show some circuitry details.
Max.
 

crutschow

Joined Mar 14, 2008
24,965
You could put two relays in series for redundancy.
But you would need to add an indicator to show when one has failed closed, otherwise the redundancy safety would be lost.
 

jpanhalt

Joined Jan 18, 2008
9,419
In my sailplane hobby, we use 12V winches and heavy duty starter relays for launching. Of course, welding of the contacts is a safety concern.

Our solution is to use two relays in series and an LED across the contacts of each. I believe that is the arrangement crutschow (post #5) is describing as well. When power to the winch is off, both LED's should lit. If not, the launch is aborted and the failed relay is replaced. We also include an emergency disconnect in case both fail during a launch.
 

MisterBill2

Joined Jan 23, 2018
6,069
There is no mention of the load current of the spot welders, but since you did mention the clear-cased relays I am guessing that it is not over ten amps. And it is obvious that you are not using contactors.
Using safety relays with an adequate contact current rating should solve the problem, BUT there may be more to it than what we know. Many Japanese products leave out information that may be very useful, such as inrush current, which is probably the cause of the contacts sticking closed. The ultimate fix may be to use adequately rated solid-state relays. But probably snubber devices across the contacts as well would be an additional insurance.
Possibly relays rated for motor starting service would be another choice, but most of those are somewhat position sensitive.
And one final question: is the relay coil voltage really dropping to zero? It may be that the PLC output controlling the spot weld relays is not switching off completely. I have seen that happen. A solid-state output triac stayed half-on for unknown reasons.
 

Thread Starter

nspr

Joined Jan 31, 2019
4
How is it actually failing? That is, what is the defect that is causing it to fail on?

If the contacts are welding the solution might be some kind of snubber circuit, if it the design of the circuit requires the relay to be energized to turn welding OFF, that's a really bad design!

Can you provide more information about the failure mode, and the actual circuit (and the ratings of the parts)?
thanks for replying,
we use omron relays which we will use them till they die so the contactor tip will burn up and stay on which in turn will energize the circuit.
 

Thread Starter

nspr

Joined Jan 31, 2019
4
There is no mention of the load current of the spot welders, but since you did mention the clear-cased relays I am guessing that it is not over ten amps. And it is obvious that you are not using contactors.
Using safety relays with an adequate contact current rating should solve the problem, BUT there may be more to it than what we know. Many Japanese products leave out information that may be very useful, such as inrush current, which is probably the cause of the contacts sticking closed. The ultimate fix may be to use adequately rated solid-state relays. But probably snubber devices across the contacts as well would be an additional insurance.
Possibly relays rated for motor starting service would be another choice, but most of those are somewhat position sensitive.
And one final question: is the relay coil voltage really dropping to zero? It may be that the PLC output controlling the spot weld relays is not switching off completely. I have seen that happen. A solid-state output triac stayed half-on for unknown reasons.
i checked with PLC that is good and checked with solid state relays now i will try to use safety relays
 

MisterBill2

Joined Jan 23, 2018
6,069
i checked with PLC that is good and checked with solid state relays now i will try to use safety relays
It was a possible source of the problem, but you have shown that it is not the source of the problem. Now I understand better, that the smaller relays control the larger contactors. So it is a failure problem of the smaller relays. There is also another way to do it, but it requires a second PLC output and a second small relay, and a program change as well.
That would be to have a second relay with normally CLOSED contacts in series with the present relay with the normally OPEN contacts. Then, when the contactor is supposed to switch off, the normally closed relay would fist operate and open the circuit, then the normally open relay would release, and after that the normally closed relay would also release, so that the circuit would be ready for the next weld cycle. You could then add a monitor function to each relay to give a warning when a contact stuck closed. Certainly this is a bit more complicated, but it could help with the problem if a safety relay is not available.
 
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