Mercury relay....failure mode?

[Rudy]

Today, the trolley motor in my automatic wash started intermittently sticking ON. Of course, now I can't duplicate it.

There's a mechanical relay on the output of a PLC that feeds a Mercury relay that feeds the 1/2 HP trolley motor.

So....either the PLC relay is sticking on, or the Mercury relay is sticking on.

Here's my question...... My wash is over 25 years old, and I've NEVER changed a mercury relay. NEVER. It seems to me, that if they'd fail, the coil would burn out...but....stick on? That seems impossible....or at least, improbable.

I'm tending to think that the mechanical relay on the PLC would be the culprit. Mechanical relays DO stick on.

Would you concur?

Ever see a Mercury relay stick ON?

 

[Randy]

Rudy, I've never had a mercury relay fail in well over 25 years of using them. It would probably be the mechanical relay that has gone bad, at least that is what I'd probably lean to.

 

[mac]

Rudy I agree about the relay. Because the mercury is a liquid, yet conducts electricity, it would be hard to see how it would stick. Your problem might be the small relay in the plc that controls it. Some plcs allow you to change the output relays. Try that.

 

[Rudy]

I agree with both of you.

I replaced the PLC output module with a spare. I'll see what happens.

I replace the relays on these output modules every now and then. It's a PCB board level repair. Not difficult if you know how to desolder etc.

$5 dollar relay, and 20 minutes....vs. new output module $150.

 

[OurTown]

Does it have the metal tab mounted on the trolley that goes past the sensor (I'm not sure what it is called) to count the times it goes around the oval? If so It might need adjusted a little closer to the sensor when it goes past. Our sensor got a crack in the housing causing corrosion inside and quit working. Once the trolley was called to go it just went around and around and would not stop. We replaced it and has worked fine ever since.

 

[Rudy]

My setup has a whisker switch, which is activated every time the trolley passes by. When this whisker switch fails, the trolley just keeps going around and around....basically until it triggers a "timeout" protection circuit. The PLC is in an "activated" state, and the bay signs are lit.

This is not what happened.

Bay was dead. PLC is not activated. No lights on. Nothing.

Trolley would keep running. Sometimes for 30 sec., sometimes for 5 minutes....all different. There had to be something "sticking" that was supplying power to the trolley motor.

It would either have to be the PLC running the machine, or a stuck mercury relay that powers the trolley motor.

Since the failure of a mercury relay is very improbable...I changed the output module on the PLC, and we haven't seen a recurrence. It doesn't mean it's fixed yet....it just means it hasn't recurred. Likely, however, it's fixed.

 

[OurTown]

I gotcha now and did not realize it was running without a wash activated.