Emergency stop malfunction

[ladybird]

I work in a brewery as an electrical engineer. we have been installing a new line of vessels as a way of upgrading to the latest technology.

At the moment we are checking the electrical connections to the pumps in the vessels to ensure that they are in proper working condition.

We have all processes being controlled by a PLC and hence some signals require 24 VDC to pass a feedback that will enable the working of others.

Recently, we encountered one major problem. We have 24 VDC providing a signal to the Start button and the Emergency stop button and to a relay that sends a signal to the PLC, and this signal energizes a 3-phase coil to supply the motor.

The main problem is, once we supply the 24 VDC to the Start-Emergency stop-Relay path, the contactor that supplies the 3 phases is not energized and then again if we choose to bypass this emergency stop, and connect the Start to the Relay, the coil is energized.

At first we suspected the 24 VDC from the transformer in the panel and hence substituted it by a power supply of 24 VDC but the problem still persisted. We couldnt come up with any solution, since the behaviour seems to be totally abnormal.

Anyone with any more tips? And please tell me where are we going wrong?

Thanks

 

[RalphChristie]

I assume the start button is in series with the emergency button and this is in series with the relay? Thus the relay gives a signal to the PLC?

It seems for me you have used the wrong contacts for the Emergency stop button. It have to be Normally Closed (for energizing the relay when you press start), and when you press stop, it must open (to de-energize the relay).


Ralph

 

[ladybird]

Thanks Ralph, but i honestly dont think that is the case. This is because if we isolate the start and connect the emergency stop directly to the 24 VDC and the relay, when we manually operate the start button on the panel, the emergency stop easily operates without flaws.

Then here comes the abnormal behaviour. If we bypass the emergency stop and connect the start to the relay directly, on swithcing on, the contactor is energized and the motor runs.

At the moment, we are in the process of replacing the cable that connects the emergency stop and the panel and if it works, then i'll let you know. But for the time being, am open to more suggestions.

Cheers.

 

[RalphChristie]

Ok

How far is the stop button, start button and relay from each other? How big is the cables if they are far apart?

If you elliminate the start button and connect 24V on the stop button, do you also use the same cable to supply the 24V?

How do you bypass the stop button? Just brige it?

The best would be to measure the 24V at your different connection points. Let someone press the buttons and meassure the voltage. Disconnect the input to the PLC for the time-being when you do the measurements. (otherwise the motor will stop/start every time you press a button)

Hope it helps

Regards
Ralph

 

[rbulsara]

Post a wiring diagram....

 

[ladybird]

Thanks guys!

Ralph, your suggestions are being implemented and some are going to be implemented first thing tomorrow.

As for the wiring diagram, let me work on that.

For now am grateful for all your contributions and the ones that are on the way.

Cheers.

 

[ladybird]

Hello all,

This has been here for sometime, and it almost went stale.

I was about to send the circuit diagram but didnt know whether to send it as an attachment or as inline diagram. But that is neither here nor there.

We finally resolve the issue. We discovered that since the behaviour was recurrent only in emergency stop circuits, we decided to do this one last thing after everything else. We checked the CB, the Relays and the Contactor. It was found that the contactor coil malfunctioned immediately after energizing the first time.

We start replacing them today and this will run for a couple of days.

Thank you guys for your contribution.

Cheers.