Help with plc motor sequence

[Joe_Jansen]

Safety relays are safety relays for a reason. Force guided contacts and redundancy to be specific. A 4PDT relay is *NOT* a safety relay, and does leave your company open to liability if the plantiff can show that the relay was part of the circuit that was supposed to protect from the accident.

Your best protection against losing a lawsuit is following the safety codes.

Not to mention the fact that if it were your hand at risk of being cut off, would you feel good knowing that the machine builder took shortcuts in the safety system? Is someone's hand worth the money you are saving?

One welded contact, and the whole relay stays closed even after the estop is pressed.

 

[OkiePC]

Before you choose any hardware for your safety circuit, you should perform a safety assessment on the machinery. Search this forum for some good info on safety assessments.

I have worked on machines that used general purpose relays for safety, but I would never install one myself, even if the risk was low. Apply a little C.Y.A. and at least use a positively guided relay designed for at least the requirement of the safety assessment.

Paul

 

[mordred]

here is a site to get you started on understanding the classifications OkiePC mentioned.
http://www.ia.omron.com/support/guide/3/safety_circuit_example.html

the attached guide is a simpler laymens example showing the circuitry but has some good info as well.

When I mentioned the E-stop concern my intention was to ensure you avoid programming the controls of the E-stop via the PLC other than monitoring. I did not expect such a discussion on the subject. Keep in mind we would still like to help you with your original problem if you can describe where you are having a problem with programming your conveyer controls

 

[leitmotif]

I agree with blamb on the ESD. The ESD indication need to take the fastest path to PLC in order to reset the program to a safe state. Some contactor take a second too long to drop out so what happens when someone push/pull the E stop rapidly would result in multiple pump/motor to drop out and start all together because the PLC hasn't seen the E-Stop yet. The result can be very bad.

I think we are starting to take this on a whole new tangent
BUT a good tangent.

I am going to post as a new topic but hopefully flag it to this one.

Dan Bentler

 

[blamb]

Eugenious - did your original question(s) get answered here? Are you still having any troubles with your motor control logic?

 

[Eugenious]

The tech at automationdirect.com

The tech at automation direct helped me out with the logic.
The only thing that's got me concerned is the safety circuitry,since I was going to use an e-stop with 1NC & 1NO and a 4pdt relay.
My plan was to wire the NC to to the relay and the relay would cut power to the the output card's common terminals.
The NO on the E-stop would be used as an input to the plc logic.
But from what I can see that is a major violation,am i right?

 

[blamb]

I would recommend that you talk to your customer who you are supplying the system to and find out what they typically do for their ESD systems. This way you'll be sure that you are supplying something that will be acceptable for them. As far as the relay used to cut power to the output card(s), just make sure that the contacts are rated properly for the load.

 

[mordred]

would be more correct to wire the NC to the Master control relay MCR which you can wire in series to pull power to the motor contactors this way the PLC cannot in any way activate a conveyer. Or pull in the contacter through a program error. Each motor should have its own contactor wiring the E-stop to disable those would be a simple matter.
As your [process does not sound like it require a controlled stop this would be the correct option. One thing many copmanies prefer on E-stops is to remove all power to components such as motors this way its a fast way to remove power in case of electical based accidents. All conveyer systems I've worked on have this scheme also plan to have a motor disconnect switch within 10 ft of any motor. With lockout capabilities this way one motor can be serviced with the lockout procedures

The main concern with your method is that many maintenance workers use the E-stops to isolate power to the motors granted this is not a correct lockout procedure however it does happen. When an emergency stop is enabled you want to make sure you isolate the 3 phase power as well as the control power. Use the other signal to deactivate the outputs in your logic and require a reset to restart. This is why I recommend the main MCR on the 3 phase circuit.

 

[mordred]

One other question in how you were intending to wire your E-stop
what will happen if an electician decides to direct pull the motor contact in due to a blown output on the PLC? will the E stop stop that conveyer? NO it will not. Unless there is a control stop or a similar problem on the risk assessment requiring a controlled stop your best and safest bet that covers the majority of the Stupid things ppl do. Is to disable all power between your Main disconnect and your main transformer. Yes this will also disable the power to the PLC assuming you are fed from a 600/460 to 120 then further stepped down for 24 volt or similar combinations. However say there is a lazy electrician or an electrical hazard that prevents someone from using the main disconnect?
Wiring the E-stop to drop the Common would require someone to run to the main breaker. However if all E-stops are setup to remove all power which all E-stops are suppose to except for control stops then there is several points where someons life can be saved

 

[Roy Matson]

Hello,Can anyone help me with this program I'm trying to develop?
I have 4 conveyor and crusher motors and here is how I want the sequence to go:
Also got an e-stop that needs to imediatly shut down whole sequence in case it gets pressed.
Thanks for your help.
Eugene.

Hi, I worked for many years in mining so I am familiar with conveyors & crushers.
For a start you don't normally have an E-stop to stop all equipment. Each conveyor will have it's own E-stop (usually near the drive) and pull-cord switches that stop just that conveyor.
Each of the E-stop and pull cord switches are wired in series and directly into their own motor control circuit. You could use an aux contact to pick up the status but what I normally see
is a 120 VAC input that picks up the voltage down stream of the button contact.
Once that conveyor stops all the ones feeding it also stop when they see the motor status go off, but these are not emergency, just a process interlock to prevent a big mess.
The crusher doesn't normally stop when the conveyors stop, it just keeps on crushing away to clear itself otherwise you will have what's known as a "bogged" crusher which may take several
hours to clear and get back running.
The crusher will be interlocked so if it stops the conveyor feeding it also stops immediately.
You might have a button somewhere that causes the system to stop but it's really a Master stop, not an E-stop and it shouldn't be labeled so.
Hope this helps.
Roy

BTW: From your timing sequence I have to assume the material only goes through the plant one time, it doesn't recirculate oversize material back to the start otherwise you will end up with a big blockage somewhere. In mining they usually just cut the feed and wait for the material to pass through then shut down all at once by stopping the last in sequence.