How to turn off a bit when power goes out?

[CLXlifestyle]

Yes, exactly right, my HMI is the one doing the latching. Unfortunately, I need to keep it this way. At the end of the day, the motors will be turned off, and then the next day, the first operator will come and manually turn on the motors through the HMI. They won't have the PLC logic in front of them to toggle on and off.

No no, what everyone is saying is to make your button momentary, so it functions like a start switch. Your PLC logic should look like this:

http://image.slidesharecdn.com/wiri...ams-and-ladder-logic-70-638.jpg?cb=1462993102

You don't need to see the logic in the PLC to make this work. They hit the button the HMI and it momentarily "blips" the start circuit, starting the motor.

 

[Dravik]

Edit - CLX beat me to it, but I like having a 2nd bit from the HMI 'enabling' the motor in case you want to sequence the startup or start it from an 'all start' button or something.
You can also add a one-shot, though its not necessary.

 

[RonJohn]

The way that you are describing it sounds like your HMI is the one thats actually doing the latching. I reccomend only having the PLC do this. Just make whatever button turns on the motors momentary, and then do the latching in your ladder logic.

Exactly! I agree.

So when the power is cycled on the PLC, it will still be latched in, thus causing the motors to start at the very beginning, which is what I want to avoid.

There are multiple ways to tackle this, but this is separate from the latch/unlatch issue because you're trying to avoid starting up multiple motors simultaneously. Sounds like this is true whether it's at power up or any other time right? Some properly arranged on delay timers could probably take care of this.

 

[SandwichMagic]

Yes, exactly right, my HMI is the one doing the latching. Unfortunately, I need to keep it this way. At the end of the day, the motors will be turned off, and then the next day, the first operator will come and manually turn on the motors through the HMI. They won't have the PLC logic in front of them to toggle on and off.

Alright, I understand you better now, but I dont see why you cant just have a stop button and a start button? Just like a 3 wire start/stop Circuit. I wish I had better experience with the ladder creation functions on this site so I could just show you, but I do not. I'll give it a shot I guess.

    Stop Start                M1 EN 

----[\]---[ ]-----------------[OTE]---

        |     |

        |M1 EN|

        |-[ ]-|

Stop and start would both be buttons on your HMI, when you press start, it starts until a) someone hits stop, or b)when power is lost.

just like a 3 wire start stop.

EDIT: Tried to see what happpens when i put it in ladder tags.

 

[James Mcquade]

alive15,

I'm sorry, but this is NOT allowed per nfpa 79 - Electrical Standard for Industrial Machinery and I would also say a few osha rules are also in violation here.

I'm going to paraphrase the rule since I don't have the book with me.

No machine / system may automatically start after the power has been reapplied from an emergency stop / power out condition.

you must use a safety relay and reset it. it in turn energizes the mcr.
The system May NOT start until the user pushes a start /cycle button !!

If someone is hung in the machine and presses the e-stop, the whole point is possible homing the machine or manually using the controls to free the person, not do more damage.

I always have a power on contact that will drop out if the plc faults or looses power in the outputs and control rungs to prevent this from happening.

regards,
james

 

[dragonx556]

This is a trick we use for HMI that do not have the momentary push button feature. It is important to put the OTU as one of the last rungs that is scanned in the main routine so that the bit is not unlatched before all logic that use that bit has executed.

 

[AGill]

You need to get rid of the HMI toggle. It should be momentary, much like a push button.

 

[jdbrandt]

Good afternoon,

I have a motor running on a machine and whenever I cycle power on the PLC, the motor stops and comes back on when the PLC turns back on. Is there a way to keep the motor off, even when the PLC comes back on after losing power? Thanks.

I really don't get discussions like this.
Can't we just agree to adhere to NFPA 79?
If the PLC loses its program, or, in this instance, goes to sleep and then comes back on line ('turns back on'), there needs to be an external reset action by somebody, because, long before that point, the ESTOP relay should have dropped out - because the PLC stopped executing its program.
Back in the old days, when PLCs were not all that reliable, we used a pair of timers, one ON delay, and one OFF delay as part of the ESTOP string, and, the PLC would turn an output ON for 2 secs, and OFF for 2 secs. The timers were both set to 3 seconds (fixed, actually), so that, when the PLC went off line for any reason, the ESTOP would drop out.
I have never understood why designers don't put stuff like this in PLC-controlled systems...and, at my age, I guess I never will.

 

[daba]

I really don't get discussions like this.
Can't we just agree to adhere to NFPA 79?
If the PLC loses its program, or, in this instance, goes to sleep and then comes back on line ('turns back on'), there needs to be an external reset action by somebody, because, long before that point, the ESTOP relay should have dropped out - because the PLC stopped executing its program.
Back in the old days, when PLCs were not all that reliable, we used a pair of timers, one ON delay, and one OFF delay as part of the ESTOP string, and, the PLC would turn an output ON for 2 secs, and OFF for 2 secs. The timers were both set to 3 seconds (fixed, actually), so that, when the PLC went off line for any reason, the ESTOP would drop out.
I have never understood why designers don't put stuff like this in PLC-controlled systems...and, at my age, I guess I never will.

There are many instances where the PLC absolutely needs to take control when it comes back on-line, and so long as it doesn't have a safety implication I see nothing wrong with that. Any areas that might be "infested" with humans should have been LOTO'd out, or the area protected by safety equipment apart from the PLC.

The notion that the PLC can't "restart" processes is often "false".

You said "when the PLC went off line for any reason, the ESTOP would drop out".... The PLC going offline is not an emergency, so why trip the ESTOP ?? ESTOP IS ALWAYS HARDWIRED !!!!

There are hundreds of installations around the world where the power supply is intermittent, so restoring power shouldn't be treated as an emergency stop, no-one hit an estop, and there were no process conditions that declared one. Estop is the wrong way to handle it. ESTOP IS ALWAYS HARDWIRED !!!!

Restoration of power should allow the process to continue where it left off if safe to do so.

I would even go so far as to state that an ESTOP can only be instigated by a human, the PLC should NEVER declare an ESTOP, any condition that isn't an Emergency is simply a condition that can be accommodated. PLCs don't (as yet) have the ability to hit one of them Red mushroom buttons.

 

[ASF]

Once more for the people in the back:

ESTOP IS ALWAYS HARDWIRED !!!!

Cue reminders about safety PLC's

 

[AGill]

Restoration of power should allow the process to continue where it left off if safe to do so.

This is just bad practice, and if ever absolutely required would be extremely rare. When the power goes out on the PLC, the system is in an uncontrolled state. Having equipment start up after a power outage isn't usually just a safety concern, but also concern for equipment damage. Brown outs, voltage drops/ spikes aren't good for electrical equipment.

How a system handles a power outage is always a design concern most often requiring intervention to resume, restart, etc.

 

[Aabeck]

How a system handles a power outage is always a design concern most often requiring intervention to resume, restart, etc.

+1 - I always force the MCR to drop if the PLC stops.

Also when the PLC starts on First Scan I make sure all controlled motors & valves are off, and I put the HMI in Manual Mode.

The operator will have to restart the MCR, then will have to put it in Auto mode (if safe to do so, otherwise get everything ready and positioned in Manual) Then start the process or cycle.

 

[James Mcquade]

AGill,

How a machine starts up after a power outage is PRIORITY # 1 !
The power may have been lost due to a blown fuse or someone could have pulled the disconnect because someone was caught in the machine.

the point is, you have no idea what the machine will do. A machine is most dangerous when power is first applied.

Someone may be caught in the machine and others are trying to get the person out, maintenance could be trying to work on something or remove a hung part. Lots of things can go wrong when power is restored.

You just can't turn the power on and allow the machine to start operating.

james

 

[AGill]

@James,
I completely agree.

 

[alive15]

Alright, I understand you better now, but I dont see why you cant just have a stop button and a start button? Just like a 3 wire start/stop Circuit. I wish I had better experience with the ladder creation functions on this site so I could just show you, but I do not. I'll give it a shot I guess.

    Stop Start                M1 EN 

----[\]---[ ]-----------------[OTE]---

        |     |

        |M1 EN|

        |-[ ]-|

Stop and start would both be buttons on your HMI, when you press start, it starts until a) someone hits stop, or b)when power is lost.

just like a 3 wire start stop.

EDIT: Tried to see what happpens when i put it in ladder tags.

Thanks dude, this sounds like it will work for sure. When the power goes out, the output will cut off, so when the power comes back on, the motor should not start. I'll use this method instead of OTLs and OTUs. Thanks again.