Timer question...

My situation is as follows: I have 2 A-B 523 VFDs that are each supplied 3 phase voltage through 2 contactors pulled in 3 seconds apart with a MicroLogix 1000 upon initial startup. In the event of a power disruption, I want to allow at least 2 minutes to lapse before the drives are able to re-energize to protect the drive. Re-energizing the drives too quickly can damage the drives.

So... I have 2 TONs (1 set for 3 seconds, 1 set for 6 seconds) and they sequence the contactors in 3 seconds apart. This is used for initial start up (say, when coming in at the beginning of the day)

What I need help with is putting in a 3rd timer: it should not allow a start up after a power outage until 2 minutes have lapsed. If more than 2 minutes of power outage has occurred, enough time has lapsed due to the power outage itself, and it should resort to the start up of 3 and 6 seconds.

Any thoughts? I thought about MOV statements along with timers but I'm open to any suggestions.

I would use the first pass bit (S:1/15?) to latch in an internal coil. This coil would run a 2 minute TON. I would then use this timer to enable your existing timer logic.

Opening and closing contactors is not the best thing to do for this, it stresses the pre-charge circuit inside of the drive..

You realize you can do this inside of the 523, right? Parameter 541 sets the Auto-Restart by selecting a number of attempts other than zero, and 542 sets the time delay between attempts. Set Drive #1 for 0.1 second, set Drive #2 for 120 seconds.

If you want 2 minutes before Drive 1 and another 2 minutes before drive 2 run, set drive 1 to 2 minutes as well, then use a run output contact from drive 1 as a permissive in the Run command line for drive 2

You already have timers that work in a 3 second interval between each other, on power up you can use the first scan bit or write your on version using ladder to server as your first scan bit.

Now quote "I have 2 A-B 523 VFDs that are each supplied 3 phase voltage through 2 contactors pulled in 3 seconds apart with a MicroLogix 1000 upon initial startup".

Initial startup meaning on its first scan. So basically you probably have logic such that on start up, to execute the timer sequence of the 3 second interval you wanted.

Problem with this is a power outage is pretty much the same thing as a first scan/ initializing of the plc because it will have to go through the first scan every time on power up. So that method won't really work, unless ofcourse their is a bit that will tell you a power outage occurred, but i don't think such a bit exists. Keep in mind of your OTE unless you use an OTL, the OTE goes back to false in a PLC go to run state, meaning when the plc goes from offline to online again.

OdinIII method can work but you will always wait 2 minutes every time the PLC powers up before your VFD's pull in.

Personally you shouldn't be having that many power outages to begin with unless, you have an operators thats constantly power of and power on the system, or the area that your are in has a lot of power outages

I don't usually let the VFD's just start on startup, i would incorporate start sequence which can have delays to avoid any power outage constantly dropping out issues.

I would also look into some parameter options on the VFD drives itself, what kind of VFD are you using?

sorry you actually mentioned the VFD type, in that case what jraef mentioned should be what you need

Gentlemen:
First of all, thank you all for your responses. Great support from you all.
Secondly, I was using contactors so that I could keep the operators from pulling the disconnect and re-energizing the drives without ample bleed down time. The contactors would serve to delay the re-energizing the drives... not start and stop the motors. I'm afraid I may have lead some of you to think that was what I was doing.
Finally, I think I will free up some I/O and let the drive take care of this task for me with the parameters suggested by jraef. I was having difficulty figuring out how the PLC would be able to detect a power outage and first scan. All of what you guys said made sense.

Thanks!

Gamecock said:

... Secondly, I was using contactors so that I could keep the operators from pulling the disconnect and re-energizing the drives without ample bleed down time. The contactors would serve to delay the re-energizing the drives...

Click to expand...

I'm not sure why you think this is a concern, did someone tell you that? The thing that stresses a drive from being de-energized and re-energized is in cycling the pre-charge circuit. On power up, the cap charging current can be very high, albeit short. So there is a resistor put in series when first energized. That resistor is shorted out with a relay or contactor on larger drives, but on little drives like this, the resistor is actually an NTC thermistor; NTC meaning that as temperature increases, resistance decreases. It saves on having to make room for an electromechanical device like a relay. Constantly opening and closing contactors ahead of the drive increases the thermal stress on this thermistor. Re-energizing before the caps bleed down completely would actually LESSEN the stress on that thermistor, hence my comment about why you think that is a problem. maybe it is a holdover from days gone by when there actually was a contactor and a fuse, so allowing it to bleed down allowed the DC fuse link to cool off and not pop on re-energizing? Don't know. But there is no DC fuse link now, I assure you.

Maybe this being a problem is something else I am unaware of, so if you know why, please let me know.

jraef,

Actually, the A-B drive manager told me that cycling the power without ample bleed down time is detrimental to the drive's pre-charge circuit.