UPs backing up UPS?

[ruble3]

We are undergoing an expansion at our plant - we were discussing the requirements for backing up PLC's (4) with UPS's - our IT guys want to use one large one, other people would prefer a separate one for each PLC - another suggestion if we went to one large one was a UPS backing up a UPS but they said this was a bad idea ( harmonics) I've seen other threads on UPS systems but didn't see any on backing up one with another ?

 

[robertjo24]

Every recommendation I ever read/heard is that it is a bad idea to connect one UPS to another UPS.
I would suggest using a separate UPS for each PLC, if the PLC's are distributed about the building or plant. This allows for maintenance of individual UPS's without disturbing the entire system. If they are all in the same general location, it may not make sense to install multiple UPS systems.
Another option, have one UPS, but individual (and isolated), battery backup for each PLC.

 

[stevenal]

Use one device. Multiple batteries scattered around the plant will ensure that one or more will be missed during maintenance, leaving you with dead batteries just when they are needed.

Why rectify > store > invert, when PLCs are available with DC power supplies? Leave off the inversion step to eliminate another failure point. An AC PLC power supply will rectify it again anyway. Use one (or two for redundancy) battery and charger, and distribute the DC circuit throughout the plant.

 

[AGomaa]

Your project is to add UPS to existing PLC, so you shall be able to specify the power requirements for each PLC, the power requirements for signals (typically you have existing power supplies for your I/O in each PLC installation), and distance between the PLC and the UPS room (if central UPS to be used)

A PLC with DC power supply is nice but check the distance between the PLC and the supply location for voltage drops etc. besides this is existing system so what do you have you may have to live with.
If power required is small and distances are long then one UPS per PLC shall be OK.
Also consider a bypass switch to bypass the UPS when in service or in case of failure.
For larger UPS you can ask the UPS supplier for redundant UPS system.

 

[illiterate]

The question is is it the right to back up a plc.
There are several different opinions.
I know that rockwell used to state in their manuals that if you use a UPs to back up the plc that the I/O should also be on the UPS so that the PLC Program woudn,t register the change of states of the inputs caused by the I/o
de-energising / energising.
Most Plc's have battery backup built in and without power on the I/o keeping the plc running is pointless

 

[AGomaa]

Normally if you lose power for PLC, you lose power to your motors, and other equipment so you may say it is pointless to have a running PLC with no power to field. The same would apply if you have UPS for your PLC and nothing for your I/O power supply. However the problem I found in a lot of places specially in manufacturing places that power cuts off for second then restore, then after one/two seconds it off again, a series of OFF/ON with second or less intervals. The result is corrupted PLC program. Regardless of the PLC brand you will use this will corrupt the program memory. A UPS is good solution for these installations.
In critical system you would like to still see the process even if power is not available for fiend by powering the I/O and the PLC from UPS.

 

[illiterate]

For Small < 1 sec power interuptions and power conditioning i would try a CVT constant voltage transformer

 

[peebee]

Re: &quot;UPS backing up a UPS&quot; -- This is a bad idea, but harmonics is NOT the main reason why it's a bad idea. The reason is that redundant devices in series REDUCE reliability, whereas redundant devices in parallel INCREASE reliability. If you wanted to use two UPS systems to increase reliability, I'd suggest investigating if the PLC could be purchased with dual power supplies, and source each power supply from a separate PLC. If that's not possible, I'd investigate a fast-transfer switch to select between UPS systems. I'd go with only one UPS before plugging one into the other.

Redundant UPS systems are probably overkill, though, for most PLC applications -- as previously mentioned, you've lost your process anyway. A cheapo UPS will keep your data logging going most of the time, and if that drops out every 5 or 10 years you can probably live with it (but maybe not).

One big UPS will fail less often than several small UPS modules, assuming they all have the same MTBF. Actually, large systems usually have lower MTBF numbers, which makes the single-module solution even more attractive. Assume for now, that all UPS systems have MTBF ratings of 200,000 hours -- therefore, as there's 8760 hours in a year, you'd expect a single-module system to fail, on average, every 22.8 years. However, if you're using four modules, you'll have modules failing four times as often -- that means one out of the four UPS systems will fail, on average, 22.8/4 = 5.7 years.

All that aside, I agree, go with the pure DC system if possible -- it's not clear to me if it is possible in your situation, given the I/O requirements, but that's already been discussed.

 

[stevenal]

If you go with the UPS, use the on line style. Folks at the local paper mill have told me that the switching in a switch mode UPS can interrupt their process for a voltage sag that they could otherwise ride through.