How to specify UPS requirements

[Daryl 1]

Hi folks.  whilst I'm now comfortable enough doing programming PLCs/SCADA systems these days, the electrical/electronic side of things still eludes me.

I'm looking for some help on how I go about specifying what size of UPS I require.  A significant change is due to occur to the electrical setup in one of our facilities and I've been asked about specifying a UPS specifically for two PLCs and an HMI.

At this point, let's make some assumptions.

Assume I need four hours at most, two seperate PLCs, one which is process control with 8 I/O modules, the other is a safety PLC that defaults outputs to fail-safe when something goes wrong and again has 8 I/O modules, and finally a standard Windows desktop PC with touchscreen monitor for the HMI.  Also assume the rest of the plant has power/UPS requirements already in place.

I'm sure this isn't just a straight forward issue, and there's probably other questions that need answered before doing a proper specification, but even a pointer in the right direction on how I go about his would be appreciated.

Also, any other advice on how to handle power cuts when using PLCs and HMIs would be appreciated.

thanks.

[Crossbow 295]

This is not even close to a cut and dry application.  There are many variables.  

My first concern is even if you put a UPS on the PLC, you will not be able to run your program if all of the inputs and outputs are not also powered by the UPS.  It's no use to keep the PLC powered on if all inputs turn off.  So chances are at least one power supply besides the PLC will need to be backed up.

So you need to figure in the current draw of the PLC power supply, current draw of the safety PLC power supply, current draw of the HMI, and current draw of any power supplies or output devices you may also want to keep alive.  The specs for those should all be available in the product manuals.

That being said, I did a panel many years ago where I had to keep a PLC (2 racks) and an HMI (12") alive long enough to unplug it from the wall (utility power) and plug it in to a generator without the machine going down.  All of my outputs (motors, motor starters, VFDs, etc. were constantly powered from a diesel generator so they did not need to be accounted in my calculations.  Just the PLC, the 24V power for the panel, and the HMI.  I went with a 1KVA UPS and I had it unplugged for over 15 minutes without loss.

[Daryl 1]

Thanks Crossbow, that's a good pointer forward with the advice on current draw and your own example.  Do the input/output cards  and CPU all count as one panel being on the same backplane?

Also, I know it's more a basic electrical question than I should be asking (my background is programming, not electrical) but with a UPS is the KVA rating used for current draw or length of time the UPS will provide power for?

thanks.

[gleblanc 1]

On 2/3/2016 at 7:48 AM, Daryl said:

Thanks Crossbow, that's a good pointer forward with the advice on current draw and your own example.  Do the input/output cards  and CPU all count as one panel being on the same backplane?

Also, I know it's more a basic electrical question than I should be asking (my background is programming, not electrical) but with a UPS is the KVA rating used for current draw or length of time the UPS will provide power for?

thanks.

The KVA rating is primarily used to determine the load that the UPS is capable of supporting.  That said, a UPS with a larger KVA rating will generally have larger batteries.  Also, most of the smaller UPS units (below 750 KVA) do not have connections for external batteries.  With an external battery connection, you can typically add additional batteries to get as much runtime as you want.  Longer runtimes also take longer to recharge the batteries after a power failure.

I've always sized UPS units so that they can clamp the full rating of whatever equipment is connected.  But using those same numbers to calculate runtimes always under-estimates.  You might be able to get a better idea of runtime by measuring the peak usage of the panel directly.