Inputs don't work on UPS power??

elevmike

Member
Join Date
Feb 2004
Location
Detroit, MI
Posts
4,012
So I just got back from a jobsite where we installed a UPS system that provides 240 vac 1 phase to our control panel. The UPS actually powers a control transformer 500va 240 - 120 vac. The plc is a AD DL 240 CPU. The PLC goes into the run mode but the AC inputs (D2-16NA) don't light up or register an input in the program. I can read voltage applied to the inputs, but can not see them in the program, and the leds don't light. On normal power all is good. Go figure??? I plan on returning to the job with a scope to look at the UPS sine wave. In the mean time any suggestions or comments would be appreciated.

Thanks, Mike.
 
I plan on returning to the job with a scope to look at the UPS sine wave.

That is most likely where the problem will be found. AD plc's do like to have a fairly clean sine wave. Problem is that many UPS systems tend to put out more of a square wave than a sine wave. Running it through a transformer helps, but in some cases it isn't quite enough.
 
sounds to me like your inputs aren't picking up a correct reference point to work correctly????????? I would say that the inputs would rectify the the applied voltage internally before sampling them so the quality of the sine wave probably would be of little significance if the processor is holding run mode and sounds like you can get online ok I reckon the power supply has gotta be clean enough I am not familiar with these plcs but when you run from the normal supply your neutral is usually bonded to the ground wire perhaps this is why it works with normal power??just a thought!
 
Last edited:
UPS does not provide proper voltage

I had this exact same problem with the 06 plc and had to use a more expensive ups that provided a sine wave and not a square wave. The minimum voltage input the plc accepts is about 90 vac, using an analog meter I was only getting about 85 volts out of the ups. As I remember a digital meter will read 120 volts until you put a load on it and then it will read 85volts.
 
For some uninspired guessing. I'm going along with netvisor on the bonding. Did the L2 leg of the xfmr get bonded to a grounding point? What type meter are you using...digital or analog...(my curiosity)? What is the measured voltage at the inputs with the UPS-xfmr powering it? And a real long shot...frequency of the UPS output?

waggs
 
I have had similar problems years ago. Several UPS power supplies (Tripplite I believe, were built from an old Motorla mobile radio power supply design with switching transistors in a typical flip-flop circuit). Because of contract wording, they suddenly had to produce inverters that would work, i.e. had a true sine wave.

I think I would want to put the O'scope on the transformer output. The transforner will help smoth out the square edges to a point, but if there are spikes on the waveform, that could really confuse the daylights out of the plc.

Also, maybe some MOV's or a line filter, to be safe. (Perhaps I'm to conservative).

Like the previous posts, we found that our problem was a square wave, with a lot of spikes, and the "real" voltage was way off, not to mention all over the scale depending on load.

Also, the frequency was stable either, depending on the load, but I can't say that this would be a tremendous factor.

I have vauge recollection of one project 8 or so years back, where we had to pull the AC input cards and convert a machine over to DC inputs do to AC line quality. This was caused by horrendous EMF interference that could not be easily filtered out, not becuase of a UPS.

UPS on elevator controls??? To control dynamic braking perhaps?

Sell them a nice standby generator!

regards.....casey
 
Last edited:
I may be stating the B#$%@ obvious but have you checked what the UPS supplies?
Where I work, only the PLC power supply (and SCADA PC) is wired to the UPS. All I/O just dropps out. Unless you specified differently, this may be what you got.

Doug
 
To further explain, it's a hydraulic elevator. Hydos are raised by a pump and lowered by gravity. When the power fails the UPS supplies 240 vac tot the main controller transformer. The PLC gets in input telling it that the car is on UPS power. The elevator is lowered to the bottom floor and the doors open, and the system shuts down.

The UPS was spicificly designed for this application. I talked to the manufacture and they told me to buy a better UPS with an inverter output. We have done this before with the same UPS and PLC with no problems. So I will have to look at the other files to see what changed. The output on the xfrmr is 500va at 127 vac. Maybe a line filter or something??? I have to do some more research.
 
Now it makes more sense.

The load is probably quite similar, so that shouldn't be an issue.

Perhaps this ONE inverter came off the line with a problem. No doubt, you probably don't have one to readily swap out, and I wouldn't want to buy one just to try it.

Line filter is probably a good choice. I've used emcor filters in nasty shop environments, but they were on the shelf. You probably have some kind available.

PLAB B: When I was a tech with Peoria Police, thw chief requested an hour notice on ALL power outages to get an on-call technician in to start up the generator and be there to babysit it. This included an hour notice for any drunk that may take out a power pole. ok....

regards.....casey
 
Mike,
Probably you have found the problem. But, you did make sure that your Neutral side of that 240/120 vac transformer was connected to your PLC (and not the original neutral from the normal power supply)? I am sure you did check that.
 
Lancie1,

Yep. The UPS puts out 240vac and drives the primary of the main controller transformer, which is grounded on the neutral, along with everyting else. The output from the UPS is a chopped square wave, and AD confirmed that the input requires a real sine wave. I cant go to DC inputs due to other issues. So the inverter on the 115vac controller circuit seems to be the best answer right now. Actually I'm going to add the inverter to the UPS, which only comes on during a power failure. I talked to the manufacture and they sent me a drawing on the changes required on the UPS to do this.
 
Just in case anybody's interested, we resolved the issue by installing a true sine wave inverter from Power Bright.

Link: http://www.powerbright.com/aps300.html

The 12dc supply connects directly to the battery and charger so it's on all the time. But the 110vac output is only connected via a "Power Failure" relay when the power fails. In essence the power is cycled on the board, so it's not a true ups system. But the end result is the inverter operates the PLCs AC inputs. (y)

Have a Great Weekend!!! Expecially all you Dads!!
 

Similar Topics

Compactlogix controller, program has 28 conveyors that use TON's to start the conveyors. The TT sounds a warning horn during start and the DN...
Replies
5
Views
86
I am converting a SLC 500 to a Compact Logix. I plan on using a Compact Logix 5380 with conversion Kit. The problem is that the analog input cards...
Replies
1
Views
116
Hi there, I'm doing some extensive testing and commissioning with a slew of new Emerson PACSystems RX3i PLCs. It would be convenient to...
Replies
5
Views
80
Problem: Our PLC can only output 4-20mA, but the actuators it needs to control, modulate based on a 0-135Ohm signal. Buying 4 or 8 individual...
Replies
7
Views
219
Hello I need to message read the entire 16 channel raw analog inputs from a 1769-L33ER Compact Logic controller to another 1769-L33ER Compact...
Replies
8
Views
231
Back
Top Bottom