Ups

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ptonsparky

ptonsparky

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We are going to install a small UPS, 350 VA, to maintain operation of a small PLC and HIM during power failure so we can ensure that all operations are cleared and information printed prior to manual or automatic shutdown of the PLC. When testing at the shop we noticed that during power failure, the PLC & HIM are not affected by the switching time of the UPS but the small control relay that is used ahead of the inputs and outputs drops out. This will not be a problem for our application but I was suprised. Is this normal for an inexpensive UPS? Our current draw during shop test was less than .1 amp at 120V.
 
Yes, this is quite normal for the "cheaper" ups systems that are "off-line" systems(1-2 millisecond transfer time).The more expensive (30-50%) are full on line systems.Electrical equipment that does not have enough electrical ride through will drop out every time this cheaper ups switches power sources.
 
If its a cheap UPS, it may be due to the frequency being out of sync when the unit switches over. so when it switches over, the relay notices the change, essentially the voltage goes to zero for a small period of time, the HMI and PLC internals may be such that they do not "notice" the drop off.

You could install a small capacitor across the coil of the relay to help maintain a voltage for when the UPS switches over. I think that will work. As far as the value of the cap, you would have to play around, make sure its not too large or else your response time of the relay will be affected,

But im not 100%, just my initial thought.
 
Yes it is normal. The type of UPS you are describing is really more of a vibrator rather than a UPS. Technically it is a standby type UPS, meaning it does nothing other than set there under normal conditions. When power fails, the control relay looses it coil supply from the input and operates. This turns on a very simple square wave oscillator, drives a pair of Darlington transistors that are fed to a step up transformer to convert the low voltage sqaure wave to a higher voltage that sort of resembles a sine wave with a lot of harmonics and noise.

These are not the types of UPS you would want to install on anything that is for critical operations. At 350 VA it isn't even suitable for a home PC.
 
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Another thing is that many of the inexpensive (and 350va is going to be "inexpensive") UPSes do not put out anything close to 120v. Unless one considers 90v to be close to 120v ...

Moral of the story -- before deploying a UPS, measure the output voltage under real operating conditions.
 
some of the 'cheaper' UPS have to be 'calibrated' by the vendor before being shipped out. which probably means that they ask the buyer what voltage is 'usual' for them and then set the UPS accordingly.
 
tallgirl said:
Another thing is that many of the inexpensive (and 350va is going to be "inexpensive") UPSes do not put out anything close to 120v. Unless one considers 90v to be close to 120v ...

Moral of the story -- before deploying a UPS, measure the output voltage under real operating conditions.
Click to expand...

So we tried a bigger load this morning. 150 watt inc lamp. Voltage stayed at 115. Fluke 87. Held good for about 15 seconds then whole thing shut down. Not real good at math but I believe 150 watt lamp is somewhat less than 350VA rating although box does also state 200 watt.

May still work, our load is very small and we only need a program scan to accomplish our task.
 
Output of cheap UPS systems are Quasi-Sine wave or square wave that are design to give an RMS value of 120V. If you are using a non-rms volt meter you will be measuring the average of the waveform and not the rms thus your low or high reading.

UPS manufactures play all type of games with battery run-time. The watt rating is usually way lower than the va rating and the battery run time stated is based on the watts. Typicall run time at full watts is usually 3-5 minutes. This is based on full charge of new batteries at 77 deg F. Anything outside of this parameter and you can expect large deviations from expected run time.
If UPS is new, I suspect you didn't let it charge for 24 hours.
 
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