Ups

jcurado

Member
Join Date
Sep 2003
Posts
49
This is a bit off topic, but hope you can help in a problem I’m facing.
We will build new equipment to an Asian country. This will be one electrical furnace (700 KW) with 32 tyristor to control the temperature.

I was told that the power supply in this country is not steady. Changes off more than 15% may occur to the 380V main supply.
I’m thinking in one UPS to supply the PLC, HMI, temperature regulators and contactors. What normally do you do in a situation like this? An office UPS should do or I must use an industrial UPS (if there is such thing).

What do you think about the tyristors? They will survive to the power changes?

Thank you for your help.

João
 
I would recommend a constantly-online UPS (not a hot-standby UPS). Its a bit more expensive.

An alternative could be a socalled voltage stabilizing transformer, we use it when there is a lot of noise or spikes in the supply (it is very effective at cleaning these out of the supply voltage). It cannot take care of even a short power failure, but in such cases the system will have to be stopped anyhow. Your application may be different.
 
By my experiences, I would lean towards a transformer. Though the UPS would give you a steady output and ride through (on a power failure), I have found some potentially dangerous issues with them acting up. Howerver, we have switched to a different brand and developed a rigorous PM schedule. But I still don't like them. I'm in the process of getting the systems powered by them switch to one of our motor generator feeds.
 
Hi Jcurado,

Changes off more than 15% may occur to the 380V main supply.

What kind of equipment you should use depends on several things

1. What is the maximum power drop (15%?) and is it a one, two and/or three fase powerdrop.

2. What is the lenght in time of your powerdrop. 0.5 sec or is it more like 30 minutes.

3. Is there a possibility to regain some power?

If your power drop is less then 30% and the time that it occours is less then one minute. You could consider a DVR (dynamic voltage restorer) it costs about 200 €/kVA.


Another option is a continous rotating flywheel together with a dieselgenerator. The minimal power is then 250 kVA. and it cost about 150000 €.

But,
what Yesper suggested is the sollution to your problem. Just install an a constantly-online UPS. Please take in account the everything else stop working, and you probably need to foresee an automatic restart.

Rudi
 
I do not particularly like a UPS on a PLC. Have had some problems. Usually use a battery charger, batteries and a 24vdc/24vdc switchmode voltage regulator. The batteries also supply the inputs/outputs etc. The switch mode regulator allows the battery voltage to drop to 19 volts and still supplies 24vdc to the PLC.
 
on ships we are mostly using a charger and batteries for the controls.
when blackout all stops but control still continues on battery.
no maintenance and 24 volt is nice without **** on it.
i never use dc/dc as they are also inside the plc.


however the contactors are all high volt types.
when power fails you wont need any heating.
i would use only one small thyristor controller and have the rest on contactors if possible.
special for ovens always start slowly as all coils are cold and need lots of amps to heat up.
 
I normally dislike UPS systems, and feel they cause more problems than they solve. However, in your case it may be the right way to go, since it will not only provide transitent protection and filtering but also eliminate brwonout problems.

However, you need to consider the implications of maitaining PLC power when the controlled equipment is down, and add logic accordingly. Another option is to have a hardwired relay that cuts the UPS from the PLC and control circuits when the main power goes out, perhaps with a time delay relay.
 
Thank you

Thank you all for your replies.
I will get be here when I have more information about the project

Fritz - I don't have answers to any of your questions, and I believe there will be none untill I make my way to the other side of the Earth to the set-up of the equipment.

Have any of you find a place with such power supply problems? I'm used to 5% but 15%... (the costumer said it could be more)

João
 
Last edited:
I use primary switch-mode 24V dc power supplies with a wide input range
This is the spec for The Seimens SITOP

Rated voltage 85-132V, 175-550V
Frequency range 50/60Hz
Power boost rating for
5A model 15A
10A model 30A
20A model 60A
40A model 120A
Input current 1·2A-9·6A (model dependant)
Output voltage 24Vdc
Output voltage range 24·0 to 28·8Vdc
Operating temp. 0 to +60°C

This should give you plenty of range on your power suppply

I have just installed a 6EP1337-3BA00 960W unit. This is powering the controls for 7 x 300A contactors.

Use a separate power supply for your PLC and HMI.
 
Oh yes,

Siemens has a range of 24 VDC UPS.
It could be an alternative to a standard 110/230VAC UPS if you can hook everything unto 24VDC.
The advantage as I see it, is that you can keep some critically important IO alive (not just the PLC and the HMI).

To João: My experience from China in particular says that the problems can be many, long power outages, very short power outages, brown-outs, spikes, noise.
I would programatically try to trap errors like contactors disengaging caused by a short power outage or brown-out.
The problem is if a short power outage or brown-out can put the PLC in an undefined state from which it cannot recover. You could add a voltage-monitoring relay to the system. This relay could then activate an audible alarm in case a brown-out has occurred. The operators must then investigate and take appropriate action.
That said, I have seen many simple control systems in China with no particular measures taken at all. The operators are simply accustomed to restart everything when required.
 
Ladder structure

There is one more thing that may be worth mentioning here.
When you design your ladder logic pay attention to what happens to the logic when power goes momentarily off and then back on. Will you lose your ladder logic generated memories?
In critical applications, provided that you don't jeopardize the safety, you might want to consider using latching relays, so that when the power comes back on, your system will restart with no hick ups.
 

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