# UPS and Step-down Transformer Capacity

Discussion in 'General Electronics Chat' started by Mr.Smoke, Oct 22, 2013.

1. ### Mr.Smoke Thread Starter Member

May 26, 2006
12
0
Dear All,

Pls help me regarding this issue, if I need an output of 120kVA which is from stepdown transformer (400V to 200) fed from UPS. The capacity of UPS should be?

Is there any relation, or UPS of 120 kVA would be enough?

Thx v much

2. ### PackratKing Well-Known Member

Jul 13, 2008
843
219
120 kVA ?? As in 120,000 watts ?? VA - Watts = same difference

Toward what end ??? Are you backing up computer power at the Pentagon or some such place ???

Where would one obtain such a huge UPS anyway... ??

Either that, or I missed the point completely...

3. ### elec_mech Senior Member

Nov 12, 2008
1,501
195
First, are you talking about a solid state UPS or a rotary (flywheel) UPS? We'll assume the former for the rest of the discussion.

Why are you using a stepdown transformer with a UPS? Is the voltage at your location 400VAC and you need to supply 200VAC to your load?

I work for a company that sells solid state UPS's and frequency converters. Whoever you decide to use, ask them if they can supply you with a UPS that accepts the input voltage you have and outputs the lower voltage that you need. While this is not a standard request for most UPS manufacturers, it is possible since solid state UPS units typically have transformers built in to them.

You could use a stepdown transformer on the output of the UPS, but you need to keep the following in mind:
• Transformers will have an in-rush current when power is first applied. The UPS will likely need to be oversized to account for this in-rush, otherwise it may shut its output off since this will be seen as an overload condition even though the in-rush lasts a second or less.
• Anytime you change power, you end up with loss. You'll need to know the efficiency. Taking the in-rush out of the equation for the moment, if the transformer is 88% efficient, the UPS will need to be 120/0.88 = ~136KVA minimum.
The transformer supplier should be able to give you both of these values. However, I hope you see it is far better and often less expensive to have this done in the UPS itself.

Additionally, if there are any inductive motor loads, the UPS will also need to be oversized to account for their in-rush as well.

Are you only needing to change the voltage or do you also need to change the frequency as well? I ask because we'll get calls from people who contact electrical supply houses who tell them a transformer will work as a frequency converter since they need to change the voltage. This is not the case.

Let us know if you have any questions or need something clarified.

My company makes them up to 500KVA, although our primary business is frequency conversion. Some big dog UPS manufacturers are Eaton, APC, Liebert to name a few.

PackratKing likes this.
4. ### Mr.Smoke Thread Starter Member

May 26, 2006
12
0
Thx v much for the reply

Why are you using a stepdown transformer with a UPS? Is the voltage at your location 400VAC and you need to supply 200VAC to your load?

Yes, and the type of UPS is solid-state IGBTs based

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Actually, we've faced overload problem that's why I'm interested in calculating capacity

Is there any reference material for the said solution "if the transformer is 88% efficient, the UPS will need to be 120/0.88 = ~136KVA minimum."

i

5. ### elec_mech Senior Member

Nov 12, 2008
1,501
195
This is basic physics, but it appears they're more efficient than I thought. You can read more here. You can also look here and here.

Also note the UPS will have an efficiency too. Ours, for example, have an efficiency of ~93%, so a 120KVA UPS under full load will require: 120/0.93 = ~129KVA of input power.

We could be a lot more help if you elaborated on this.