UPS Rating

[global9000]

Hi,

I have a 3phase 208V 15KVA ups rated for 0.8 p.f. This is an older unit (1994) and the manfacturer was bought out by another company and I am unable to get any support on it.

Does this rating mean I can supply a 0.8 PF load up to 15KVA and a 1.0 PF load up to 12KW? I'm assuming that the unit is only rated to supply 12KW of real power and anything above that could lead to overheating.

 

[RichardMi]

I believe you have interpreted the ratings correctly.

 

[ScottyUK]

Sounds a reasonable assumption.

Who is the manufacturer? There have been a lot of buyouts but most of the companies will support the legacy products. Often it is little more than a rebranding exercise when the original company gets absorbed by the multinational.


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Sometimes I only open my mouth to swap feet...

 

[waross]

If it was a generator you would be correct.
For a UPS it is possible that it may handle 15 KVA at 100% PF, but have commutation problems with a load of any magnitude at less than 80% PF. The implications of poor power factor, and the supply of real power are quite different between generators and UPSs.
The KVA rating of a generator is determined by the maximum allowable current times the rated voltage(over 1000). It is the limit of acceptable heating. The KW rating is limited by the available power of the prime mover. If the prime mover is over powered, it can safely deliver KWs equal to rated KVAs into a unity power factor load.
I'm hoping for more comments here. I may be on the verge of learning something new.
respectfully

 

[alehman]

I think the OP has correctly interpreted the rating.

waross - I'm not sure I'm understanding your statement "the supply of real power are quite different between generators and UPSs." Are you saying the main difference is due to the commutation problems? I agree with your comments about generators. Additionally, generators have a power factor capability curve which takes into effect stator and rotor heating effects.

 

[waross]

In a generator the supply of real power is limited by the power of the prime mover. I see quite a few gensets in the 25KVA to 100+KVA range that are factory reconnected and rerated for single phase. The KVA drops By 1/3, the power factor rises from 80% to unity and the KW equals the KVA. The prime mover now has enough power to smoke the alternator if the current is not limited by the main breaker.
Example;
A three phase set rerated for single phase:
3 PH. 1 PH
75 KVA 50 KVA
PF 0.8 PF 1.0
60 KW 50 KW
The prime mover has enough power to supply 60 KW, whether on three phase or single phase. The alternator can only safely supply 50 KW on single phase.

In a UPS, the real power output is dependent on the allowable discharge rate of the batteries. The other components are current limited.
This is the basis of my comments regarding real power from a generator versus a UPS.
There are others on this forum who are better qualified than I to comment on commutation issues with a poor power factor.
I hope to hear from them soon.
respectfully

 

[ScottyUK]

Commutation troubles is a throwback to the days of thyristor-based UPS designs. These are getting rarer today now that fast high power switches such as IGBTS are available: most UPS systems use a PWM output to synthesise a decent representation of a sinusoid.

Crest factor is something which is definitely worth looking at if the UPS will be working at the top end of its capacity. A CF lower than 3 would set alarm bells ringing for me - if the UPS can't deliver the high peak current which typically occurs just before the voltage crest then the voltage distorts.


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Sometimes I only open my mouth to swap feet...