Single Phase and 3 phase

[StuMurr]

Can anyone help as I'm getting in a muddle. I have a UPS system with a 3 phase output rating of 400V, 50kVA 3 phase. This 3 phase supply feeds distribution boards. Each phase is then split to feed single phase loads of 230V. My question is what is the VA rating on each single phase? On the drawings I have each phase has a rating of 50kVA/3 i.e. 16.67kVA but in this instance the current doesn't add up. Am I being thick here? I'm trying to calculate with a route 3 but am getting no where, can anyone help with this frustrating calc?

 

[DanDel]

I'm assuming this is a wye(star) system, since you say the 3-phase voltage is 400V and the single phase voltage is 230V.
The VA rating for each phase would simply be the individual phase's current multiplied by the individual phase's voltage. Just don't forget that the voltage for each phase in this case is 230V.

 

[peebee]

Yup. 16.67 kVA per phase is an average and would be a relatively good guess, but to get better accuracy you'd need to measure the individual phase currents and multiply each one by 230/1000 to get the individual kVA on each phase.

Using "route 3" (acutally "root 3&quot, you'd take phase current * 400 /1000/root(3). Note that 400/root(3) = 230 so this is the same exact calc.

 

[jbartos]

Suggestion: The currents are vectors with some angles between them. They do not arithmetically add up but vectorially, yes. To circumvent their vectorial additions, add the power in VA=WATT + jVAR and when you obtain the total, VAtotal=WATTtotal + jVARtotal, calculate the total current Itotal since the voltage is known and it is assumed to be equal a constant.

 

[busbar]

Consistent with the comments of others, if the UPS load is limited to 72.5A per phase without harmonics, and all other nameplate conditions met, the UPS should not be overloaded. {50kVA/230V/3 = 72.5A}

 

[peebee]

busbar, I strongly disagree.

Most electronic UPS loads now have a power factor close to 100%. Most UPS systems have a power factor of 80 or 90%. That means most UPS systems can only be loaded to 80 or 90% of their kVA rating (equating to 100% of their kW rating) before they go into overload.

Same as with generators, its the kW loading, not the kVA loading, that is usually breached first with a UPS system.

 

[busbar]

So noted, peebee. No argument here. I stand corrected.