1) What is the difference between "KVA rating" and "KW rating" of UPS?
The equation is KW= KVA *PF. In general KW is active or useful power and KVA is the vector sum of active power and reactive power. The equation is kW= KVA *PF.
2) What is the Input Power Factor (P.F) of the UPS at different loads?
Power factor is a function of phase displacement angle between voltage and current and distortion factor. It is calculated as a ratio of real power to apparent Power drawn by load. PF is low at lower loads and goes on improving at higher loads. PF is also lower at higher input voltage and goes on improving as voltage reduces. Input Power Factor is independent of type of load for double conversion technology.
For nonlinear loads the power factor is low due to the presence of high percentage of harmonics present in the load current.
3) How to arrive at correct UPS capacity for a given load?
Estimate steady state (normal running) KVA of the total load. Determine inrush KVA of the largest load in the total load distribution. The rated KVA capacity of the UPS should be 85% of the sum of above two [Steady state KVA + In rush KVA].
This is under the assumption that all loads are not switched on at the same instant and largest load is started while all remaining loads are running
4) What is the % total harmonic distortion (%THD) of the UPS output voltage and how it is measured?
Total Harmonic Distortion is measured using Distortion meter. For linear load it is less than 1% typical and less than 2% maximum. For nonlinear load with crest factor 3:1 it is less than 5%.
5) Is UPS output isolated from its input?
Yes, double wound isolation transformer of full load UPS rating is used at inverter output, which ensures complete isolation from input.
6) Can the UPS output neutral be earthed?
Yes, provided bypass isolation transformer is used or the operation of static switch is permanently disabled.
7) Why there is no alarm or indication for any fan failure?
Fan failure is not directly indicated because on failure of fan heat sink temperature of UPS will increase and this rise is depends upon percentage of load on UPS. Fan failure indirectly indicated as 'Over temperature'.
