kVA vs. Watts
kVA vs. Watts
(OP)
If I have a single phase 120 VAC circuit that only powers 5 DC power supplies and 3 AC relays, what is my Power factor used to calculate kVA (kVA*Pf=Watts)? Is it 1 since I have no inductive or capacitive loads? Is this considered a resistive load? The circuit's max pull is 23AMP.






RE: kVA vs. Watts
However, for minimal loading and in the absence of any motor loads, as in your case, the inductive and capacitive effect of alternating current on the system impedance is considerably small. It is also safe to assume that the power factor is unity (1) but for accuracy, you may also consider the inductive characteristics of relay coils.
RE: kVA vs. Watts
The power supplies may have some capacitive contribution due to internal capacitors or inductive contribution due to internal transformer. Likely the power supply power factor would be lower under low load than high load (since power factor is a relative measurment of reactive vs real load).
One more complication introduced by the power supplies is harmonics... which cannot be classified as inductive or capacitive... but which may lower your power factor if you use the definition of p.f. = Watts / (VrmsIrms). The reason is that the harmonic currents increase Irms without contributing to watts. That doesn't fit neatly with the definition that p.f.=cos(angle between volt and current), but that second definition is based on assumption of no harmonics. Some people use a definition that the power factor [Watts / (VrmsIrms)] is composed of the product of a cos(theta) type of power factor and a harmonic power factor.
RE: kVA vs. Watts
just to play devils advocate here - 5 dc power supplies??? wouldn't happen to have transformers in the front end would they?? ==>> lagging pf big time??
ac relays ??? the ultimate in coils of conductors === >> very inductive ??
It may be that this load COULD be very inductive. The manufacturer will give you the va if you ask . Sometimes it is in the literature distributed with the units
Let us know how it goes
Don
RE: kVA vs. Watts
VA1 = W1 + jVAR1
VA2 = W2 + jVAR2
...
VAn = Wn + jVARn
-----------------------
PFtotal = Sum(W1..Wn)/Sum(VA1..VAn)
This does not account for harmonics; however, it may be a reasonably approximate power factor.