kVA vs. Watts 1

[santa]

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.

 

[Gene1203]

Theoretically, AC systems whatever the load maybe, shall have a power factor due to the line and load impedance that is a function of the frequency. That is why VA (volt-amperes) was introduced. The watts is for DC systems.

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.

 

[electricpete]

As Gene mentioned, the relays have some inductive contribution (although primarily resistive).

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.

 

[don01]

hi guys
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

 

[jbartos]

Suggestion: The total power factor, PFtotal, can be measured by an oscilloscope or power factor meter, if you have things built. Else, technical specifications of the power supplies and relay shall include it. Once you obtain them, then calculate
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.