Power factor relation with load

Question

I was reading about power factor in electronic devices and I found charts showing that usually the power factor varies (usually decreases) with the load. So if you have a device that either works with 0% or 100% of load it's somehow easy to develop a power factor corrector (passive) but if your device varies the load from 0% to 100% it gets more complicated.

My question is: the device can be seen as a Z load. This Z (impedance) load is composed by a R (resistance) part with a X (reactance) part. So if my device varies the load by changing the relation between X and R (for example keeping same capacitors and inductors values but varying the resistance somehow), of course we then have a different final Z value so we have a different PF. But is it possible that in some cases the X varies like R so the resultant Z (and therefore the PF) remains the same for every load?

Furthermore, is it possible to map the device PF x Load curve and create a PFC that follows the same curve (even though if you change something in the device you have to do it again and considering simple devices where there's just a single dial to vary the load)?

Answer 1

It is already what active (continuously tracking the complex power) compensation systems do.

Furthermore, is it possible to map the device PF x Load curve and create a PFC that follows the same curve (even though if you change something in the device you have to do it again and considering simple devices where there's just a single dial to vary the load)?

It is possible, why not, and also a good idea for few states. If you have a machine with 2 power states, then you need 2 pre-calculated reactance exclusively selected by a simple reactive power detection logic. Or it can even somehow be commutated by the dials on the machine itself.

Answer 2

But is it possible that in some cases the X varies like R so the resultant Z (and therefore the PF) remains the same for every load?

Take a look at this phasor diagram: -

For power factor correction (aka resonant tuning) the capacitor is chosen so that it's reactive current totally cancels the inductive reactive current and yields just a small current (I, due to R) taken from the supply (U).

So, if only R varies then a fixed value of C cannot keep power factor at unity because the inductive reactive current will change.

This is a specific case of a parallel tuned circuit where the resonant impedance (i.e. impedance where the phase angle is zero) is dependent on all three components at a particular operating frequency: -

\$\omega_0 = \sqrt{\dfrac{1}{LC}-(\dfrac{R}{L})^2}\$

Furthermore, is it possible to map the device PF x Load curve and create a PFC that follows the same curve (even though if you change something in the device you have to do it again and considering simple devices where there's just a single dial to vary the load)?

I have no idea what you mean.

Answer 3

is it possible that in some cases the X varies like R so the resultant Z (and therefore the PF) remains the same for every load?

Very much possible. Imagine a plant with several identical machines, an office full of computers with non-PFC power supplies, or a hall with several groups of LED lights using similar drivers.

is it possible to map the device PF x Load curve and create a PFC that follows the same curve?

You don't really compensate for the power factor, you compensate for reactive currents which drive your power factor away from 1. Whether the reactive current in your load increased twofold because its internal R changed or because you have connected a second identical load is irrelevant, you'll still need to apply twice as much correction.