Power factor

Status
Not open for further replies.
mbrooke said:
Ok, lets say C goes down, does voltage not drop across a series R?
Click to expand...

Yes
as C gets smaller its Z (actually Xc) gets larger ( Xc = 1/(j 2 Pi f C) )
So total loop Z increases
Therefore current gets smaller (assuming a fixed V, i = V/Z)
since current gets smaller so does Vdrop = current x R
 
Last edited:
Ingenieur said:
Yes
as C gets smaller its Z (actually Xc) gets larger ( Xc = 1/(j 2 Pi f C) )
So total loop Z increases
Therefore current gets smaller (assuming a fixed V, i = V/Z)
since current gets smaller so does Vdrop = current x R
Click to expand...

Makes perfect sense. :) However, what would the power factor be in such a case? Take a simple Home Depot 4 watt LED for example.
 
mivey said:
I thought we were talking equal C & L? If we have a net series RC or RL circuit then as the reactance increases then R sees a lower voltage.
Click to expand...

We are, but also what happens when each value changes in relation to the other. However, I can picture parallel X and C, and series X and R, but I still can not understand what happens when X and C are in series. For example, I have never seen a reactor HID ballast with a cap in series for PF correction...
 
mbrooke said:
R is resistive impedance in ohms and X capactive impedance in ohms?
Click to expand...
Yes........but....
In power circuits, capacitors are often used for power factor correction to compensate for lagging power factor.
Industrial loads typically have lagging power factor. The current lags the voltage. This is usually expressed as as a ratio - kW/kVA. Sometimes they are referred to as real and reactive power respectively.

The result is higher current than would be needed for the kW. Larger conductors and more losses. Bigger transformers, switchgear, the whole nine yards if you get my drift. Things that are dimensioned based on current rating.

Capacitors are often used as a fix or improve this situation. Power factor correction. PFC.
In this application, it is not their impedance that is defined. It is kVAr, the r meaning reactactive. Being leading power factor, it compensates for the lagging power factor.
 
Besoeker said:
Yes........but....
In power circuits, capacitors are often used for power factor correction to compensate for lagging power factor.
Industrial loads typically have lagging power factor. The current lags the voltage. This is usually expressed as as a ratio - kW/kVA. Sometimes they are referred to as real and reactive power respectively.

The result is higher current than would be needed for the kW. Larger conductors and more losses. Bigger transformers, switchgear, the whole nine yards if you get my drift. Things that are dimensioned based on current rating.

Capacitors are often used as a fix or improve this situation. Power factor correction. PFC.
In this application, it is not their impedance that is defined. It is kVAr, the r meaning reactactive. Being leading power factor, it compensates for the lagging power factor.
Click to expand...


I agree, however in a dropper ballast, does a capacitor not actually make power factor worse?
 
mbrooke said:
I agree, however in a dropper ballast, does a capacitor not actually make power factor worse?
Click to expand...
A series capacitor instead of a transformer can make the power factor worse. But that power factor is on the capacitive side. It will partially compensate for low PF on the inductive side from motors.
It will also be closer to a pure displacement (phase) power factor reduction than the distortion (harmonic) low PF caused by capacitor input rectifiers and power supplies.
 
GoldDigger said:
A series capacitor instead of a transformer can make the power factor worse. But that power factor is on the capacitive side. It will partially compensate for low PF on the inductive side from motors.
It will also be closer to a pure displacement (phase) power factor reduction than the distortion (harmonic) low PF caused by capacitor input rectifiers and power supplies.
Click to expand...


Thank :) I had a suspicion it would be worse. But how does phase displacement tie into harmonics?
 
mbrooke said:
Thank :) I had a suspicion it would be worse. But how does phase displacement tie into harmonics?
Click to expand...
It doesn't really. They are two different things. Displacement and distortion.

Examples.

A common (cage) induction motor usually has a lagging power factor. The current lags the voltage. Capacitors on the supply can be used to compensate for this because the have a leading power factor to cancel out the lagging power factor of the motor. Hence the term power factor correction or PFC.

Distortion, the other flavour, is caused by non-linear loads. Things that draw non-sinusoidal current. VFDs are a common example that gets attention. As a system designer, you know that going in and the design needs to be compliant with applicable standards. The solutions for this are just a tad more complex than slapping some PFC capacitors on the supply. Actually, naked PFC is best avoided in this situation. It can bite you in the bum.

Passive or active harmonic can be used. Multi-pulse rectifiers. Active front VFDs is another. Such solutions obviously come at a cost. Good system design is essential.

But I might mention in passing another issue that tends to slip under the radar. We live in a world where there is an ever increasing load of non-linear devices, much of it at domestic level. Chargers, computers, televisions, CFLs. Such devices generally fall below the level that has to be addressed. But the sheer number of them from every household causes significant harmonic distortion on the supply.
 
Status
Not open for further replies.
Top