I'm a little confused here. Are you asking about Power Factor or harmonic distortion?
VA (volts x amps) are what you are supplying. The watts (V x A x cosine angle) is how efficiently your load works. The VAR's (volts X amps x sine angle) are how much reactive energy the load supplies.
In resistive loads, the voltage and current peak simultaneously (as Winnie said).
In inductive loads (motors, ballasts, etc.) the counter electromotive force (emf) created as the magnetic field builds and collapses across the winding opposes the current that is creating it, causing the rise in current to occur behind (lag) the voltage. So the farther apart the two peak, the less efficient the use of the VA. When they get to 90 degrees apart (cos = zero, a perfect inductor), the watts are zero regardless of the VA. The sine of the angle would be 1, so VAR's would equal the VA.
In a capacitive circuit, the opposite occurs, but from electrostatic forces rather than magnetic ones. In any event, the current peaks AHEAD of the volt, with the same effect of being less efficient.
Since the two are opposite effects, they cancel each other out. That is why you see capacitor banks installed next to large motor loads. A motor (inductive) load running at 100 VARs with a 100 VAR capacitor bank connected to it will bring the source VA back to unity (100% PF).
Say you have a motor pulling 10 amps at 100 volts with the angle between the current and voltage being 30 degrees.
100 volts X 10 amps = 1000 VA
cosine of 30 degrees = .83
sine of 30 degrees = .5
100 volts x 10 amps x cos of .83 = 830 watts
100 volts x 10 amps x sine of .5 = 500 vars
Now, if you put a 500 var capacitor in parrallel with the motor, the vars cancel each other out. Since the source now only has to supply the VA to match the watts, the current from the source can drop to 8.3 amps, saving transformer capacity, line losses, voltage drop, etc.
Harmonic distortion. If you are familiar with SCR's, you know that they turn on and off almost instantly. Picture a voltage sine wave feeding a resistive load in your mind. It is smoothly building and dropping to zero at 60 cycles. So if the SCR turned on halfway through this wave, you would see the current wave form go from zero to maximum almost instantly, creating a wave form that appears chopped. If the SCR did this several times in the cycle, the current waveform would have a very sawtooth appearance.
Now, you know from ohms law that whenever you apply a load to a voltage source, you will get a voltage drop across that source. So in the scenario above, each time the SCR fires, there will be a corresponding distortion of the voltage sinewave as the sudden current flow causes a small drop at the source. As long as this drop is insignificant, nobody sees it. But when it becomes larger, you start seeing the same distortion on the voltage at the source, and anyone connected to that source starts having that distortion applied across their loads. And if that particular distortion happens to create a third harmonic, then you can start getting neutral problems in 3 phase systems.
Did that make any sense?
