Since my job is related to led drivers and ballasts i get confused with some questions. I see that the capacitor that is placed after diode bridges decreases the power factor and when we remove it we can achieve a higher pf. So i think that our power factor is a leading pf.
How do you compensate leading power factor?
Also, why we know most of the loads as inductance loads? and why do we just have capacitor banks, not inductors bank?
The capacitor in a power supply following the bridge rectifier is not causing a phase shift in the current. Instead, it is contributing to harmonic distortion of the current waveform, which is a different method of creating a poor power factor.
When you remove the capacitor, the conduction through the rectifier is more continuous. When you add the capacitor, the conduction now occurs only in short pulses at the peak of the voltage waveform.
Putting an inductor in parallel with such a load would solve nothing — you would now have a lagging current component PLUS a harmonic distortion component.
However, putting an inductor in series with such a load can have benefits. It will filter out the higher-frequency components of the harmonic distortion by forcing the rectifier diodes to switch on and off more slowly. Note that it will still cause a lag in the current relative to the voltage, but you can probably find a "sweet spot" value for the inductance that balances the two effects to create the best overall power factor.
Traditionally, large electrical power consumers (factories) 'spend' most of their power consumption driving large inductive loads (like motors).
Inductive loads cause a lagging power factor (since the current in the circuit 'lags' behind the voltage).
In order to compensate for these inductive loads, these factories can install power-factor correction equipment which, in its simplest form, is a bank of capacitors connected up to the power lines in parallel with their existing inductive loads with the size of the bank of capacitors calculated to approximately offset 'lag' caused by the inductive loads.
As you've noticed, LED drivers (and most other electronic devices running from the mains) have large-ish smoothing capacitors in them and yes you are correct - this does cause a leading power factor (opposite to inductive loads).
So - why don't we need inductor banks to compensate for these?
A couple of reasons:
I guess that it is cheaper to buy high power factor led drivers than try to fix lots of low power factor drivers, but if you are in a huge trouble (already bought, installed and got complaints), let us think. I'm not thinking about dimmable drivers.
Solution #1: about 23 years ago I had problems with low power factor and harmonics when designing a 2x32W 220V electronic ballast for fluorescent lamps. Since the load could be considered almost constant, the solution was to add a passive LC elliptical filter to the input tunned in the 4th harmonics (240Hz in a 60Hz grid) in order to attenuate strongly the 3rd and 5th order harmonics; 7th, 9th and above not so much.
Cons: needs you to find the calculation, not useful for small quantities (many of them with their inputs tied together would be better to lower the amount of filters), the DC voltage over the electrolytic capacitor inside the driver will drop about 20%, need someone who makes the inductor. Inductor if not built properly will generate "hummmm noise".
Pros: the PF will rise close to 0.96 and harmonics will be lowered.
Solution #2 - Will work only if the components of the driver circuit (electrolytic capacitor, mosfet, integrated circuit and other) can handle 400VDC (if your mains is about 220VAC). If your mains is about 120VAC, then you can work with output voltages close to 300VDC, but always watch out for the other internal components of the driver.
1)Tie the inputs of the AC drivers of a room together (in two or more sets); 2)Know the input power of them together; 3)Build a circuit (or ask somebody to do it for you) using a high power factor integrated circuit (plus other components: mosfet, inductor, etc.). L6562, FAN7527B are some of the good examples of ICs and have some suggested circuits in their datasheets. 4)The output of the circuit (power factor with DC voltage output) will be connected to the inputs of the led drivers. Don't worry where to connect the positive or negative: there is a rectifier bridge inside your drivers that can handle this problem.
CONS: find somebody to do it for you if you won't, distribute DC to the drivers, the wall switch must turn on/off the PFC circuit (not the DC outit worksput of it!), when making maintenance DC is more dangerous than AC at the same voltage value.
PROS: power factor close to 0.98 and extremely low harmonics. This circuit makes no "hummmm" noise.
Solution #3: Valley-fill circuit. Search the web. It is not miraculous but sometimes it works.
CONS (that I remember from 15 years ago): Power factor close to 0.92, current harmonics about 25 to 30%. Weird DC output voltage. Necessary to know if your driver will handle the DC waveshape without stroboscopic effect or overheat, must change the input circuit inside the driver (oh my...).
PROS: low cost.
Best regards
