What is power factor correction?

Question

What do these things do...something about effeciency...and how are they connected?

Answer 1

Power factor correction units are usually banks of capacitors connected in star, and the will be switched in as they are required they are controlled by a device that monitors the power factor.

  you need to keep the power factor as close to 1 as possible other wise the electricity board will fine you for having a bad power factor, as this means they will be supplying  the same power but with higher current, so loading up the overhead lines and substation transformers.

Answer 2

the power factor is a measure of how in phase the current and voltage waveforms are. for a perfectly resistive load these are in sync and the power factor is 1.

however most electrical load is from motors and heater meaning the load is inductance based (coils in electric motors have inductance) in which the voltage lags behind the current in the waveform.

this can reduce the power factor from 1.0 (pure sine wave) to 0.7 or so.  this is bad as electrical transmission is not as efficient where the power factor is not 1.

to correct this the utility connect capacitors in parallel with the load (normally on the switching transmission liners near power stations and city outskirt sub stations),  this causes the current to LEAD the voltage and increase the power factor.

the utility switches in increasing amounts of capacitance until either maximum capacity is reached (BAD) or the power factor is around 0.95 to 1.0 (sine waves in near sync)

Answer 3

In DC circuits the power is simply the Volts X Amps however in an AC circuit the Volts won't always be in phase with the Amps and as such a power factor is used to calculate the Power

Answer 4

When you start a motor, the inrush of current causes the voltage and current in the supply line to go out of phase.

Voltage and current perfectly in phase is a pf of 1.0, out of phase is less than 1.0

The real power required to start a motor is the area under the curves of both the voltage and current waves. Keeping them lined up reduces the power needed to start an appliance.

The power company has to supply the real power, so an inefficient power factor makes their job harder. There are companies that make equipment for that purpose. The power company may also require large industrial users to buy/lease equipment for that purpose to protect the power company's euipment from surges.

Boost capacitors can be added by consumers (who know what they are doing). Whole house air conditioners are the first appliance to check, esp in the summer when line voltage may be low from high demand and the a/c may short-cycle trying to keep up with the heat.

Answer 5

Okay so i think you've got plently of answers for what power factor is (the phase angle of Volts Vs Amps)and power factor correction (PFC) being, bringing the current back in phase (or as near as).

But just in case your intrested in how it can be done - huge F**k off capacitors are not the answer for small appliances.

Yes some companies use huge banks of capacitors to bring back the power factor. but is primerily used on large machinery.

I believe the law in the uk is that any electrical product above 20W needs to to have a power factor of 0.9 (90% if your that way enclined)

but you can't go around shoving large capacitors into our home gadgets or lighting etc as they cost loads of dosh (relitively speaking) and are quite large.

For switched mode power supplies (sometimes called chopper PSU)

The way we do it. lets see if i can explain this and make sense....hmmm a challange indeed for me.

for a boost convertor;

basically you switch your input on and off using pulse width modulation (PWM) normally, in most cases a rectified sine wave signal is used as a controll signal. at the peak input volts you want the most current so you switch at a slower frequancy and give your pwm a lower duty cycle. as you follow the your sine wave voltage down you increase the frequency and duty cycle corrospoding to the required inductor current.

During the off time energy stored in the inductor will be dumped into a relatively small capacitor.

nope i think i've failed, I think I need a diagram to explain.

Look up boost convertor on wiki

http://en.wikipedia.org/wiki/Boost_conve...

that should hopefully give you a few images to explain the basic componant of a boost convertor.

below is a link for a datasheet for a PFC IC - if your not enclined to read it all then scroll to page 8 it has a diagram showing what i've been trying to explain. The mosfet is basically the switch as shown in the diagrams on wiki.

http://www.onsemi.com/pub_link/Collatera...

There are even more ways to achieve PFC activily each one taking a long time explain

Answer 6

It's a (large) capacitor put across the AC line to compensate for inductive loads.

for more info

http://en.wikipedia.org/wiki/Power_facto...

http://en.wikipedia.org/wiki/Power_facto...