# electricity

Discussion in 'Physics' started by tauqeer, Jan 24, 2008.

1. ### tauqeer Thread Starter New Member

Jan 23, 2008
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VIcosθ in the electric AC power cosθ is the power factor what is meant by power factor and why it is not a sinθ or a tanθ

2. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
The power factor is the ratio of the real power (P) and the apparent power (S) and scales the real power dissipated in an AC circuit dependant on the levels of real and reactive power "dissipation" (actually ideal reactive components don't dissipate power in the way resistive or real components do, but simplicity of this explanation we shall leave the description as that). It is defined as:

$Cos$$\theta$$=$$\frac{P}{S}$

And the apparent power is defined in vector form as:

$S^{2}$$=$$P^{2}$$+$$Q{2}$

Where Q is the reactive power.

In the absence of reactive power the apparent power equals the real power:

$S = P$

And consequently $Cos$$\theta$$=$$1$

This means the power dissipated is merely the product of the voltage and current, which we know is correct.

Are you happy with the terminology of real and reactive power?

Dave

3. ### integrityco New Member

Feb 1, 2008
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Dave - did a great job explaining PF...In it's simplest and most applicable sense PF is the ratio of the power you buy from your supplier(utilities) and what you actually use. Truepower is what you actually use while apparent power is what the supplier sends you.. Power that is not used yet consumed is considered wasted power (reactance)...

DRC

4. ### rwmoekoe Active Member

Mar 1, 2007
172
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when a load is totally reactive (cos phi = 0) the phi (phase shift between the voltage and the current) is 90 degrees.
if you count the power for this load (power = voltage between x current through the load) the result will be cycling plus and minus all the time. plus means you use the power, minus means you give back the power. so, the power is not wasted, it's given back.
that's why a load with cos phi below 1 can be compensated with a given value of capacitor parallel to the load without any losses. in the electric power industry this is done through the capacitor bank.
the drawbacks of below 1 power factor loads is that they cost the electric company the maintenace capacitors for compensating the power factor, and the unnecessary high current that's dissipated as heat through the power lines all the way from the power station to the consumers.