Mathematical Vs Power Engineering convention (Power Factor) 1

[ppaya]

I see that some of us are more familiar to mathematical convention. Therefore motor load is described as "consuming WATTS" and "Generating VARs (or consuming negative VARs)". This means power factor is negative and lagging.

Engineers would conviniently say motor load consume both. Power factor is lagging, and positive.

There seems to be both conventions in use. Any thoughts what convention to apply ?

 

[electricpete]

Looks like good stuff, cuky.

A minor correction to my earlier comments - in a pure reactive circuit the direction of instantaneous power reverse every quarter (not half) cycle.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[stevenal]

We happen to live in a world where many loads are inductive. In another world where loads are capacitive and shunt reactors are required for voltage support, I'm sure the convention is reversed.

The original post spoke of negative power factor. No such animal. The ratio of the magnitude of real power to the magnitude of apparent power is always positive. Some meters will apply a - sign after the calculation to indicate when current lags the voltage. I understand others use a + sign to indicate the same situation. No convention here.

 

[cuky2000]

Stevenal, I am not disagreeing with your statement regarding negative power factor. However, I found interesting the interpretation of power factor sign from the Instruction book of the Power Quality Analyzer Fluke 434:
<sub>"Interpretation of Power Factor when measured at a device:
· PF = 0 to 1: not all supplied power is consumed, a certain amount of reactive power is present. Current leads (capacitive load) or lags (inductive load).
· PF = 1: all supplied power is consumed by the device. Voltage and current are in phase.
· PF = -1: device generates power. Current and voltage are in phase.
· PF = -1 to 0: device is generating power. Current leads or lags.

If you see negative power or power factor readings and you are connected to a load, check to make sure the arrows on your current clamps are pointing towards the load."</sub>

 

[davidbeach]

Consider if you add capacitors to a transmission system at a certain location you will almost invariably increase the voltage at that location (and adding inductance decreases the voltage). It is easy to imagine that adding a "source" increases the voltage and adding a "load" decreases the voltage.

In the usual application of line capacitors that is true; shunt capacitors will have a tendency to cause a voltage rise at the point of application, particularly on lightly loaded lines. However, it is also possible to add series capacitors into a line to counteract some of the inductance, reducing line impedance and these would have an effect more of reducing line voltage drop rather than a rise as such.