Strengthen knowledge on power factor

[samEEEf]

We know - The power factor is defined as the ratio of the real power flowing to the load to the apparent power feed into the load. So, we can calculate power factor as below -

Measure voltage & current to the load and calculate VA of the load.
Measure power (P) drawn by the load using a wattmeter.
So,
Power factor = P/VA


Power factor depends on the type of load. If the load is purely resistive pf is 1 and this value can be as low as zero if the load is purely capacitive/inductive.

So, power factor is related to the loads only. It is related to source? Can we use this term with inverters?

 

[zorro]

Hi Sam,

For nonsinusoidal waveforms, there are other terms related to harmonics.
Imagine a sinusoidal source delivering power to a load.
Now, allow the source to have in addition a component of voltage at some harmonic, and suppose that the load does not allow current flow at this higher frequency.
The rms (effective) voltage of the source increased and then the apparent (VA) power too, but the active (real) power did not.
I hope this example helps you.
Regards

Z

 

[samEEEf]

Thanks zorro for your such helpful post.

I've an Inverter in my house having rated VA 500. How can I measure the power factor my Inverter?

 

[FvM]

Now, allow the source to have in addition a component of voltage at some harmonic

I doubt, that there's a commonly used definition (respectively a reasonable at all) of power factor for non sinusoidal voltage sources.

 

[zorro]

For sinusoidal or nonsinusoidal waveforms, the power factor is defined as follows:

PF = (Actual active power) / (Max. active power attainable with the same Vrms and Irms)

Clearly, PF is between 0 and 1. The denominator is Vrms*Irms .
PF attains the value 1 only when the load is purely resistive and has the same value at all the frequencies involved.
Regards

Z

 

[FvM]

I agree, that the standard definition for power factor PF = real power/apparent power can be applied to non-sinusoidal sources, too. In so far, I have to agree to the consideration in post #2.

On the other hand, power factor correction, particularly for high power systems, is usually referring to the grid voltage fundamental, and intentionally trying to achieve a sinusoidal load current, even in case of a distorted voltage waveform.

 

[samEEEf]

Is it possible to measure the phase difference between voltage and current using an Oscilloscope? If yes, how?

 

[zorro]

Is it possible to measure the phase difference between voltage and current using an Oscilloscope? If yes, how?

Assume pure sinusoidal waveforms. Two different methods:
a) Voltage and current in two different V channels, trigger with one of them. Look at zero-crossings.
b) Voltage and current in XY mode. Look at the Lissajous figure.
Regards

Z

 

[samEEEf]

I can see voltage waveform in Oscilloscope. How can I observe the current waveform?

 

[FvM]

You need a current transformer or a shunt resistor to measure the current. I have been waiting for this question since your first oscilloscope related post.

 

[samEEEf]

You need a current transformer or a shunt resistor to measure the current. I have been waiting for this question since your first oscilloscope related post.

:smile:


Sorry to keep you waiting for a long time. ;-)

Now, could you please show both the processes in brief?

 

[FvM]

Current transformer (CT): A small transformer, mostly with one turn primary winding Current transformer - Wikipedia, the free encyclopedia
For oscilloscope measurements, the secondary winding of the CT has to be terminated with a resistor, e.g. 100 ohm. As an advantage, the current transformer provides isolation from the primary circuit. Disadvantage: no measurement of DC currents.

A special CT design has a split core and can be clamped on a conductor without disconnecting it.

Shunt: A low ohmig resistor inserted into the current path of the load. It's voltage drop is proportional to the current. Shunt (electrical) - Wikipedia, the free encyclopedia

 

[atripathi]

You can also use current probes to directly see the current, these probes comes with clamp mechanism and you can directly observe the waveform at scope. **broken link removed**

 

[ALERTLINKS]

It is strange to see specifications of comercial made ups including pure sine wave state power factor equel to 0.6. Why pfc is not inbuilt.