Power Factor calculation

studiot

Joined Nov 9, 2007
4,998
A traditional crossed coil wattmeter compensates for power factor, or may be calibrated in phase angle or power factor.

In the lab alternatively we can use an impedance bridge sucha s a schering bridge to separately measure the resistive and reactive components.
 

The Electrician

Joined Oct 9, 2007
2,833
how can we determine leading or lagging power factor by wattmeter or by any other meter in lab?
please help me
Have a look at this:

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

You measure the load current and voltage separately, multiply the two values and that gives you apparent power S.

Then measure the true power drawn by the load with a wattmeter; that gives you active power P.

The power factor is given by PF = P/S
 

WBahn

Joined Mar 31, 2012
26,398
Have a look at this:

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

You measure the load current and voltage separately, multiply the two values and that gives you apparent power S.

Then measure the true power drawn by the load with a wattmeter; that gives you active power P.

The power factor is given by PF = P/S
Though this only gives you the magnitude of the power factor, and not whether it is leading or lagging.
 

WBahn

Joined Mar 31, 2012
26,398
You can connect wattmeters to read VAR as well as watts. If you measure VARs, then whether it is positive or negative will tell you leading or lagging.
 

The Electrician

Joined Oct 9, 2007
2,833
i m not asking the method to calculate power factor i wish to know the method to find leading or lagging pf
Connect a capacitor sized to draw a current of at least 30% (but not more than 95%) of your load current in parallel with the load. Measure the power factor with the capacitor in place as I described in post #3.

If the PF improves (gets closer to 1), then your load PF (without the capacitor) is lagging. If the PF gets worse then your load PF is leading.

Alternatively, use an oscilloscope to examine the phase relationship between load current and voltage.
 

WBahn

Joined Mar 31, 2012
26,398
How would one connect wattmeters to read VARs?
First, I am assuming something that is perhaps not a valid assumption, and that is that we are talking about three phase power here.

The key is that there is nothing intrinsic about a wattmeter that makes it magically measure real power. It responds to the product of the voltage across one coil and the current through another coil -- in other words it is simply evaluating a mathematical relationship and displaying the result. Normally, you connect them so that the voltage coil is measuring the voltage across the load through which the current going through the current coil is going. But you don't have to do that, particularly in a multiphase system. By choosing which voltage and which current you apply to the wattmeter's coils, you perform the same math, but the relationship means something different.
 
First, I am assuming something that is perhaps not a valid assumption, and that is that we are talking about three phase power here.
Sounds to me like this is a lab exercise for the OP, so I would assume single phase. How would you connect wattmeters to measure VARs in a single phase system?
 
First, I am assuming something that is perhaps not a valid assumption, and that is that we are talking about three phase power here.

The key is that there is nothing intrinsic about a wattmeter that makes it magically measure real power. It responds to the product of the voltage across one coil and the current through another coil -- in other words it is simply evaluating a mathematical relationship and displaying the result. Normally, you connect them so that the voltage coil is measuring the voltage across the load through which the current going through the current coil is going. But you don't have to do that, particularly in a multiphase system. By choosing which voltage and which current you apply to the wattmeter's coils, you perform the same math, but the relationship means something different.
It's not this easy in a polyphase system if you want accurate results under all conditions. My Electrical Engineers Handbook says:

"Any reactive power measuring device which depends on cross-connection of the voltage circuits will read incorrectly if both the voltages and the currents are unbalanced."

This is likely the first lab where the OP is learning to make these sorts of measurements. Surely he wouldn't be learning on a polyphase system at first.
 

WBahn

Joined Mar 31, 2012
26,398
It's not this easy in a polyphase system if you want accurate results under all conditions. My Electrical Engineers Handbook says:

"Any reactive power measuring device which depends on cross-connection of the voltage circuits will read incorrectly if both the voltages and the currents are unbalanced."

This is likely the first lab where the OP is learning to make these sorts of measurements. Surely he wouldn't be learning on a polyphase system at first.
I have no idea what kind of lab the OP is taking -- it may well be a power lab since few other labs at most colleges ever deal with AC power circuits at all. Now, if he's taking a lab at a trade school then I don't know, but it would seem that there would be a good chance you are correct. I'm not sure what the handbook is referring to or whether it applies to what I am talking about -- I lack the context to determine that. Surely there are techniques that only work if the system is balanced.
 
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