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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567
I don't know if others have posted this before but I came across a simple way to determine the power factor for your devices: motors, transformers, etc. It can be done with just a simple amp and voltmeter. This is useful if you want to know how much you need to correct in order to reduce current draw while running highly reactive components. Instead of drawing 8-10amps you may only need to draw 3A.
The device being measured has a Rload and Xreactive component which need to be determined. Use a variac as the testing voltage so you can keep the voltage low. Connect a current meter from the variac to a small (like 1-10ohm) resistor/10-20W. Connect this resistor the the load. Connect a voltmeter in front of the resistor to ground and after the resistor to ground. Slowly increase the variac until you have a suitable voltage on V1 and V2 without exceed R's power limit. The formula is:
Registered Member #142
Joined: Sat Feb 11 2006, 01:19PM
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I'm assuming you would have to drive that with a sine wave. For measuring ordinary circuit elements like chokes with a few microhenries inductance, how accurate is your method using 60 Hz drive? I've tried measuring small inductors at 60 Hz with metods similar to yours and couldn't get acccurate results, because the reactance at 60 Hz is so low that the inductor's resistance kind of swamps it. I haven't tried your method but was wondering if it works as well for a 40 uH toroid as it does for a motor or 60 Hz transformer with inductance in the henries.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567
I've only tried it for my 15kv/60ma transformer and a motor. I will try it with a few inductor/resistor combinations of values close to the ones you mentioned and report back to you. I am only doing this with the 60hz from my wall outlet and using a variac to slowly increase the voltage until I get close to my series resistor's wattage limitation. I guess I can double up two 10ohm/10W in parallel to increase my accuracy.
Also, if you are trying to bench test components values, I would use a function generator (if available) and use a higher frequency to drive up the inductive reactance. Then, just measure the voltages the generator puts out before and after the series resistor.
EDIT: I tried this method on an inductor of 2.6mH and a R of 0.8ohms. I used a series resistor of about 0.4ohms. I used a frequency of 1000Hz. I determined experimentally a R of 1ohm and L of 2.1mH. If I was more deliberate I probably could have gotten better accuracy.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567
I used a sinusoidal to mimick the wall current. While this method should work well for benchtesting components, I can just use an LC meter. I am finding this is very useful for testing items where the true reactance is only evident when they are in use. For example, the inductance I measure on an unpowered motor is different than that which I see when it is moving and generating a back-emf. I can meaure its true reactance in service. The same for a transformer or an active LC circuit.
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Easy way to determine Reactive load and power factor correction with volt/ampmeter
