How do I measure...

[Tom in NH]

Can anyone tell me how to measure or calculate the impedance of

a) the output of my Xantrex 3000+ inverter

b) the input of any inductor coil

Thanks in advance.

--Tom

[commanda]

Do you know Ohm's law? Good.

Measure the open circuit voltage.

Put a load on it.

Measure the load voltage & current.

The current through the internal impedance

is the same as the load current.

So the internal impedance is the difference

between the two voltages, divided by the current.

A better way is to measure the load voltage & current

at two different load points. Use simultaneous

equations to solve for internal impedance.

The inverters use feedback to control the output voltage,

so the impedance should actually be quite small.

The impedance of an inductor is proportional to frequency.

Xl = 2 * pi * f * L

where Xl is in ohms

f is frequency in Hertz

L is inductance in Henry's

Amanda

[Victor]

 Impedence is the vector sum of the resistance and the reactance

 or the sq root of (R^2 +(2pie Lf)^2) for a circut with resistance and inductance.

 Assuming that you want the impedance at 60 hz (it varies with frequincy) you could measure the inductor impedance directly  by using a low voltage transformer and or a series resistor to keep the current down . Measure the voltage across the inductor and the current through it then use ohms law to get the impedance at that frequincy. E=IZ

 I don't know how to measure the inverter Impedance.

 Hope this helps

Best

Victor

[Victor]

Hi Amanda,

 I know I'm nit picking here but 2 pi f L is the reactance. The impedance includes the resistive componant.

Best

Victor

[commanda]

I was assuming he meant a fairly large inductor,

which would have minimal dc resistance.

But yes, you are correct.

I thought vector math and the square root of minus one

might be a bit too much information.

Amanda

[jimjjnn]

Why do you need to know the impedance?? You may be able to get that from Xantrex if you talk nice to them.

That puts you in the scary area of electricity/electronics if you aren't really mathmaticaly inclined. I hated math in college as we always ended up with a professor that used every blackboard in the classroom to demonstrate Ohm's law and and always got the wrong answer. We always had the answer in less than a minute. This was when the first  electronic calculators

just came out. Still used slide rule.

 AH! Memories.HEHE

[finnsawyer]

Oh, aren't you going to tell him about the wonders of the complex plane?  How the impedances of a pure inductance and a pure capacitance fall along the positive and negative imaginary axis respectively?  Probably wouldn't do him too much good.  Still, that's the way to analyze complex impedances.

[Peter V]

While I don't know for sure, I'll bet the Xantrex adjusts it's output voltage to account for sagging under load, so you won't be able to determine it's output impedance this way.

Asking Xantrex is probably your best bet.  

Though I am curious as to why anyone would want to know?

[mkseps]

I suggest that you do the following:

Place a variable resistor in sries with a non polarized capacitor.  The capacitor should have a fairly high value, perhaps 1 mfd and the variable resistor perhaps 10 ohms.  Place this across the inverter output and adjust the variable resistor until the AC voltage across it is identical to the voltage across the capacitor.  Now disconnect this circuit and measure the DC resistance of the variable resistor.  That value is the same as the capacitive reactance of the source.  You may have to play around with parts values to obtain readable values.

Gene

[mkseps]

Correction:

When the voltage output of the inverter is equally divided across the resistor and capacitor, then the input impedance is 2 times the resistor value>

Gene

[Tom in NH]

Hey, you folks are good! I think you all have given me something to start with. In answer to the question of why do I want to know this stuff, I'm thinking of building a low pass filter to connect to the output of my inverter. I've been reading how the clipped square waves of the inexpensive inverters are really sinewaves with a lot of harmonic frequencies added in. If you filter out the harmonics with a low pass filter, voila, you have a sine wave. And I need a sine wave to run my microwave oven. I've been playing with this really cool downloadable filter software called "Elsie," by James Tonne (The student version is free). It is telling me my filter designs have dramatically different characteristics when the source output impedance changes just the tiniest amount. So I want to make sure I'm not too far away from reality before I actually build the filter. Thanks all for your very straightforward, concise, and practical solutions. If you see a flash on the northeastern horizon, don't worry, it's not nuclear armageddon. It's only me burning my eyebrows!

[Flux]

Best of luck with the filter, it's not easy. You may be able to make one that suits the microwave and leave it wired to the microwave. I don't think you will make a general purpose filter that will run everything.

The inverter end will be low impedance but the microwave end depends on what is in the microwave. If it is the old transformer type it will have a lot of inductive reactance and this could well be part of your filter inductance. If it is the modern inverter type it could look like anything depending on the design but may well look to be capacitive.

Flux

[Nando]

Heck, why in God's name you need the Xantrex impedance ?.

To measure a dynamic converter you need a set up with an accurate AC Voltmeter.

measure the AC output without any load, then measure the output voltage with increasing load current ( like 15 % increments), until you attain about 75 % of maximum current.

The AC voltmeter need to be very sensitive to about 2 or 3 decimal places -- hard to find a good one.

The input of any inductor coil ??

You meant the impedance of an inductor ( inductance )

Take an Oscillator, let's say 100 KHZ, place a variable resistor in series with it, then place the inductor under test, measure the voltages at the junction of the resistor and inductor; and the output of the Oscillator ( resistor), when the AC voltmeter reading of the resistor and inductor junction is exactly one half the oscillator output AC voltage.

Then apply the formula 2*pi*F*L= Xl= impedance of course the internal resistance of the inductor may be added to the impedance measurement, but one can measure the resistance using a good DVM and take that value away from the impedance measurement.

Regards

Nando