[AMPS] conjugate match

Wed, 3 Sep 1997 09:13:47 +0000

> From:          Peter Chadwick <Peter.Chadwick@gpsemi.com>
> Date:          Wed, 3 Sep 97 07:59:11 +0000

> Tom,
> 
> I'm in deep disagreement with you on conjugate matching.  Not that it
> matters - it should lead to a lively discussion! But, when you say:

Anything is better than beating one's head against a rock about grid 
dissipation, hi.

> > Non-dissipative source resistance 
>> is often confused with a dissipative resistance like a resistor.
> 
> What's this new 'non dissipative resistance'? My text books don't accept
> a non dissipative resistance - even a matched transmission line is
> dissipative.

It's not new at all Peter. It's the result of the unfortunate 
selection of the term ohms to describe impedance. The mechanical 
engineers were smarter, they never defined friction using the same 
terms and names as torque and RPM. Torque and RPM are the 
"non-disspative resistances" while friction is the dissipative 
"resistance" parameter.  When you climb a hill in your car, you 
conjugately match the "non-dissipative resistance" (the real part of 
the engine's impedance) to the load presented by the rear wheels.  

When we want to extract maximum power from water falling over a 
water-wheel, we conjugately match the wheel to the water. If we 
mismatch it, the water doesn't boil. The energy transfer just drops.

The IEEE Dictionary of electronic terms defines resistance as "the 
real part of an impedance".

Suppose we have a point in a complex system with a sinewave of a 
RMS value of 300 volts and one ampere, and the voltage and current 
are in phase with each other. What is the impedance at that point? 

Is the 300 ohms dissipative?

If we perturbed that system with the tiniest series voltage and 
measured the current change, what would be the result? It would look 
exactly like 300 ohms!

I never ran into a problem with this designing charging systems for 
automotive applications, we knew the goal was to conjugately match 
the alternator to the load by varying the field current. For some 
reason radio engineers want the real part of an impedance to be 
dissipative.

Edison went through the very same stuff when he proposed a 
power transmission system using a regulated generator. Many 
physicists claimed the maximum efficiency could only be 50% unless 
the generator was capable of many many times the desired output.

Fortunately, Edison was right.

73, Tom W8JI

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