Corcom EMI/RFI filters

[Grainger49]

   .  .  .    I'm a little confused, why doesn't it appear as a short when placed across the neutral and live?

I can see why this is perplexing.  I mean, it looks just like a short, doesn't it?  But it isn't.

The difference between the terms resistance and impedance is frequency.  DC has no frequency, it never cycles.  So when you measure with a meter it measures [DC] resistance.  But impedance has a resistive component and a reactive component.  That reactive component changes with frequency.  So at 60 Hz the impedance of a choke is much higher than just the resistance of the windings.

[mcandmar]

Excellent explanation, thank you Grainger.

So if i understand this right, as the frequency goes up the impedance goes down, therefore the choke looks like a low impedance path at higher frequencys.  So on an AC line its effectually just providing a bypass for the HF keeping them away from other devices on the line?

[Grainger49]

Not quite...

Even though 60 Hz is a low frequency, as the frequency goes up the impedance of an inductor goes up.  So at very high frequencies it is a brick wall.  This is why they are great in a power supply.  They will not pass high frequency pulses or high frequency dips.  Variable speed drives in industry notch the power line when they turn on. 

Thinking back to older cars, a choke (coil in your ignition) resists changes in current through it.  In an ignition system the coil "charges" from the 12V battery when the points close.  When the points open the coil kicks 10,000 volts to the spark plug. 

In a power supply a choke suppresses voltage spikes and fills in notches.

In a crossover a series choke passes low frequencies and a parallel choke shunts low frequencies. 

In crossovers capacitors do the opposite.  A series cap passes highs and a parallel cap shunts highs.

[mcandmar]

That much i can follow, series wired in DC supplies, crossover theory, and especially car ignition coils as i have been on the receiving end a few times.  I still don't really understand what it does across an AC line as it would be parallel to the load, i.e. a power transformer.

[butchbass]

To add a little more the formula for inductive reactance is:

RL= 2(Pi)FL
RL=inductive reactance
Pi=3.1417
F= frequency in cycles/sec
L= inductance in Henries
So you can see inductive reactance goes up as the frequency goes up or if the inductance goes up, therefore more resistance to a particular frequency.

Capacitance  reactance works inversely:

RC=1 divided by 2(Pi)FC
RC= reactive capacitance
C=capacitance in farads
Reactive capacitance decreases as frequency increases or capacitance increases.

I apologize for the awkward way I wrote the formulas. I know something of AC power because I worked in a power plant for my career and learned now Rl and Rc affected the main generator. I still have not mastered computer symbols.

[butchbass]

And  a little more theory.
 A voltage is generated when there is relative motion between a conductor and a magnetic field. That is why generators have to rotate.
Since AC current alternates it creates a magnetic field in a coil that goes from a + to - polarity. It does this across the conductors of the coil ( you now have a conductor with relative motion to a magnetic field) so  a voltage that opposes the original voltage is generated in the coil. This causes a resistance to the flow of the original current source that is called inductive reactance since we are not dealing with a DC current.

I hope this helps.

[braubeat]

I do not know if this actually would work.
How about getting two identical electric motors and hooking them togeather with a rubber belt and some big heavy cog wheels. One is input 120 vac the other would output 120 vac. Seems like that would clean things up at a reasonable cost.

Michael

[butchbass]

This will not work. Unless specifically designed a motor will not function as a generator.  There are parameters such as frequency and voltage regulation that have to be considered.

[Grainger49]

    .  .  .    two identical electric motors and hooking them together with a rubber belt and some big heavy cog wheels. One is input 120 vac the other would output 120 vac.  .  .  .   

The system looks like this:

A synchronous AC motor hard coupled to an alternator generates AC that is free from all line problems.

I have installed two of these systems.