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Demonstration of Ferranti Effect in transmission line

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Mithun_K_Das

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In electrical engineering, the Ferranti effect is an increase in voltage occurring at the receiving end of a long transmission line, above the voltage at the sending end. This occurs when the line is energized, but there is a very light load or the load is disconnected.

As far as I know and found after study, this effect is only found in 3 phase line. But my question is, how can I generate this effect in single phase line? I just need to demonstrate the effect really exists.

Any suggestion?

Thanks in advance.
 

FvM

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As far as I know and found after study, this effect is only found in 3 phase line.
Transmision line theory clarifies that the effect can occur in a single phase line as well.

To see the effect, the line length must be in a wavelength order of magnitude, e.g. thousands of kilometers at 50 Hz.
 

chuckey

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Small size ? How small? For instance you could scale it by 1,000,000 and use a 50 MHZ signal source. The wave length would be 60 m, so a measurable effect should be seen at 8m. Or you could model your transmission line by using lumped inductors and shunt capacitors but you might have a problem convincing your pupils that a piece of overhead cable is the same as a mass of coils and capacitors.
Frank
 

Warpspeed

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That should certainly work.
Build an open feed line of two parallel bare wires energized at one end with a high frequency signal generator, properly impedance matched into the line.

Have something that clearly indicates RF voltage that can be moved along the line.
The voltage will be seen to rise as you move further away from the source.

Terminate the far end with a suitable resistor, and the voltage will then be the same all along the line.

Its a classic demonstration, and if done at 100 Mhz or more, the whole thing then becomes a more practical class room size.

At 100 Mhz one wavelength is three metres, and the effect is best demonstrated with a quarter wavelength line (around 750mm).
 
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mtwieg

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If you want to do it at lower frequencies, consider making a lumped element transmission line using a long LC ladder.
 

Mithun_K_Das

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Should I try with increasing the line frequency? such as 10KHz or 100KHz? Because generating at MHz range will not be easy. Also space is a matter for presentation.


So if I try not to represent the voltage is going up but only the wave shape will it be ok? As when capacitors are added, the wave shape changes a little bit.
 

FvM

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So if I try not to represent the voltage is going up but only the wave shape will it be ok? As when capacitors are added, the wave shape changes a little bit.
I presumed so far you are takling about sinewave excitation. Then capacitors (transmission line modelled by a lumped LC circuit) won't change the waveform.

I don't know which equipment you have available for the demonstration. Some kind of generator and an oscilloscope are obviously necessary. A standard waveform generator has a typical frequency range up to 10 MHz. So the demonstration can be made with 5 m coax cable (electrical length about 1/4 wavelength at 10 MHz). The generator must have a lower impedance than the transmission line, can be faked by a parallel resistor if your generator has 50 ohms output.
 

Warpspeed

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Should I try with increasing the line frequency? such as 10KHz or 100KHz? Because generating at MHz range will not be easy. Also space is a matter for presentation.
.
The effects you wish to demonstrate only occur over very long power transmission line distances, hundreds or thousands of Km.

To shrink that down to work bench size needs a frequency increase of the same order.
Even at 100 Khz one full wavelength is still 3 Km
At 100 Mhz it is only 3 metres.
A frequency a bit lower tha 100 Mhz would be workable.

The ideal thing would be a small radio transmitter with a few watts of output, and a light bulb indicator.
Even right at the back of the class, the simplicity of the setup would make for a pretty dramatic and convincing demonstration.

I believe there are 16,000 licensed Ham Radio operators in India. See if you can find a local club or contact.
Someone there might be able to help you to assemble some suitable equipment.
http://www.hamradio.in/index.php?sid=51c17b8a0a6c8cbfefb33e9ee2b25896
There is also a Forum at that site.
That may be your best bet.
 
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mtwieg

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For going the lumped element route, here's an example:

330uH inductors: http://www.digikey.com/product-detail/en/2318-V-RC/M8892-ND/775431
1uF film capacitors : http://www.digikey.com/product-detail/en/R46KN410045N2M/399-11741-ND/5141849

Buy ten of each, connect them together, and you have an approximate transmission line with a delay of 182us, equivalent to the delay of 38km of coax cable. You could get large voltage gains at a frequency of just 1kHz. As an added bonus, you could easily measure the voltage at different points along the line, which you couldn't do with a coax cable.
 

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