Hi,

I need to calculate the resistance in AC of a steel conductor that is not round. Is there a way to get it from the DC resistance and the shape of the conductor ? Or is there a way to measure it without the self-inductance ?

Thanks.

Just now, baltoche said:

Hi,

I need to calculate the resistance in AC of a steel conductor that is not round. Is there a way to get it from the DC resistance and the shape of the conductor ? Or is there a way to measure it without the self-inductance ?

Thanks.

The short answer is yes, but it rather depends upon what you are trying to do. It is not as simple as you think.

If you would like to tell us more about your problem we can help further.

Hi,

I'm working for the railways. One of the project works at 60Hz but I've only got the electrical resistance in DC while I need the resistance in AC at this frequency. If I measure it by impressing an AC current, then I'll measure the self-inductance with the resistance. Unless there is a measurement method I don't know.

I tried to approach the problem with another method, by calculating the skin effect thickness but I don't know about the magentic permeability of my rails (high carbon rate).

Just now, baltoche said:

Hi,

I'm working for the railways. One of the project works at 60Hz but I've only got the electrical resistance in DC while I need the resistance in AC at this frequency. If I measure it by impressing an AC current, then I'll measure the self-inductance with the resistance. Unless there is a measurement method I don't know.

I tried to approach the problem with another method, by calculating the skin effect thickness but I don't know about the magentic permeability of my rails (high carbon rate).

OK so if I understand you correctly you wish to measure the resistance of a length of rail section ?

You should be aware that electrical resistance does not change with frequency - IOW there is no such thing as AC resistance per se.

If AC is involved it has a frequency dependant property called impedance, which is a compound quantity of the resistance and the ac reactance.

Commonly instead of V_dc = I_dcR (ohm's original law) we have V_ac = I_acZ

One method of measuring the resistance is to use a variable frequency generator and measure the ratio of V_ac / I_ac

For a range of frequencies

and plot a graph of the result.

You can then extrapolate this graph back to zero frequency (ie DC) to obtain the resistance component.

This technique is often used to get around two effects that can upset the measurement.

One is polarisation and the other is the effects of contact potentials or contact resistance with the measuring equipment.

Does this help ?

@baltoche

Have you given up ?

I would recommend a variation on the 4 terminal measurement technique

https://en.wikipedia.org/wiki/Four-terminal_sensing

The Wiki version is DC but you can replace the battery with an ac generator.

A simple way to effect the measurent is to insert a non inductive high resistance (in comparison to the rail resistance you are measuring) in series with the source to cause it to be a constant current source.
Measuring the voltage then give a direct readout of the resistance or impedance.
In the days of vacuum tube meters this method was commonly used to achieve a linear resistance/ impedence scale.

Hi

Sorry for the delay (I had'nt been notified you responded).

• I confirm I wish to measure the AC resistance of a piece of rail.

• Regading AC vs DC resistance, it is right that it should be the same resistance. However, due to the skin effect, the surface where he current flows is reduced in AC, affecting the resistance. I consider his is not part of the reactance but well the resistance.

• I do agree with your method consisting in making the frequency vary, but all measurements in AC will include both the DC resistance, the skin effect and the reactance. Therefore, I cannot get rid of the reactance itself and I cannot get the resistance in AC (with the skin effect).

• When you write "A simple way to effect the measurent is to insert a non inductive high resistance (in comparison to the rail resistance you are measuring) in series with the source to cause it to be a constant current source. Measuring the voltage then give a direct readout of the resistance or impedance.", do you mean that I inject an AC current and I measure the voltage drop accross both the rail and the non-inductive resistance ?

Thanks for your support.