Wire derating at high frequencies

[aditya_kr8]

Hi,


how do we derate a PVC wire at higher frequencies.

Normally we consider the ampacity rating of the system where the frequency would be 50/60Hz and select the wire. but how do we select a wire for higher frequencies (<=400Hz).

Is there any formulae which would determine the selection of the wire.

Normally i know that the skin effect comes into picture at higher frequencies. so how do we select a wire in this regard?

i also found that litz wire solves the skin effect but how is this different with PVC wire?

 

[SOG38]

Wire impedance at frequencies higher than 50/60 Hz

Wire impedance at frequencies higher than 50/60 Hz

The wire characteristics are different for copper vs. aluminum and stranded vs. solid.
My practice has been to use only stranded wire and I have not had any issues.
I have worked with frequencies up to 10 KHz. It makes a difference if it in magnetic and non-magnetic pipe/ducts. It is different with 3 single conductors or a 3 conductor cable. Years ago I found impedance data for the various types of conductors
(This is a good source http://www.arcadvisor.com/reference.html#CopperData ). You also need to look at circuit breaker and fuse impedance at the higher frequencies. I have used Z Tot = R2+Z2 R and Z from the tables
Z is raised by the ratio of High Freq /to 60 or 400/60 = 6.666
I think what you will find is that the impedance will only increase maybe 2% at 400 Hz.
Hope this helps.

 

[petersonra]

There is not much effect in smaller wire sizes at all for 400 Hz. I do a fair amount of design for such systems and I do not derate wire for 400 Hz up thru 1/0.

The effect gets worse as the wire size increases, so if you needed much larger wires, it could become an issue.

If you have long runs of wire, use non-ferrous conduit or cable trays.

 

[steve66]

There is not much effect in smaller wire sizes at all for 400 Hz. I do a fair amount of design for such systems and I do not derate wire for 400 Hz up thru 1/0.

The effect gets worse as the wire size increases, so if you needed much larger wires, it could become an issue.

If you have long runs of wire, use non-ferrous conduit or cable trays.

I would agree. 400 Hz is a very low frequency.

I believe Litz wire is only used for much higher frequencies.

 

[Besoeker]

We use Litz wire in commutating chokes that have a resonant frequency of 10kHz upwards.

FWIW, here's a scanned page page from a rather old book that sits on the shelf above my desk at home.

 

[petersonra]

We use Litz wire in commutating chokes that have a resonant frequency of 10kHz upwards.

FWIW, here's a scanned page page from a rather old book that sits on the shelf above my desk at home.

I have a similar chart from an IEEE publication from the 50s. The numbers are almost identical to those on your chart except they go out to 3 decimals.

 

[jim dungar]

It seems many "electricians" are taught about skin effect as if it is a daily occurrence for them.
Not only is it not very important below the MHz range, the NEC tables already account for it, and the stranding used to make up normal circuit conductors is for all intents electrically equal to solid conductors

 

[Besoeker]

I have a similar chart from an IEEE publication from the 50s. The numbers are almost identical to those on your chart except they go out to 3 decimals.

That the conductor is given in circular mils suggests that the source might have been the same. It's from a British book but circular mils have not been used here as long as I can remember. And that stretches back to before that edition of the book was printed.

 

[beanland]

Harmonics

Harmonics

Skin effect comes into play when there are significant harmonics present. The higher frequencies travel in a reduced skin on the conductor. As a result, they contribute more to heating and losses than the same current at lower frequencies. There is a complex relationship between impedance and frequency as this effective resistance increases with increasing frequency.