AC Resistance vs. DC Resistance

[K8MHZ]

At 60Hz the skin effect is negligible for small wires. What is a prox effect?

For figuring voltage drop and conductor size adjustment, resistance and impedance are the same thing, aren't they?

From Wiki:

Proximity effect:

In a conductor carrying alternating current, if currents are flowing through one or more other nearby conductors, such as within a closely wound coil of wire, the distribution of current within the first conductor will be constrained to smaller regions. The resulting current crowding is termed as the proximity effect. This crowding gives an increase in the effective resistance of the circuit, which increases with frequency.

Also, a correction. My figures were for ohms per kilometer. AC reactance / resistance for 1000 feet would be .054 vs. 2.0 for three conductors in PVC conduit.

 

[K8MHZ]

Originally Posted by ggunn
At 60Hz the skin effect is negligible for small wires. What is a prox effect?

For figuring voltage drop and conductor size adjustment, resistance and impedance are the same thing, aren't they?

One difference between reactance and resistance is that reactance can be cancelled out, whereas resistance cannot.

 

[Frank Restly]

Besoeker

Besoeker

The issue is the operating frequency. Because the mechanical contractor is using a direct drive motorized fan, he would like us to provide a higher frequency output from the VFD to the motor (90 Hz) than what we typically provide (60Hz). If this was a belt driven fan, the mechanical contractor could simply change the belt disc ratio to obtain the operating speed he would like.

 

[ggunn]

From Wiki:

Proximity effect:

OK, but that is a geometrical effect which I don't think is taken into account in Table 9. For example, wouldn't this effect be different for single phase and three phase conductors in close proximity, or for multiple conductor sets, or for conductors with different thicknesses of insulation?

 

[K8MHZ]

OK, but that is a geometrical effect which I don't think is taken into account in Table 9. For example, wouldn't this effect be different for single phase and three phase conductors in close proximity, or for multiple conductor sets, or for conductors with different thicknesses of insulation?

Having looked at the chart, I am now wondering why it's even there. The differences between AC and DC resistance are nearly nill in the table. And yes, as I understand prox effect, it would apply to conductors that were very close, like in a cable or compacted in a raceway.

If the NEC is not intended to be a design manual, what is the point of Table 9? As pointed out earlier, the difference between 60 Hz and DC is almost noting. The OP wanted to know if bumping up to 90 Hz would make a significant difference in resistance. It may not, but there may be a difference in reactance to be considered. I'm not up to doing the math, but I don't think it would matter much.