Conductor impedance

[drbond24]

How do I calculate the ground impedance of VFD cable that has three ground wires? The cable consists of three conductors and three bare grounds wrapped in a copper tape shield. The grounds are in continuous contact with the copper shield.

Does the tape shield connect the bare grounds in parallel? If so, the resistance would be 1/3 of one ground, but I'm not sure it's that easy.

Thanks

 

[coulter]

dr - A couple of questions.

...The cable consists of three conductors and three bare grounds wrapped in a copper tape shield. ...

Are you saying the bundle of three conductors and three grounds are wrapped in copper tape, or just the three grounds are wrapped separate from the three conductors? I've seen plenty of cables where the individual conductors are wrapped with a copper tape shield, but never where the equipment bonding conductors are wrapped with a conductive tape.

How do I calculate the ground impedance ... If so, the resistance would be 1/3 of one ground, but I'm not sure it's that easy. ...

If the conductors are connected to the same bar at both ends, and they should be, then they are in parallel. The copper tape does not change that - except the copper tape reduces the resistance

carl

 

[petersonra]

How do I calculate the ground impedance of VFD cable that has three ground wires? The cable consists of three conductors and three bare grounds wrapped in a copper tape shield. The grounds are in continuous contact with the copper shield.

Does the tape shield connect the bare grounds in parallel? If so, the resistance would be 1/3 of one ground, but I'm not sure it's that easy.

Thanks

Why would you care? It just needs to be low enough to trip the branch circuit OCPD in the event of a fault.

If it matters to you, I would be inclined to get a piece and measure it. Bearing in mind that you used the word impedance rather than resistance, a simple Ohmmeter is not going to work, as the impedance will vary with frequency.

 

[coulter]

... Bearing in mind that you used the word impedance rather than resistance, a simple Ohmmeter is not going to work, as the impedance will vary with frequency.

dr -
I saw that too, but you used the word "resistance" the second time. My guess is you aren't differentiating between impedance and resistance.

carl

 

[drbond24]

more info

more info

The three conductors and three grounds are twisted together, then the whole assembly is wrapped in copper tape.

I care because my supervisor told me to calculate it. Don't know why he wants it. I intend to verify the calculation with a measured value, but I do have to calculate it.

 

[drbond24]

oops

oops

I did slip and use the wrong word. I need impedance, not resistance. It's variable frequency drive cable.

 

[coulter]

dr -

... I care because my supervisor told me to calculate it. Don't know why he wants it. ...

Absolutely a good reason.

The three conductors and three grounds are twisted together, then the whole assembly is wrapped in copper tape. ...

Hopefully she wants wants the DC resistance. and not an AC impedance.

If so, then I would take the cross sectional area of the tape - figure a resistance. Figure the equivalent resistance as the parallel combination of the three grounding conductors with the copper tape resistance.

If he wants a 60Hz AC impedance then come back here.

carl

 

[drbond24]

impedance

impedance

I need AC impedance.

 

[winnie]

I bet you need the AC impedance, and that you need it at multiple frequencies, in particular the drive switching frequency.

I would start by contacting the cable manufacturer to find out if they have test data.

-Jon

 

[drbond24]

good advice

good advice

That is very good advice. Ironically, in this case, I am the cable manufacturer. I have to figure this out so that when someone else takes your advice and calls me, I'll have the answer.

I dug out some textbooks and am making progress with the calculation. The only problem I have is that the capacitive impedance is Zc = 1/jwC, but C is a very small number. When taking the inverse, you get a HUGE number which doesn't look good for an impedance value.

 

[LarryFine]

It's variable frequency drive cable.

Keep in mind that you'll end up with an impedance curve, rather than a single number, if you need to cover a frequency range.