Solid or stranded wire for motors on VFD's 2

[unwiredo]

Does it matter if the wire feeding a motor from a VFD is stranded or not. I would think the more strands the better.
This is relative to the same wire gauge for comparison.

The stranding gives more paths for the current to flow so the theory goes.

thanks

 

[ScottyUK]

Hi toygasm4u,

The linked documents don't appear to address the use of solid versus non-insulated-stranded conductors and why one should be superior to the other. I don't disagree with you about reflected wave problems on long leads, but those are transmission line effects and are fundamentally different to skin and proximity effects. It has little to do with solid versus standed inner conductors and everything to do with the insulation material, cable screen and physical geometry chosen in construction of the cable because these determine the properties of the transmission line.

Good links by the way. Off-topic a little but interesting. Thanks.


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If we learn from our mistakes I'm getting a great education!

 

[dpc]

I'm having a hard time understanding why solid wire would present that much difference in transmission line characteristics at these frequency compared with stranded wire.

I understand the issues with standing waves, bus overvoltages, etc, but I'm not seeing how solid vs stranded effects this.

I have used solid and stranded wire in HF and VHF antennas and there is very little difference in between the two assuming the same physical dimensions and effective cross-sections.

 

[itsmoked]

The standing wave/reflected wave issues should be primarily related to the capacitance. If the insulation is a different material, or a different thickness, or the mutual line capacitance ends up being different once pulled thru conduit then you could very well see a different resonance response between the two species of wire. I would not say that was caused be the copper being stranded or not stranded but rather the differences in everything surrounding the copper, and the proximity of the copper once installed.

These issues are common and central in communications since they're using high frequencies just like this odd power application - VFDs.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[controlsdude]

I have to ask the stupid question.

Why would anyone pull solid core conductor wire instead of stranded wire for power leads?

I have yet to be at an install and seen an electrician pull solid core wire for VFDs or motor leads. I would consider it a bad install.

 

[Skogsgurra]

Sorry to jump in so late. I didn't realise that there could be such diverse opinions on this simple matter.

First: There is very little HF current in the motor cables. Some is there because of cable capacitance and leakage capacitance in the motor. But the dominant current (I would say 95 - 99 %) is motor fundamental frequency. This is simply because the motor inductance smoothes the current. So litz would not be of any use. The sole reason for using stranded wire is because it is easier to handle. Ever tried to bend and connect a solid 185 or 240 mm2 wire?

Second: The reflection (aka standing wave, which it really isn't) depends on differences in wave impedance between inverter (close to zero ohms), cable (in the 10 - 100 ohms range) and the motor winding (several kiloohms). It has absolutely nothing with stranded, solid or litz wire to do.

Third: The capacitance between a wire and the surrounding world is mostly dependent on the equivalent wire diameter. Stranded wires tend to have a slightly larger equivalent diameter than solid wires. So there should be a slightly larger capacitance in the former wire. But the effect is not big and hardly worth disputing.

toygasm4u, I am sure you do good business in your company. But it seems that the business idea could be improved. The problems you describe are not caused by the solid wires - which I am surprised to hear about. No one uses them. For purely "mechanical" reasons.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

Thank you Gunnar.



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If we learn from our mistakes I'm getting a great education!

 

[burnt2x]

We can all agree that stranded wires have more "surface area" than a single solid wire. If we base our analysis on that premise, and granting that AC current (especially with high frequency carrier waves)tends to flow at the outer surface, logic dictates that stranded wires could have higher current carrying capacity for the same length, of same size of wire.
It can also be seen in all wires and cable manufacturers' ampacity tables that the wire's DC resistance is slightly lower for a stranded wire for the same size.

IMHO, as to the OP, we should specify stranded cables as opposed to "solid wires", aside from the mechanical handling advantages stranded cables present.(especially on on longer than specified links between motor and VFD)

 

[ScottyUK]

Logic... so how about explaining the physics?

It doesn't matter how much surface area the conductors have if all the surfaces are touching each other. That's the whole point: if they touch each other they aren't separate conductors. Why do the high frequency users use Litz wire in their power transformers? Why do we use laminated busbars for high current high frequency equipment? Why do you think generator and transformer manufacturers go to the trouble of using expensive Roebel conductors? Because using plain conductors is boring? Because they have too much money?

Fill a length of hose with copper dust and use that as a conductor. Lots of surface area, but it will still experience skin and proximity effect in the same way as a solid conductor of the same diameter, because skin and proximity effects are a function of the physical geometry of the conductor and the frequency of the current.

I am amazed at how many people have an opinion on this topic yet can't be bothered to read up on the subject. Seems like the drives world has more than its fair share of people with an opinion which they can't back up with any known science, just like the audio community with their magical gold plated mains plugs which make their amplifiers sound better. Same pseudo-scientific crap.

/rant


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

Scotty, Can I interest you in some Monster Cables for your stereo?


BTW you are 100% here on your assessment. I fully agree in all respects. It's the surface area/geometry.


And now..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Hi Keith,

What about this then!

http://www.noteworthyaudio.co.uk/diy/page17.html

I haven't laughed so much since the last conservative government got booted out of office...


By the way, where did you get an emoticon of a madman chopping up a horse with a lightsaber?


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If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

I didn't laugh, Scotty - I almost threw up.

If that company were in Sweden, I know someone that would sue them. Someone that I see every morning.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[burnt2x]

Scotty,
1. Geometrically, round wires always leave voids when packed into a single conductor. Your assumption that "if all the surfaces are touching each other" is therefore invalid. There are in fact surfaces that do not touch inside the stranded conductor.
2. A physicist that you are, how does your physics explain the differences in ohmic values of solid vis a vis stranded conductors? Please enlighten us.
3. Any opinion could be wrong, as the word means. Yours, included. The point I raised was an offshoot of actual investigation of data: "that stranded wires have higher ampacity compared to solid wires of the same size". For sure, there is physics behind that.

The best performing conductor will be based on your application and requirements. Both solid and stranded types have advantages and disadvantages.

Back to the OP, we use stranded wires for the following reasons:

    1. flex life, 
    2. ease in terminating, 
    3. Power capacity edge over solid, 
    4. Length of conductor less sensitive to temperature,
       flexure and other environmental conditions.

 

[ScottyUK]

They are conductors. One touches its neighbour, which in turn touches its neighbours. Thus they are all electrically in contact with each other over anything more than a few inches of cable. Do you need a picture to help explain that? In fact, don't take my word for it, try it with a multimeter and prove it for yourself.

Different ohmic resistances per unit length are due to differences in the cross section of copper in the conductor. I thought that would be pretty obvious.

We actually agree on something! Any opinion could be wrong, but far greater minds than mine have developed the theory of skin and proximity effect, and others have proven via documented experimentation that the theory of skin and proximity effect is a valid representation of the observed behaviour in the real world.

We're going to agree on something else too: "The best performing conductor will be based on your application and requirements. Both solid and stranded types have advantages and disadvantages." That statement is most certainly true, but high frequency behaviour is not one of those reasons.


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If we learn from our mistakes I'm getting a great education!

 

[electricpete]

I don't have any horse in the race and don't work with this stuff, and don't know the answer to the question.

But on the subject of high frequency losses, it seems that stranded wire is better than solid, but worse than Litz (as might be expected).

http://thayer.dartmouth.edu/inductor/papers/stranded.pdf

Special winding construction, using litz wire or foil, is often necessary to control eddy-current losses in high-frequency transformer and inductor windings. The high cost of these techniques is a major limitation in developing high-powerdensity, high-efficiency components for power electronics. A much lower-cost alternative is simple stranded copper wire with uninsulated, bare strands. In addition to the wire cost advantage arising from avoiding the insulation process, there is a component-manufacturing cost advantage arising from the easier termination of bare strands. The loss with uninsulated strands will certainly be higher than in true litz wire with individually insulated strands. However, the high-frequency loss can be substantially lower than in solid wire—the separation into strands impedes eddy currents, even if it does not completely stop them. Anecdotal evidence has supported the idea that this can work well in some applications. However, until now, there has not been a model available to predict the eddy-current losses in stranded wire with uninsulated strands, which we will refer to simply as stranded wire for the remainder of this paper. Thus, it has not been possible for a designer to evaluate this lowcost alternative and determine whether it is a good choice for a particular application, and it is almost never used.

Whether this difference is significant in VFD applications, I don't know.

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Eng-tips forums: The best place on the web for engineering discussions.

 

[ScottyUK]

Thanks for the link, ePete.

Don't think I have a horse in the race either. Keith killed it earlier on...


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

Wasn't me!! I just used my cell phone to snap a picture as I drove by.

BTW: I think that guy was using a stick. These are light sabers.




Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Some observations.
>Capacitance. I see three basic constructions.
1> Solid wire with the insulation formed onto the surface of the wire.
2> Stranded wire with the insulation formed onto the surface of the outside strands. This will have a greater contact area per unit length and thus greater capacitance.
3> Stranded wire with a wrap of sheet plastic to make it easier to strip the cable. The Insulation is formed on top of this wrap and is in proximity to the top of the outside strands. The capacitance would be less than the base solid wire.
However, These slight differences may be insignificant when compared to differences resulting from varying insulation thickness and proximity to a grounded surface.

I think that the differences in opinion and observed effects are much more due to eddy currents than to skin effect. Skin effects are the result of magnetism and magnetism does not respect stranding or strand insulation. Stranding and Litz wire reduce skin effect slightly by increasing the overall diameter. However stranding and insulating greatly reduce eddy currents.
There are some effects that are dependent on both frequency and current. Skin effect due to current and eddy currents are two of these effects. Audio and radio people see skin effects and eddy currents at high frequency. Power people see skin effect at high current levels.
Hence my wishful suggestion to replace audio cables with a copper tube with similar cross sectional area of copper.
IPS buss works well for high currents, why not for high frequencies?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

An interesting thing noted in Standard Handbook for Electrical Engineers, 14th Ed, By Donald G. Fink, H. Wayne Beaty, Page 4-14"

Increase in Resistance Due to Stranding. If it were true that no current flows from wire to wire through their lineal contacts, the proportional increase in the total resistance would be the same as the proportional increase in total weight. If all the wires were in perfect and complete contact with each other, the total resistance would decrease in the same proportion that the total weight increases, owing to the slightly increased normal cross section of the cable as a whole. The contact resistances are normally sufficient to make the actual increase in total resistance nearly as much, proportionately, as the increase in total weight, and for practical purposes they are usually assumed to be the same.

If you read carefully, he sad there are two ways to view the dc resistance of stranded wire, depending on whether we assume the strands act as if they are insulated or whether they act as if they are solidly bonded. The way which more closely reflects the actual dc resistance behavior is the model where the strands are insulated from each other. He mentions "lineal" contact meaning a line along a cylinder in 3-d, or a point on the circular circumference in 2-D. From this geometric perspective we can see why there is resistance between strands due to the very small contact area created by this type of contact (in fact it would be infinitessimatelly small if not for conductor surface roughness and deformation under contact pressure which tends to flatten the circular diameter at point of contact) . If we apply this same concept to eddy losses, we expect some physical reduction in eddy current losses due to the insulating effect of the small contact area between strands.

I suspect waross is correct in his comments.

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[Skogsgurra]

Interesting as it may be. I think that this thread has left reality.

First, as I said some time ago, because there is very little HF current in the motor cable and second because the OP asked "Does it matter if the wire feeding a motor from a VFD is stranded or not. I would think the more strands the better".

Compared to the real reason to use stranded wire, the second and third order effects are hardly worth discussing.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[burnt2x]

epete,
Very good points! I only have my 12th edition here. You must be poring over that thick book for so long!
I think I missed the exact term "lineal contact" between strands.
Thanks for the efforts. Star to you!