Solid or stranded wire for motors on VFD's
Solid or stranded wire for motors on VFD's
(OP)
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
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





RE: Solid or stranded wire for motors on VFD's
Basically, we specify stranded wire for everything except 120 V receptacle and lighting circuits.
Who is trying talk you into solid wire?
RE: Solid or stranded wire for motors on VFD's
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RE: Solid or stranded wire for motors on VFD's
ht
It's all about flexibility. Motors are used because they make things move. Even though the inherent soft starting capabilities of a VFD will reduce torsional stresses on the conductors, movement means vibration. Stranded conductors flex with it, solid wire can transmit the vibration to the device terminals, eventually loosening them. I don't know of any industrial plants (with engineering standards) that would allow solid wire on anything other than lighting and office outlets.
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RE: Solid or stranded wire for motors on VFD's
RE: Solid or stranded wire for motors on VFD's
Or am I missing something here?
RE: Solid or stranded wire for motors on VFD's
RE: Solid or stranded wire for motors on VFD's
Check out Litz wire. It exists specifically because ordinary stranded wire doesn't bring any benefit for HF usage. Laminated busbars with insulation between layers exists for the same reason. The wiki entry isn't bad:
http://en.wikipedia.org/wiki/Litz_wire
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If we learn from our mistakes I'm getting a great education!
RE: Solid or stranded wire for motors on VFD's
RE: Solid or stranded wire for motors on VFD's
RE: Solid or stranded wire for motors on VFD's
RE: Solid or stranded wire for motors on VFD's
Rather, I suspect that the wire braiding is to make the cable more flexible. The Pirelli stuff almost requires a conduit bender to get it around a corner but Belden and Olflex braided shield cable is much easier to bend.
Many drive manufacturers suggest grounded metallic conduit as a first step in containing drive/motor lead noise. That's certainly not stranded!
RE: Solid or stranded wire for motors on VFD's
This point is a slight tangent to the original post but my response was to highlight the simple benefit of braided screen.
RE: Solid or stranded wire for motors on VFD's
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RE: Solid or stranded wire for motors on VFD's
I was under the impression that braided ground cables for HF grounding used a braid to avoid the increased inductance of a conventional cable with twisted strands. Bare cable may develop a surface corrosion that will impede current flow between strands, forcing the current to spiral down the individual strands increasing the inductive reaction to a high frequency signal.
I have suggested to audiophiles that instead of outrageously expensive special stranded speaker wires they would be better served by solid conductors (no oxidation impeding current transfer between strands after time) or even better served by insulating lengths of copper tubing.
Bill
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"Why not the best?"
Jimmy Carter
RE: Solid or stranded wire for motors on VFD's
Even though most people will recommend a shielded VFD cable for any drive motor combination, it should be noted that shielded is the worst performer for cable charging and common mode currents. The only true benefit of shielded is RF immunity for nearby equipment, and the reduction of ground circulating current in the VFD motor system. The shield, when terminated on both sides provides a low impedance return for the HF vs. the basic bond lead.
It has been my experience simply from a VFD and motor performance perspective, that the best performer is good ol' 600V stranded in conduit. The lack of symmetry in the run keeps capacitive effects low, and if the conduit is up to snuff, your RF mitigation will be minimal.
The secondary concern for solid core is at the motor termination end; solid core has a tendency to suffer insulation scraps at the entry point and in your bends. While your average 460V RMS won't find this pin-hole, the VFD's HF Switched DC output... will. At longer lead lengths, this becomes a greater concern, as the impedance mismatch in the cable will sometimes produce twice buss at the motor terminals (reflected wave). For a 460V drive, this is approaching the 1400Vp mark. Insulation pin-hole "found".
RE: Solid or stranded wire for motors on VFD's
My understanding now is that the stranded wire is much better because it carries the RF current better and requires less current to drive the wire and therefore is more efficient.
Wow who would have though there is so much detail in just type of wire.
Appreciate the feedback
Thanks
RE: Solid or stranded wire for motors on VFD's
So how does stranded wire without inter-strand insulation perform better than solid wire? Let's have some facts and analysis, not speculation and hearsay!
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RE: Solid or stranded wire for motors on VFD's
I think you are right to be suspicious of this reasoning.
The main reason skin effect is not significantly different is that the circulating currents flow around the outside strands as they are in contact with one another.
There is some higher resistance between strands but this is not very significant at VFD frequencies.
And I also question the idea of "charging current" being different for the two. The surface area may be slightly higher with stranded but those surfaces face each other and their impact on capacitance would be less.
What all this has to do with VFD's i am not sure of. They operate far down in the frequency region where most of this stuff is unimportant.
RE: Solid or stranded wire for motors on VFD's
I have worked closely with Rockwell Automation in Mequon Wisconsin, Delta, ABB, and others regarding low HP applications of this type. Without releasing customer service records, I can tell you that VFD installations which utilize solid core wire for motor leads yields us an 80~90% failure rate. The most common mode of failure is a buss overvoltage and or motor overload due to HF feedback on the buss, and motor insulation breakdown. I get on planes and fly around the country to resolve these kinds of issues. It is all purely a matter of DV/DT effects on the total system impedance which includes motor and motor lead set. While there are output filters that resolve these issues, the price point is an uncomfortable figure to look at, thus, we avoid solid core at all costs.
While my "in a nutshell" explanation may have been a little rough around the edges, my company would literally be out of business due to warranty issues if it weren't true. I would recommend for anyone to have a conversation with a VFD design engineer or project manager to discuss the nature of these phenomenon.
I'm supposed to be working, but here are a few good reads:
h
http://www.ab.com/drives/techpapers/ieee/pcic.pdf
http://www.belden.com/pdfs/Techpprs/capwmtp.pdf
No speculation required.
~Mark
RE: Solid or stranded wire for motors on VFD's
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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RE: Solid or stranded wire for motors on VFD's
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.
RE: Solid or stranded wire for motors on VFD's
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
RE: Solid or stranded wire for motors on VFD's
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.
RE: Solid or stranded wire for motors on VFD's
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Solid or stranded wire for motors on VFD's
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RE: Solid or stranded wire for motors on VFD's
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)
RE: Solid or stranded wire for motors on VFD's
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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RE: Solid or stranded wire for motors on VFD's
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
RE: Solid or stranded wire for motors on VFD's
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!
RE: Solid or stranded wire for motors on VFD's
If that company were in Sweden, I know someone that would sue them. Someone that I see every morning.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Solid or stranded wire for motors on VFD's
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:
CODE
2. ease in terminating,
3. Power capacity edge over solid,
4. Length of conductor less sensitive to temperature,
flexure and other environmental conditions.
RE: Solid or stranded wire for motors on VFD's
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!
RE: Solid or stranded wire for motors on VFD's
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:/
Whether this difference is significant in VFD applications, I don't know.
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RE: Solid or stranded wire for motors on VFD's
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!
RE: Solid or stranded wire for motors on VFD's
BTW: I think that guy was using a stick. These are light sabers.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Solid or stranded wire for motors on VFD's
>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
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"Why not the best?"
Jimmy Carter
RE: Solid or stranded wire for motors on VFD's
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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RE: Solid or stranded wire for motors on VFD's
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Solid or stranded wire for motors on VFD's
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!
RE: Solid or stranded wire for motors on VFD's
This subject seems to have nine lives and has become almost comical.
Is anybody really thinking about this???????
RE: Solid or stranded wire for motors on VFD's
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RE: Solid or stranded wire for motors on VFD's
I did not ignore it Skogsgurra's comments. I responded to comments which implied there would be absolutely no high frequency derating for solid conductors as opposed to stranded conductors.
And the last comment is somewhat ironic. As we know, stranded wire has higher dc resistance per unit length than solid conductor of the exact same circular mil cross section (or same weight) due to the very reason that there is an insulating effect between the strands (so the effective length of the conductor is longer since it has to spiral around following the strand instead of traveling straight).
I have the highest respect for the author of these comments. But in this case I believe he missed the mark and I was weighing in on the side of someone whose arguments I believe were unfairly criticized.
Do you think my comments were inappropriate?
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RE: Solid or stranded wire for motors on VFD's
Keith Cress
kcress - http://www.flaminsystems.com
RE: Solid or stranded wire for motors on VFD's
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RE: Solid or stranded wire for motors on VFD's
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RE: Solid or stranded wire for motors on VFD's
I appreciated the link, and it was mainly for my benefit I think. Thanks.
I am not 100% sold on the minimal contact betwen strands theory although I can see the reasoning behind that argument. I am pleased that you can back up what you are saying with some references. That puts you in a minority of one so far.
Assuming the lineal contact theory has some merit I was surprised at the very wide spread of results obtained by the researchers.
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RE: Solid or stranded wire for motors on VFD's
And, electricpete, I particularly appreciate and respect your comments in this forum. My shouting should not have been directed at you but at those who absolutely will not stop the discussion even in the face of data that says the issue is moot.
Again, my apologies.
RE: Solid or stranded wire for motors on VFD's
Perhaps I overreacted just a weeee bit to Dick's comments
Thanks guys for not over-reacting to mine.
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RE: Solid or stranded wire for motors on VFD's
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RE: Solid or stranded wire for motors on VFD's
Tarnish not withstanding, I will still use stranded wire for motor connections with or without a drive.
Bill
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"Why not the best?"
Jimmy Carter
RE: Solid or stranded wire for motors on VFD's
GTiz.