VFD SHIELDED POWER CABLE

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Jim1959

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Longmont, CO
I haven't worked with VFD driven motors a lot, a spec I'm bidding calls for shielded power conductors to the motor. What is the purpose of the shielding?
 
Besoeker

Besoeker

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Location
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Jim1959 said:
I haven't worked with VFD driven motors a lot, a spec I'm bidding calls for shielded power conductors to the motor. What is the purpose of the shielding?
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To stop the switching frequency affecting other conductors.
 
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GoldDigger

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1. Because even though the current output through the motor closely resembles a square wave the voltage on the output wires is a controlled train of variable width square DC pulses at a high frequency called the switching frequency.
The square edge is equivalent to the waveform containing lots of high harmonic energy which, because of the high frequency, easily couples through stray capacitance into any nearby wires.
The shielding contains that.
2. Because the transmission line impedance of the output wires is pretty much guaranteed not to match the load impedance of the motor windings. The resulting standing waves (with peak voltage up to several times line voltage), if the line is not shielded, will be even more likely to induce (not implying induction) stray voltages in nearby wires.
 
Jraef

Jraef

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If it is just those 3 conductors (+G) in the conduit, then the shielding effect is the same, as long as the conduit is properly bonded (at both ends). The VFD cable however also provides for a consistent geometry in the triplexing (twisting) of the cables and that too helps to mitigate the standing wave generation. Generally though, I recommend one or the other, but both doesn't hurt.

But to GoldDiggers point, if you have multiple VFD outputs in the same conduit, or they share space with other conductors in a cable tray or trench, then you absolutely need the shielding.

Side note: for this purpose, you ground the shield at BOTH ends, unlike shielded SIGNAL wires that are grounded at one end. Different purposes for the shielding. With the VFD output cables, you are making a "Faraday cage" with the shielding. Think of your VFD as a local radio transmitter and the output cables are the antenna. Remember, the "FM" in FM Radio = Frequency Modulation...
 
Jraef

Jraef

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Some of this I know you know Besoeker, I'm explaining it all for those that don't.

THHN uses thermoplastic insulation, liquid PVC applied to the wire and heat set to solidify. In that process, it gets tiny voids in it, bubbles if you will. When working with sine wave power, these are irrelevant. But on the output of a VFD, the DC pulses cause capacitive coupling and standing waves that reflect back and forth and increase the voltage. That higher voltage subjects the insulation of the wires to corona discharge stress. The voids in the PVC reduce the cable's ability to withstand this and eventually it fails, leaking to ground or to adjacent cables. This is something that has taken years to come out but I have personally seen it now several times, starting about 5 years ago, on installations that were 5-20 years old. One time when I had to pull the cables out, you could see the burn-through marks at regular intervals, indicating the wave length of the standing waves. It was interesting, but of course represented the failure of the THHN.

The better choice being touted now is XLPE insulation, which you get with XHHW cable. It's essentially heat shrink tubing put over the wire and shrink into it, no voids. It typically has higher voltage ratings too, 100V or even 2000V.
 
Besoeker

Besoeker

Senior Member
Location
UK
Jraef said:
Some of this I know you know Besoeker, I'm explaining it all for those that don't.

THHN uses thermoplastic insulation, liquid PVC applied to the wire and heat set to solidify. In that process, it gets tiny voids in it, bubbles if you will. When working with sine wave power, these are irrelevant. But on the output of a VFD, the DC pulses cause capacitive coupling and standing waves that reflect back and forth and increase the voltage. That higher voltage subjects the insulation of the wires to corona discharge stress. The voids in the PVC reduce the cable's ability to withstand this and eventually it fails, leaking to ground or to adjacent cables. This is something that has taken years to come out but I have personally seen it now several times, starting about 5 years ago, on installations that were 5-20 years old. One time when I had to pull the cables out, you could see the burn-through marks at regular intervals, indicating the wave length of the standing waves. It was interesting, but of course represented the failure of the THHN.

The better choice being touted now is XLPE insulation, which you get with XHHW cable. It's essentially heat shrink tubing put over the wire and shrink into it, no voids. It typically has higher voltage ratings too, 100V or even 2000V.
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Thank you Mr J.

As I said, we use SWA PVC or XLPE.
 
K

keith gigabyte

Senior Member
Location
New Kensington, Pa
Mtw

Mtw

Did a lot of vfd work in mid 90's always used mtw on load side because that's what the old timers used never asked why. No cap issues possible reason? Also experienced the shielded cable issue only once. Most installs were single vfd. This was a bank of chilled water pumps for a data center already in operation. I got the call vfds tripping intermittent but able to reset. Changed to shield no probs. Shared a trough. Not sharing conduits
 
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GoldDigger

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Location
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Besoeker said:
Please excuse my ignorance. What is mtw?
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Machine Tool Wire. A type of wire which is, I believe, characterized by high strand count for high flexibility and a tough wear resistant flexible insulation.
In that respect it would be similar to DLO (Diesel Locomotive) wire and is NOT a wire type found in NEC tables for building wiring.
It can be used for building wiring if it is also categorized as a different UL wire type that is in the NEC tables, or it can be used for internal wiring in a product, listed or unlisted, that is wired as a unit by the manufacturer and is subject to different evaluation criteria.

For example, based on its insulation and construction some MTW may also be listed as TFFN (fixture wire) or as THHN and can be used in building wiring where Chapter 3 allows those wire types.
 
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GoldDigger

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ptonsparky said:
and I thought I asked this question before any of the responses above. My connection must be dang slow or you guys are super fast.

Thank you.
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More likely is that you have your configuration set to display ten posts per page and you hit the, fortunately infrequent now, vBulletin bug that prevents display of the second page of a thread until after you have posted to it.
 
Besoeker

Besoeker

Senior Member
Location
UK
GoldDigger said:
Machine Tool Wire. A type of wire which is, I believe, characterized by high strand count for high flexibility and a tough wear resistant flexible insulation.
In that respect it would be similar to DLO (Diesel Locomotive) wire and is NOT a wire type found in NEC tables for building wiring.
It can be used for building wiring if it is also categorized as a different UL wire type that is in the NEC tables, or it can be used for internal wiring in a product, listed or unlisted, that is wired as a unit by the manufacturer and is subject to different evaluation criteria.

For example, based on its insulation and construction some MTW may also be listed as TFFN (fixture wire) or as THHN and can be used in building wiring where Chapter 3 allows those wire types.
Click to expand...

So many variations.........
 
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