VFD Cable Supplier Recommendations

Hello,

I'm in the process of fitting a VFD to my lathe. The VFD manufacturer states I need to use 3.5mm² shielded twisted pairs cable between the motor and VFD.

Does anyone know where I can purchase 2M of this special cable? I've tried RS, Farnell and general searches on the web but to no avail.

Thanks in advance

That sounds an odd spec for a power cable for a VFD to motor, do you have any details of the VFD?

Mike

When I connected my lathe and mill to their VFD's I used standard shielded/screened cable , for you 2M connection I feel you could use standard shielded / Screened cable.

I used 4 Core 1.5mm2 - SY Multi Flex Cables , 18Amp capacity more than enough for a 1 HP motor

3.5mm2 is an odd specification the standard sizes are 1.5 , 2.5 , 4.0 6.0

I got mine from TLC Electrical Wholesale https://www.tlc-direct.co.uk

For the signal cables for the Remote Control Box [ Stop Start Direction] I made twisted pair cables by using hook up flexible wire and twisting them together using a battery drill. I've also used Ethernet Cat5 Cable [4 twisted pairs] successfully

This company has some interesting info on VFD cable

https://www.elandcables.com/media/39064/variable-frequency-drive-vfd-emc-2yslcy-cable.pdf

VFD Info

Steve

As Mike said that is an odd type of cable, so odd that it probably does not exist.

Screened twisted pairs are usually used for signal wires rather than power cables, if you say what the VFD model is and the motor size someone will be able to answer the question easily.

Ian P

Spec for the SB 1024 says no more than two horse power motor. That would not require such large cross section conductors for its three phases.

This **LINK** is the cable I got for a 1.5kw motor and Invertek VFD. I went with 2.5mm wire but 1.5mm would have been just as fine. You don't mention the power for the vfd and motor?

The motor wouldn't, but the cable requirement might be related to keeping EMC levels down.

Although VFD's produce power good enough to fool a motor it's being fed genuine 3-phase, the waveform is synthesised. Full of high-speed switching artefacts, and there's plenty of radio frequency power in the mix. High frequency AC behaves very differently from low frequency AC, such as radiating from relatively short lengths of cable. The purpose of using fat section cable might be to keep the RF impedance low, perhaps matching it to reduce reflections, and improving the efficiency of the shield.

I used 2.5mm² 4 core SY (from TLC as it happens), which is good for 24A, and should work fine. 6mm² shielded is available if fatter is essential.

I wondered if two different cable requirements are mixed up. The controls are likely to call for twisted pair, and the usual shielded power cable is needed by the motor. Not a special twisted-pair shielded power cable!

Dave

SY cable has a braided steel layer which is designed for mechanical protection, CY has a copper screen which is intended for suppression. SY is probably fairly effective at controlling radiated noise but it’s primary function is mechanical protection.

Mike

Doubtful. Radio waves, from an capacitance/inductance system (aerial) also depends on the length of the antenna rods. I doubt very much that choosing a slightly bigger cross section would make any difference at all. Anything radiated would likely be at any random frequency.- likely dependant on the length of the lead, if it is not properly screened.

It’s OK twisting pairs, but for three phase there would be a spare one, so you are likely correct that the poster is confused.

Twisted pairs are not suitable for reducing interference on 3 pahse circuits like VFD to motor. For twisted cable to be effective the signal on the conductors should be balanced. For two core thai is easy to understand the fields from the conductor carrying current out are cancelled by the fields caused by the current coming back. You can see this with a compass, a few feet of wire and a AA cell. lay the wire in a big loop with the compass 1/4 way along the wire. Connect the cell for a second (yes a short circuit) and the compass will deflect. Now bring the last 1/4 of the wire and lay it close to the fist part without moving the first wire or compass. Connect the battery for a second and the compass deflection will be much less. Twist the two wires together as they pass the compass and there will be no deflection.
For 3 phase you need 3 wires twisted together. If you use
In practical terms adding a 4th wire carrying little current (the ground conductor) to the twist it has little effect, However using two twisted pairs.causes an imbalance because 2 have balanced surrents and one does not.

As others have mentioned a length of 2.5 mm 4 core SY cable (i also use TLC for short lengths ) https://www.tlc-direct.co.uk/Products/MF2dot5slash4.html Is perfectly adequate for your application.

Robert G8RPI.

If you have a local CEF you could save postage

Sy cable

I'm not convinced fatter wires would make much difference either, but the frequency at which radiation is most efficiently radiated depends on the inductance of the motor windings as well as the wiring. Whatever the resonant frequency happens to be isn't primarily determined by the length of the wires, because their inductance and capacitance are small compared with the rest of the circuit, including the motor.

Loading coils are often added to antennas reduce the amount of metal needed in the air because there isn't enough real estate or weight carrying capability to support a full-size resonant aerial.

The loading coil in the photo above isn't only an inductance calculated to make a short antenna resonant: equally important, it's designed to transform the antenna's impedance to match the 50 ohm coaxial feeder cable. (Black Wire at the base.) If the feeder and antenna impedances aren't matched standing waves appear on the outside of the coaxial cable's shield and it radiates like the clappers - it's bad. Something like that might explain the VFD cable's odd specification, it's part of an EMC strategy. Or might be a misprint...

Dave

Edited By SillyOldDuffer on 22/06/2022 21:28:22