Shielded vs Unshielded Cable for VFDs

..."Like many engineering solutions, VFDs present not only benefits, but also drawbacks. For example, the same fast switching rate of the transistors inside a pulse-width modulated VFD that can accommodate an abrupt speed change in a motor (and offer precise control of processes) is also capable of generating unwanted noise in the drive system cable and in the drive itself.

Electrical energy flowing in the cable contains frequencies as high as 30 MHz. If this high frequency energy is not contained within the cable, it can radiate out to interfere with the proper operation of nearby equipment.

Examples include electronic equipment, less than-robust, or commercial grade Ethernet systems and simple instrumentation wires— even circuits that have absolutely nothing to do with the VFD system itself.

This noise emission can sometimes be difficult to track down and eliminate and is likely the single most significant problem associated with VFD systems today. Unless a proper cable shielding design is present to control it, noise emission from a drive system cable can disrupt plant and factory operations.

Moreover, because a longer cable radiates more noise, the length of cable runs must be limited."...

http://www.belden.com/docs/upload/vfd_choosing_wp.pdf

Thanks for the reply

So it is actually the opposite of what I was thinking. Its not to protect the signal to the motor from the VFD, its to protect other signals from the spurious emissions of the output of the VFD.

Maybe you explained this and didn't fully understand but why then a longer allowable run for unshielded cable?

The way I interpreted your answer is because the shield can only do so much because as the length increases the more noise output from the VFD cable we can generate

Also what are your thoughts on PLC analog outputs and inputs?

I have a system I'm fooling with where the longest run I have is say 3 feet.

I know it depends on the instruments but this is just a for fun project I'm speaking of and not work place critical. I have access to some belden cable but trying to decide if its required for a "for fun" project

The link I provided gives a much longer and more precise answer than I am willing to commit to....

Sure I can understand that

Thanks for the link. I was just curious if I was understanding at least the basic point you were trying to make.

Any thoughts on the short run Analog PLC wiring?

Well it's really a crapshoot as to whether your vfd or vsd is going to cause any interference with other control wiring...it depends so much on a myriad of factors....It doesn't hurt to take some precautions and know what tools are at your disposal to correct any problems...

http://control.com/thread/1026194952

Gee, I'd long forgotten that thread I started 13 years ago! A lot of water's gone under the bridge since then and we now have many hundreds of VFD / PLC combinations running great without interference problems. As usual, the secret to success is good equipment plus good sense and experience.

BTW, we do install the VFDs and PLCs together in the same electrical panel without any special separation. However, we're careful to keep low voltage wiring at safe distance from the VFD output cables.

Zvi

As long as the technology is still in use, the same questions get asked again and again...

The reason a longer run is permitted with unshielded cable versus shielded is because the unshielded cable has lower losses due to the reduced capacitance. The link to the Belden cable by SE is an EXCELLENT reference. It is spot on.

Here is the excerpt describing the mutual capacitance between phases on the motor cabling.

"Capacitive coupling and cable charging are effects which consume some of the drive output currents in capacitive interaction with either the other motor leads, or other adjacent cable systems. These phenomena can consume drive power and result in reduced motor torque, drive overload trips, and induced voltages on adjacent cable systems. The keys to reducing the cable losses associated with capacitive coupling and cable charging are: Having the lowest practical cable capacitance, and ensuring that motor lead sets are effectively shielded from each other to prevent capacitive interaction. Capacitive charging losses are proportional to the length or run, and the number of conductors with which a given set of motor leads can capacitively interact. A worst case scenario is when multiple motor leads with high capacitance (THHN for example) are ganged together in conduit."

In this case, adding a shield over the conductors adds additional 'plates' to the inherent capacitance of wires laid alongside each other. Twisting the conductors helps control EMI by reducing the "loop size" and reduces inductance, but it also increases the capacitance as well.

Good luck with your quest.

GA

You beat me to it by a wide margin, well done.

Isn't your comment basically the complete opposite of what the OP actually said....?

Did you (or maybe I!!) not read it fully?

The primary purpose of the shielded VFD cable is to Pye the the environment from the EMI/RFI that would emit from the cables. Think of the output cables as relatively powerful local FM radio broadcast antennae. FM = Frequency Modultaion, it's not a big leap when you think about what the output of a VFD is doing.

The other benefit is that by maintaining a consistent geometric relationship between the conductors, you reduce the cable capacitive effects that lead to standing wave creation, which leads to reflected wave voltage spikes and motor insulation damage. But when you get longer and longer, the capacitances creep up again because of shear distance. That's where unshoelded cable IN STEEL CONDUIT begin to look better again, BECAUSE the asymmetry starts to help.

But bottom line if you are going to use STEEL conduit and are rigorous about bonding it and maintaining it, you don't need the shielded cable. In some parts of the world, people don't use STEEL conduit, for them the shielded cable is much much more important. Also, if you want to run the VFD output cables in a raceway with any other cables, you must use the shielded cable, but generally it's best to avoid that if st all possible.

Instrumentation cable shielding is different, the shield there is to prevent stray signals from getting INTO your wires.

Instrumentation cable shields generally should always be grounded at one end only. VFD power shielded cables must be grounded at BOTH ends.

Pye?

OK, it wasn't just me.

Huh, weird. Was supposed to be prevent.

Shielding has multiple purposes. In the context of your questions, one is to prevent unwanted electrical noise from being radiated from the conductors within the shield, the other is to prevent unwanted outside electrical noise from being picked up by the conductors within the shield.

Thanks for all the replies much appreciated!