Different shield types and noise attenuation

Is the PWM wave on the twisted pair essentially symmetrical (both lines swinging through equal and opposite voltages)?

Is the line feeding a floating load?

I'll bet that your shield will not be effective, regardless of what it is made of, unless the signals are symmetrical. If they are not, they can be made more symmetrical by adding a heavy duty common mode choke at the source end. Then the shielding will be a lot less critical.

I'd take the other side of that bet if the load is shielded by its metal case and the shield of the cable is AC connected to that case.

The cable core vs its shield looks like a low impedance coax in that situation. Return current for the high frequencies will be flowing in the shield. This transmission line will be quite lossy at high frequencies.

Are you designing the cable or choosing from available materials?

If selecting from available wire style, check it's spec, particularly voltage vs frequency handling capability.

Seriously shielded cable will not rely on just one kind of material, and high voltage types will often depend on foam/air type dielectrics to reduce dissipation. As mentioned elsewhere - two antiphase conductors plus shield offer distinct advantages in reducing outward-bound radiation.

'Slim' doesn't describe it.

RL

Well actuallty its a twisted triple and not a pair. The signals come from a

The edges will not be symetric and the three wires are PWM approximations of three sine waves at 120 degrees apart.

As I stated in a previous reply this is for a 3 phase motor driver so the cable is actually a twisted triple. The shield is grounded on the driver side and at the motor case which is 5 feet away. The grounding at the motor end is to collect the currents that capacitively couple from the windings to the stator and allow them to return through the shield to the driver where they originated.

The wires is actually wtisted shielded triple and not pair. Its for a 500V three phase motor drive application with a PWM motor driver. The signals are not complementary so I woun't get the advantge of field cancellation.

by slim I meant the physicical OD of the cable is smaller than the OD of a braided design.

On Jun 7, 6:29 pm, "Mook Johnson" wrote: [.....]

In that case the shield will greatly reduce the radiation from the cable.

Adding a lossy common mode choke at the driver end will also help.

put a L-C low pass before the coax that'll fix those rise times, - all of a sudden shielding will be less of an issue.

Bye. Jasen

Not only will the shielding work better, but those fast edges will not ring at the motor and generate corona discharge that will eat the motor insulation.

I see nothing here about a three phase motor control, that only comes out in later posts. What is he HP or kW rating of the motor? It makes a difference. As stated before by MooseFET you need a (maybe lossy) common mode choke. Be very careful in handling the grounding connections, it is imperative that there be neither shielding gaps nor ground loops.

All that said, braid is the most variable, due to variations in braid density. Copper foil is the best but the most expensive. Polymer coated foil is almost as good, but metalized foil may not be as good as mediocre braid. That was the test lab results about 20 years ago, i cannot think of any for there to be any significant change in the results. Also given 500 V operating voltage, and the reasonable expectation that significant power (over 1 kW) is involved the shield material will not have significant impact on the total cable diameter.