[Mook Johnson]

I know of three different types of shield used for say twisted shielded pair
wire.

1) braided copper
2) copper foil
3) Metailzed poly film

I may be forced to use the metal poly film but can find information about
how well it shields noise compared to braid.

Anyone have experience with these different shield type and can comment on
the noise atteniation.

The internal conductors will have a 500V PWM signal with 200nS fise and fall
times. I'd like the cage in that dV/dt as best I can but need the slim
design of the poly shield vs the braid.

 

[John Popelish]

I know of three different types of shield used for say twisted shielded pair
wire.

1) braided copper
2) copper foil
3) Metailzed poly film

I may be forced to use the metal poly film but can find information about
how well it shields noise compared to braid.

Anyone have experience with these different shield type and can comment on
the noise atteniation.

The internal conductors will have a 500V PWM signal with 200nS fise and fall
times. I'd like the cage in that dV/dt as best I can but need the slim
design of the poly shield vs the braid.

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.

 

[MooseFET]

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.

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.

 

[legg]

I know of three different types of shield used for say twisted shielded pair
wire.

1) braided copper
2) copper foil
3) Metailzed poly film

I may be forced to use the metal poly film but can find information about
how well it shields noise compared to braid.

Anyone have experience with these different shield type and can comment on
the noise atteniation.

The internal conductors will have a 500V PWM signal with 200nS fise and fall
times. I'd like the cage in that dV/dt as best I can but need the slim
design of the poly shield vs the braid.

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

 

[Mook Johnson]

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.

Well actuallty its a twisted triple and not a pair. The signals come from a
3 phase triple half bridge motor driver with a 500V bus.

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

 

[Mook Johnson]

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.

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.

 

[Mook Johnson]

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

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.

 

[MooseFET]

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.

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.

 

[Jasen]

I know of three different types of shield used for say twisted shielded pair
wire.

1) braided copper
2) copper foil
3) Metailzed poly film

I may be forced to use the metal poly film but can find information about
how well it shields noise compared to braid.

Anyone have experience with these different shield type and can comment on
the noise atteniation.

The internal conductors will have a 500V PWM signal with 200nS fise and fall
times. I'd like the cage in that dV/dt as best I can but need the slim
design of the poly shield vs the braid.

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

 

[John Popelish]

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.

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.

 

[joseph2k]

I know of three different types of shield used for say twisted shielded
pair wire.

1) braided copper
2) copper foil
3) Metailzed poly film

I may be forced to use the metal poly film but can find information about
how well it shields noise compared to braid.

Anyone have experience with these different shield type and can comment on
the noise atteniation.

The internal conductors will have a 500V PWM signal with 200nS fise and
fall
times. I'd like the cage in that dV/dt as best I can but need the slim
design of the poly shield vs the braid.

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.