Pig tails EMC

[Jako13!]

What is meant by pig tails for EMC?

 

[DPG]

I'm assuming they mean the practice of coiling the CPC in the form of a spring just before the termination.

 

[Lucien Nunes]

Agreed, coiling the CPC adds inductance and reduces the effectiveness of the CPC as a sink for RF interference. The text appears to have been translated, not very well, from another language. I wonder what the name for a pigtail is in the source language.

 

[DPG]

Agreed, coiling the CPC adds inductance and reduces the effectiveness of the CPC as a sink for RF interference. The text appears to have been translated, not very well, from another language. I wonder what the name for a pigtail is in the source language.

Yes, those instructions don't inspire much confidence in the product!

 

[Jako13!]

Oh okay another question while I’m on this topic is the manufacturer states to (ground the screening of the motor cable both ends motor > drive ) however they supply the cable with an exposed screen one end and the other end which they have heat shrunk. What is the benifit/disadvantage of earthing just one end or earthing both ends of the screen?

 

[David Prosser]

Yes, those instructions don't inspire much confidence in the product!

to be fair Xylem (Flygt) are one of the best submersable equipment manufacturors.

What is the equipment you have to install Jako ? Is it one of the Hydrovar or Aquavar VSD.

 

[pc1966]

If you want a cable's braid screen to be effective then you need a low inductance connection (as other have said). The ideal is to use the equivalent of a SWA gland that provides a 360-degree connection from the shield to the (assumed!) metal box so you have the least possible inductance.

The "pig tail" is any sort of wire used to connect the braiding to earth. Obviously worse if coiled, but even a straight 5-10cm length has a massive effect at HF/VHF frequencies!

If you are using SY or CY cable for the motor connections then you can get glands for that sort of cable to both restrain and ground the shield, for example:

20S CXT SY Gland Pack (2 Per Pack)

For use with all types of screened flexible wire braid [CY/SY]

www.cef.co.uk

Sometimes SWA glands will do, but they don't always allow clamping of the much thinner braid on the SY/CY style of cables.

(Yes, it is often discussed how SY, etc, are not BS standard and generally not UV resistant so should not be used for fixed power, etc, but for the output of VFD they are often a very good choice due to the braid screening and relative flexibility).

 

[pc1966]

Here is some EMC simulations. OK it is 500MHz so quite a bit above your typical VFD EMC region, but they are also looking at just 1cm long "pig tails". At 50MHz the same would more or less apply for 10cm, etc:

EMC drawback of shielded cable pigtail

www.rfemcdevelopment.eu

 

[Jako13!]

to be fair Xylem (Flygt) are one of the best submersable equipment manufacturors.

What is the equipment you have to install Jako ? Is it one of the Hydrovar or Aquavar VSD.

Yeah it’s the hydrovar David?

 

[David Prosser]

Yeah it’s the hydrovar David?

We don't use them but I've seen them on some booster pumps and was curious as to how you got on with them ? We use ABB normally which are pretty easy to set up.

 

[Jako13!]

We don't use them but I've seen them on some booster pumps and was curious as to how you got on with them ? We use ABB normally which are pretty easy to set up.

If you want a cable's braid screen to be effective then you need a low inductance connection (as other have said). The ideal is to use the equivalent of a SWA gland that provides a 360-degree connection from the shield to the (assumed!) metal box so you have the least possible inductance.

The "pig tail" is any sort of wire used to connect the braiding to earth. Obviously worse if coiled, but even a straight 5-10cm length has a massive effect at HF/VHF frequencies!

If you are using SY or CY cable for the motor connections then you can get glands for that sort of cable to both restrain and ground the shield, for example:

20S CXT SY Gland Pack (2 Per Pack)

For use with all types of screened flexible wire braid [CY/SY]

www.cef.co.uk

Sometimes SWA glands will do, but they don't always allow clamping of the much thinner braid on the SY/CY style of cables.

(Yes, it is often discussed how SY, etc, are not BS standard and generally not UV resistant so should not be used for fixed power, etc, but for the output of VFD they are often a very good choice due to the braid screening and relative flexibility).

So best practice is 360degree at both ends of the motor cable?

 

[pc1966]

So best practice is 360degree at both ends of the motor cable?

Yes, if you can use glands that clamp the braid and they are on metal boxes (ideally using Earthing / Piranha nuts for good connetion to box, not relying on a cm or so of banjo) then you get the best EMC protection.

If the box is plastic (or otherwise non-conductive / not-connected) then you have your earth flying lead anyway so the majority of EMC benefits are lost. But the braid (or armour) obviously has to be earthed/chassis connected for safety by some means.