Encoder Line in parallel with High Voltage VFD

[CroCop]

I've got a machine tool that runs a servo, along with a variable speed drill. The servo input card is being damaged when the VFD fires up from time to time. It's always when the VFD starts.

Also got a moving piece that runs VFD (480 V) lines in parallel with encoder lines.

Besides not doing this (running in parallel over long distances) what can I do to minimize noise from the VFD getting into the low voltage encoder cabling?

 

[DickDV]

Place a dv/dt filter on the output terminals of the VFD's before the motor leads. This will block the highest frequency components from entering the motor leads and will reduce the radiated and coupled energy that transfers from the motor leads to adjacent conductors.

Don't take this, please, as an absolute guarantee that your problem will be solved. There are other possible causes but, with the above change, certainly, the most likely causes will be eliminated.

If the problem persists, then you can go after more obscure causes. Further, the dv/dt filters are not terribly expensive either so I would start with them.

 

[Peter Nachtwey]

I learned something

I have never heard of dv/dt filters. However, it appears to be another name for a low pass filter that can handle the drive/motor current. I did a google search to find out what these filters do exactly. The manufacturers don't seem to be very forthcoming with what the filters do, exactly. I was looking for the filter type, number of poles and the corner frequency.

Have I told anybody today that I hate noise problems?

 

[CroCop]

I googled the dv/dt filter too.
Is there another common name?
The info I found wasn't exactly forthcoming.

 

[jraef]

The plain dV/dT filter will help, but an RFI filter may also be needed for this particular problem, especially if the VFD is not using shielded conductors. Here is a link to a dV/dT filter from TransCoil Inc., one of the market leaders in this technology here in the US.
http://www.transcoil.com/#klc

If you scroll down the page you will see info on their RFI filters too.

 

[hagos]

Hi folks,

Here is a useful link for dv/dt filters .

http://www.automation.com/store/pid12830.php


Cheers
Hagos

 

[DickDV]

A dv/dt filter is basically a line reactor with an additional rc network to enhance the selectivity of the reactor's frequency characteristic. It is slightly more expensive than a simple reactor and, not surprisingly, only slightly better at suppressing motor lead noise problems.

In my view, it is a good first place to start with the problems mentioned above.

 

[DickDV]

I forgot to mention---dv/dt filters are made by the usual reactor manufacturers, TCI, MTE, and Hammond, as well as others likely.

 

[Doug-P]

If practical, run the high and low voltage conductors at right angles to each other.