noise of VFD on DAQ board output

[jftak01]

Hi:

I have connected a VFD(Variable Frequency Drive) to a PC which has a PCIe  6320 DAQ board. I have several thermocouple connected to SCC module to read temperature.

When the VFD is off and it is disconnected from electricity outlet, I can read temperatures very well (about 1 deg. error).

but when The VFD is on, temperature reading has about 4-5 deg error (fluctuation).

I tried to get rid of VFD noises by using  a filter in labview for lowpass and defferent cut off frequencies. But it doesn't work!!!

How can i get rid of VFD noises???

thanks.

-Jeff 

[Kyle_A.]

Hi Jeff,

Does your noise look consistent with a 60 Hz power line signal?  When you used your low pass filters, at which frequency did you choose the cutoff to be?  Is there any way to increase the distance between your VFD and your thermocouple measurements?  Temperature measurements are generally sensitive and more prone to EMI from external sources.  If you are able to use the VFD on another PC that may help solve this issue.

Also, take a look at this forum post for other helpful tips:  http://forums.ni.com/ni/board/message?board.id=110&message.id=7852&requireLogin=False

[jftak01]

I don't think so. How can i find out the frequency of noise??

I chose cut off frequency from 5-50 hz,, but none of them had a effect!!

thermocouple are far enough way,(2 meter).

I have to explain more detaile:

my PCI DAQ is installed on a pc and i have connected RS-485 port of VFD to the same PC using a cable.(to read motor parameter).

and also i have connected the ground pin of RS-485 port to the body of PC to make the grounded the same.(because without doing this grounding i can not read VFD data in PC, and the reading process would stop working because of the changing frequency of VFD)

it's better to read all of data in just one PC for my project.(because i have to control and analysis them simultaneously) 

I think i have to use ferrite filter..

could you please give me more detail about it???  what's  ferrite filter?

thanks a lot. 

[Kyle_A.]

Hello Jeff,

The easiest explanation about what a ferrite filter is can be found on Wikipedia.  Basically, it acts as a physical low pass filter for blocking high frequencies on your cabling.  There are several models and brands that can be found on the web.  

[Wayne.C]

jftak01,

Make sure that the VFD is properly grounded.  The noise in this case is going to be pretty high frequency.  In this case I would start by optically isolating the 485 connection.  That ground connection you had to make to get the 485 to work is probably creating a ground loop.  You could test this by running the drive with the 485 disconnected and see what happens with your temp readings.