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Common Mode Noise From Vsd


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#1 chaterpilar

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Posted 21 May 2010 - 06:13 AM

We have 4 ABB make ACS 550 series drives (30kw) in a dairy facility.

There are few computers on the same location which constantly have harddisk failures...we measured the common mode noise and found 5 volts.(output of UPS)

The online (APC make) UPS does filter some of the noise but still some noise is seen at its output.

I propose,

1.Replacing the existing unarmoured cable (VSD to motor) into armoured cable and the armour grounded at both sides.

2. Putting isolation transformer at the output of the UPS.

3. Putting line chokes for the VSD.

Are there any other known culprits which cause common mode noise... ( I have used Fluke 435 and found THDVN going as high 233% range 50-233%)

Are there any other things which can be done to protect the computers.


Any ideas appreciated.

#2 marke

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Posted 22 May 2010 - 06:13 AM

Hello Chaterpilar
VFDs do create a lot of switching noise and this noise is capacitively coupled inot the frame of the motor.
It is important to get the switching noise from the frame of the motor back to the frame of the VFD.
The best way to do this, is to use a proper EMC type screened cable with the screen connected to the motor frame via a screened gland, and connected to the frame of the motor using a screened gland or clamp.

The high frequency noise travels on the surface of the earth return, typically penetrating around 0.2mm inot the cable, so the important issue is maximum conductor surface area.

The addition of an EMC filter on the input of the VFD will improve the problem further, but will not work until the output is connected to the motor via a screened cable.

Best regards,
Mark.

#3 chaterpilar

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Posted 22 May 2010 - 06:09 PM

Thanks Marke for your usual prompt and precise answer.

Cheers and have a nice day.

#4 chaterpilar

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Posted 23 May 2010 - 05:59 PM

Hi Marke,


Just a quick check..does the incomer cable to the VSD also needs to be screened type..?

And will Isolation transformer help in reducing the noise..?



Cheers.

#5 marke

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Posted 23 May 2010 - 07:43 PM

A screened cable is not necessary on the input unless you are using an input EMC fiter, in which case use a screened cable between the VFD and the filter unless it is installed very close to the VFD.

An isolation transformer may assist, but I expect that due to the high capacitance between the primary and secondary, it will have reduced affect.

Best regards,
Mark.

#6 chaterpilar

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Posted 24 May 2010 - 05:24 PM

QUOTE (marke @ May 23 2010, 10:43 PM) <{POST_SNAPBACK}>
A screened cable is not necessary on the input unless you are using an input EMC fiter, in which case use a screened cable between the VFD and the filter unless it is installed very close to the VFD.

An isolation transformer may assist, but I expect that due to the high capacitance between the primary and secondary, it will have reduced affect.

Best regards,
Mark.



Thanks Marke.




#7 chaterpilar

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Posted 11 August 2010 - 10:44 AM

We use 3 pole isolators between VSd and motor. If we put a screened cable, then is it Ok to pigtail the screen and join at isolators,or it is better to use a straight uninterrupted run of cable to the motor.

cheers.

#8 marke

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Posted 12 August 2010 - 07:07 AM

Hello Chaterpillar

NEVER NEVER use pigtails on motor screened cables!!!

You can fit an isolator in a metal box and gland the cable into and out of the box with screened glands.
As a compromise, put a metal plate across the bottom of a plastic box and gland into and out of the box via the metal plate.

If this is still not possible, clamp the screens together by overlapping the cables and putting a single hose clip around both cables.

Using a pigtail, is like a water pipe system using 6 inch pipe with a step down to 1/4 inch pipe and back up again. The smal pipe dictates the flow or hydraulic impedance.

Best regards,
Mark.

#9 chaterpilar

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Posted 13 August 2010 - 05:06 AM

Thanks Marke, i had heard about the pitfalls of pigtails, but wanted to confirm from an expert. Your alternative suggestions are excellent.

Cheers.

#10 ABBdrives

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Posted 06 September 2010 - 02:14 AM

With ABB drives imprpoer grounding wil exposed your system to high level of common mode noise. You need an armour cable with solid ground at both drive and motor end. This should eliminate most of your problem. An armour cable with symmetrical ground is even a better option.






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