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Old July 15th, 2007, 11:20 AM   #1
lalion
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Unhappy VSD Noise Headache

Hi,

Im having noise problems from a VSD that has been installed and running fine in the current system for couple months now.

To start, the customer originally asked to have speed control of outfeed conveyor of log scanning system. Easy solution put in a VSD, because motor is on far end of conveyor and cable run (80m unshealded motor) was already in place I decided, instead of re-running Sheilded motor cable and place VSD at Control room we would mount VSD in cabinet right at the motor and wiring in to existing motor cable.

As I said, this worked perfect for 2 months, then suddenly log scanning system (located 1/3 down convayor) started playing up, we have proved that it is caused by this VSD, I feel noise must be getting back thru power cabling from vsd. Ive replaced internal Filter but no luck yet, my next option is to install external RFI Filter, but this isn't cheap and supplier wont lend me one to test with. My other option is to run sheilded motor cable back, but at 55Amps this will be more expensive.

Ive read few other noise related posts and there seems to be different types/causes of noise. I guess my question is, am I on the right path, is there any easy way to analyze?

Thanks for all advice,

Leon
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Old July 15th, 2007, 11:30 AM   #2
rsdoran
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Personally I would first check for grounding issues, that is probably the #1 cause of issues like this, something may have worked loose OR someone bonded something that has created a loop.

In many situations it may not be directly from the power/motor leads but could be AC riding on DC, whether its the control voltage or communication signals. May want to look at those using a scope.

Others will offer more detailed advice.
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Old July 15th, 2007, 01:15 PM   #3
OkiePC
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What Ron said, plus, physical separation.

If you can move the sensitive wiring further from the motor lead wiring, every inch of increased distance will help...
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Old July 15th, 2007, 01:21 PM   #4
DickDV
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lalion, there are three different sources of noise problems in VFD systems. The first is input power supply harmonics caused by the non-linear way the input diodes use current. This is pretty easy to fix usually by the addition of input lead line reactors. If this is your problem, it can be more severe at light loads on the motor or even when the drive is idle--not running the motor at all. This problem doesn't usually behave for a time and then suddenly rear up its ugly head.

The second source of VFD noise has to do with the high frequency pulses on the motor leads. These tend to spray out into the surrounding air and cause havoc whereever sensitive equipment is not thoroughly grounded and sometimes even then. Damage to motor insulation can also result from ringing and reflected wave problems on the motor leads but, if I understand you correctly, the drive is right next to the motor. It is unlikely that motor lead noise is the problem with such short leads. If you want to be sure, simply add 5% motor lead reactors in the motor leads right at the drive and that should be enough to muzzle any noise on such short leads.

The third problem is a nasty bugger and rsdoran mentions it. Known as common mode noise, it is the drive output pulses getting into your ground system and going places they shouldn't, such as your scanning system. This problem is always much more likely to occur if your power network to the drive is a floating delta source (no neutral). The surest solution for this is to install a drive isolation transformer with a grounded wye (star) secondary. You might find that simply adding drive output reactors will solve this problem but you are having an extremely lucky day if they do.

You also might find that your scanning system problem will go away with the addition of an isolation transformer feeding it. In this case you would want the secondary to float so do not provide any ground for a neutral. I don't like any power system that floats because of some associated personnel hazards and, further, if it fixes the problem for the scanner, it might just move the problem to something else in your plant. The pulses in the ground are going to find some way back to the drive whether it is thru your scanner or elsewhere. I like the drive transformer best but it does cost money.

I'd start with a drive input reactor if your drive doesn't already have one, then add a motor lead reactor, then the drive transformer in that order.

Good luck. Hope I've helped.
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Old July 15th, 2007, 03:06 PM   #5
Nick B
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If you suspect is comming from the output. Install a dV/dT filter.

Nick
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Old July 19th, 2007, 10:09 AM   #6
lalion
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thanks for all the comments so far,

to keep everyone up to date..
First gone through and checked all earth cables, and ran extra earth cable between vsd and motor, as suggested by drive rep. Next I placed an isolating transformer in front of scanning system, both with its UPS in place and bypassing the UPS, ive even pulled the power from UPS and allowed UPS to power scanner system, thus completely isolating these circuits. Still no luck.

Followed the data cables back from scan-head to control room and found that they are ran through metal conduit all the way and physically separated from other power cables. Speaking to the people who installed the scanning system, i found that they had a similar problem with noise on the coax data cables and had to run the data in metal and run sheilded motor cable for vsd's. This seems to be where Im heading, now that ive been told there is a 4 week wait to get an RFI filter in. But first I plan to find some spare motor cable and run temporary to vsd, well away from any data cable, and from this hopefully prove whether its RF noise. And Im also going run the 3m of cable from vsd to motor, just to be sure.

question for DickDV, the line reactors you talk about, is a choke the same thing?

thanks again.
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Old July 19th, 2007, 04:48 PM   #7
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Yes, three chokes wound on a common lamination. MTE, TCI, and Hammond are well-known manufacturers.
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