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#65391 - 05/02/06 06:54 AM VFD RFI/EMI
jkochan Offline
Member

Registered: 01/19/05
Posts: 61
Loc: Phoenix, AZ USA
Posted this on the technical/applications thread also:
Within the last 60 days our cooling tower Lakewood TDS meter's
operation became erratic . We have linked it to the operation of the
Danfoss V.F.D.s. When the drives are bypassed, meter operation is
normal and TDS is right on the money when checked against a handheld.
As the drives ramp down TDS rises, and vice versa. Both drives/towers
exhibit the same effects on the meter. Power was checked with a true
RMS meter and no voltage spikes or harmonics are evident. We also ran
the meter on UPS clean power with no change in results. The meter and
sensor are wall mounted outside in the tower yard approx. 40 ft. from
the drives which are in the chiller mechanical room. The copper
sampling pipe is grounded immediately upstream from the meter to the
tower earth ground arrestor system. At the meter location the TDS can
drift by as much as 1000 in a three hour period. Using a second
Lakewood and sensor placed in the tower basin we have found that the
the effect is still present but to a lesser degree. The closer you
move the second meter toward the first the greater the deviation. No
new equipment has been added and the system has been running trouble
free for over six years. Lakewood pronounced the meter to be in
perfect working order. Has anyone had any similar experiences? Any
input is appreciated.Thought of the RF issue but consider it unlikely
to be radiating from the cabinets. Why would both units develop the
same problem at the same time? Both units individually create the
same issue when the other is off line. What about a high impedance
ground connection? Wouldn't that have the effect of allowing noise
from the drive to radiate from the power leads instead of draining it
away? If so would the connection be local or might it extend back
toward the service entrance through the various MCC's? I'll contact
our electrical contractor for input, but what tests or procedures
should be implimented to track down this problem?

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#65392 - 05/02/06 10:14 AM Re: VFD RFI/EMI
LarryC Offline
Member

Registered: 07/05/04
Posts: 776
Loc: Winchester, NH, US
I think you are on the right path.

Assumptions:
1) TDS = Total Dissolved Solids Probably milliVolt level signal, possibly unbalanced.

2) Installed TDS system is OK. Similar results to handheld unit. Also similar unit in sump reads fine.

3) Problem is related to VFD’s. Problem goes away when VFD’s are bypassed. Problems tracks either VFD operation.

4) Amount of interference is location dependent. As sump TDS unit is moved closer to installed unit, the amount of interference increases.

5) The sampling pipe is grounded right by the sensor.

My guess is that you are getting noise being coupled into the conduits around the motor leads, and it is seeking a path to ground thru the sampling pipe. I would bet that you will find a high impedance path to ground from the motor conduits. Check for any current flowing in the sampling pipe just before the ground connection.

Was any maintenance done recently? Any painting, inspections, replacements of tiles, overflowing of the system, etc? Have there been any recent lightning strikes in the area?

What is “the tower earth ground _arrestor_ system” Is that a grounding grid?

LarryC

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#65393 - 05/02/06 11:08 AM Re: VFD RFI/EMI
jkochan Offline
Member

Registered: 01/19/05
Posts: 61
Loc: Phoenix, AZ USA
Larry, The arrestor system is am aluminum braided loop around the top of the enclosure with lightning rods which connect to a copper braid running down each side of the brick enclosure through the slab to earth ground. It is tied to the conduit at two points prior to entering earth. I don't have an RFI meter but I can pick up the "hash" around the conduit and in the genral area of the meter with an AM radio. It is the same hash that can be detected next to the drive enclosures themselves. There has been no significant maintenance or changes to the system othe than the usual monthly cleaning and we haven't seen rain here in over a month. I have inspected the conduit which runs from the motors through liquid tight to IMC with threaded connectors to the diconnects through IMC back to the drives and can see no visual signs of failure. Since most drains are open at one end and grounded at chassis at the source would temporarily disconnecting the liquid tight from the IMC prove or disprove a loop anntenna theory?

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#65394 - 05/02/06 12:21 PM Re: VFD RFI/EMI
LarryC Offline
Member

Registered: 07/05/04
Posts: 776
Loc: Winchester, NH, US
“I can pick up the "hash" around the conduit and in the general area of the meter with an AM radio. It is the same hash that can be detected next to the drive enclosures themselves.”

How is the grounding at the Drives?

____ASSUMING THE ARRESTOR SYSTEM AND THE PROTECTIVE EARTH ARE SUPPOSED TO BONDED TOGETHER____

How is the bonding between the arrestor system and the service entrance? What is the voltage between the neutral feeding the VFD’s and the arrestor system?

Using the radio, check for hash along the motor conduits especially in the area of the ground system bonding. If the hash does not change where the bonding occurs, I would suspect that the bonding connection is not secure.

Try a temporary jumper from the conduit bare metal to the copper braid. See if the hash changes. When you are dealing with RF noise, the long run of pipe looks like a high impedance path.


“Power was checked with a true RMS meter and no voltage spikes or harmonics are evident.”

Where? The Service entrance, the TDS equipment, the input of the VFD’s, or the output of the VFD’s?

LarryC

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#65395 - 05/02/06 05:21 PM Re: VFD RFI/EMI
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
This is just a possibility that I would like you to consider. Many analog to digital converters have jumper selected filtering for 50 or 60 Hz. You would naturally select the filtering "notch" based on your line frequency. A VFD, ramping down would be steering the frequency of your potential line noise source farther below the center notch frequency. Of course, if you really had a phase controller, instead of a VFD, noise would be through the roof. You might ask them if they have different model #s, jumper settings, programming, Et,c. for 50 and 60 Hz. If so, there might be quite a challenge, like an active tracking filter, required.
Good luck,
Joe

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#65396 - 05/02/06 09:50 PM Re: VFD RFI/EMI
Paulusgnome Offline
Member

Registered: 06/15/04
Posts: 56
Loc: Christchurch, New Zealand
I don't know the answer here, but I do have some knowledge of how TDS meters work, and this might be useful.
To measure TDS, the meter actually measures the conductivity of the water, as this is directly proportional to the TDS. Alas, it is not possible to simply apply a DC voltage to a couple of probes because of polarisation effects, so the measurement is made using an AC voltage and some kind of synchronous rectifier.
Unsurprisingly, this measurement is easily upset by stray voltages and currents in the water. Drive harmonics seem a likely villain here.
Now, as to why our poster is having problems, it seems to me that we need to identify just what it is that has changed since the TDS readings started to play up. And this is not clear from the original post.
I hope the above is of some help.
Mark Monson
_________________________
Mark aka Paulus

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#65397 - 05/03/06 09:57 AM Re: VFD RFI/EMI
jkochan Offline
Member

Registered: 01/19/05
Posts: 61
Loc: Phoenix, AZ USA
Thanks to all for the input. The problem appears to be RFI radiating from the conduit. After locating the radio noise along the conduit run, I used a pair of jumper cables and grounded the conduit to the lighting arrestor earth ground in the tower yard as far from the affected area as I could manage, which was about 15 feet off the ground and 10 feet from the tower and first meter. Not great surface area contact but enough to have an effect. Both meters now read approx the same and though there is still some drift it has been reduced substantially. The original meter which is the furthest down stream from the grounding clamps is now more accurate than the meter in the tower. We are having our grounds checked by our contractor on Fri. There have been no changes to the system, so it will be interesting to see what has happened. Since both drive paths are affected, and since there are two additional drives fed by the same MCC which are not affected, I’m anticipating that the problem will be located between the drives and the MCC.

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#65398 - 05/03/06 10:05 AM Re: VFD RFI/EMI
LarryC Offline
Member

Registered: 07/05/04
Posts: 776
Loc: Winchester, NH, US
I'm glad to hear that is getting under control.

LarryC

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#65399 - 05/03/06 07:52 PM Re: VFD RFI/EMI
LarryC Offline
Member

Registered: 07/05/04
Posts: 776
Loc: Winchester, NH, US
jkochan:

Its been bugging me why the problem all of a sudden showed up. Something triggered this!

“Within the last 60 days our cooling tower Lakewood TDS meter's operation became erratic.” “There has been no significant maintenance or changes to the system other than the usual monthly cleaning”

“No new equipment has been added and the system has been running trouble free for over six years.”

SOMETHING HAPPENED WITH IN THE LAST 60 DAYS.

I would suggest examining EVERYTHING that was done in the last 60 days. My guess is that you will find that the grounding of the conduits was disturbed. Check for structural modifications, cutting and welding, addition of _anything_ to the brackets that the conduit is mounted to, somebody cleaning up the area, new paint, apparently unrelated actions like paving, new siding, additional lighting, new signs, etc.

It could be that the grounding path was unintentional but it worked. Now that somebody ‘fixed it’ to the way it should be, and your problem showed up.

The other thing to check is the RF grounding of the motors themselves. Are the bodies of the motors isolated from the grounding system? Are the motors on vibration mounts? How about the output shaft couplings? Has there been any mechanical alignments, checks, or repairs with in the last 60 days?

For the system to work fine for six years and all of a sudden develop a problem in the last 60 days is a bit unusual. Poke around and try to figure out what changed.

LarryC

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#65400 - 05/05/06 03:12 PM Re: VFD RFI/EMI
jkochan Offline
Member

Registered: 01/19/05
Posts: 61
Loc: Phoenix, AZ USA
All grounds tested .2 ohms or less between the fan, all the way back to the service entrance.In addition, our BAS went berserk every time we opened or disturbed a ground anywhere. It apparantly relies heavily on grounding for communication and is very touchy. Even loosening a ground screw connection and teaking the wire was detected and we received a communication alarm. This would seem to indicate that the grounding system is fine or we would have had other problems. more infor will follow... thanks for all the input...it's helping.

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