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#130425 - 05/02/06 06:49 AM VFD FRI/EMI
jkochan Offline
Member

Registered: 01/19/05
Posts: 61
Loc: Phoenix, AZ USA
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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#130426 - 05/02/06 06:50 AM Re: VFD FRI/EMI
jkochan Offline
Member

Registered: 01/19/05
Posts: 61
Loc: Phoenix, AZ USA
Sorry... that's supposed to be RFI

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#130427 - 05/02/06 06:52 AM Re: VFD FRI/EMI
jkochan Offline
Member

Registered: 01/19/05
Posts: 61
Loc: Phoenix, AZ USA
Sorry again... the whole thing didn't take for some reason.....here it is:
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?

Top
#130428 - 05/05/06 04:12 AM Re: VFD FRI/EMI
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
OK,
What sort of cabling have you used between the VFD's and the load?.
I'm thinking Shielding here.
All VFD cabling here in New Zealand are required to have double sheilding and a series of drain wires in the cable as well as EGC (Electrical Grounding Conductors).
5 point sheilding.
Variable Frequency Drives are rife for causing all sorts of problems in other electrical equipment.
At the end of the day, it's pretty simple.
Screen your gear and cables properly.
Believe me I've been there before!.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#130429 - 06/19/06 02:01 PM Re: VFD FRI/EMI
jraef Offline
Member

Registered: 07/21/04
Posts: 91
Loc: San francisco, CA, USA
I agree, that is the first place to look. Think of it this way: PWM from a VFD output if a modulation of frequency, a.k.a. FM radio transmission. If the output conductors are free to radiate through unshielded cable or non-ferrous conduit, they make a very effective transmitter. I see this problem a lot in water installations because people use PVC conduit to avoid corrosion problems, unaware of the fact that on a VFD output you must then use shielded cable inside the conduit.

A TDS meter is essentially a very very sensitive conductivity meter. To be that sensitive it will pick up all kinds of stray EMI, from an RF source or otherwise. I've had guys call me up on their walkie-talkies to tell me that their TDS meter is malfunctioning, then when I get there it's OK. I point out to them that it only malfunctions when they key up their mike! I've started to see that with come cell phones now too.

If that wiring checks out OK, then the next likely source is it being emitted from the line side. Most VFD manufacturers sell EMI filters that are connected to the line side of the drive for that very reason. Just because you don't read harmonics doesn't mean it isn't emitting RFI.
_________________________
JRaef

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#130430 - 06/19/06 03:04 PM Re: VFD FRI/EMI
jkochan Offline
Member

Registered: 01/19/05
Posts: 61
Loc: Phoenix, AZ USA
Thanks for the reply. Here is what's going on to date. Grounds have been tested back to and including the ECG. All grounds were at 0.02 ohms or less. THD prior to filtering the input was 1.26%, 1.69 and 1.86 on 1,2, and 3 respectivley with the highest mag being 7.07 V. Drive outputs were a mess as expected. If you are interested in the testing report I can try to post a link. 6 AWG welder's cable was used to ground motor casings to the conduit (IMC) and from the drives to the conduit, and filters to the cases. Each was tested open on one end to act as a drain and connected at both ends for shield as recomended by two different E.E.'s. One drive has both an output and input filter installed(Transcoil V1K21A00 and KRF32ATB respectivley) There has been no improvement with these filters. We have moved the meter within the Tower yard to locations approx. 15 to 20 feet from the original location, run it on clean UPS power by extension cord and on line voltage from a panel served by a separate transformer and have run the meter without it's ground....briefly. We are now considering a completely different meter and a huge upgrade in filter. I'm trying to get authorization for additional testing. I'm thinking spectrum analyzer. I hate the fact that they are trying to fix this by throwing parts at it without knowing what they're aiming for.

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#130431 - 06/19/06 03:18 PM Re: VFD FRI/EMI
jkochan Offline
Member

Registered: 01/19/05
Posts: 61
Loc: Phoenix, AZ USA
PS: Trumpy thank you for replying. Here in the US of A, sheided cable is the exception rather that the rule....go figure. Probably saved a whole $150.00 by running TTHN in conduit because the rest of the building was wired with it and that made it convenient. We're at $1600.00 + and climbing. Some savings. I 'm sorry not updating sooner. I've had a huge chiller problem while this was going on and just got the sagafragit thing back on line. Your input is very much appreciated.

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#130432 - 06/19/06 03:20 PM Re: VFD FRI/EMI
jkochan Offline
Member

Registered: 01/19/05
Posts: 61
Loc: Phoenix, AZ USA
That's THHN...got to run...another fire to put out. Thanks

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#130433 - 06/20/06 09:31 AM Re: VFD FRI/EMI
jraef Offline
Member

Registered: 07/21/04
Posts: 91
Loc: San francisco, CA, USA
I'm slightly confused by this statement, sorry.

"Each was tested open on one end to act as a drain and connected at both ends for shield as recomended by two different E.E.'s."

So is the IMC conduit grounded at each end? The motor is grounded to the conduit and nowhere else? It appears to me you may have a ground loop issue here with respect to the conduit-as-shield.

Want a cheap test? Go buy a 40' chunk of shielded VFD cable from Belden or someone and run it temporarilly on the floor, shield grounded at both ends.
http://bwccat.belden.com/ecat/jsp/Index....ed&P6=undefined
_________________________
JRaef

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#130434 - 06/20/06 11:38 AM Re: VFD FRI/EMI
jkochan Offline
Member

Registered: 01/19/05
Posts: 61
Loc: Phoenix, AZ USA
Both engineers, one at the drive mfg. and one from the meter mfg. recomended that we ground the motor casing to the conduit at one end and the conduit to the vfd casing at the other end using 4 awg welding cable, lugs and grounding clamps. This was to insure that there was a good connectionfrom motor to vfd case and hence to ground for the conduit. The explanation was that this would trap any radiated signal and take it to ground. Why the ground wire which runs the entire length with the motor leads doesn't do this, I don't know. The drive engineer said that if that didn't take care of it, to disconnect the 6 awg at the motor end and leave the vfd case end connected. the explanation was that the broadcasted signal would be picked up on the conduit and drained back to the case of the vfd. For the record the conduit is IMC outside the building and EMT inside the building.

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