EMI from transformer flickering monitors.....help

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maddawg-

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Hey guys,
An owner asked us if we could fix a problem they have with computer monitors flickering in a computer classroom because of a 150kva trans on the otherside of the wall in the electric room. Only the computers directly behind the tranny are flickering and moving the transformer isn't a real option. Anybody have this problem and know any fixes.....we have installed copper meshes before but that was during construction and this building is already done.

Thanks for any info,
Scott
 
Re: EMI from transformer flickering monitors.....help

If moving the transformer isn't an option, what about moving the computer?
 
Re: EMI from transformer flickering monitors.....help

no dice....its a computer lab, so theres no where else to put them
 
Re: EMI from transformer flickering monitors.....help

Then it was, regrettably, a bad design from the start. :(

Making this problem go away is going to be expensive. Once the owner gets over that fact, then the possibility of moving one or the other is going to become less of an ?impossible solution.? A third possibility is to construct a magnetic shield. You would need someone with a special expertise to make it work, and I am not that someone. I cannot tell you if you need to coat the entire transformer room with sheets of copper, or whether a single half-wall sheet of thin steel would do the trick. Try looking for a local consulting firm with that specialty.

There is, of course, another solution: live with it. The owner has a tough call to make.
 
Re: EMI from transformer flickering monitors.....help

LCD displays might be the cheapest solution.

I also have to wonder if changing the rate on the monitors might make a difference. If the monitors are set at 60Hz, maybe the flicker would not be as noticable if they were set to 85 Hz.

Ever see a video tape of a low rate monitor where the monitor seems to flash? The flashing is caused by the 60Hz rate of the monitor being the same as the film. Big budget films use either a higher frame rate for the movie, or a higher refresh rate on all the monitors to avoid this.

Steve
 
Re: EMI from transformer flickering monitors.....help

I agree with charlie b, this is a blunder on the designer's part :eek: . You already have all the viable options for remedy. Move the transformer, move the puters, shield the transformer, or possible use LCD. All expensive, lets us know how it turns out.
 
Re: EMI from transformer flickering monitors.....help

Hello Scott-

You may have already considered these, but here are a few things that might help reduce the impact of the problem:

1) As previously mentioned, consider changing the frequency of the monitors.

2) Take a peek at that 150Kva transformer, verifying proper grounding. I've seen plenty of transformers that have an incorrect connection between the primary and secondary neutrals. Normally this will not be an obvious direct connection, but is made through various connections to the grounding system. An incorrect interconnection between the two sides can increase radiated EMI levels.

3) Double check the branch circuit supplying the equipment for incorrect N-G bonding or N-G wires reversed. Same for the panel/sub-panel.

4) Consider pulling an oversized neutral and an oversized EGC to the receptacles supplying the monitors and CPUs that are experiencing the problems.

5) Find out if the transformer has ever had maintenance performed on it. Degradation= elevated EMI.

Hope these help.


Mark
 
Re: EMI from transformer flickering monitors.....help

While improper grounding i.e. neutrals grounded downstream from the transformer resulting in ground current can and does cause EMF and resulting flicker, from what you describe it appears to be EMF from the transformer normal operation.

As mentioned different monitors (flat screens)

Raising the transformer, utilize a gauss meter and not the distance you need to be to get below 10 milligauss, if the distance is possible to obtain move/raise the transformer.

Shielding is possible and depending on where you are located, I know of several contractors that specialize in shielding.


If your interested email me.

Increasing the size of the EGC and grounded conductor to the utilization outlets will not resolve this issue and would be a waste of your customers money.

Degradation of the transformer may have a impact on the field but there is little you can do as part of an EPM (other that verifying proper connections, torque of connections and cleaning) that would resolve this issue. If degradation is an issue, insulation break down or lamination separating the transformer should be replaced.
 
Re: EMI from transformer flickering monitors.....help

LCD displays might be the cheapest solution.
Click to expand...
This worked for us and it costs very little, just the difference in cost from a standard monitor to an LCD one. They will not trash the old monitors but will use them somewhere else; therefore, it is only the difference in cost. :D
 
Re: EMI from transformer flickering monitors.....help

check how the secondary wires (from the xf) are brought to the MCB. i suspect said wires are laid out along the wall directly behind the PCs.. we, at meralco, had high success of reconfiguring the cable set-up in the xf vault such that the secondary wires were relocated at the opposite side of the xf vault wall (farther from the PCs)...but then again, buying LCDs can be less costly....

i've read in an article that monitors with refresh rate of 85Hz can already be affected by EMI for fields as low as 10 mG, yet some suggested for a higher refresh rate...can somebody pls enlighten me on this and the reference site? ty
 
Re: EMI from transformer flickering monitors.....help

Have you taken a look at what Mike has posted on his web site?

At the Forum Homepage, click on Electromagnetic Fields, click on Articles, click on Electromagnetic Interference by Karl Riley- one of the leading experts in the field.

You can also get there by clicking on the link below:


http://www.mikeholt.com/technical.php?id=powerquality/unformatted/EMIKarl&type=u&title=Power%20Quality%20Article

Karl does an excellent job describing the difference between radiated and conducted EMI, dispels common myths, and provides a ton of troubleshooting tips.

Best wishes,

Mark
 
Re: EMI from transformer flickering monitors.....help

Originally posted by steve66:
LCD displays might be the cheapest solution.

I also have to wonder if changing the rate on the monitors might make a difference. If the monitors are set at 60Hz, maybe the flicker would not be as noticable if they were set to 85 Hz.

Ever see a video tape of a low rate monitor where the monitor seems to flash? The flashing is caused by the 60Hz rate of the monitor being the same as the film. Big budget films use either a higher frame rate for the movie, or a higher refresh rate on all the monitors to avoid this.

Steve
Click to expand...
Can you tell me how to change refresh rate?

Thanks
 
Re: EMI from transformer flickering monitors.....help

jkim, If your monitor has the capability, there should be a "Menu" button on the front of the monitor. Press the button and its in there somewhere depending on what brand monitor you have. This is where you also can change contrast, brightness, screen position, refresh rate, etc.

Dave
 
Re: EMI from transformer flickering monitors.....help

Right click on the desktop - select properties, select settings, select advanced, select monitor, go to drop down menu and select the refresh rate desired. If you don't like it, you can always go back to the original setting.
 
Re: EMI from transformer flickering monitors.....help

I ran into this same problem many years ago when I was repairing computers. There was a bank of transformers on the roof above a new computer monitor. When the elevator would operate the monitor would jiggle.

I could recreate their jiggle in my shop by holding the small motor of my heat gun's fan close to their monitor. Find a small ac powered motor and move it near the monitors. If they start to jiggle, then you have confirmed my hunch that this is a magnetic problem and not the more general case of EFI. (BTW, my customer solved their problem by moving the workstation out from under the elevator's transformers.)

EMI can be easily filtered, magnetics is a bit more tricky. Magnetics though not much taught operates much like voltage and current in that it has force and flow. The lines of flux form loops, ..circuits if you like. Different magnetic materials conduct magnetic flux differently and the term for this is reluctance (similar to resistance). Soft iron is to magnetic flux like copper is to current. It conducts easily. One thing you can do with a magnetic field is to shunt it out using soft iron. The flux doesn't go away, it just gets redirected to a new physical space. Consider for instance erecting a wall of soft iron between you and the transformer, and not just where you are affected, but the whole wall. Use soft iron not steel, and thickness matters. Have you ever looked at the soft iron used in relays, transformers and motors? Note how its built up in layers. This is to reduce what are called eddy currents. Anyway I could go on and on. Magnetics is really quite interesting if you can find a good book on it. But I suggest moving or changing the equipment as others have already suggested.

-- zero
 
Re: EMI from transformer flickering monitors.....help

Switching from CRT to TFT seems to have resolved the screen jitters on my PC. In past experience I do not believe RF cages (copper) were an absolute answer to EMI.

rbj, Seattle
 
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