We have a bank of solar inverters mounted to the wall outside our lab. Is there any practical way to shield against the EMR?
I was thinking about medium gauge steel sheeting along the adjoining internal wall with a ground connection. Would this be adequate for the frequencies involved?
The inverters are in steel boxes, but we are still getting interferrence.
Thanks,
Mark Harris
Could be coming out along the cables.
Sylvia.
On a sunny day (Thu, 05 Jun 2014 22:45:11 -0700) it happened Jeff Liebermann wrote in :
mm yesterday my buck regulator from ebay arrived. Wired it to solar panel, and started charging the sealed lead acid battery. Set voltage to 14.4V (for 12V battery) and current limit to a few amp. Was still charging when the sun went down.. less current. Not to much worried about optimal power point, plenty of power... Converter uses ring core, did manage to receive some weatherfax at 8038 kHz while it was running, will have to do a noise test, thing runs at about 100 kHz. Did not screen anything anywhere, its open on the lab table. mm more later :-)
We do have a line filter installed. It appears the EMI is radiating from the actual inverter circuit, or wires as you suggest. I didn't think of that.
If the wires are radiating, it should show on the CRO. I will check tomorrow.
Steel sheet is cheaper than aluminum, so we would opt for that. For these low frequencies, am I correct in thinking it needs to be grounded?
If we use steel, how thick does it need to be to screen down to 60Hz?
Mark Harris
What is your evidence / sensitivity to interference? Does the inverter meet Part 15 (not faked numbers -- but how would you tell)? Can you tell by how much, and at which frequencies, it is interfering?
Tim
-- Seven Transistor Labs Electrical Engineering Consultation Website:
at least 1 inch thick, oh, you don't want > 120dB reduction?
Seriously, be careful, if you're having problems at 60Hz, those sheets can 'suck' in fields from lines/transformers and actually make 60Hz stuff worse.
Come back and tell us what FREQUENCIES !
And MOST IMPORTANT OF ALL, how MUCH reduction you want.
For what it's worth, I finally gave up shielding from those poorly designed SMPS, just started turning them off in the lab. That was ALL the monitors [old style] within 150 feet, even the great Tektronix scopes [newest models] sadly had to leave on the PC supply, but at least we knew what frequencies it contributed
You raise an interesting point. Assume those items were purccased with part of the operating specification was stated to 'meet' EMC requirements, not 'comply' with EMC regulations; you have a legitmate basis for returning the product, reducing the cost of the product, or ?? based upon the product NOT meeting the 'advertised' specifications when sold.
The interesting dichotomy is between 'meet' and 'comply' Meet says the product will be within limits. BUT! a product that complies can be outside those limits as a 'statistical anomaly' For example, if you're the manufacturer and are testing your product, it must not radiate outside the limit, but if you're an auditing agency; the product is allowed to be outside. Again all due to those statsicial inferences.
forgot to remind that it's always best to stop emanations AT the source. If possible you should apply your shielding/filtering at and around those inverters.
By applying shielding right at the source you close the magnetic paths with far less material, also that lets the inverse cube function take effect and almost no fields get very far. [think of magnetic fields like sound: once made, difficult to get rid of. And like sound, it's far easier to silence by placing the whole noise maker in a box, rather than erect some stupid wall, because that wall has to be pretty large to be as effective as wrapping the noise in a small box.] Shielding at the source prevents secondary reradiation, too. If it's the mains reradiating, ... need extra RFI/EMI AC mains filters, the multi-stage kinds, not those simple single stage thingies from Corcom.
Do you have a spectral plot of the noise in your lab? I've got some around here of a fairly 'quiet' lab for you to compare. Forgot to mention, even the SMPS's in the new LCD monitors required us to turn them off if they were within 20 feet.
Get some large clamp-on ferrites. Bigger = better. If you can do that safely and without running afoul of laws et cetera, you could disconnect everything and slide on real toroids. But be very careful, big solar inverters can kill and cause other harm.
As for the ferrite material I'd combine a high-perm variety like "J" or somthing for the low MHz range and stack another #43 ferrite in series. The #43 can be smaller. That's how I've quietet down a lot of noisy gear where I wondered how it ever passed EMC or whether it truly did.
Yep. But mos of all make sure the individual sheets connect to each other. If only one of them electrically floats you'd be back to square one.
For 60Hz you'd need u-metal. Expensive, heavy, and must be properly annealed after bending. Are you really sure you need that? Typically those inverters will radiate at their fundamental (high kHz or low tens of kHz) plus harmonics. Usually only the odd harmonics.
-- Regards, Joerg http://www.analogconsultants.com/
I wonder why you say steel cheaper than aluminum - steel is a lot heavier, so a lot of the advantage goes away. And you have to paint steel. As I understand what you are doing, you are emitting DC from the box. It makes sense to suppress any high frequency stuff right there. For almost a century now, DC supplies have had a large cap plus an inductor on their output. I've seen dual capacitors in parallel for different freq. suppression.
Is there any better way to clean up a DC supply?
What I've measured in labs in northern CA, new wiring inside conduit. The ELF is around 8 to 15 nT at 60Hz, and the capacitance to the wiring is around 15femtoFarads. In building dating 1900's the ELF was more like
300nT, due to the old 'knob and tube' wiring.Most of the harmonics I've seen are indeed ODD dominating. The B-field rolls down at 1/f to around 2kHz, which translates into a coil picking up the fields 'flat' out to 2kHz. Then the fields drop dramatically.
What is really irriating is that in the US the emanations below 150kHz aren't usually controlled that much so you'll see some very energetic fields from SMPS at 120kHz. And now with those CHEAP ballasts that 'claim' to comply you'll find 30kHz getting out of those lights and into your wiring and into your lab projects.
It should be grounded, but not for EMI/RFI protection. Rather for safety. Touch the case to anything inside, you want the fuse to blow, or maybe melt a hole in the steel. You don't want your body to be the conduction path to ground. More important than grounding is the need to make the case act like one solid piece of metal. That means paint on paint (an insulator) is not going to be very useful. At higher RF frequencies (roughly >50 MHz), grounding wires look more like inductors than grounds. If you're not sure if additional grounding (scraping paint, adding finger stock, braid, rivets, screws, etc) are going to help, just grab some aluminum foil, and wrap the box in foil. Punch some holes for vents and wires, but make it as contiguous as possible with tapes overlapping seams. If that magically fixes your unspecified EMI/RFI problem, then work on the case shielding. If it does nothing, as I predict will happen, don't bother with the packaging and concentrate on the wires going in and out (including any control and monitoring wires).
You do not need or want shielding at 60 Hz. It would be like shielding the entire building because of the 60 Hz from the power lines in the walls. You unspecified EMI/RFI problem is caused by the roughly 100 KHz switching frequency used to assemble the 60 Hz sine wave. Since you're not going to be doing magnetic shielding, you do not need to worry about thickness.
Any reason you've failed to disclose the maker and model, as well as the devices which are experiencing interference? If your devices are sensitive to conducted radiation via the power lines, you can probably improve the situation with a simple power line filter.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
That's why I wanted to know the manufactory and model number. I keep running into imported junk that doesn't meet Part 15, has faked FCC ID numbers, or occasionally appears to be a counterfeit. If the latter, I send information and photos to the company support people, who can usually tell me if it's theirs or something else. I once found a counterfeit Morningstar solar controller. The package was an almost perfect clone, but the circuitry was totally different. I checked with the company. Apparently, the problem is well known:
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
60Hz fields are kind of normal these days and electronics must be able to handle those. Not just in knob-and-tube situations. Our church is around 15 years old and a 60Hz nightmare. Eight powerful circuits going to tons of fluorescent fixtures in the ceiling, each wired so a switch is on either side of a wall. The neutrals run along someplace else. If you have even the slightest asymmetry in the audio ... BROOOOP ... *POCK* and everyone in the congregation will be wide wide awake.
Or you keep wondering why the WWVB clock is always showing "no signal".
-- Regards, Joerg http://www.analogconsultants.com/
Thanks to everyone for their advice. I didn't realize inverters were such a universal problem.
Rather than fiddle with shielding, I am going to ask management to move them away from our location, and install better quality line filters.
Hopefully, that will be an affordable solution, since they already saved a bucket of money by buying cheap inverters.
Mark Harris
It's still not sinking in. The inverters are generating your unspecified noise. However, your unspecified lab equipment is probably picking up that noise through the power lines. It's conducted radiation, NOT something picked up over the airwaves or delivered by induction. Moving the source farther away isn't going to do much if the same power wires are delivering the EMI/RFI directly to your unspecified lab equipment. You could put the panels and inverters a mile away, and most of the EMI/RFI will still arrive at your doorstep.
Also, careful what you ask for. Management might find it cheaper and easier to move you and your lab, instead of the inverters.
I suggest you first ask for better filters. Then clamp the biggest ferrite beads you can find, that work at about 100KHz, on the INPUT cables. Maybe find someone with some EMC experience that has the right test equipment (portable spectrum analyzer) to identify the source and method of delivery. If the easy fixes fail, only then should you mention moving mountains and solar inverters.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
LOL!!!!
Because it is. even by volume.
Or plate it, sheet steel is commonly available in a variety of protective finishes.
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