Derek:
I saw the same kind of problem you describe, only it was on a large, yellow, diesel-powered earth mover in Peoria. It was 1968, and I was using a Stoddart NM-30 with tuned dipoles. (The more things change, the more they stay the same!) I did some hand-probing, and found that the noise was coming from the oil filter! Turned out that the problem originated with the engine lubricating oil flowing through the oil filter (not a 1-quart sized filter, but more like a 5-gallon pail size). The mechanical guys did two things; first, they replaced the metal & paper filter element inside the big filter pail with a plastic filter element. Then they did a thorough oil change of the engine (they did several flushes with a very light oil, then filled the engine with the correct heavy grade). The result was like night & day. Those rapid, almost ESD-like noises went away completely. They explained to me that they figured that the culprit was tiny metal particles suspended in the oil. The particles had been picking up a charge due to flowing through the engine, and they were then discharging (as what we now call an ESD event) as they hit the metallic oil filter. (I think those guys may have seen that problem before. <g>) Now, I don't know how this explains your big, high-pressure oil pump noise, but it does show how flowing oil can create electronic noise! Wonder if your supply of oil might have some metal particles in it? (I suppose it's also possible for the pump impeller to get charged by the passing oil, thus discharging to the internal pump housing or through the shaft bearings.) If you start messing with oil additives, be very sure to consult with the pump designers and the oil manufacturer. You want to be sure you don't gunk up or destroy the pump and valve internals. Materials that pour at 14 psi may do surprising things at 10,000 psi. Ed Price [email protected] WB6WSN NARTE Certified EMC Engineer Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 Military & Avionics EMC Is Our Specialty > -----Original Message----- > From: [email protected] [mailto:[email protected]] On Behalf Of Derek > Walton > Sent: Monday, May 03, 2010 7:04 AM > To: IEEE EMC Discussion Group > Subject: Puzzle with noise source > > Good day folks, > > I have been investigating a radiated noise issue with an electrically > driven Hydraulic pump. The INDUCTION motor is directly coupled to the > pump which generates 10,000 psi at relatively low flow rates. The motor > sits above the lid of a plastic tank: inside the tank sits the pump and > the oil. > > NORMALLY, induction motors make little noise, so I was surprised to see > emissions over the class A limit. In a nutshell I found: > > The noise looks like brush noise, but there are no brushes. It is not > in > sync with the power frequency, nor does it look like pole noise from > the > slots on the rotor. > > The motor when run seperately on the bench while initally noisey, > becomes quiet as a mouse after bot 30 seconds, ever since it has been > very quiet. A second motor exhibited this same performance. > > So, suspicions return to the hydraulic pump. Wrapping the plastic tank > in AL foil ( even with nothing really to bond to ) knocks emissions > down > 10dB plus. The only thing in the tank is the pump and oil. So, is it > possible for pumps to charge and discharge while moving the oil? The > noise we see in real time on the SA is like rapid ESD events. To prove > or eliminate this, is there a static disapative oil, or oil additive? > > Any other ideas of what could be the cause? > > We have eliminated the room as the source. > > Many thanks for suggestions, > > Derek Walton > L F Research - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
