Nick, Let's not lose sight of the original question:
"We have a customer that is concerned about how our product, laboratory equipment, will respond to electromagnetic disturbances from a high speed train that runs close to their lab. The customer states that the disturbance will be around 0.7-1.2 m Teslar." The B-field and H-field are related by the permeability, in this case, of free space. This relationship holds for all frequencies, therefore there is a constant relationship between flux density and field strength. >From the perspective of designing a product to be immune from magnetic disturbances, the desired result is to limit the amount of flux passing through the victim loop and therefore reducing the amount of noise current in that loop. The effects of this noise current depend on the impedance of the victim circuit. The actual design solution is dependent on the characteristics of the victim circuit. >From the original question, EN61000-4-3 covers frequencies from 80 MHz to 1 GHz and EN61000-4-6 covers 150 kHz to 80 MHz, while EN61000-4-8 covers power frequency magnetic fields. In either case, the .7 mT requirement is substantial and more info from the customer and more info about Joe's product is required to determine if there is a real potential problem. -----Original Message----- From: Nick Rouse [mailto:nickjro...@cs.com] Sent: Friday, February 08, 2002 4:08 PM To: emc; Mike Cantwell Subject: Re: Teslars??? To be pedantic, you are mixing units of two different quantities there Mike. Tesla and gauss are units of magnetic flux density, the B field Ampere/metre and oersted are units of magnetic field strength, the H field. Only in a vacuum does 1A/m generate a flux density of exactly 0.4 pi µT In air its pretty close but in ferromagnetic materials it can be thousands of times bigger. The field strengths mentioned in the original question are not all that outrageous. Most transformers, motors and generators with electrical steel laminations operate at peak flux densities of 1.4T to 1.7T within the core. At a boundary between two materials of different permeabilities the tangential component of H and the normal component of B are the same either side of the boundary. The amplitude relative permeability of electrical steels near their maximum working flux density is only about 300 to 800. So with the flux flowing along the core at a flux density of say 1.5T and an amplitude relative permeability of 500, you get a flux density close to the core surface of 3mT. While you are in the near field (distances small with respect to the size of the magnetic circuit) this will not drop very fast. in the far field it drops according to the inverse cube dipole law. Things are even worse at the corners. The flux does not turn smart right angles just because the core does and so the flux is not parallel to the core. This increases the flux density close outside the core. Worse yet are the effects of gaps. Laminations in transformers are commonly made in two parts that fit together to form the complete lamination. C & T shape or E and I shape. This is done so that the winding can be put on the bobbin first and then the core built up. Where the two parts of the lamination meet up there is always a small gap the size of which depends on the quality of the laminations and the care with which the core is assembled. Good practice is to alternate the orientation of the laminations so that the gaps do not align but in some cheap devices the laminations are stacked up with the gaps aligned.These devices also tend to be those using poorly cut laminations. Gaps of half a millimetre are not unknown.they also tend to push the flux density up closer to the limit increasing the external flux even more because of the lower amplitude relative permeability Gaps in rotating machines can be even bigger . Since the flux crosses the gap almost normally the flux in the gap is the same as in the core 1.4T -1.7T At the edge of the core this flux bulges out into a fringing field. Fortunately this enters the far field dipole law at distances large compared to the gap length and width. Even so fields of tens of mT can be found within a few centimetres of the gap. So take a large poorly built transformer or solenoid and push the core hard up against the equipment housing and you could well exceed 0.7 mT nearby. Several metres from a train is a bit less likely but not impossible These figures should be borne in mind the next time you read about the dangers of the magnetic field from overhead power lines. I have several times seen building site welders sitting on their transformers with their testicles dangling over the gap and I haven't seen welders dropping like flies. Nick Rouse ----- Original Message ----- , From: "Mike Cantwell" <mike.cantw...@flextronics.com> To: "emc-pstc (E-mail)" <emc-p...@majordomo.ieee.org> Sent: Friday, February 08, 2002 2:53 PM Subject: RE: Teslars??? > > > I think the units you're looking to compare to would be Amps/meter. The > conversion from Tesla to A/m is: > > 1 A/m = 1.26 uT = 0.0126 Gauss > > Therefore, a field of .7 mT converts to 555 A/m !!! (it is also equivalent > to 10 Gauss) > > Assuming that your customer is referring to power frequency magnetic fields, > this would be substantial, to say the least. > > I would recommend that you question your customer a little further as to how > they determined this field strength requirement. It seems high by a factor > of about 1000. > > Good luck, > Mike > > -----Original Message----- > From: marti...@appliedbiosystems.com > [mailto:marti...@appliedbiosystems.com] > Sent: Thursday, February 07, 2002 4:22 PM > To: emc-p...@majordomo.ieee.org > Subject: Teslars??? > > > > We have a customer that is concerned about how our product, laboratory > equipment, will respond to electromagnetic disturbances from a high speed > train that runs close to their lab. The customer states that the > disturbance will be around 0.7-1.2 m Teslar. > > Can someone please explain what the unit Teslar is and how that unit > relates, or if it relates, to the immunity tests of EN 61000-4-3 Radiated > immunity, or any other immunity test. > > Has anyone ever had a similar concern from a customer dealing with this > type of disturbance? > > Your responses are appreciated. > > Regards > > Joe Martin > EMC/Product Safety Engineer > Applied Biosystems > marti...@appliedbiosystems.com > > > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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