> I had SERIOUS problems with my X200 VFD and RS485 bus- I "mostly" fixed it. > > Here's the thing- yes, RS485 is differential, but the VFD is probably > NOT opto-isolated input. Differential conveys strong noise immunity- > but ONLY when both A and B wires' voltages are within the input range of > the VFD's bus driver, which might be -0.5v to 7v relative to the ground > on the VFD driver chip. Outside that range, it will NOT function. > > The X200 confuses me greatly. Yes is has an A and B RS485 terminals, > but no ground on the RJ45 jack. If you don't have a ground to connect > to, this problem can easily come up, and DID. I had to take apart the > VFD to measure this- like >20v of noise between the PC ground and the > VFD driver. That will NOT work, and didn't.
You just hit the major problem with switched power electronics, common mode voltages. A common mode choke will increase common mode impedance. Increased impedance will not by itself decrease voltage but if some current could be conducted away the higher impedance will however lower voltage. One problem is even though ground resistance is close to zero impedance is not. I remember I have read a value of 50µH as maximum power grid inductance but are not totally sure this is correct and particular not for all frequencies. Common mode voltage source is capacitance between switch power electronics conductor with a "square" voltage and ground. It is probably correct to think about it as a capacitor connected to ground which is switched between the two rectified voltage potentials. There is capacitor between: "square" voltage between inverter transistors and cooling fin. In electric motor between phases and ground. Different insulation barrier technologies are more or less tolerant to common mode voltage. I think capcitive insulation barriers like these used in Texas Instruments ISO7421 are tolerant against common mode voltage. I would still be happy to show my trick to turn gold to lead if someone is interested? Nicklas Karlsson ------------------------------------------------------------------------------ Transform Data into Opportunity. Accelerate data analysis in your applications with Intel Data Analytics Acceleration Library. Click to learn more. http://pubads.g.doubleclick.net/gampad/clk?id=278785471&iu=/4140 _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
