> > Gene Heskett <[email protected]> wrote: > > > On Saturday 02 April 2016 07:04:14 Nicklas Karlsson wrote: > > > > > 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. > > > > > > ... > > > Given all that, I do not see how, in a noisy industrial environment, > > > or even here at the Heskett's home camp, it can be error free unless > > > an optical translator, bidirectional, is used at BOTH devices > > > terminals. > > > > I am not sure about the optical translators. Why however I can't tell > > for sure. > > > > > ... > > > > > > The question then seems to be, is who makes these rs485 to opto > > > fiber (or even to RJ45 jacks & cat5 or cat6 cable since it doesn't > > > have this common mode noise problem that I am aware of), > > > bidirectional translators and at what cost. > > > > Ethernet which use the RJ45 connector have tranformer for insulation. > > I have experienced communication errors over Ethernet then > > communicating with inverter. > > > > > > Nicklas Karlsson > > Well, in any event, it seems to me like it is time to get out the scope > and see if the interfering noise can be found. If the noise is in the > VFD, I'd certainly contact the vendor for a solution. Failing that, I > believe I'd get another VFD from a diferent source, and if it works, > then some bad publicity about the first one and lack of support should > be related here on this list. This is not the only list of course, but > bad publicity should cause a down tick in sales. And that does send a > message. You bought the product to USE it, and if its not usable, warn > the rest of us.
You could also read if there is something in the manual about a shielded cable and how it should be connected. 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
