On Sat, 2 Apr 2016 11:34:25 -0400 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 ------------------------------------------------------------------------------ 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
