On Sunday 03 April 2016 06:21:38 Nicklas Karlsson wrote: > On Sat, 2 Apr 2016 11:34:25 -0400 > > Gene Heskett <ghesk...@wdtv.com> 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. On the mix-n-match 1.5HP I just bought, there is a common point, but its labeled as the ccw end of a manual speed control potentiometer. And I have been assured by the vendor that using that for a logic ground, I can feed the PWM straight into the arm of the pot as its well filtered internally for just that use. It has 0-5 volt input range, and a 0-10 volt range as separate terminals. If that terminal exists on yours, then I would connect that terminal to the logic ground of your RS-485 driver circuitry. That should solve any common mode out of range errors. That's generally going to be the reason when the noise has exceeded the active range of the loads input comparators. That range would be from say .5 volts above the logic ground, to .5 volts below the + supply rail of the receiver. I have seen some chips claim to be able to go all the way to the rails, but I would not put a 5 nines dependable rating on such a circuit. The reason for that is the substrate diodes in the receiver IC, will have a measurable turnoff recovery time, and could output the wrong logic level during that time. That would generate edge triggered noise that could easily upset the receivers intention of sampling in the center of each baud of data coming in. A 5 to 9 element chebyshev filter on the comparators output could reduce that, but would have to be retuned according to the baud rate expected each time it was changed. And while it coud be done, the BOM additions would be several dollars for a fixed tuned filter. Tuning it would also be a hard to solve problem. Super-sampling the comparators output at say 5x the baud rate, and feeding that to a 5 input majority circuit would also work. However thats inside the VFD and 100% the vendors problem, not something that you could interpose between the devices. Its obvious that I am less than impressed with the whole RS-485 premise when it does not mandate a common reference connection. However, there might be a clue you could make use of in my mental meanderings on this. I hope there is. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ 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 Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
