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VFDs, Noise, and RS-485

Discussion in 'Electronic Design' started by Nobody, Jul 28, 2005.

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  1. Nobody

    Nobody Guest

    I have a setup that consists of a variable frequency drive (VFD) and a PC.
    The two talk to each other using RS-485. (The VFD output is 3-phase, with a
    high frequency square wave riding on a 100 Hz sinusoid carrier wave.)

    I'm running into some major VFD noise problems, and I don't know how to
    solve them.

    1. The noise problem is so bad that once the VFD starts running, it kills
    the RS-485 signal.

    2. The noise problem also kills our ability to use a hall current probe to
    sense the current through one of the 3-phase output lines.

    I've tried grounding everything. I'm using shielded cables and toroids for
    the communication and measurement lines.

    On the RS-485 end, we've tried isolated and non-isolated devices (with
    proper termination resistors) on the PC.

    The noise won't go away! Argh!

    Any tips would be appreciated.

    - Nobody
  2. Terry Given

    Terry Given Guest

    No its not, its a high-frequency variable duty cycle "square" wave,
    whose duty cycle is modulated such that the average output voltage is a
    100Hz sinusoid. Who's VFD? (Danfoss is my guess, they like the term VFD)

    Drives are nasty. Firstly, ensure you have some form of a co-axial feed
    from the drive to the machine such as neutral screened cable, with the
    screen well bonded to the drive *AND* the machine. This is exactly wrong
    for an instrumentation cable, but right for the drive. Herewith an
    excerpt from a report I wrote for a customer experiencing this sort of
    problem in Nov 2003 with a 400kW rock crusher:

    "The existing hardware operates in an extremely adverse environment,
    both physically and electromagnetically. The physical environmental
    hazards are primarily temperature, vibration and fine particles, which
    should be assumed to be conductive due to crusher plate abrasion.
    Electromagnetic hazards are primarily caused by the motor drives used in
    the rock crusher itself; drives generate significant EMI, and are
    responsible for the bulk of EMI problems in industry. Ideally drives and
    motors should be connected using “screened” cable, typically steel-wire
    sheathed. The drive cable screen should be firmly bonded to both the
    drive and the motor – note that this is exactly the wrong thing to do
    for an instrumentation cable.

    Motor drives generate huge high-frequency (HF) current spikes which flow
    through the motor winding capacitance to the motor chassis. Both the
    drive and the motor chassis are earthed, so these HF currents flow
    through the earth connection and back to the drive. The amount of this
    HF energy which radiates away is directly proportional to the size of
    the physical loop the current flows in. Connecting the cable screen as
    above ensures a good high-frequency connection between the drive and
    motor – basically it minimises the physical size of the loop the HF
    currents flow through, thus greatly reducing the amount of HF energy
    which radiates away"

    Once you have done this (minimised the source of emissions) you can then
    reduce the susceptibility of your setup. Areas to look at are:

    - optically isolate the RS485 link. This is necessary to break the earth
    loop from PC to drive (which, by Norton, carries some of the HF
    currents). A moot point if you are running the PC from batteries, but of
    vital importance when mains powered

    - keep wiring loops small. twist, twist, twist. ground plane, ground
    plane, ground plane. Drives spew out vast quantities of H-field, which
    your loops will happily receive. Technically this reduces the coupling,
    rather than the susceptibility, but who cares as long as it works right?

    - correctly terminate the RS485 cable

    - use high permeability common-mode chokes on the various dangly wire

    As to why your hall effect sensors dont work, I suspect its poor
    layout/decoupling, or perhaps ratshit DCCTs. The VFD has 2-3 inside, and
    they work fine. The problems I've come across with current transducers are:

    - capacitive coupling from the drive output. the fix is simple, a
    grounded electrostatic shield around the cable as it passes thru the
    DCCT. The ground connection must be low inductance. In practice drive
    manufacturers sample in the middle of each pulse, thereby avoiding the
    switching edge noise.

    - variation of current as a function of cable position, and/or adjacent
    metalwork. sign of a crappy DCCT....

    Make sure your DCCT supply (supplies) are well decoupled right at the
    DCCT. Again, twist the wires (stranded CAT5 cable is great for this sort
    of job, esp. STP). Also look at how you measure the DCCT results, and
    reduce/remove loop area - if with a scope, use a coax connection to the
    scope/probe rather than a 6" ground clip.

    Read Linear Technologies AN47.


  3. Joerg

    Joerg Guest


    Can you give us a first name?
    As Terry said it is probably a PWM signal that actually runs the drive
    and not just controls it. The ripple you see would then be the stuff
    that didn't get smoothed out in the output filters of the converter
    unit. One thing you could look at is filtering some more at the source
    but you may no be able or allowed to modify that, let alone get inside
    the converter.
    I usually prefer current transformers. You can filter the output of
    these since you are then dealing with isolated and low level signals.
    Running the output of the current transformer differentially to the
    system where there would be another transformer may also be an option.
    But don't ever move the resistor away from the current transformer. Else
    havoc can happen should somebody disconnect the cable.
    At these power levels you'd have to be a real RF expert to get it
    running as is. Or hire one ;-)
    This situation really calls for transformer isolated transmission, not
    just differential receivers/drivers. The required transformers would be
    pretty small and in a pinch can be made by hand. Toroids, no pot cores
    and the like. Making your own transformers instead of relying on some
    "isolation device" or transformer in a can has the advantage that you
    know how good and symmetrical it is.

    Regards, Joerg
  4. Nobody

    Nobody Guest

    Nice response, Terry.

    I'm using a Delta VFD-A. They no longer have this model on their website.

    I don't know how fesable it will be to use screened cable. The problem is
    that I'm using the drive to test a motor for production, and I may be stuck
    with whatever cables come pre-attached to the motor.

    As for the RS-485 isolation, this hasn't helped as much. I was using an
    isolated PCI RS-485 with built-in terminator, and I was still getting noise
    problems from the VFD when it was running.

    Let me see what I can do, and I'll post a follow-up.

    - Phillip

    (Sorry for the abuse of nettiquite!)
  5. Nobody

    Nobody Guest

    I'm definitely NOT an RF guy. I'm just a poor firmware coder who got
    hoodwinked into this VFD fiasco.

    When you mention transformer isolation, you are suggesting that use one
    transformer at each end, correct? I'll try that tomorrow.

    As far as current transformers... I think we tried using a current
    transformer and combined it with various types of LPFs. The problem we were
    having was that the DMM we were using would pick up the VFD noise. Noise
    Noise Noise! Argh!

    - Phillip
  6. Terry Given

    Terry Given Guest

    Hi Phillip,

    there's a clue in there - DMM. Can you perhaps take a few photos of your
    setup, and post them to a.b.s.e ( alt.binaries.schematics.electronic ).

    It sounds to me like you have some form of CT who's output is monitored
    by a DMM. The great thing about DMMs is that the leads make fantastic
    loop antennas, and excel at picking up noise. On several occassions
    doing drive EMC tests, I have been bitten by attaching DMMs to the
    D.U.T. - in one instance, we gave up on the DMM to measure smps voltage
    (which went up 40% when we turned on the 40W transmitter beside it), and
    just listened to the fan speed instead! Dis-connecting the DMM actually
    made the job easier.

    The problem is to do with the size of the physical loop formed by the
    leads. This loop is a one-turn inductor, whose inductance is
    proportional to the loop area. And inductor converts current into H
    field, and vice-versa (terrible description, but good enough here). When
    some H field impinges on your loop, the amount of current induced is
    again proportional to loop area. The DMM is quite high impedance, so
    tiny induced currents can result in large measurement errors
    (alternatively, one does not have to supply much current to apply a
    voltage across the DMM terminals).

    Lets just say, for arguments sake, you have a CT with a 10 Ohm burden
    resistor. This will be connected to the DMM leads - my fluke 12 leads
    are 1.5m long, enough to make a loop about 1m in diameter, which is
    sizeable (0.72m^2 or so). what to do?

    well, for a start, cable tie the leads together (say every 6" or so).
    Now the loop area drops to a minimum of 1.5m * 2Ti where Ti is the
    insulation thickness, say 1.5mm. so the area drops to 0.0045m^2, about
    160 times smaller than before. In practice the loop area will be bigger
    than this, but such an approach can reduce the loop area by 50 - 100x.

    That will help, but is it enough? If not, the next step is to twist the
    wires. If the physical dimensions of the twisted "loop" are very much
    smaller than the wavelength of the offending H-field (they will be) then
    adjacent twists have the same H-field across them. However the twisting
    of wires ensures that the current induced in one loop is in the opposite
    direction to the current induced in the adjacent loop, so the two
    cancel. This in effect reduces the inductance even further (and is why
    pretty much all comms cables are twisted, eg ethernet, phone etc).

    This "minimise loops" concept applies everywhere in your wiring
    (especially PCB layout). Eventually, the loops can be minimised no
    further - eg the DMM inputs are a fixed distance apart, and away from
    the DMM internal circuitry - aint much you can do about that. So at some
    point the unavoidable loops dominate noise pickup, and no further gain
    will be had from twisting wires.

    once the loops are optimised:

    If the CT has a 10 Ohm burden resistor, then paralleling another (say)
    1k Ohm across it will have a negligible effect on the measurement. So
    slapping a 1k directly across the terminals of the DMM wont bugger up
    your measurement, but will make it a lot harder to force a noise voltage
    across the DMM terminals.

    In practice, I would use a BNC-to-banana converter plugged into the DMM,
    with a 50 Ohm coax thru-terminator plugged into that (I have a whole
    bunch. I normally get people to use a T and an end-terminator, as its
    about 10x cheaper than a thru-terminator, and unless you are at very HF,
    its good enough), and a 50 Ohm coax cable to the CT, which has a BNC
    socket soldered to the burden resistor, and bugger all loop between the
    R and the CT (and the R + BNC). Have a look at a pearson CT...

    (and yes, I'd have to re-do my CT output scaling calcs, cos 50 aint >>
    10, but the maths is easy)

    why dont CM toroids help? the noise picked up by your loop is
    differential mode not common mode.

    why dont LPFs help? the DMM is picking up noise on its leads, which
    effectively gets added to the LPF output.

    If it makes you feel any better, everyone else has these problems with
    VFDs. An amusing anectode:

    A customer of ours built a large computer-controlled sawmill - really
    cool, a big bank of paralleled PCs solved a travelling salesman-like
    problem to optimise the log cuts to maximise return in real-time based
    on market data. Fancy sensors, lasers etc. They had a fancy position
    sensor that is basically a plunger with a magnet on the end, in a pipe
    fed GHz RF at the other end. Some miraculous electronics & a dead white
    guy meant they could have an absolute accuracy of 0.1mm or so over tens
    of meters. But it used H field to work.

    They had a lot of big drives (>= 250kW), and every time the drives
    turned on the whole system went apeshit - rams firing off, 12' radial
    arm sawblades whizzing out and back, that kind of thing. Because all the
    position sensors started firing off nonsense, and their controller wasnt
    very robust (no built-in bullshit detector). The cables were screened,
    but the screen was *NOT* connected to the motor, only the drive.

    So they got hold of our service dept, who told them to tie the cable
    shields to the motor as well. Their engineer was a young (my age)
    instrumentation guy, and thought that was wrong, so for 3 months he
    didnt do it. Eventually, after repeated calls to the service dept. he
    did, and *voila* all the problems went away.

    This guys manager wanted to know what was wrong with our drives and why
    this fix worked (and why his guy didnt implement it for so long), so I
    got sent there (with our service manager), to explain why it worked and
    that *all* drives behave the same. In the nicest possible way, I had to
    explain to the dozen or so customers why this guy was wrong - actually I
    just explained the physics behind the interaction, and the physics
    behind this guys erroneous decision, which was right for
    instrumentation, but wrong for drives. And man, did his boss rip him a
    new one afterwards, it was embarassing to see.

    No harm was really done though, because during this 3-month period they
    were commissioning the entire plant. It was only when all else worked
    and the drives had become a bottleneck that the heat went on this guy,
    who finally did what we asked.

  7. From bitter experience, (of something similar in aircraft),
    I can support Terry's absolute demand that there should be
    a low impedance connection between the Earth at the VFD and
    the Frame of the Motor. Also the cabling should be screened,
    (and twisted if possible).

    I don't have much experience of industrial CT's but aircraft
    3-phase CT assemblies are normally mounted inside an alloy
    screened box, craftily arranged so that the screening extends
    down the holes of the CT's without forming shorted turns.
    This avoids capacitive coupling between the feeders and the
    secondaries of the CT's. The secondary currents are taken
    off via screened twisted pairs, with those screens down to
    Earth at the destination only.

    In one instance I have seen a screened cable jacketed with
    a second outer braid. The outer was Earthed at the source
    and the inner at the destination.
    The RS485 isolator should be close to the PC (probably as
    above already), cabled as 3 (or 5) wire, so that there is
    a direct 0v-0v connection between the isolator and VFD.
    This keeps the CMV as low as possible.

    Cabling should also be done in screened twisted pair, with
    the screen connected to the 0v at the VFD.

    120 ohm termination at each end, and each receiver should
    have a local differential R-C low pass filter. RS485 comms
    can work ok with a surprising amount of low pass slugging.
    Waveforms can look awful on the scope, but the receivers
    can still sense the correct logic ok. Run the comms at
    the lowest speed possible.

    BTW: Shielded Cat5e comms cable is good for both comms
    and instrumentation.
  8. Terry Given

    Terry Given Guest

    Thats the safe way to do it. The twisting on the CT cable takes care of
    H-field, and the single-ended shield deals with E-field. I can see why
    they wouldnt muck about in aircraft :)
    that doesnt strike me as a good idea for a drive output cable, but I
    have seen it in instrumentation. Betcha its a pain to terminate :).
    Tsaliovich's book has some pretty pictures in it re. cable shielding
    Yep, I'd agree with all of that. To be honest I have never had a problem
    with RS485 and drives, but to be fair I started work at a drive
    manufacturer, and they already knew all this stuff, which I picked up
    very quickly - its nice to not have to make all the mistakes yourself.

    So I went and found a whole different bunch of mistakes to make....

  9. Flaky comms on site, (in front of the customer), is a
    pain in the bum, and the most expensive place have to
    sort the problem. It is generally cheaper to do it
    properly in the first place, back at base.
    Do tell.... Earth Management on big systems is a
    hard lesson to learn.
  10. Joerg

    Joerg Guest

    Hello Phillip,
    It's the other way around for me. I am an RF/analog/EMC guy who
    sometimes ends up digging through firmware, like when my usual tools
    found another bus contention and everybody thought it just can't be so.
    Then I probably feel a similar pain.
    Yes, pretty much like Ethernet. It is important to wind them carefully
    and bifilar. Bifilar means twisting primary and secondary wires about
    two twists per inch and then winding them onto the core together. For
    signals in the MHz range I use toroids of #43 ferrite material and for
    stuff below a MHz mostly #77 material (Fair-Rite, bought via Amidon).
    The required number of turns increases with circuit impedance and with
    how low in frequency the signals can be. Use wire with a beefy
    insulation for better capacitive isolation. Not quite the stuff
    electricians use, maybe half the insulation thickness and much thinner wire.

    Good quality LAN transformers can also work. But they would have to be
    from a reputable mfg such as Murata.
    As Terry said DMMs are not a good tool here. Their leads pick up noise,
    create loops and the internal circuitry of a DMM might not be designed
    to handle large electro-magnetic interference loads. If you have the
    time try the same with an old-fashioned analog meter. The really old
    kind, no battery and certainly no electronics in there.

    In fact I had a DMM die on me after an RF susceptibility test and I was
    only blasting about 100 watts or so in an RF cell. It was a good name
    brand in a fancy holster, not some hobbyist version.

    Regards, Joerg
  11. Joerg

    Joerg Guest

    Hello Phillip,
    If you really are stuck with regular cable there may be another option.
    It's not as good as shielded and twisted cable but at least a little
    progress: Run it through metallic conduit. Best would be copper tubing
    where the pieces are pre-soldered together and grounded.

    I don't know how good the flexible metallic EMT is (the stuff that looks
    like a large shower hose). Probably not very good for RF. If it has to
    be flexible you can also try thin corrugated copper tubing as is used
    for connecting a gas appliance to the gas line (hardware store).
    Those boards sometimes use isolator chips. While these may isolate well
    enough to pass safety muster they may not be that good in isolating RF
    loops. Transformers are the thing for data transfer in a noisy environment.

    Regards, Joerg
  12. Joerg

    Joerg Guest

    Hello Tony,
    Composite structure? I almost pulled my hair out debugging one of those.
    Luckily I didn't have much hair left ;-)
    Or whatever the cable spec says it should have.
    Just make sure to use the good stuff that doesn't turn into a toxic
    plume in a fire. I think that's what killed the people at the
    Duesseldorf airport.

    Regards, Joerg
  13. Terry Given

    Terry Given Guest

    Hi Joerg,
    hear hear. I've had some hilarious hardware/software arguments too - me
    and the s/w guy basically saying "its your fault". Usually its s/w,
    perhaps 75% of the time - I suspect because its so easy to be careless
    with s/w, and so hard to spot (not looking is also the most common
    technique used for s/w peer reviews and testing).

    That we can usually do calculations to prove the efficacy of our designs
    is a huge bonus to us, along with spice, design-rule checking etc. A
    whole host of reasons not to write firm/software.....
    bugger. I havent killed a DMM yet, but I did have one of the leads on my
    fluke 87 break while I was testing a 50kW regenerative rectifier.
    Suddnely the sytem went bonkers, it took about half an hour to figure
    out my DMM was lying and that it all worked fine. Funny how we always
    assume the worst with complex things. Customers are like that too - if
    something is wrong it must the the drive, but is usually the wires :)

    a few years back I did my limited electrical registration (allows me to
    fiddle with 3-ph fixed wired 400Vac doodads). The instructors were at
    pains to recommend test-measure-test with DMMs for exactly that reason,
    a strategy I immediately adopted. I fitted a relay to a VFD in a milking
    shed yesterday for the local sparky, who laughed at me when I did the
    TMT. But I'm still here.....

    we had a showering arc generator for EMC susceptibility, and every time
    we turned it on the R&D/production door swipe-card died, so all foot
    traffic had to go thru the power lab. The mfg eventually fixed it. The
    first time the SAG was powered up, the photocopier died too, so we
    bunged a line filter in front of it and that kept it alive. Phone
    conversations were tricky with the SAG on too :)

    The SAG is a great little toy, easy to make, fun to use and highly
    destructive. Rather than using a coupling clamp, we attached a
    multimeter probe to the output, and probed directly to the I/O
    terminals. We could write our names on the alodised metalwork with the
    spark. One product I designed had no clamping on one input, just an 0603
    cap. the SAG made the cap flash over, emitting bright flashes - caught a
    glimpse, turned the lights out and voila, weakness glows in the dark.
    BAV99 to the rescue....

    If you're interested, I posted the PDF showing how to make one to abse.
  14. Joerg

    Joerg Guest

    Hello Terry,
    When I tested one of my designs for defibrillator safety the PBX system
    quit and some computers froze up. I got dirty looks despite the fact
    that I had placed a huge wallpaper on an easel at the entrance asking
    people to back up a lot that morning.
    Or BAT54 duals if you need to cap before the substrate diodes wade too
    deep into the muck.
    Cool. I am surprised they recommend plywood for the applicator side. In
    California the fire marshall would have a beef with that. Maybe it isn't
    as hot and dry in Australia as it is here. The transformer could
    probably be scrapped out of one of those bug zappers. Is "AS" Australian

    Regards, Joerg
  15. Terry Given

    Terry Given Guest

    to quote JT, BWAHAHAHAHAHA
    ayup. You've done this before.... I once managed to design a circuit
    that occassionally triggered the parasitic supply SCR in a lattice CPLD.
    The numbers would burn off the chip, and a little dome would swell up in
    the centre of the PLCC. oops.
    I used a neon xfmr fed from a step-down xfmr to get the right voltage
    (it was free). When I say "I" I mean my slave ^H^H^H^H^H^H technician.
    He did a much nicer job than I would. I think we made the frame from
    formica, but your right, plywood might not be such a good idea. Mine is
    in a big Al case, so it can catch fire inside if it wants to :)

    Yes, AS/NZS. We often pinch the standard numbers from europe/usa....
  16. Terry Given

    Terry Given Guest

    Hi Tony,

    oh yeah, and I've sure seen some terrible comms links. 50m of RS232
    cable, both devices earthed with a bitwise protocol sans parity - one
    bit = one task = noise sample & hold & act erroneously :)
    Now thats something I'm interested in hearing more about...

  17. I have been in some 'interesting' sw/hw fights myself! I was working
    tool roads a few years back, and they had an interesting 'hardware'
    problem. Every weekend, around 2-3 in the morning, the system went
    crazy. It started a network cascade and every toll system on the road
    went down. It was 'obviously' a hardware fault, so they called in some
    network experts to solve the problem. They put a network sniffer on the
    system, and watched the traffic...

    Sure enough, early sunday morning, there was nothing, nothing, nothing,
    and then a huge cascade! What was the cascade? I bunch of 'Where are
    you?' messages! They programmers had built in a "If I don't hear from a
    system for 'x' minutes, check to see if they are still there..."
    routine. They had NOT built in the stuff to QUIT asking once it got an

  18. Joerg

    Joerg Guest

    Hello Terry,
    On large systems and even on Phillip's drive system one often has to
    accept the rule that there simply is no common potential between
    individual modules. From a 50/60Hz point of view, hopefully yes, but for
    anything much above that all bets are off.

    Regards, Joerg
  19. Terry Given

    Terry Given Guest

    Hi Charlie,

    Beautiful. And fits nicely with my theory that many programmers never
    test anything, and seldom if ever think about failure modes. I bet
    you've seen numerous systems hang when no response is received...

    the funniest one I have had was a drive we designed using an existing
    micro & software, with all new hardware. We re-arranged all the ports,
    and vandalised the software to suit. All worked spiffingly well, until
    about 3 days after the product launch, when a customer complained to the
    service dept his drive kept tripping when he used the 4-20mA input.
    After the inevitable argument with Mark, I did some tests and found that
    at about 10mA the drive tripped. I configured the input as +/-10V, same
    problem at about 0V. It happened when the uC ADC pin reached about 2V or
    so, coincidentally the logic threshold. So I looked at the original
    product, where that pin was the emergency stop switch input. Armed with
    this ammo I re-started and easily won the argument. Mark went and had a
    nosey, and came back a few minutes later looking sheepish, with the
    problem solved. He'd re-written the I/O code, but had forgotten to
    remove the old E-stop code (all the rest was gone). The 80C196 was set
    to use that pin as an ADC, but the digital input SFR still worked :)

    (best not to think about the 3 goes I had my tech have at getting a LED
    to light up a few weeks later)

    the worst one was when a programmer set up a command to shut down the
    comms link, and save that state to EEPROM so it wouldnt turn back on
    again. The unit is gooped and screwed into an IP68 box with only the
    comms link & power coming out, and to get it to talk again the lid had
    to be removed (12 screws) so we could access the diagnostic serial port.
    That wouldnt be so bad, but another programmer was upgrading a customers
    screen (1000 units) and sent out a (untested) broadcast "shut up
    forever" command. Luckily the broadcast didnt work so well, and he only
    "killed" a few hundred modules. But a guy in a climbing harness had to
    undo a thousand screws to remove the crippled modules, then a few
    thousand more to revive them on the ground, and of course put it all
    back together again.

    We made ourselves look like complete idiots to the customer so much for
    the "image quality upgrade". We had the screen back and running that
    day, which kept him happy, but if it had been a game day we would have
    been in the shit, it took hours to fix. When asked why he saved the
    state to EEPROM, the programmers reply was he had a choice (to save or
    not to save) and "just" chose to save it. The server was updated that
    day (*about 15 minutes after we tracked down the root cause) to never
    allow a broadcast of that command - until then it was transparent, you
    type it, it does it. The firmware was also hurriedly modified to remove
    the built-in self-distruct command :). And the programmers got an abrupt
    lesson in "thou shalt not **** with the customers equipment"

  20. I have read here before about this business of putting different
    signals into the drive. Why is this? I think last time someone
    mentioned (sphero perhaps) injecting a 3rd harmonic or something of
    the like.


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