EMI filters for servo drives
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  1. #1
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    Default EMI filters for servo drives

    Working on a servo motor here for an in-house project and have noise issues when dealing with mV signals for the load cell on the same system. What are your preferred EMI filters for the drive side?

    The project is still on the work bench currently and I know the noise issue will get reduced with better grounding and shielding for components, but it won't fix it entirely from other equipment we've done.

    The drive/motor is an import 240v single phase 4kw unit, so it isn't anything large. The signal cable for the cell is shielded cable, but is is conducted EMI through the equipment, with it connected the values float +/-25lbs with the drive energized and without it powered it is +/-1.

    Do you have any preferred source for drive EMI filters on servo motors or even the same for VFD drives? I can find stuff online but everything is a call for quote and before I go down the path of calling multiple people thought I'd ask here.

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    I would FIRST look at the low level signal circuit and see what sort of shielding and filtering would help IT.

    First, it's cheaper,

    Second you probably do not know for sure what is causing the interfering signal, but you sure as heck know what it is getting into. There is no guessing about what it is affecting, so fix that first.

    Plus, a load cell is not usually a high frequency signal, so you can often filter the blue blazes out of it and still get sufficient response time, etc.

    Regardless of previous experience, it should be possible to get the signal cleaned up enough to work fine. Usually, the problem is caused by incorrect shielding and grounding. Start by forgetting all the stuff about grounding one end only, etc, etc. That is generally only true for certain types of signal.

    You need a continuous shield, any break in the shield is a place that magnetic fields can get in. I have built successful loop antennas which were wire wound inside of a loop of copper tubing..... it just had one thin cut in it, but that cut changed it from a shielded system to a good loop antenna.

    Also, anything that is bread-boarded is going to be horrible, in general. You will not know much until you get stuff in good shielded enclosures with coax going to them through good coax connectors. Nothing goes into or out oof the box except through a coax connector, possibly with a filter on it as well. Any wire going in through a hole is an antenna, so it must either be shielded, or it must go in through a filter that is itself shielded from the rest of the innards..

    You may beed to decide what frequency the interference is at. Sometimes it is not clear. And lots of components are of little use above rather low frequencies, they do not act like what they are supposed to be at higher frequencies. Capacitors turn into resistors and inductors, inductors turn into capacitors, both components turn into tuned circuits. You have to look at the impedance curves.

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    As JST says, shielding the sensor system is probably easier, BUT - we've had some problems at the lab' where I work with laser interlocks interfering with low-level measurements, source discovered by using a search coil and spectrum analyser to find the culprit - an AM portable radio can do a good job if you're not blessed with so much testgear?
    The reverse is possible too, feed the search coil from a source of hefty square-waves and "probe" the sensor cabling looking for where it's most susceptible, dealing with it when you find it.

    Interference filtering VFDs is usually series inductors on the VFD-Motor wiring, much the same as used for protecting old motors / coping with long cables, any capacitance across the VFD output is a no-no as it can kill output devices - downstream of the inductors is relatively safe, but you have to ensure you don't build in a resonant circuit that... kills the output devices.

    Is the load cell, wiring and input electronics balanced and screened or unbalanced (single signal, in a shield) - balanced is far better at rejecting common-mode (read "induced from other nearby wiring") interference, which is why pro audio in studios and onstage is always balanced (and the unbalanced instrument leads pick up all the hash and bug sound engineers to death!)

    Dave H. (the other one)

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    I highly doubt trying to kill this with a filter on the servo side will work.
    Bob

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    Thanks for the input guys, It'll come off the test bench this week so I'll get to work on permanently wiring it up here soon.

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    Also, make sure you are using a separate clean power supply for the sensor/signal conditioning unit.

    We integrate load and torque sensors with large 480V servos and have learned a lot about grounding, cable routing, shielding, and drive isolation over the years. Everything from individually shielded twisted pairs for all analog signal cables, proper VFD rated shielded cables, zinc plated backpans with proper single point grounding, etc.

    A [0.01-0.1] uf capacitor on the force signal output from the signal conditioner can do wonders to knock down the noise as a last resort . . . depends on whether this is just for a read out or if you are using the force signal to close a force loop.

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    Quote Originally Posted by JST View Post
    Capacitors turn into resistors and inductors, inductors turn into capacitors, both components turn into tuned circuits.
    Masterfull summary....I love the way you said that !

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    Lightbulb EMI dV/dt filters for servo motors and VFD

    Quote Originally Posted by rbent View Post
    Working on a servo motor here for an in-house project and have noise issues when dealing with mV signals for the load cell on the same system. What are your preferred EMI filters for the drive side?

    The project is still on the work bench currently and I know the noise issue will get reduced with better grounding and shielding for components, but it won't fix it entirely from other equipment we've done.

    The drive/motor is an import 240v single phase 4kw unit, so it isn't anything large. The signal cable for the cell is shielded cable, but is is conducted EMI through the equipment, with it connected the values float +/-25lbs with the drive energized and without it powered it is +/-1.

    Do you have any preferred source for drive EMI filters on servo motors or even the same for VFD drives? I can find stuff online but everything is a call for quote and before I go down the path of calling multiple people thought I'd ask here.
    Please take a look at my article on the subject in Interference Technology Magazine: Mitigating EMI Issues In Servo Motors and Variable Frequency Drives | Interference Technology We have a line of dV/dt filters (just got the patent) that use differential, common mode and ground filtering: Servo / VFD Motor Filters | OnFILTER These filters are used often in semiconductor industry where EMI causes serious disturbance of operation.

  12. #9
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    Lry's hope you stick around after this self promotion, and provide ongoing information, as well as this one "advertisement".....

    Your site does show the common mode ground current issue, , which is a useful point that many miss. A good reason for a filter, anybody's filter, if competently designed. I'll leave your post and links up for now, and see what happens.


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