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Drive Unit and AM Radio "Feedback" Implications

Discussion in 'Model S: Driving Dynamics' started by Evbwcaer, Oct 22, 2015.

  1. Evbwcaer

    Evbwcaer Member

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    My DU is making the noise and I really don't care....Anyway, as I read somewhere else on the forum, if you tune to AM 1100ish-1300ish you will "hear" the DU milling. I am wondering:

    -Do those with healthy DUs get "hear" anything around AM 1200?

    -Is the solution to use noise cancellation? Has anyone have a DU mechanically fail?
     
  2. AmpedRealtor

    AmpedRealtor Active Member

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    What you are hearing in the AM band is not the "milling noise", but an electrical interference. The milling noise is mechanical and would have no way to find its way into your speakers! LOL!
     
  3. Max*

    Max* Autopilot != Autonomous

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    Why do you have to put him down for asking a question, instead of just correcting his misinformation?
     
  4. AmpedRealtor

    AmpedRealtor Active Member

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    Sorry, where was I putting anyone down? Did you miss the part where I corrected his misinformation?
     
  5. Max*

    Max* Autopilot != Autonomous

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    I'm sorry, when you laugh out loud at someone asking a question, you're actually laughing with him, not at him. My mistake.
     
  6. AmpedRealtor

    AmpedRealtor Active Member

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    I believe you are reading far, far too much meaning into a laugh. My goodness... go outside and enjoy the sunshine. And if I must explain my "LOL" to you, the smile police, then let me say that I laugh when I find something funny. There is no judgment in laughing! You seem to be on a tirade today, especially in the drive unit thread. Maybe you need a break to give you some perspective.
     
  7. Evbwcaer

    Evbwcaer Member

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    OP here, integrity in tact, I still want answers to my questions and don't think Amped has put anything to rest.

    -Are cars without the milling noise hearing the "feedback" on AM 1200?

    Whether "electrical interference" or vibrations of the same amplitude as AM 1200, Tesla does use a microphone of some type when diagnosing DUs, so there is some validity to the concept.
     
  8. CHG-ON

    CHG-ON Still in love after all these miles

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    Yikes. I'm leaving.
     
  9. cwerdna

    cwerdna Active Member

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    You want to hear AM radio noise? Listen to BMW i3 AM Radio Interference - YouTube (skip to ~0:46) from a BMW i3.

    That's probably why the AM radio is disabled (inaccessible) from the factory.
     
  10. EarlyAdopter

    EarlyAdopter Active Member

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    #10 EarlyAdopter, Oct 22, 2015
    Last edited by a moderator: Mar 13, 2016
    Or any gas car. Spark plugs generate a tremendous amount of radio frequency noise.

     
  11. kennybobby

    kennybobby Member

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    #11 kennybobby, Oct 23, 2015
    Last edited: Oct 24, 2015
    If the source of the milling noise is due to stray electrical currents passing thru the bearings and eroding or fluting the raceways, then there is an arcing or spark discharge that would radiate rf energy and cause EMI, which could be detected on an AM radio. The amplitude of the bearing current rf would be tiny compared to the rf emitted by the PWM switching of the inverter, but the inverter is covered with a metal cover (faraday shield). It may require poking around with an E-field oscilloscope probe in suspected areas to find a bearing signal. The metal case of the gearbox might block the rf signal also.

    The fluted raceways would then generate mechanical vibrations and acoustic noise that could be detected with a microphone or accelerometer. FFT or PSD spectral analysis of the recorded signal could be used to identify characteristic bearing frequencies, such as the ball-pass frequencies at the inner and outer raceways, the cage or ball-train frequency, and the ball-spin frequency. The energy or vibration amplitude at a given frequency can be used to determine whether the damage had progressed to a level that requires replacement, etc.

    If the root cause source of vibration were due to axial shaft loads in the gearbox, then the same holds true about using a microphone to record the sound. The highest energy frequencies can be used to identify the source of the vibration. LOL!
     

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  12. mknox

    mknox Well-Known Member

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    I'm still not 100% convinced it's mechanical... it sure sounds electrical to me. I've heard all the arguments back and forth, but it just doesn't pass the "smell test" in my mind.
     
  13. Evbwcaer

    Evbwcaer Member

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    Can anybody answer the question if a Tesla without audible DU issues gives AM feedback? Thanks
     
  14. EarlyAdopter

    EarlyAdopter Active Member

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    Yes. They all do.

    My low VIN (3200) P85 had no DU issues at 2 years old and, yes, you could tune to a low unused AM station and hear the RF feedback. They are not related.
     
  15. scottm

    scottm Active Member

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    #15 scottm, Oct 24, 2015
    Last edited: Oct 24, 2015

    Mike, I'm not convinced you're wrong either. Let me say it another way... you might have a good sense of smell. Or maybe hearing. I agree there's an electrical component to this sound.

    Here's why... using your right foot on the pedal you can change sound characteristics by quick choices of pedal position inputs, and you'll hear the sound respond far faster than the car physically responds by speed changes or even the feeling of thrust hitting your butt. Like a real quick sharp little jab and release of the pedal rewards you with screech variatiions but no driving dynamic change that is equally matched to the noise. You get a sense of power drivers pouring it in to the motor is what is making a complaint.

    To me, this is an electrical driver sound that is doing PWM with gobs of power that is shaking inductors. It's like I want to cement some coils down to the printed circuit board more. I just can't imagine a rotor with the inertia of the one in a Tesla can change its rotational speed to make its bearing rotational sounds change pitch that fast. So these are not rotationally tied sounds is my conclusion.

    But this doesn't mean the bearings are not also doing something nasty in a non-rotationally related way. i.e. they could just be chattering in their races. Vibrating as they rotate. Even within spec limits and tolerances of bearing clearances.. there is room between ball and race... and therefore vibration is possible. And I believe they could respond in their vibration frequency as fast as you can modulate the pedal. Because bearing balls are low mass in comparison to the rotor. The balls have become "speakers".

    And if Tesla is finding that a bearing change removes the noise.. it points to mechanical issue (audible vibration) coupled with (induced by) the issue of being electrically driven. The bearing makes no sound when power is taken away (lift the pedal) because the electrical component is removed (reversed). Electrically induced Vibration stops. And if the races or balls have been worn, you'd never know it and cannot hear it.. these are silent bearings again.

    New bearings have tighter tolerances (less space) between ball and race, so we don't hear the sound on new cars. But same frequency sounds could still be there at very low amplitutde, too quiet to hear. As the bearings get into their groove, so to speak, after many rotations and "loosen up" a bit... the sound moves in louder. More room to shake. But bearings could still be well within permissible limits... just making noise you can hear now. Louder as it ages more. But well within spec.

    Micro pitting. Vibrations may wear the ball or race or both because it is like hammering metal on a tiny scale... flattening spots on the ball or race, or tiny bits of material fly off. This may cause bearings to wear faster than they otherwise would, tolerances opening up beyond spec.

    Tesla knows this. Bearings are a wear item. Moreso on these crazy powerful high frequency driven things. Noise is just a nuisance while bearings remain in spec. But noise is an indicator of opening gaps. And like any other application, bearings making noise is an indicator of something wearing, not going in a good direction, and needs to be replaced before a failure. There is probably tons of "room" between what we hear as loud sounds from the DU and when a failure is likely to happen.

    Tesla is selling a silent experience, but they can choose to stop doing that. It would be interesting to hear bearings that have just exceeded ther spec and actually need replacement to prevent failure. Maybe we're hearing that already? But maybe we're still hundreds of thousands of miles within that. The real determining factor is how loud can people stand it.
     
  16. Evbwcaer

    Evbwcaer Member

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    I am also not convinced and am looking for any case of a mechanical failure of the drive unit. If the milling sound was a result of faulty bearing/gears/backlash/etc., then surely we should hear about mechanical failures, not just annoying sounds.

    Has anybody had a drive unit failure or can anybody link to a case of one?
     
  17. AmpedRealtor

    AmpedRealtor Active Member

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    Sit in any Model S, past or present, with or without DU issues, and tune the AM radio to the appropriate band. You'll hear the whine through the speakers, which is surely electrical in nature. It is not indicative of a DU problem. All of the cars do this. In fact, AM radio interference in EVs is so pronounced that BMW doesn't even offer AM on the i3.
     
  18. wdolson

    wdolson Active Member

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    #18 wdolson, Oct 24, 2015
    Last edited by a moderator: Mar 13, 2016
    What you are probably hearing is electrical noise generated by the PWM motor itself. PWM motors generate a ton of electrical noise, it's one of their downsides. If the AM in the car isn't swamped with noise, it indicates Tesla is doing something to shield the passenger compartment from it, but some leaks through. The body of the car might be acting as a Faraday Cage to filter a lot of the noise out, but the i3 has a worse problem because the body is mostly made of carbon fiber which doesn't conduct electricity.

    The test would be to drive a car with the milling noise problem and one without under the same conditions with the AM radio in the car detuned and see if the AM noise is higher in the car with the milling DU. It is still possible the bearing issue is adding to the electrical noise.

    - - - Updated - - -

    You get that if the AM radio isn't grounded correctly. In my current ICE the radio's ground went bad and AM became unusable. I thought about fixing it, but it wasn't worth tearing the car apart.
     
  19. kennybobby

    kennybobby Member

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    This is not true-- carbon fiber does indeed conduct electricity and it will function as a faraday cage to block rf.
     
  20. wdolson

    wdolson Active Member

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    It varies depending on the type of carbon fiber. Some are very conductive and some are a good insulator. One process of making carbon fiber coats the individual fibers with nickel, which makes it conductive, I think that's a fairly common one.

    I don't know what the i3 carbon fiber's qualities are.

    In any case, aluminum is one of the best conductors in existence, though the conductivity is going to vary depending on the alloy used.
     

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