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    Some Thoughts on EMI in Auto Electric Systems

    I read a short article on EMI that I thought some of you might find interesting. The author is Charles Hansen, an electrical engineer. He begins by noting that there are three potential sources of electromagnetic interference (EMI) in car alternators. The first is the noise due to the full-wave three phase rectification of the alternator stator (stationary armature) output AC voltage. The rectified output is a pulsating DC voltage that because of its three phase source is at a low variation. Since a large capacitor is installed on the diode heat sink that acts in conjunction with the inductive reactance of the stator windings, a very effective low-pass (LP) EMI filter is formed.

    The second source of EMI is the integral solid-state voltage regulator, which uses a power transistor to provide pulse-width modulated (PWM) current to the alternator stator (the rotating field). This module also incorporates LP filtering.

    The third source is the brush noise from the field slip rings. These brushes are used to deliver the voltage regulator field current to the rotating field windings. Since the slip rings are a smooth surface rather than the series of bars and insulators characteristic of the commutator of a brush-type DC motor or generator, noise is a minor factor. Thus the modern alternator is an extremely low EMI producing device.

    To further amplify these findings, the author was involved in a program instituted by NASA in the early 1990's to investigate the use of automotive components in private aircraft. He ran extensive testing on a Ford alternator from 2000 RPM (idle) to 12,000 RPM at various load conditions and with various battery conditions from new to near-dead. He noted the lack of any real EMI problems with modern alternators.

    Hansen goes on to say that because of the proliferation of microprocessors in modern automobiles, much effort is made to ensure that control systems and sensors are not susceptible to EMI sources. Although the author does not reveal the make of his own vehicle he notes that the following components have shielded wiring: L&R knock and O2 sensors, primary and secondary crankshaft and throttle position sensors, MAF sensor, fuel pump control module, ABS/TCS wheel speed sensors, six airbag deployment accelerometers and line-level signals from the dash head unit to the door and rear audio amplifier units.

    Finally, although the alternator can be ruled out as a source of EMI, Hansen notes that the vehicle itself can be a source of interference due to the wide use of frame and engine ground connections that eliminate many of the ground return wires. This can give rise to ground loops that can produce electrical noise. Consider a situation in which different engine and frame ground points differ from each other and from the battery negative terminal in terms of electrical potential. In this case, a ground current will flow (ground loop) between points of differing potential possibly giving rise to noise and interference.

    After reading the article, it occured to me that given the possibility of ground loops, a ground wire kit might be the solution to eliminating the problem. The first step is to choose a common ground point-either the battery negative terminal or the ground plate on the driver's side of the battery. Then using a DMM set to measure ohms, determine whether a potential difference exists between selected ground points (head, block, intake manifold selected frame points, etc.) and the common ground point. If such a potential difference is found, run a no.4 ground wire between the two points. Don't use the daisy chain method of wire connection as this will provide little, if any, benefit. Other benefits such as brighter headlights and less dimming, as accessories come on line, have been discussed in other threads. For those of you who experience noise problems that you haven't been able to resolve in your audio systems, a ground wire kit may just be the answer.

    Reference: Audio Express Magazine, Nov. 2004, pgs. 52-53
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    #2
    The first two thirds of that is too complicated for me, but at least the last part is a good idea. I know there have been discussions about better grounding from the amp/sub to the frame of the car being helpful in audio, but I don't remember hearing anything about the main grounding system. Thanks for the info!

    Comment


      #3
      awesome find. Thanks for the info d00d.
      The T3 BP MX-3 conversion has begun, and is taking forever & will kill me.

      Comment


        #4
        Why exactly is the daisy chain method not usefull?
        Visit My Garage

        Comment


          #5
          Originally posted by Iceman112k3
          Why exactly is the daisy chain method not usefull?
          With the common ground method, all ground cables are in parallel yielding the lowest possible resistance since each grounded point has its own individual path to a common ground. The daisy chain method is actually a series circuit. In a series circuit, all resistances in the circuit add. For example, assume you want to ground points A, B and C and that each of these points is .1 ohm above battery ground. Now in a common ground circuit, the only resistance between A and ground will be the wire resistance which in a no.4 gauge wire, given the relatively short distance involved, can be considered negligible, thereby bringing A's potential down to that of common ground. Likewise for B and C.

          In the daisy chain method, linking A to B to C and then to ground places the resistances in series with each other so that for example A will 'see' the resistance of B and C interposed between itself and ground rather than just the resistance of the 4 gauge wire. Also, if A, B and C happen to be at different potentials from each other, a ground current will flow between say A and B along the ground cable (in an attempt to equalize the potentials) and result in what's known as a ground loop. Ground loops are often the cause of noise and interference and are what we were trying to eliminate in the first place by using the common ground method. In other words, a daisy chain ground system may yield little or no improvement and could even make things worse compared with a common ground system.

          I hope this answers your question. If not, let me know and I'll try again.

          02 DX Millenium Red
          Last edited by goldstar; 10-21-2004, 12:20 PM.
          02 DX Millenium Red - The Penultimate Driving Machine
          MP3 Strut Tower Bar kit; Cusco Front Lower Arm Tie Bar
          MSP Springs, Struts, Stabilizer Bars, Trailing Links, #3 Engine Mount
          Kartboy Stabilizer Bar Bushings; Nyloil Shifter Bushings; Red Line MT-90 Gear Oil
          MP3 Shifter, Knob and Aluminum Pedal Set
          Suvlights HD Wiring Harness; Osram Night Breaker H4 Bulbs; Exide Edge AGM Battery
          Summer: 5Zigen FN01R-C 16 x 7" Wheels; Yoko S.drive 205/45-16s
          Winter: Enkei OR52 16 x 7" Wheels; Falken Ziex ZE-912 205/45-16s
          Modified OEM Air Intake; Racing Beat Exhaust System; Techna-Fit SS Clutch Line
          Denso SKJ16CR-L11 Extended Tip Spark Plugs; Magnecor Wires
          Power Slot Front Brake Rotors; Techna-Fit SS Brake Lines; Hawk HPS Pads
          Red Line Synthetic Engine Oil; C/S Aluminum Oil Cap
          Cyberdyne Digital Gauges: Tach; Ambient Air Temp; Voltmeter

          Comment


            #6
            So waht your saying is rather then A---->B---->C------->Ground point it should run something like:

            A------------>Ground
            B------------>Ground
            C------------>Ground
            etc.

            correct?
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            Comment


              #7
              Originally posted by Iceman112k3
              So waht your saying is rather then A---->B---->C------->Ground point it should run something like:

              A------------>Ground
              B------------>Ground
              C------------>Ground
              etc.

              correct?
              Yes, absolutely correct.

              02 DX Millenium Red
              02 DX Millenium Red - The Penultimate Driving Machine
              MP3 Strut Tower Bar kit; Cusco Front Lower Arm Tie Bar
              MSP Springs, Struts, Stabilizer Bars, Trailing Links, #3 Engine Mount
              Kartboy Stabilizer Bar Bushings; Nyloil Shifter Bushings; Red Line MT-90 Gear Oil
              MP3 Shifter, Knob and Aluminum Pedal Set
              Suvlights HD Wiring Harness; Osram Night Breaker H4 Bulbs; Exide Edge AGM Battery
              Summer: 5Zigen FN01R-C 16 x 7" Wheels; Yoko S.drive 205/45-16s
              Winter: Enkei OR52 16 x 7" Wheels; Falken Ziex ZE-912 205/45-16s
              Modified OEM Air Intake; Racing Beat Exhaust System; Techna-Fit SS Clutch Line
              Denso SKJ16CR-L11 Extended Tip Spark Plugs; Magnecor Wires
              Power Slot Front Brake Rotors; Techna-Fit SS Brake Lines; Hawk HPS Pads
              Red Line Synthetic Engine Oil; C/S Aluminum Oil Cap
              Cyberdyne Digital Gauges: Tach; Ambient Air Temp; Voltmeter

              Comment


                #8
                meh, thats the way I ran my grounding wires anyways. Now I know why it works so well.
                The T3 BP MX-3 conversion has begun, and is taking forever & will kill me.

                Comment

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