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
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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