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FCP Euro Service Kits

FCP Euro Kits

Photograph of an alternator from a MercedesOver a period of time during the 60s and 70s, the electrical generator in most cars was replaced with what is called an alternator. So called because of the alternating current which it generates, the alternator was a great advancement in charging system technology because it's design makes it much more durable than a standard generator. Most of the reason behind this has to do with the brushes you find in both generators and alternators. Where a generator uses a set of carbon brushes and a commutator to transfer the entire generated current, an alternator only uses the brushes and slip rings to supply a small current to the rotor which is used to induce current in the stator, or stationary windings,in the casing of the alternator. This reduces the wear on the brushes immensely and is one of the biggest reasons for the transition to alternators all those years ago. This article is primarily focused on Bosch and similar alternators, but many other manufacturers use similar design practices, and the general function will be the same for practically all alternators.

Alternator Diagram

The basic principle of an alternator is as follows

  1. A small current is applied to the windings in the rotor of the alternator via a set of brushes connected to the voltage regulator. This creates an electric field around the rotor.

  2. As the rotor spins and the electric field moves with it, a current is induced in the stator windings found on the case of the alternator. Current induction is a fascinating topic, but unfortunately is a bit too dense to discuss here - I encourage anyone interested to learn about how induction takes place, and the physical laws governing it.

Current before and after passing through the diodes (rectifier). The bottom image shows 3 rectified AC waveforms together, similar to the output from the diodepack in an automotive alternator. Current before and after passing through the diodes (rectifier). The bottom image shows 3 rectified AC waveforms together, similar to the output from the diodepack in an automotive alternator.

  1. The current induced in the stator windings is alternating, and of course increases in frequency as the rotor spins faster. The field passing through the stator windings as the rotor spins generates a wave of current through each adjacent set of stator windings. These waves have to be turned into direct current, and this is done by means of the diode pack. The diode pack is what we call a rectifier- it forces all of the current to flow in one direction, essentially turning AC into DC. These waves will be bumpy, as pictured below, but there are several separate windings in the stator, each one being out of phase with each other. This makes the “bumps” much less severe. This bumpiness is called “ripple” and can be smoothed out in a number of ways, including through careful design of the rotor.

  2. The voltage output from the diode pack can be pretty high. It has to be brought back down to the stable 14-or-so volts we need. This is done by modulating the current in the rotor windings. By decreasing the current in the rotor windings, we decrease the voltage that is induced in the stator windings. In this way, we can closely control the alternator's output. And since the control current for the rotor windings is so small, the brushes are not worn as easily, and rarely need replacement.

A typical Bosch voltage regulator including carbon brushes. A typical Bosch voltage regulator including carbon brushes.

In order to begin charging, a small current has to be available at the rotor windings from the get-go. Nowadays, this may be computer controlled, but in the olden days, a very clever solution was put into place. Earlier Bosch systems involved a battery light indicator on the dash board that was in series with a point on the voltage regulator that would provide an initial current to the rotor. As the engine started up and the stator windings began producing a voltage, the difference in voltage between the windings and the battery would drop, eventually turning out the battery light. However, if a failure was present somewhere in the system, and the stator windings stopped producing adequate voltage, the voltage differential would light the warning indicator lamp, letting you know that your battery was not charging. This is indeed a very elegant solution to a serious problem! Many systems also had a resistor in parallel with the lamp. This is so that the alternator could still charge if the battery indicator lamp had burned out (a real problem with my '93 Volvo 940).

Troubleshooting

When troubleshooting a malfunctioning alternator, the first thing to do is always to ensure that all connections are properly made, tight, and of low resistance at connections. It often helps to have an ohmmeter to check for resistance in wires and connections. Once a strong B+ connection and ground connection have been established, and all other connections are firm and not corroded, one should check for output voltage. This can be accomplished by checking the battery voltage while running. If the voltage is high, typically over 14.5 volts or so, a malfunctioning regulator is very likely. If the voltage appears to be close to the battery nominal voltage (battery voltage with the engine off), the alternator may not be charging at all. This can be caused by a failure in the windings, regulator, or diodes. If this is the case, the regulator can often be changed easily, but the other components will often warrant a rebuild or replacement. Keep in mind that the alternator will not start charging unless there is an initial field current applied, usually by the battery lamp on the dashboard. If this lamp does not light up, it can be an indication of a burned out lamp, a poor connection, or a damaged regulator. The lamp is a very important diagnostic tool that cannot be overlooked.

In short, alternators are generally trouble-free devices with a long service life as long as they are well cared-for. With a little experience in using a multi-meter and a basic knowledge of the alternator's function, you can save yourself a lot of time and trouble by bypassing the mechanic!

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Written by :
Dan Bullmore

Dan Bullmore is a physicist and engineer from Houston, TX. Preferring the old to the new, Dan has owned many examples of Mercedes and Volvo vehicles and has devoted much of his time to maintaining and understanding them.


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