About Car Alternators

Having an understanding of the function of an alternator and the effect on the vehicle in the event of a failure is highly important. The multitude of vehicles stranded every day on the highway due to this problem proves a serious lack of understanding the consequences of this type of failure. In most of the cases, the operator tries to jump start the vehicle with another power source only to find that either the engine will not respond or if it does it will only run for a few minutes before failing again.

History

For many years, starting with the first cars produced, a generator was the electrical power source used for charging the battery and running the vehicle. Generators had a major drawback inasmuch as the voltage would increase with the speed of the engine. They would not charge the battery unless the engine was running at a relatively high RPM. It would generally take 2000 or more RPM to produce over 12 volts. A 12-volt battery will not stay charged unless a voltage higher than output of the battery is obtained. The drain of voltage and amperage by lights and other equipment put an even heavier load on the generator.

With the introduction of the alternator a relatively high voltage could be obtained at an idle doing a lot better job of keeping a charge in the battery under all conditions of load and temperature. Low temperatures reduce the amperage of a battery. The amount of amperage the battery contains relates to the length of time and ability of the battery to operate the starter motor in all temperature conditions. A look at the top label on a battery will state how many cold cranking amps the battery is capable of in freezing weather. The designation is CCA-550, which interprets to 550 cold cranking amps a 0 degrees. The higher the cold cranking amps the stronger the battery.

An alternator has the ability to charge a battery in cold weather much more efficiently with other electrical drain than the older generator.

Types

Alternators come in many different amperage ratings as well as manufacturers. It is important to know what type as posted on the alternator that is in place. Different manufactures have different connectors and usually will not adapt to a different system. It is also vitally important to get the right amperage for the vehicle since the amperage determines how much of a load the alternator will handle and still maintain a charge. This is related to the amount of accessories that are present in the vehicle. If the amperage of the alternator is not known, mention to the parts supplier the number of accessories such as power windows, seats, air conditioning, door locks and so on and he can determine the choice.

Identification

Pay particular also to the type employed in the vehicle in several categories before purchasing another alternator. First thing is to look at the belt and see if it is a single "V" groove belt or is it a serpentine belt with multiple grooves. If it is a serpentine belt count the number of grooves. The same alternator will fit many applications, but the pulley on the alternator could have between three to five grooves. Only the right pulley will work on the car its intended for. Next look at the location of the main battery wire and the plastic field connector. Depending on the application they could be located at any clock position on the back of the alternator. Only one will work. Before you leave the auto parts store it is wise to take it out of the box and match it up with the existing alternator, checking that these items are the same.

Considerations

If an after market stereo with an amplifier is installed in the vehicle, it is necessary to increase the output of the alternator. The amount of amperage that the amplifier draws is necessary to install the correct alternator. If the radio amplifier draws 60 amps, an alternator with 120 amps or better is necessary. Anything less would tend to overheat the alternator by making it operate at too close to its maximum capacity.

Function

The alternator case is made up on a non magnetic aluminum case which is split in two pieces. The Stator is three sets of windings of copper fixed to the walls of the case and is connected by two wires to a rectifier. The center of the alternator is the field armature which is an iron core with wrapped around iron poles that surround the armature. On both ends of the armature there is an extension protruding out with slip rings. The armature rides in two pressed in bearings located in both halves of the case. When power is applied to the field through the use of brushes on the slip rings the field is energized. When the armature or field surrounding it is energized, it becomes an electromagnet and has a north and south pole. As it rotates every 120 degrees it energizes a separate stator winding and creates a current with multiplies with each succeeding stator winding. This current is an alternating current unusable for the car so it goes through a rectifier. This contains six diodes, two for each stator winding. The diode only allows the current to go in one direction converting the alternating current to usable direct current.

Benefits

With the understanding as to the operation of the alternator it is extremely important to know that an alternator can have several types of failures and what to do if this occurs. An alternator can fail if the bearings fail. This is indicated be a grinding or high-pitched whine. The most common failure is the rectifier or diode banks. Usually they only fail on one bank at a time which reduces the amperage that it puts out as well as the voltage.

The car will still be able to charge without a large load on it but will be slower to charge when the lights and air conditioning are on. Any indication that the battery is not fully charged, such as having a harder time starting, the alternator should be checked.

Simply take a voltmeter and check the voltage across the battery terminals with the engine running and the lights and air conditioning on. The voltmeter should show in excess of 13.5 volts if the alternator is good. If so, the battery must be tested as it is suspect. If the voltmeter shows less than 13.5 volts, the alternator is starting to go bad and needs to be replaced. If the alternator light lights up on the dash, it means that the alternator is bad also. Any time this is the case, the battery should also be tested. The battery starts the car and the alternator takes over when running.

The car will run without the help of the battery once started. If the car stalls on the road while running and will not turn over or try to start the alternator is at fault. Once the alternator fails the battery will take over and operate the car until it dies. It takes hours to charge a discharged battery enough to last a short time so jumping the car with a bad alternator is a waste of time.