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Mechanics Corner: Electrical system team players

Jim Schlund Published on 02 February 2011
Up close of an engine (alternator)

The battery, starter and alternator are the three electrical team players. Without them, nothing will happen. In past columns, we have discussed the battery and the starter. Now we’ll move on to alternators.

Batteries only last a certain period of time and you can only make so many starts. Not only that, when the battery starts the engine, the energy taken from the battery storage has to be replenished. The alternator is the part that restores energy back into the battery in a safe manner.



How the alternator works
The alternator works as a series of mechanical processes. The alternator consists of insulated wires, coils and laminated metal. The three major players inside the alternator are the rotor, stator and rectifier.

The rotor, which is turned by a belt and pulley, rotates inside the alternator. This is nothing but a rotating magnetic field. It’s not just one field though, it has pole shoes that surround a coil inside. By putting voltage into the coil, the magnetic field is intensified.

This high magnetic field has north and south poles. The field also contains pole shoes that lace in between each other and the poles. The magnetic field makes a loop from one pole to the other inside of the rotor.

The next part that jumps into action is the stator. The stator attracts and collects the electrons created from the rotation of the rotor. The stator fits around the outside of the rotor, and doesn’t move. It is made of laminated metal and copper.

The copper-insulated wiring in the stator attracts the electrons that are given off. The negatives and positives then attach to the stator, creating an alternating current. The process continues on to the rectifier.


AC to DC power
Rectifiers play a vital role because they convert AC to DC power. Rectification means that negative electrons are collected and taken to a ground. The positive charge is also collected, but becomes the energy output. The positive flow then goes to whatever part/accessory needs it or returns to the battery.

What protects the system?
Let’s compare this to a football game. What makes a football game run smoothly? Referees.

Alternators have voltage regulators, which in this case are the referees. The regulators monitor the voltage going into the battery, the output of the alternator so that it doesn’t go too high under normal conditions, as well as the load amount.

Keep in mind this is assuming that the engine is operating under normal conditions, meaning the battery is just a little low, but not completely discharged.

When the alternator is only running the engine and handling a mild load, it functions properly. A normal start doesn’t take a lot out of energy from the battery either. This will allow the alternator to restore energy to the battery at a safe rate while driving down the road, even if you do have the lights or accessories on.

Brushless alternators
Brushless alternators use induction as a means of transferring energy. In these, one magnetic field is next to another. Energy is transferred from one to another without a connection between the rotor, stator and rotor field control coils. It’s brushless.


Diodes also play a significant role in electrical systems. Diodes are useful if you want to have a directional current, where current flow goes in only one way. If you want it to be directional to a certain voltage, then you use a Zener diode.

In what instances would diodes be used? Diodes are used in rectifiers, when you only want the negatively charged flow to go one way, and keep it from flowing to another circuit.

One thing to keep in mind is that electricity will take the path of least resistance. If it can’t go one way, it’ll go another, because you are dealing with high voltage.

What to avoid
Vehicle accessories, such as radios and powered windows, consume electricity. They cause the alternator and battery to work harder.

Adding additional accessories to a vehicle or piece of equipment will cause the alternator to handle a heavier load and creates more work for the battery.

Checking alternators
I believe it is beneficial to make sure that the alternators are yielding the maximum energy output. In order to do this, you have to load the system. There’s a special tester that is used to monitor the current flow, amperage and voltage output. This test has to be done by a properly trained technician.  PD

The alternator is the part that restores energy back into the battery in a safe manner. Photo courtesy of Jim Schlund.

Jim Schlund  

Jim Schlund
Retired Diesel
Mechanics Professor
College of Southern Idaho