How Does an Alternator Work?
In order to understand how the alternator does its job, it’s helpful to first know how it’s put together.
This coconut-sized part usually sits near the top of the engine and consists of an outer housing covering a stator and rotor. The housing is made of aluminum because it cannot be magnetized and dissipates heat well.
The rotor has roughly 10 to 20 magnetic poles sticking off of a center prime mover like petals of a flower growing off of a stem. These magnetic poles alternate North and South.
The rotor is powered by the turning crankshaft of a running engine via a belt (usually the serpentine belt in modern vehicles), which connects to the prime mover and turns the entire rotor (following the previous example, the flower spins). As the poles move in this way, magnetic flux is generated.
Brushes, which produce the magnetic field in each pole, are located near the poles and send them a direct current via slip rings around the prime mover. This power comes from an external power source or from small generators attached to self-excited alternators.
The stator assembly is made up of a system of copper coils. Most often vehicles use a three-phase assembly, where there are three sets of copper coils spaced 120 degrees out of phase of each other. This makes three times the electricity as one set would. As the rotor spins, the magnetic flux generated creates alternating current in the copper wire.
At the other end of the alternator, a diode (also known as rectifier) converts the AC power into DC power which is the format that can actually be used by the vehicle and stored in the battery.
Older vehicles have a voltage regulator between the alternator and the battery to make sure the battery doesn’t get too little or too much electricity. This can harm the battery or other electrical components. In newer vehicles, the ECU does this job.
Since cooling is critical to the proper function of an alternator, there are vents on the front and back to dissipate heat and cooling fans that are also powered by the serpentine belt.
Several terminals on the back of the unit connect the alternator to the electrical system in order to turn it off and on and send data from sensors to the ECU.
How to Load Test Your Alternator
To give you more information about the health of your alternator, testing it with a voltmeter (which is a function on a multimeter) is helpful.
With the engine of the vehicle off, a good and fully charged battery should measure 12.6 to 13.2 volts. With the RPM of the vehicle around 1k, the voltage should still be between 12 and 14 volts. Then start turning on the accessories in the power and rev the engine – the voltage should not drop below 12 or above 14 volts.
- If any of your numbers are out of range, the alternator may be bad.
- If the voltage is too high, the voltage isn’t regulated which can be frying the battery.
- If the voltage is too low, the alternator can’t provide the battery with enough charge which isn’t good for the battery.