What is an Alternator?

S. Mithra

An alternator is part of the charging system of your car that produces electricity for many devices. A type of generator, it transforms mechanical energy into electrical energy. Although a car's battery supplies some electricity, most of the electrical mechanisms within the vehicle require the this device's steady stream of power.

Alternators draw electrical power directly from an automobile's engine.
Alternators draw electrical power directly from an automobile's engine.

Alternating current (rather than direct current) gives the alternator its name, because this is the type of electricity it creates. It's mainly a mechanical device, concealing a pulley, wheels, brushes and wires, which hooks to the crankshaft and runs to the battery. This way, the gasoline powers the engine to turn the crankshaft, which in turn connects to the alternator, which converts that motion into current whenever the car is running. The resulting electricity operates the cooling fan, headlights, windshield wipers, radio, defogger, and air conditioning.

The alternator is part of the car's charging system.
The alternator is part of the car's charging system.

To be more specific, this part is very efficient at producing a constant, high voltage, even when the car is idle, because of how it works. The belt (from the crankshaft) connects to a pulley system, called the rotor, so that when the belt is turning, it moves magnets across a special surface, called a conductor. Moving magnets in the stator generate an electrostatic field, otherwise known as electricity. This alternating current is controlled by a voltage regulator to keep the voltage steady. Another part, called the diodes, convert alternating current into direct current that flows on to the battery and other components.

Even if an alternator dies or malfunctions, the car will run for a while directly off of the battery until all of its power is sapped. This makes it hard to tell when the device is having problems. Sometimes, a harsh noise or intermittent headlights will give it away. Drivers should check to make sure the belts are not cracked, or improperly tightened, before replacing the entire part. A rebuilt alternator can be a reliable, but less expensive, option if it must be repaired.

If an alternator dies or malfunctions, a car will run off of the battery until all of its power is sapped.
If an alternator dies or malfunctions, a car will run off of the battery until all of its power is sapped.

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Discussion Comments


@fishybarbar: Time to change the battery it sounds shot.

@anon23244: In a way, yes.

@anon22262: Alternator causes a magnetic induction which is vital for your engine to have. I'm not quite sure it it's 'part of the engine' or not, though.

@anon7830 - take it for a drive for a bit, sometimes they do that because it needs to become regulated within the electrical system, it's hit and miss until it's regulated then it should be fine. If not, it was a poor installment job.

@realniyi: Something is wrong with the setup in the electrical wiring or you need a new battery.

@anon1824: Depends on what kind of alternator you want -- a rotating-armature, rotating-field, one phase, three phase...?

Some of these are not easy for me to answer, I will admit. I'm not exactly an expert on the subject but I know a few things. I gave you all my educated guesses but don't take it as law; I might be wrong.


What kind of an alternator is on most vehicles: a three phase or a single phase?


how often should a car alternator be rewound?


my battery keeps losing charge. once i've jumped it it will start for the rest of the day then the next day it won't start again? help please.


is the alternator related to physics?


Is the alternator considered part of the engine?


I just had the alternator on my 1998 Honda Accord replaced to the tune of $400, based on advice from a mechanic who rebuilt the car's transmission. A day later, I started the car and the fuel engine light and battery light came on twice while I was driving. What made it alarming is the engine light was RED, as opposed to the yellow light that comes on to signal an engine problem or to remind the driver of the car's maintenance schedule.

I called the shop and was asked the question, 'What did the engine light say?' which I thought had to be the most stupid question as, unless I had the tools to diagnose the problem, I would have NO way of knowing what the engine light is trying to say.

This was the latest in a series of huge expenses for this car, and I'm at my wit's end. What does it mean when the engine light and battery light come on - in red - immediately after the alternator that was supposed to address electrical issues was installed?


What determines the type of battery I buy for my car?

My car battery is flat. If I connect another battery to the terminals and start the engine, then replacing my battery (dry cell) it will start. What is causing this?


What is the size of an alternator to generate 15v ?

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