It's dead, so you plug it into the cigarette lighter so you can call for help. Guess what? No power. It's past midnight, and you're stranded on the side of the road with snowflakes for company. What you've just experienced is an example of an alternator breathing its last breath. Your first thought might have been the battery is dying. In a sense, you would be right because the battery and the alternator are related, but the battery tends to get all the press. This article aims to explain the mechanics of alternators, how you can diagnose problems and what you can do if you have a bad alternator.
Read the next section to learn some background information about alternators and the war of the currents. Sign up for our Newsletter! Mobile Newsletter banner close. Mobile Newsletter chat close. Mobile Newsletter chat dots.
The voltage regulator oversees the power the alternator makes. The diode rectifier converts the voltage from the alternator into a form that can be used by the battery to recharge. Alternators give off a lot of heat and need to be cooled to operate efficiently.
What does an alternator do? As we know, the alternator provides your car with the bulk of its electricity and helps recharge the battery. But to do all of that, the alternator must first turn mechanical energy into electricity. The process of producing electricity begins with the engine. Electricity is made as the rotor spins. This is an integrated circuit board which monitors the output of the alternator and varies the current flowing through the electromagnet to control its strength.
The strength of the electromagnet can be used to vary the output of the alternator. Electricity is the flow of electrons in a wire. The copper wire is made from millions and millions of copper atoms. Each atom has a free electron. This is an electron which is able to move freely between other atoms.
It does move to other atoms by itself but this occurs randomly in any and all directions which is of no use to us. We need lots of electrons to flow in the same direction and we do that by applying a voltage difference across the two ends of the wire.
This forces the electrons to flow. If we reverse the battery, the electrons flow in the opposite direction. When electricity passes through a wire, an electromagnetic field is generated around the wire. If we place some compasses around the wire and pass a current through it, the compasses align with the magnetic field.
If we reverse the direction of current, the magnetic field reverses and the compasses change direction. If the wire is wrapped into a coil, the magnetic field becomes stronger. Each cross section of wire still produces an electromagnetic field, but they combine together to form a larger, stronger, magnetic field. The electromagnet generates a north and south pole, just like a permanent magnet, and we can see that by again using some compasses.
If we increase the current to the coil, the electromagnetic field increases. We can also do the opposite.
If we pass a magnet through a coil of wire, a current is generated in the coil. The dial on the ammeter indicates a current flowing in a forward direction, this is therefore generating a DC or direct current. When the magnet stops moving, the dial returns to zero. When the magnet is moved in the opposite direction, the current flows the opposite way and the dial indicates a reverse current.
If we move the magnet in and out repeatedly, the current will therefore alternate between flowing forwards and backwards. This is how AC or alternating current in generated. The current is alternating in direction. Instead of using a permanent magnet, we could use an electromagnet. As we move this in and out, it will also generate an AC current in the coil. But with the electromagnet we can adjust the current and voltage to vary the strength of the magnetic field, this lets us control how much current is generated in the coil.
Instead of moving a magnet in and out of a coil, we can generate a current much easier by rotating the magnet and placing the coils around this. The strongest part of the magnetic field is at the ends where the magnetic field lines converge. You can see the magnetic field lines by sprinkling iron filings over the magnet. With the magnet between the two coils, there is no current generated, but as the magnet starts to rotate, the strongest part of the magnetic field gets closer and closer to the coil.
Then the magnet starts to move away from the coil, so the magnetic field begins to decrease and so does the current of electrons until it reaches zero again. Now the opposite end of the magnet begins to get closer to the coil and this pulls the electrons in the opposite direction, again to a maximum point and then decreases back to zero. So, if we plot this current on a chart, we get a sine wave with the current flowing in the positive and then negative regions.
This setup gives us a single phase, AC supply. But, we have all this empty space between the coils, which seems a bit of a waste. So, what can we do with this space? Well, we can add more coils and create more phases to provide even more power. If we place another coil degrees rotation from the first phase, this will give us a second phase.
Because the coil is at a different angle, so it will experience the change in intensity of the magnetic field at a different time. The current is therefore going to flow forwards and backwards at a different time. That gives us another sine wave, which occurs at a different time. We still have empty space here, so we can add another set of coils at degrees from the previous to create a third phase. If we used just a single phase, then for every rotation of the magnet, half the time the current is flowing forwards and half the time the current is flowing backwards.
But with three phases, we always have a phase which is flowing forwards and always have one which is flowing backwards. Which means we can utilise this to provide more power. Instead of having 3 separate coils and 6 wires, as the phases are always switching between forwards and backwards, we can connect the ends of the coils together. The current will then flow freely between each coil as it changes direction.
Now, we are producing 3 phase AC electricity. But, all our electrical circuits and components within the car use DC or direct current. So, we need to convert AC to DC, and for that we use a full bridge rectifier. This is essentially just 6 diodes connected in pairs and wired in parallel.
So, with a single phase supply, for every turn of the magnet, current will only flow for half of the turn, the other half will be completely blocked. If we connected each of the 3 phases separately to a diode, then the current will flow or be blocked at different times. Therefore, we can combine the phases into a block of diodes, and only the phase nearest its maximum will be allowed to pass through. Giving us a slightly rough DC output. To smooth this out we can connect a capacitor which will basically absorb electrons and then eject electrons automatically to maintain a constant output.
This gives us a constant DC supply. By the way we have covered diodes, capacitors and power inverters in great detail previously. OK, so we now have a DC output. But, if the magnet is connected to the engine, and the car speeds up, then the magnet will spin faster and that will increase the output voltage and current.
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