What.. alternates by them to recharge the battery? How is it recharging? In a gas car? While its off? I feel like an idiot!
In: Engineering
Alternating refers to the type of current (AC) and not the mechanical motion of the alternator itself.
First there were generators (dynamos) that were operated directly by shafts on engine, later by drive belts – they use an electrical property called induction to generate current by rotating a magnet inside a coil of wire. That current is used to recharge the vehicle’s battery. This current is called direct current (DC.) A generator is basically a moving shaft with magnets (called the rotor) suspended inside of a large non-moving coil of wires (called the stator.) Carbon “brushes” allow for contact between the rotor and stator.
One really cool thing about generators is that if you play a Uno reverse card and instead feed the current into the generator, it will rotate, and this one neat trick can be used to start the engine in the absence of an electric starter motor. Early cars often had a single motor which would be used as a starter motor to turn the engine over, and then once running, it functioned instead as a generator to recharge the storage battery.
In the mid 1960s, the alternator was made popular for cars. There are two key differences: an alternator produces current that alternates (AC power) just like the power that you’d get from your home power outlet, and alternators cannot produce current until they have power supplied to them – this is known as a “field” current. It powers up the magnetic field that is used to make induction work. Unlike generators, alternators are dependent on the field current to operate. If you remove the field current supply to an alternator, you can spin it as fast as you’d like, but it will not produce any power.
The alternating current is then run through a component known as a “diode rectifier” which converts that AC power into DC power that is then used to power your vehicle’s electrics and charge the storage battery while the car is running. After you turn it off, the storage battery provides the current needed to energize the alternator’s field circuit and also to turn the starter motor, allowing the engine to start up again.
An alternator still uses a stator and a rotor, just like a generator, but it will not rotate on its own if you supply power to it – it is entirely dependent on being driven by the engine via a belt, geared shaft, or whatnot. Therefore, you normally can’t use it as a starter motor.
So why did they switch to alternators? Mainly due to reasons of cost, weight, and maintenance, along with the ever-increasing demands of electrical systems in cars to power more and more things. Generator brushes wear out, and generators themselves tend to be larger and bulkier than alternators. With most cars by then having dedicated DC starter motors, the folks who built cars were able to replace those bulky generators with lighter and smaller alternators that can produce more current and require less mechanical energy to turn.
Both alternators and generators originally required a device called a voltage regulator to regulate the amount of current flowing into the battery and electrical system. This was another point of failure and added more weight and parts to the car’s electrical system. Eventually it was realized that by varying the field strength (the current delivered from the battery to the alternator’s field terminal) that the power produced by the alternator could be internally regulated, allowing car makers to eliminate the external voltage regulator entirely, saving more money and eliminating a common point of failure.
DC generators output based on speed and at idle don’t produce enough voltage to charge a battery. AC voltage can be increased with a transformer and then rectified to a rippled DC with just a set of diodes. So the alternator can charge the battery at any engine speed.
It’s called an alternator because it generates alternating current (AC) voltage. Then uses rectifier diodes to convert the AC to 12v DC which is what your vehicle’s electrical system is designed to store and use.
AC generators are used instead of DC because it’s much easier to regulate the voltage output under load. Old cars used DC generators but the output power was very limited and they didn’t generate full voltage at engine idle.
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