When being introduced to electricity and how it works, most people learn that electric current flows from a negative pole to a positive one. That's actually true, however, only for DC (direct current) electricity, and DC is but one of two possibilities. AC (alternating current) is the other.
Instead of traveling from one pole to the other, AC current oscillates between a pair of terminals – the hot and the neutral – changing direction with a frequency characteristic of the generator that produces it.
AC generators work because of electromagnetic induction, whereby a changing electric field produces a magnetic field, and vice versa. In an AC generator, also known as an alternator, a spinning rotor generates current in a coil, and the direction of the current reverses with each half-turn of the rotor. One of the main uses of an AC generator is to produce electricity for mass consumption.
A key advantage of an alternator is that it works with a device called a transformer, which can both increase and decrease voltage. This is the reason why AC generators power the bulk of the world's electricity grid, at least for now.
Uses of an AC Generator
The principle behind an AC generator is simple. An external energy source, such as moving water or steam produced by fossil fuel combustion or controlled nuclear fission, spins a rotor and the rotation generates AC current in a coil winding. The electricity is basically ready to use as soon as you connect the coil to a load.
Small gasoline generators can supply enough power to run household appliances, and large hydroelectric, coal-powered and nuclear-powered turbines ones can power entire cities. When it comes to large-scale electricity production, AC power generation has a distinct advantage over DC.
Transformers Reduce Transmission Loss
By using a transformer, you can increase the voltage of AC current to many thousands of volts, making long-distance transmission along power lines feasible. At the point of use, you use another transformer to reduce the voltage to a usable level. Transformers only work with AC power, because they also rely on electromagnetic induction.
Without the increase in voltage, power loss to electrical resistance and magnetic leaking would make long-distance power transmission impractical. If DC power generators supplied the electric grid, there would have to be more power stations, and each station would only be able to supply a limited area. The landscape would be dotted with mini-power generation stations instead of the large centralized stations in existence today.
Alternators that Generate DC Current are Called Dynamos
It is possible to generate AC power with an alternator by attaching a commutator to the rotor, which prevents the current from changing direction as the rotor spins. This turns the alternator into a dynamo, and one of the advantages of a dynamo is that you can use it to charge a battery.
Increased efficiency is one important advantage of an alternator over a dynamo, though, so dynamos are usually used in reverse as motors for battery-powered toys and power tools and not to charge batteries in automobiles.
The Hazards of AC Power Generators
Generating AC power with an alternator is inherently no more dangerous than using a battery, but when the voltage of a large-scale AC generator is stepped up to several thousand volts, it becomes extremely hazardous. Thomas Edison famously made this point by having stray animals electrocuted in an effort to convince investors to back DC power development. AC generators and transformers have to be heavily insulated to make them safe.
The flow of electricity through generator and transformer coils produces resistive heat, and this creates another problem. If the heat becomes extreme during, for example, an accidental power surge, a transformer or generator coil can burn out or get hot enough to damage the electrical insulation or start a fire. This type of accident does occur from time to time, and it is a potential cause of wildfires.