Alternators are usually designed with a rotating magnetic field and the armature is stationary so that no brushes or slip rings are required to draw power.
The magnetic field of the synchronous generator comes from a permanent magnet or an electromagnet. Large synchronous generators used in power plants can change the output voltage and reactive power by controlling the excitation system (electromagnet).
When the asynchronous generator is stationary, it does not generate a magnetic field by itself, and uses the air gap between the stator and the rotor to rotate the magnetic field to interact with the induced current in the rotor winding to generate electricity. Asynchronous generators require capacitors or synchronous generators to provide reactive power to operate, so they are usually not self-starting, ie, they are not activated by external power.
AC power is widely used for power transmission because AC transmission is more efficient than DC transmission under the previous technical conditions. The power dissipated by the transmitted current on the conductor can be obtained by Joule's law (P = I 2R). Obviously, to reduce the energy loss, it is necessary to reduce the current transmitted or the resistance of the wire. Due to cost and technology limitations, it is difficult to reduce the resistance of currently used transmission lines (such as copper wires), so reducing the transmitted current is the only and effective method. According to P=IU (actually active power), increasing the voltage of the grid can reduce the current in the wire to save energy.
The easy-to-accelerate characteristics of AC power are just right for high-voltage transmission. Using a simple step-up transformer, the AC can be raised to thousands to hundreds of thousands of volts, resulting in minimal power loss on the wires. In the city, a step-down transformer is generally used to reduce the voltage to tens of thousands to several thousand volts to ensure safety, and then reduce to the mains voltage or the applicable voltage supply appliance before entering the household.
With the development of power electronics, more and more long-distance transmission uses high-voltage direct current (HVDC), and the DC power factor is 1, which is more efficient. In Japan, the Kyutake River Shizuoka structure line is 50 Hz east and 60 Hz west, and there are three frequency conversion substations in Shizuoka and Nagano, and the submarine HVDC is connected between Honshu and Hokkaido and Shikoku.