Generators are divided into DC generators and AC generators. The latter can be divided into synchronous generators and asynchronous generators. Synchronous generator is the most commonly used in modern power stations. This kind of generator is excited by DC current, which can provide both active power and reactive power and meet the needs of various loads. Asynchronous generator has simple structure and convenient operation because it has no independent excitation winding, but it can't provide reactive power to the load, and it also needs to draw lagging magnetization current from the connected power grid. Therefore, asynchronous generators must be connected in parallel with other synchronous motors or with a considerable number of capacitors. This limits the application scope of asynchronous generators, which can only be used in small automatic hydropower stations. Before 1950s, DC generators were mostly used as power sources for urban trams, electrolysis, electrochemistry and other industries. However, DC generator has commutator, which is complicated in structure, time-consuming, expensive, prone to failure, difficult to maintain and less efficient than AC generator. Therefore, since the advent of high-power controllable rectifier, there has been a trend to replace DC generator with DC power obtained from AC power supply through semiconductor rectification.

According to the different prime movers used, synchronous generators are divided into three types: turbine generators, hydro-generators and diesel generators. They are similar in structure, except that small motors use permanent magnets to generate magnetic fields, the general magnetic fields are generated by DC current excitation coils, which are placed on the rotor and armature windings on the stator. Because of the low voltage and low power of the excitation coil, there are only two outlet terminals, which are easy to be led out through the slip ring; However, the armature winding has high voltage and high power, and adopts three-phase winding with three or four leads, which is convenient to be sleeved on the stator. The armature (stator) core of the generator is laminated with silicon steel sheets to reduce iron consumption. The rotor core can be made of an integral steel block because the passing magnetic flux is constant. In large motors, because the rotor bears strong centrifugal force, the material for manufacturing the rotor must be high-quality steel.

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