什么是"同步"发电机， 同步转速是如何确定 What is a "synchronous" generator and how is the synchronous speed determined

发电机是发电厂的心脏设备，发电机按其驱动的动力大致可分为水轮发电机(水力)和汽轮发电机(蒸汽)。

发电机在正常运行时，在发电机定转子气隙间有一个旋转的合成磁场，这个磁场由两个磁场合成:转子磁场和定子磁场。所谓"同步"发电机，就是指发电机转子磁场的转速(原动机产生)与定子磁场的转速(电力系统频率决定)相等。

转子磁场由旋转的通有直流电的转子绕组(磁极)产生，转子磁场的转速也就是转子的转速，也即整个发电机组的转速。转子由原动机驱动，转速由机组调速器进行调节，这个转速在发电机的铭牌上都有明确标示。定子旋转磁场由通过三相对称电流的定子三相绕组(按120°对称布置)产生，其转速由式确定(式中:p为转子磁极对数；f为电力系统频率；n为机组转速)。从式中可见，对某一具体的发电机，其磁极对数是固定不变的，而我国电力系统的频率也是固定的，即50Hz(也称工频)可见每一具体的发电机的定子旋转磁场的转速在发电机制造完成后就是"定值"。当然，电力系统的频率并不能真正稳定在50Hz的理论值，而是允许在这个值的上下有微小的波动，也即定子磁场在运行中实际是在额定转速值的周围动态变化的。转子磁场为了与定子磁场同步也要适应这个变化，也即机组的转速作动态的调整。如果转速不能与定子磁场保持一致，则我们说该发电机"失步"了。

The generator is the heart equipment of a power plant, which can be roughly divided into hydro generators (hydraulic) and steam turbine generators (steam) according to the power it drives.

During normal operation of the generator, there is a rotating composite magnetic field between the air gap of the generator's stator and rotor, which is composed of two magnetic fields: the rotor magnetic field and the stator magnetic field. The so-called "synchronous" generator refers to a generator where the rotational speed of the rotor magnetic field (generated by the prime mover) is equal to the rotational speed of the stator magnetic field (determined by the frequency of the power system).

The rotor magnetic field is generated by the rotating rotor winding (magnetic pole) with direct current, and the speed of the rotor magnetic field is also the speed of the rotor, that is, the speed of the entire unit. The rotor is driven by the prime mover, and the speed is adjusted by the unit governor. This speed is clearly indicated on the nameplate of the generator. The rotating magnetic field of the stator is generated by the stator three-phase winding (symmetrically arranged at 120 °) passing through the three-phase symmetrical current, and its speed is determined by the formula (where p is the number of rotor magnetic poles, f is the frequency of the power system; N is the unit speed). From the formula, it can be seen that for a specific generator, the number of magnetic pole pairs is fixed, and the frequency of China's power system is also fixed, that is, 50Hz (also known as power frequency). It can be seen that the speed of the stator rotating magnetic field of each specific generator is a "fixed value" after the generator is manufactured. Of course, the frequency of the power system cannot truly remain stable at the theoretical value of 50Hz, but allows for slight fluctuations around this value, meaning that the stator magnetic field actually dynamically changes around the rated speed value during operation. In order to synchronize with the stator magnetic field, the rotor magnetic field also needs to adapt to this change, that is, the speed of the unit needs to be dynamically adjusted. If the speed cannot be consistent with the stator magnetic field, we say that the generator is "out of step".