Why is the no-load air gap magnetic field of synchronous generator sinusoidal?

The air gap magnetic field mentioned in the textbook of electrical machinery is sinusoidal when it is running at no load, which should be an idealized assumption and simplification when writing a book. For simplicity and convenience of students' study.

When the motor is teaching material analysis, the most traditional motor model is used. The stator and rotor are represented by two concentric circles, and the air gap is the annular part between the concentric circles. If it is a uniform magnetic field, the magnetic lines of force are parallel everywhere in space, that is, parallel straight lines.

In this case, the lengths of different parallel straight lines passing through the ring are different, that is, the lengths passing through the air gap are different. However, because the synchronous generator is DC-excited and the magnetomotive force generated by a single conductor is the same and constant, the magnetic field intensity in the air gap calculated by Ampere's loop theorem is not the same everywhere, which contradicts the assumed uniform magnetic field, so there is no uniform magnetic field in the air gap.

Synchronization characteristics:

The external characteristics of a synchronous generator generally refer to the change curve when the internal potential of the generator terminal voltage is constant and the load current changes, mainly to test the longitudinal synchronous reactance of the generator, that is, the internal impedance of the generator, which is an important index of the load capacity of the synchronous generator.

However, synchronous generators mostly use thyristor fast excitation and damping windings, and their longitudinal synchronous reactance is mostly transient, far less than the steady-state value. In addition, due to the adjustment of the excitation system, external characteristics can be artificially created, which can be positive or negative.

The positive outward characteristic is that the terminal voltage decreases with the increase of load current, and the negative outward characteristic is that the terminal voltage increases with the increase of load current. Generally, the excitation system can be adjusted within the range of plus or minus 15%.