Working Characteristics Of Generator
Jul 05, 2018

Working characteristics of generator:

The main characterizations of synchronous generators are no-load and load operating characteristics. These characteristics are an important basis for users to choose generators.

Armature reaction:

When the generator is connected with symmetrical load, the three-phase current in the armature winding will generate another rotating magnetic field, called armature reaction magnetic field. The speed is exactly the same as that of the rotor, and both rotate synchronously.

The armature reaction magnetic field and rotor excitation magnetic field of synchronous generator can be approximately assumed to be distributed according to sinusoidal rule. The spatial phase difference between them depends on the time phase difference between the no-load emf E0 and armature current I. Armature reaction magnetic field is also related to load conditions. When the load of the generator is inductive, the armature reaction magnetic field plays the role of demagnetization, which will lead to the voltage reduction of the generator. When the load is capacitive, the armature reaction magnetic field ACTS as a magnet to increase the output voltage of the generator.

No-load characteristics:

When the generator is not connected with load, armature current is zero, which is called no-load operation. At this point, the three-phase winding of the motor stator only has the no-load electromotive force E0 (three-phase symmetry) induced by the excitation current If, and its size increases with the increase of If. However, due to the saturation of the magnetic core of the motor, the two are not proportional. The curve that reflects the relationship between no-load electromotive force E0 and excitation current If is called the no-load characteristic of synchronous generator.

Load operation characteristics:

Mainly refers to external characteristics and adjustment characteristics. The external characteristic is the relationship between the terminal voltage U and the load current I of the generator when the speed is rated, the excitation current and the load power factor are constant. The adjustment characteristic is the relationship between the excitation current If and the load current I when the speed and terminal voltage are rated and the load power factor is constant.

The voltage change rate of synchronous generator is about 20 ~ 40%. General industrial and household loads require that the voltage remain essentially constant. Therefore, with the increase of load current, the excitation current must be adjusted accordingly. Although the change trend of adjustment characteristics is exactly opposite to that of external characteristics, for inductive and pure resistive loads, it goes up, while for capacitive loads, it generally goes down.

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