Operating Characteristics:
The performance of a synchronous generator is primarily characterized by its no-load characteristics and load operating characteristics. These characteristics serve as important criteria for users when selecting a generator.
No-Load Characteristics:
When a generator is not connected to a load, the armature current is zero; this state is known as no-load operation. In this condition, the only electromotive force (EMF) present in the stator's three-phase windings is the no-load EMF ($E_0$)-which is three-phase symmetrical-induced by the excitation current ($I_f$). The magnitude of $E_0$ increases as $I_f$ increases; however, due to magnetic saturation in the machine's iron core, the relationship between the two is not strictly proportional. The curve illustrating the relationship between the no-load EMF ($E_0$) and the excitation current ($I_f$) is known as the synchronous generator's no-load characteristic.
Armature Reaction:
When the generator is connected to a symmetrical load, the three-phase currents flowing through the armature windings generate a secondary rotating magnetic field known as the armature reaction magnetic field. Its rotational speed matches that of the rotor, meaning the two fields rotate synchronously.
Both the armature reaction magnetic field and the rotor's excitation magnetic field can be approximated as having a sinusoidal spatial distribution. The spatial phase difference between them depends on the time-domain phase difference between the no-load EMF ($E_0$) and the armature current ($I$). The armature reaction magnetic field is also influenced by the nature of the load. When the load is inductive, the armature reaction magnetic field exerts a demagnetizing effect, causing the generator's terminal voltage to drop; conversely, when the load is capacitive, the field exerts a magnetizing effect, causing the output voltage to rise.
Load Operating Characteristics:
These primarily refer to the external characteristics and the adjustment characteristics. The external characteristic describes the relationship between the generator's terminal voltage ($U$) and the load current ($I$) when the rotational speed is at its rated value and both the excitation current and load power factor remain constant. The adjustment characteristic describes the relationship between the excitation current ($I_f$) and the load current ($I$) when the rotational speed and terminal voltage are at their rated values and the load power factor remains constant.
The voltage regulation rate of a synchronous generator is approximately 20% to 40%. General industrial and household loads require the voltage to remain essentially constant; therefore, the excitation current must be adjusted accordingly as the load current changes. Although the trend of the adjustment characteristic is the inverse of the external characteristic-rising for inductive and purely resistive loads, and generally falling for capacitive loads-this adjustment is necessary to maintain stable voltage output.
