Armature Reaction of Diesel Generators Under Symmetrical Load

Many people have a limited understanding of the armature reaction of diesel generators under symmetrical load. This article will systematically organize and elaborate on the relevant principles and mechanisms, aiming to provide a clear reference for learning and practical applications. We welcome corrections and suggestions if there are any inaccuracies.

When a diesel generator is running under no-load condition, there is only a synchronously rotating rotor magnetic field. This field induces three-phase electromotive force in the armature winding, so the terminal voltage U of each stator phase is equal to the no-load electromotive force E₀. After the stator is connected to a symmetrical three-phase load, a second armature magnetic potential is generated. The excitation magnetic potential and the armature magnetic potential interact to form the resultant magnetic potential in the air gap under load, establishing the air gap magnetic field under load. At this time, although the excitation current remains unchanged, the air gap magnetic field is different from the original excitation magnetic field. Therefore, the electromotive force induced in the air gap is no longer E₀. Among the various factors that cause U to differ from E₀, the influence of the armature magnetic potential plays a decisive role.

Neither the rotor magnetic field nor the armature magnetic field is stationary, so it is necessary first to clarify the motion relationship between them. The speed of the armature magnetic field is generated by the alternating current in the stator winding, which forms a rotating magnetic field with p pole pairs. The relationship between them is expressed as: n₁ = 60f/p. The speed of the rotor is determined according to the requirement of the electromotive force frequency generated in the stator winding, and the number of pole pairs is equal to that of the stator. This indicates that the speed of the fundamental armature magnetic potential is equal to that of the rotor magnetic potential. In addition, the direction of rotation of the fundamental armature magnetic potential is determined by the three-phase armature current, which in turn is determined by the rotation direction of the rotor magnetic potential. It is not difficult to see that the direction of rotation of the fundamental armature magnetic potential must be consistent with that of the rotor. The fundamental wave of the armature magnetic potential and the fundamental wave of the rotor magnetic potential rotate in the same direction and at the same speed, and they are in a relatively stationary state in terms of spatial position. These two magnetic fields do not change with time, and this state remains the same under any circumstances. Therefore, any moment can be selected for analysis when studying armature reaction.

Based on the above analysis, it is this relative stationary state that enables the motor to generate a stable air gap magnetic field and average electromagnetic torque, realizing electromechanical energy conversion. This is a basic condition for ensuring the normal operation of electromagnetic induction type rotating electrical machines. The spatial relative position between the fundamental armature magnetic potential and the fundamental excitation magnetic potential determines the armature reaction. In the analysis, it is assumed that the air gap is uniform, and both the spatial vector and the time phasor are fundamental sinusoidal quantities.

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