# Principle of Synchronous Generator

Aug. 24, 2022

How a synchronous generator works？

1. Establishment of main magnetic field: DC excitation current is passed through the excitation winding to establish an excitation magnetic field with alternating polarities, that is, the main magnetic field is established.

2. Current-carrying conductor: The three-phase symmetrical armature winding acts as a power winding and becomes the carrier of induced potential or induced current.

3. Cutting motion: The prime mover drives the rotor to rotate (input mechanical energy to the motor), and the excitation magnetic field between polar phases rotates with the shaft and cuts the stator windings in sequence (equivalent to the conductors of the windings cutting the excitation magnetic field in reverse).

4. Generation of alternating potential: Due to the relative cutting motion between the armature winding and the main magnetic field, a three-phase symmetrical alternating potential whose magnitude and direction change periodically will be induced in the armature winding. AC power is available.

5. Induced potential RMS: From Chapter 11, it can be known that the RMS of each phase induced potential is

6. Frequency of induced potential: The frequency of induced potential is determined by the rotational speed n of the synchronous generator and the number of pole pairs p, namely

7. Alternation and symmetry: due to the alternating polarity of the rotating magnetic field, the polarity of the induced potential is alternated; due to the symmetry of the armature winding, the three-phase symmetry of the induced potential is guaranteed.

Synchronous Generator Structural Model

1. Like other types of rotating electrical machines, synchronous generators are composed of a fixed stator and a rotatable rotor. They are generally divided into transition-type synchronous motors and pivot-type synchronous motors.

2. Figure 15.1 shows the structure model of the most commonly used transition-type synchronous generator. The inner circle of the stator core is evenly distributed with stator slots, and three-phase symmetrical AC windings arranged according to certain rules are embedded in the slots. The stator of the synchronous motor is also called the armature, and the stator core and winding are also called the armature core and the armature winding.

3. Pairs of magnetic poles made of a certain shape are installed on the rotor core, and excitation windings are wound around the magnetic poles. When a DC current is applied, a distributed magnetic field with alternating polarities will be formed in the air gap of the motor, which is called the excitation magnetic field (also known as the excitation magnetic field). called main magnetic field and rotor magnetic field).

4. The air gap is located between the inner circle of the armature and the magnetic pole of the rotor. The thickness and shape of the air gap layer have a significant impact on the distribution of the internal magnetic field of the motor and the performance of the synchronous motor.

5. In addition to the transition-type synchronous motor, there is also a pivot-type synchronous motor, whose magnetic poles are installed on the stator, and the AC windings are distributed in the slots on the surface of the rotor. The rotor of this synchronous motor acts as an armature. The three coils AX, BY, and CZ, which are staggered by 120 electrical degrees in space, represent three-phase symmetrical AC windings.

Synchronous generator synchronous speed

1. Considering the quality of power supply, the frequency of the AC power grid composed of many synchronous generators in parallel should be a constant value, which requires that the frequency of the generator should be consistent with the frequency of the power grid. The frequency of the power grid in my country is 50Hz.

2. In order for the generator to supply power frequency power of 50Hz to the grid, the speed of the generator must be some fixed value, which is called the synchronous speed. For example, the synchronous speed of a 2-pole motor is 3000r/min, and the synchronous speed of a 4-pole motor The speed is 1500r/min, and so on. Only when it runs at the synchronous speed, the synchronous motor can run normally, which is also the origin of the name of the synchronous motor.

Synchronous generator operation mode

There are three main operation modes of synchronous motor, namely, operation as generator, motor and compensation machine. Operation as generator is the most important operation mode of synchronous motor, and operation as motor is another important operation mode of synchronous motor. Synchronous motor The power factor of the motor can be adjusted. In the occasions where speed regulation is not required, the application of large synchronous motors can improve the operating efficiency. In recent years, small synchronous motors have begun to be widely used in variable frequency speed regulation systems. Synchronous motors can also be connected to the power grid as a Synchronous compensation machine. At this time, the motor does not carry any mechanical load, and sends the required inductive or capacitive reactive power to the grid by adjusting the excitation current in the rotor to achieve the purpose of improving the power factor of the grid or adjusting the grid voltage.

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