How to Distinguish Permanent Magnet Generator from Excitation Generator

Generator sets have been widely used in banks, schools, airports, hospitals, real estate and other fields, but many users lack understanding of generator sets. In order to enhance the understanding of generators for users, the following explains the permanent magnet generators and excitation. The difference between generators. 

The generator can be divided into a permanent magnet generator and an excitation generator. The biggest difference between the permanent magnet generator and the excitation generator is that its excitation magnetic field is generated by a permanent magnet. The permanent magnet is both a magnetic source and a component of the magnetic circuit in the motor. The excitation power unit provides the excitation current to the rotor of the synchronous generator, and the excitation regulator controls the output permanent magnet generator of the excitation power unit according to the input signal and the given regulation criterion.

The excitation power unit supplies an excitation current to the synchronous generator rotor; and the excitation regulator controls the output of the excitation power unit based on the input signal and a given adjustment criterion. The automatic excitation regulator of the excitation system plays a considerable role in improving the stability of the parallel unit of the power system. In particular, the development of modern power systems has led to a trend of lowering the stability limit of the unit, and has also led to the continuous development of excitation technology. The excitation system of the synchronous generator is mainly composed of two parts: a power unit and a regulator (device).

The excitation power unit is a portion of the excitation power source that supplies a DC excitation current to the rotor winding of the synchronous generator, and the excitation regulator controls the output of the excitation power unit according to an input signal required by the control and a given adjustment criterion. The entire system consisting of the excitation regulator, the excitation power unit and the generator itself is called the excitation system control system. The excitation system is an important part of the generator, which has a great impact on the safe and stable operation of the power system and the generator itself.

The main functions of excitation system are:

1. Improving the static stability of generator in parallel operation.

2. Improving transient stability of generators in parallel operation.

3. Reactive power distribution among generators in parallel operation.

4. When there is a fault inside the generator, de-excitation is carried out to reduce the degree of the fault loss.

5. Maximum excitation limit and minimum excitation limit for generators according to operation requirements.

6. The excitation current should be adjusted accordingly according to the change of generator load, and the voltage at the terminal of the generator should be maintained as the given value.

1. Excitation mode of DC generator power supply

The generator with this excitation mode has a special DC generator, which is called a DC exciter. The exciter is coaxial with the generator. The excitation windings of the generator obtain DC current from the exciter through the sliding ring mounted on the big shaft and the fixed brush. This excitation mode has the advantages of independent excitation current, reliable operation and reducing self-consumption of electricity. It is the main excitation mode of generator in the past few decades, and has more mature operation experience. The drawback is that the speed of excitation regulation is slow and the maintenance work is large, so it is seldom used in units above 10MW.

2. Excitation mode of AC exciter power supply

Some modern large-capacity generators use AC exciter to provide excitation current. The AC exciter is also installed on the generator shaft, and the output AC current is rectified to supply the generator rotor excitation. At this time, the excitation mode of the generator belongs to his excitation mode, and because of the use of static rectifier, it is also called the static excitation, and the AC auxiliary exciter provides the excitation current. AC auxiliary exciters can be permanent magnetic machines or alternators with self-excited constant voltage devices. In order to improve the speed of excitation regulation, the AC exciter usually adopts the generator of 100-200HZ, while the AC sub exciter adopts the generator of 400-500HZ. The DC excitation winding and the three phase AC winding of the generator are wrapped in the stator slot. The rotor has only teeth and slots and no winding, like a gear. Therefore, it has the advantage of reliable operation, simple structure and convenient manufacturing process. The drawback is that the noise is high and the harmonic component of the AC potential is larger.

Self-excited excitation system of generator can be divided into several forms.

The excitation system with excitation power supplied by the generator itself is called self-excited excitation system. The AC exciter is a rotating machine in the excitation system, while in the self-excited excitation system, the excitation transformer and rectifier are all static components, so the self-excited excitation system is also called the all-static excitation system.

Self- excited excitation system can be divided into the following forms.

1. Self-shunt excitation system

Self excitation sstem, self-shunt excitation system, which obtains excitation power from synchronous generator voltage. As shown in figure 1, the wiring diagram of the self-shunt excitation system. Alternator current from a synchronous generator is converted to the required voltage by an excitation transformer (some low-voltage small capacity units are introduced directly from the machine end). from thyristor or power diode rectifier to DC, supply excitation winding to establish magnetic field. Automatic regulator automatically adjusts excitation current according to generator output voltage variation.

2. Self-excitation system

The self excitation system, which obtains excitation power by both voltage and current of synchronous generator, is called self compound excitation system. According to the excitation current compound position, there are DC side recombination mode and AC side compound mode. Medium and small capacity diesel generator sets are mainly AC side parallel complex uncontrollable excitation system, as shown in figure 2. The excitation transformer is connected in series with a reactor and connected in parallel with the excitation converter. The output of the excitation transformer is superposed and then rectified by a silicon rectifier to supply the excitation of the synchronous generator.

The excitation transformer is connected in series with a reactor and connected in parallel with the excitation converter. The output of the excitation transformer is superposed and then rectified by a silicon rectifier to supply the excitation of the synchronous generator. This excitation system can reflect the voltage, current and power factor of generator, also called uncontrollable phase compound excitation system.

If in addition to the parallel excitation transformer there is an excitation AC (or series transformer) in series with the generator stator current loop the combination of the two constitutes the so-called self-compound excitation mode. There are four combined programmes:

1. DC side parallel self-compound excitation mode

2. DC side series self compound excitation mode

3. Ac side parallel self-compound excitation mode

4. Ac side series self compound excitation mode