Introduction of synchronous motor

            Introduction of synchronous motor

Synchronous motor, like induction motor (namely asynchronous motor), is a commonly used AC motor. The synchronous motor is the heart of the power system. It is a component that integrates rotation and static, electromagnetic change and mechanical movement, and realizes the transformation of electrical and mechanical energy. Its dynamic performance is very complex, and its dynamic performance has a great impact on the dynamic performance of the entire power system. Features: during steady state operation, there is a constant relationship between the rotor speed and the grid frequency n=ns=60f/p, where f is the grid frequency, p is the pole pair of the motor, and ns is called synchronous speed. If the frequency of the power grid is constant, the speed of the synchronous motor in steady state is constant and independent of the load. Synchronous motor is divided into synchronous generator and synchronous motor. In modern power plants, synchronous generators are the main type of alternators.

working principle：

When a synchronous motor is working, three-phase symmetrical current is introduced into the three-phase winding of the stator, and DC current is introduced into the excitation winding of the rotor. When three-phase alternating current is introduced into the three-phase symmetrical winding of the stator, a rotating magnetic field will be generated in the air gap. When DC current is introduced into the rotor excitation winding, a static magnetic field with constant polarity will be generated. If the magnetic pole pair of the rotor magnetic field is equal to the magnetic pole pair of the stator magnetic field, the rotor magnetic field rotates synchronously with the stator rotating magnetic field due to the magnetic pull of the stator magnetic field, that is, the rotor rotates at a speed and direction equal to the rotating magnetic field, which is the basic working principle of synchronous motors.

Operation mode

There are three main operation modes of synchronous motor, namely, it operates as generator, motor and compensator.

As a generator, it is the main operation mode of synchronous motor.

Operation as a motor is another important operation mode of synchronous motor. The power factor of synchronous motor can be adjusted

Synchronous motor

Synchronous motor

When speed regulation is not required, the application of large synchronous motor can improve the operation efficiency. Small synchronous motor has been widely used in variable frequency speed regulation system.

The synchronous motor can also be connected to the power grid as a synchronous compensator. At this time, the motor does not carry any mechanical load, and it sends the required inductive or capacitive reactive power to the grid by adjusting the excitation current in the rotor, so as to improve the power factor of the grid or adjust the grid voltage.

Like other types of rotating machines, synchronous generators are composed of fixed stator and rotatable rotor. Generally, it is divided into rotary synchronous motor and pivot synchronous motor.

The most commonly used is the rotary synchronous generator. The inner circle of the stator core is evenly distributed with stator slots, which are embedded with three-phase symmetrical windings arranged regularly. The stator of such synchronous motor is also called armature, and the stator core and winding are also called armature core and armature winding.

The rotor core is equipped with a pair of magnetic poles which are made into a certain shape. The magnetic poles are wound with excitation windings. When DC current is applied, a scattered magnetic field of polar phases will be formed in the air gap of the motor, which is called the excitation magnetic field (also called the main magnetic field and the rotor magnetic field).

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 each stator phase winding in sequence (equivalent to that the conductor of the winding reversely cuts the excitation magnetic field).

Characteristic：

During steady state operation, there is a constant relationship between the rotor speed and the grid frequency. If the frequency of the power grid is constant, the speed of the synchronous motor in steady state is constant and independent of the load.

classification

According to the different excitation modes, synchronous motors can be divided into electrically excited synchronous motors and permanent magnet synchronous motors.

Folding electric excitation

Its rotor is made of an obvious pole type. The magnetic field coils installed on the magnetic pole iron core are connected in series with each other and have alternate and opposite polarity. Two leads are connected to two slip rings installed on the shaft. The magnetic field coil is excited by a small DC generator or battery. In most synchronous motors, the DC generator is installed on the motor shaft to supply the excitation current of the rotor pole coil. Since the synchronous motor cannot be started automatically, a squirrel cage winding is also installed on the rotor for starting the motor. The squirrel cage winding is placed around the rotor, and its structure is similar to that of asynchronous motor.

When the three-phase AC power supply is connected to the stator winding, a rotating magnetic field is generated in the motor. The squirrel cage winding cuts the magnetic line of force and generates induced current, which makes the motor rotate. After the motor rotates, its speed slowly increases to a speed slightly lower than that of the rotating magnetic field. At this time, the rotor field coil is excited by DC to form certain magnetic poles on the rotor. These magnetic poles attempt to track the rotating magnetic poles on the stator, thus increasing the speed of the motor rotor until it rotates synchronously with the rotating magnetic field.

Folding permanent magnet

The synchronous motor with the rotor not excited can be applied to single-phase power supply and also to polyphase power supply. In this kind of motor, one kind of stator winding is similar to the stator of split phase motor or polyphase motor, and has a squirrel cage rotor, and the surface of the rotor is cut into a plane. Therefore, it belongs to the salient pole rotor. The magnetic pole of the rotor is made of a kind of magnetized steel and can always maintain magnetism. The squirrel cage winding is used to generate the starting torque. When the motor rotates to a certain speed, the rotor pole will synchronize with the current frequency of the stator coil. The polarity of the salient pole is induced by the stator, so its number should be equal to the number of poles on the stator. When the motor turns to its proper speed, the squirrel cage winding will lose its function. The rotor and the magnetic pole keep up with the stator magnetic pole to keep it synchronized.

Differences

Differences in design between synchronous motor and asynchronous motor

The biggest difference between synchronous motor and asynchronous motor is whether their rotor speed is consistent with the stator rotating magnetic field. The rotor speed of the motor is the same with the stator rotating magnetic field, which is called synchronous motor, and vice versa.

In addition, the stator windings of synchronous motor and asynchronous motor are the same, but the difference lies in the rotor structure of the motor. The rotor of asynchronous motor is short circuited winding, which generates current by electromagnetic induction. The rotor structure of synchronous motor is relatively complex, with DC excitation winding, so it needs external excitation power supply to introduce current through slip ring; Therefore, the structure of synchronous motor is relatively complex, and the cost and maintenance cost are relatively high.

Difference in reactive power between synchronous motor and asynchronous motor

Compared with asynchronous motors, they can only absorb reactive power. Synchronous motors can emit reactive power and also absorb reactive power!

The difference between synchronous motor and asynchronous motor in function and purpose

The speed of synchronous motor is synchronous with the electromagnetic speed, while the speed of asynchronous motor is lower than the electromagnetic speed. As long as the synchronous motor does not lose step, the speed will not change regardless of the load, and the speed of asynchronous motor will change with the change of the load.

Synchronous motor has high precision, but it is complex in manufacturing, high in cost, and relatively difficult to maintain. Asynchronous motor, though slow in response, is easy to install and use, and cheap at the same time. So synchronous motor is not widely used as asynchronous motor.

Synchronous motors are mostly used in large generators, while asynchronous motors are almost used in motors.