Synchronous motor content

Synchronous motors are a common AC motor like induction motors. The characteristics are: during steady-state operation, there is a constant relationship between the rotor speed and the grid frequency n=ns=60f/p, and ns becomes the synchronous speed. If the frequency of the grid remains unchanged, the speed of the synchronous motor is constant in steady state and has nothing to do with the size of the load. Synchronous motors are divided into synchronous generators and synchronous motors. The AC machines in modern power plants are mainly synchronous motors.
Working principle
Establishment of the main magnetic field: The excitation winding is passed with a DC excitation current to establish an excitation magnetic field with alternating polarity, that is, to establish the main magnetic field.
Current-carrying conductor: The three-phase symmetrical armature winding acts as a power winding and becomes the carrier of the induced potential or induced current.
Cutting motion: The prime mover drags the rotor to rotate (inputs mechanical energy into the motor), and the excitation magnetic field with alternating polarity rotates with the shaft and cuts the stator windings of each phase in sequence (equivalent to the conductor of the winding cutting the excitation magnetic field in reverse). [2]
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 with periodic changes in magnitude and direction will be induced in the armature winding. The AC power supply can be provided through the lead wire.
Alternation and symmetry: Due to the alternating polarity of the rotating magnetic field, the polarity of the induced potential alternates; due to the symmetry of the armature winding, the three-phase symmetry of the induced potential is guaranteed. [2]
I. AC synchronous motor
AC synchronous motor is a constant speed drive motor. Its rotor speed maintains a constant proportional relationship with the power supply frequency. It is widely used in electronic instruments, modern office equipment, textile machinery, etc.
II. Permanent magnet synchronous motor
Permanent magnet synchronous motor belongs to asynchronous start permanent magnet synchronous motor. Its magnetic field system consists of one or more permanent magnets. It is usually installed with permanent magnet poles according to the required number of poles in the cage rotor made of cast aluminum or copper bars. The stator structure is similar to that of asynchronous motor.
When the stator winding is connected to the power supply, the motor starts to rotate according to the principle of asynchronous motor. When it accelerates to the synchronous speed, the synchronous electromagnetic torque generated by the permanent magnetic field of the rotor and the stator magnetic field (the electromagnetic torque generated by the permanent magnetic field of the rotor and the reluctance torque generated by the stator magnetic field) pulls the rotor into synchronization, and the motor enters synchronous operation.
Reluctance Synchronous Motor Reluctance synchronous motor, also known as reaction synchronous motor, is a synchronous motor that uses the unequal reluctance of the rotor's quadrature axis and direct axis to generate reluctance torque. Its stator is similar to the stator structure of the asynchronous motor, but the rotor structure is different.
3. Reluctance Synchronous Motor
Evolved from the cage asynchronous motor, in order to enable the motor to generate asynchronous starting torque, the rotor is also equipped with cage cast aluminum winding. The rotor is provided with reaction slots corresponding to the number of stator poles (only the salient pole part is used, without excitation winding and permanent magnet) to generate reluctance synchronous torque. According to the different structures of the reaction grooves on the rotor, it can be divided into internal reaction rotor, external reaction rotor and internal and external reaction rotor. Among them, the reaction groove of the external reaction rotor is opened on the outer circle of the rotor, so that the air gap in the direction of the direct axis and the quadrature axis is not equal. The internal reaction rotor has grooves inside, which blocks the magnetic flux in the quadrature axis direction and increases the magnetic resistance. The internal and external reaction rotor combines the structural characteristics of the above two rotors, and the difference between the direct axis and the quadrature axis is large, which makes the power of the motor larger. Reluctance synchronous motors are also divided into single-phase capacitor operation type, single-phase capacitor starting type, single-phase dual-value capacitor type and other types.
4. Hysteresis synchronous motor
The hysteresis synchronous motor is a synchronous motor that uses hysteresis materials to generate hysteresis torque. It is divided into inner rotor hysteresis synchronous motor, outer rotor hysteresis synchronous motor and single-phase shaded pole hysteresis synchronous motor.
The rotor structure of the inner rotor hysteresis synchronous motor is hidden pole type, the appearance is a smooth cylinder, there is no winding on the rotor, but there is an annular effective layer made of hysteresis material on the outer circle of the core.
After the stator winding is connected to the power supply, the rotating magnetic field generated causes the hysteresis rotor to generate asynchronous torque and start rotating, and then automatically enters the synchronous operation state. When the motor is running asynchronously, the stator rotating magnetic field repeatedly magnetizes the rotor at the slip frequency; when running synchronously, the hysteresis material on the rotor is magnetized and permanent magnetic poles appear, thereby generating synchronous torque. The soft starter uses a three-phase anti-parallel thyristor as a voltage regulator, which is connected between the power supply and the motor stator. This circuit is like a three-phase fully controlled bridge rectifier circuit. When the soft starter is used to start the motor, the output voltage of the thyristor gradually increases, and the motor gradually accelerates until the thyristor is fully turned on. The motor works on the mechanical characteristics of the rated voltage, achieving smooth starting, reducing the starting current, and avoiding starting overcurrent tripping. When the motor reaches the rated speed, the starting process ends, and the soft starter automatically replaces the thyristor that has completed the task with the bypass contactor to provide the rated voltage for the normal operation of the motor, so as to reduce the heat loss of the thyristor, extend the service life of the soft starter, improve its working efficiency, and avoid harmonic pollution in the power grid. The soft starter also provides a soft stop function. The soft stop process is opposite to the soft start process. The voltage gradually decreases and the number of revolutions gradually drops to zero, avoiding the torque shock caused by free stop.