Today we will learn about synchronous motors in motors:
Synchronous motor and induction motor is a common ac motor. In steady-state operation, there is a constant relationship between the rotor speed and the power grid frequency n= NS = 60F/P, and NS becomes synchronous speed. If the frequency of the power grid is constant, the speed of the synchronous motor is constant and has nothing to do with the load. Synchronous motor is divided into synchronous generator and synchronous motor. Synchronous motors are the main alternators in modern power plants.
The working principle of
The establishment of the main magnetic field: the excitation winding through the DC excitation current, the establishment of the polarity alternating excitation magnetic field, that is, the establishment of the main magnetic field.
Current-carrying conductor: a three-phase symmetrical armature winding that acts as a power winding and acts as a carrier of induced potential or induced current.
Cutting motion: the prime mover drives the rotor to rotate (to input mechanical energy to the motor), and the magnetic field with polarity alternating phase rotates along with the shaft and sequentially cuts the stator windings of each phase (equivalent to the conductor of the winding reversely cutting the magnetic field).
Alternating potential generation: Due to the relative cutting motion between the armature windings and the main magnetic field, the armature windings will sense a three-phase symmetrical alternating potential with periodic changes in magnitude and direction. Through the lead wire, can provide ac power supply.
Alternation and symmetry: due to alternating polarity of rotating magnetic field, the polarity of induced potential changes; Due to the symmetry of the armature winding, the three-phase symmetry of the induced potential is guaranteed. [2]
I. AC synchronous motors
Ac synchronous motor is a constant speed drive motor, its rotor speed and power frequency maintain a constant proportional relationship, is widely used in electronic instruments, modern office equipment, textile machinery and so on.
Permanent magnet synchronous motors
Permanent magnet synchronous motors are asynchronous start permanent magnet synchronous motors, whose magnetic field system consists of one or more permanent magnets, usually in the interior of a cage rotor welded with cast aluminum or copper bars, according to the required number of poles embedded with permanent magnets. Stator structure is similar to asynchronous motors.
When the stator winding is connected to the power supply, motor with principle of asynchronous motor starting regulated, rev to synchronous speed, generated by the permanent magnet rotor magnetic field and stator magnetic field of synchronous electromagnetic torque, the electromagnetic torque generated by the permanent magnet rotor magnetic field and the magnetic resistance torque of the stator magnetic field to produce synthesis) will pull in synchronous rotor, into synchronous motor running.
Reluctance synchronous motor Reluctance synchronous motor, also known as reactive synchronous motor, is a synchronous motor that produces reluctance torque by using the rotor cross axis and straight axis reluctance. Its stator structure is similar to that of the asynchronous motor, but the rotor structure is different.
Reluctance synchronous motors
Evolved from the cage type asynchronous motor, in order to make the electric function produce asynchronous starting torque, the rotor is also provided with cage type cast aluminum winding resistance. The rotor is provided with a reaction groove corresponding to the number of stator poles (only the convex pole part of the role, no excitation winding and permanent magnet), used to generate reluctance synchronous torque. According to the structure of the reaction groove 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 opens the outer circle of the rotor, so that the air gap between the direct axis and the intersecting axis is unequal. The internal reaction rotor has grooves, so that the magnetic flux in the direction of the intersecting axis is blocked and the magnetic resistance is increased. The internal and external reaction rotor combines the structural characteristics of the above two kinds of rotor, the difference between the straight axis and the cross axis is large, so that the motor's force energy is larger. Reluctance synchronous motors are also divided into single-phase capacitor running type, single-phase capacitor starting type, single-phase double-value capacitor type and other types.
Four, hysteresis synchronous motor
Hysteresis synchronous motor is a synchronous motor which uses hysteresis material to produce hysteresis torque. It is divided into inner rotor hysteresis synchronous motor, outer rotor hysteresis synchronous motor and single phase cover pole hysteresis synchronous motor.
The rotor structure of internal rotor hysteresis synchronous motor is hidden pole type, the appearance is smooth cylinder, there is no winding on the rotor, but there is a ring effective layer made of hysteresis material on the outer circle of the iron core.
After the stator windings are connected to the power supply, the rotating magnetic field generated causes the hysteresis rotor to produce asynchronous torque and start the rotation, and then pull itself into the synchronous operation state. When the motor runs asynchronously, the stator rotating magnetic field repeatedly magnetizes the rotor at the slip frequency. During synchronous operation, the hysteresis material on the rotor is magnetized and a permanent magnet magnetic pole appears, resulting in synchronous torque. The soft starter uses three opposite parallel thyristors as the voltage regulator, which is connected between the power supply and the motor stator. Such a circuit as three - phase fully - controlled bridge rectifier circuit. When using a soft starter to start the motor, the output voltage of the thyristor increases gradually, and the motor accelerates gradually until the thyristor is fully turned on, and the motor works on the mechanical characteristics of the rated voltage to achieve smooth start, reduce the starting current, and avoid starting overcurrent trip. When the motor reaches the rated revolution and the starting process is over, the soft starter automatically replaces the thyristor with bypass contactor to provide rated voltage for the normal operation of the motor, so as to reduce the thermal loss of the thyristor, prolong the service life of the soft starter, improve its working efficiency, and avoid harmonic pollution of the power grid. Soft starter also provides soft stop function, soft stop and soft start process is the opposite, the voltage is gradually reduced, the revolution gradually drops to zero, to avoid free stop caused by the torque impact.