Synchronous motors, like induction motors, are commonly used AC motors. Their key characteristic is that during steady-state operation, the rotor speed and the grid frequency maintain a constant relationship: n = ns = 60f/p, where ns is the synchronous speed. If the grid frequency remains constant, the synchronous motor's speed in steady state is always constant and independent of the load. Synchronous motors are divided into synchronous generators and synchronous motors. Modern power plants primarily use synchronous motors in their AC motor systems.
I. Working Principle
Establishment of the main magnetic field: A DC excitation current is passed through the excitation winding to establish an excitation magnetic field with alternating polarities, that is, to establish the main magnetic field.
Current-carrying conductor: The three-phase symmetrical armature winding acts as the power winding, becoming the carrier of induced electromotive force or induced current.
Cutting motion: The prime mover drives the rotor to rotate (inputting mechanical energy into the motor), and the excitation magnetic field with alternating polarities rotates with the shaft and sequentially cuts each phase of the stator winding (equivalent to the conductors of the winding cutting the excitation magnetic field in the opposite direction).
Generation of alternating electromotive force: Due to the relative cutting motion between the armature winding and the main magnetic field, a three-phase symmetrical alternating electromotive force with periodically changing magnitude and direction will be induced in the armature winding. AC power can then be provided through the leads.
Alternating polarity and symmetry: The alternating polarity of the rotating magnetic field causes the polarity of the induced electromotive force to alternate; the symmetry of the armature winding ensures the three-phase symmetry of the induced electromotive force.
II. Types of Synchronous Motors
(I) AC synchronous motor
An AC synchronous motor is a constant-speed drive motor in which the rotor speed maintains a constant proportional relationship with the power supply frequency. It is widely used in electronic instruments, modern office equipment, textile machinery, and other fields.
(II) Permanent Magnet Synchronous Motor
Permanent magnet synchronous motors belong to the category of asynchronous starting permanent magnet synchronous motors. Their magnetic field system consists of one or more permanent magnets, typically housed inside a cage-type rotor made of cast aluminum or welded copper bars, with the required number of permanent magnet poles installed. The stator structure is similar to that of an asynchronous motor.
When the stator winding is powered on, the motor starts and rotates according to the principle of asynchronous motor. When it accelerates to synchronous speed, the synchronous electromagnetic torque generated by the rotor permanent magnet field and the stator magnetic field (the electromagnetic torque generated by the rotor permanent magnet field and the magnetic reluctance torque generated by the stator magnetic field) pulls the rotor into synchronization, and the motor enters synchronous operation.
Reluctance synchronous motors, also known as reactive synchronous motors, are synchronous motors that generate reluctance torque by utilizing the unequal magnetic reluctance between the rotor's quadrature and direct axes. Their stator structure is similar to that of asynchronous motors, only the rotor structure is different.
(III) Reluctance Synchronous Motor
Evolved from the co-cage asynchronous motor, the rotor also features cage-type cast aluminum windings to generate asynchronous starting torque. The rotor has reaction slots corresponding to the stator pole number (only the salient pole portion functions, without excitation windings and permanent magnets) to generate reluctance synchronous torque. Based on the structure of the reaction slots, it can be divided into internal reaction rotors, external reaction rotors, and internal-external reaction rotors. In external reaction rotors, the reaction slots are cut into the outer circumference of the rotor, making the air gap unequal in the direct and quadrature axes. Internal reaction rotors have internal grooves that obstruct magnetic flux in the quadrature axis direction, increasing reluctance. Internal-external reaction rotors combine the structural characteristics of the above two types, with a significant difference between the direct and quadrature axes, resulting in greater motor power. Reluctance synchronous motors also come in various types, including single-phase capacitor-run, single-phase capacitor-start, and single-phase dual-capacitor types.
(iv) Hysteresis Synchronous Motor
A hysteresis synchronous motor is a synchronous motor that operates by generating hysteresis torque using hysteresis materials. It is classified into internal rotor hysteresis synchronous motors, external rotor hysteresis synchronous motors, and single-phase shaded-pole hysteresis synchronous motors.
The rotor structure of the internal rotor hysteresis synchronous motor is salient pole type, and its appearance is a smooth cylinder. There are no windings on the rotor, but there is an effective ring layer made of hysteresis material on the outer circle of the iron core.
After the stator windings are powered on, the resulting rotating magnetic field causes the hysteresis rotor to generate asynchronous torque and start rotating, subsequently pulling itself into synchronous operation. During asynchronous operation, the stator rotating magnetic field repeatedly magnetizes the rotor at the slip frequency; during synchronous operation, the hysteresis material on the rotor is magnetized, creating permanent magnet poles, thus generating synchronous torque. The soft starter uses three anti-parallel thyristors as voltage regulators, connected between the power supply and the motor stator. This circuit resembles a three-phase fully controlled bridge rectifier circuit. When starting the motor with a soft starter, the thyristor output voltage gradually increases, and the motor gradually accelerates until the thyristors are fully conducting. The motor operates at its rated voltage mechanical characteristics, achieving smooth starting, reducing starting current, and avoiding overcurrent tripping. Once the motor reaches its rated speed, the starting process ends, and the soft starter automatically replaces the thyristors with a bypass contactor, providing the rated voltage for normal motor operation. This reduces thyristor heat loss, extends the soft starter's lifespan, improves its efficiency, and prevents harmonic pollution of the power grid. The soft starter also provides a soft stop function. The soft stop process is the opposite of the soft start process. The voltage gradually decreases and the speed gradually drops to zero, avoiding the torque shock caused by free stop.
