A synchronous motor is an AC motor whose working principle is that the rotor speed is always equal to the speed of the rotating magnetic field of the armature, that is, there is a strict and constant relationship between the rotor speed and the frequency of the armature current.
I. Working Principle of Synchronous Motor
Basic structure of synchronous motor
Synchronous motors have a stator core with three-phase symmetrical windings and a rotor core with DC excitation windings.
Establishment of the main magnetic field
A DC excitation current is applied to 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 windings serve as power windings, and the gearbox becomes 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.
Generation of alternating electric potential
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.
II. Main Operating Modes of Synchronous Motors
1. Generator: This is the most important operating mode of synchronous motor.
2. Electric Motor: This is another important operating mode of synchronous motors. The power factor of a synchronous motor can be adjusted. In applications where speed regulation is not required, synchronous motors using large bevel gear reducers can improve operating efficiency. In recent years, small synchronous motors have begun to be more widely used in variable frequency speed control systems.
3. Compensator: The synchronous motor is 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 power grid by adjusting the excitation current in the rotor, so as to improve the power factor of the power grid or regulate the voltage of the power grid.
III. Comparison of Two Types of Synchronous Motors
1. Brushless DC Motor: Its starting point is to replace the stator poles of a brushed DC motor with a rotor equipped with permanent magnets, effectively turning the rotor armature of the original DC motor into the stator. Brushed DC motors rely on a mechanical commutator to convert direct current into an approximately trapezoidal AC wave, while a BDCM directly inputs a square wave current (which is also actually a trapezoidal wave) into the stator. Its advantage is that it eliminates the need for a mechanical commutator and brushes; this is also known as electronic commutation. To generate constant electromagnetic torque, the system must input a three-phase symmetrical square wave current into the BDCM, and the induced electromotive force in each phase of the BDCM must be a trapezoidal wave; therefore, the BDCM is also called a square wave motor.
2. Permanent Magnet Synchronous Motor (PMSM): Its design principle is to replace the excitation windings on the rotor of an electrically excited synchronous motor with permanent magnets, thus eliminating the need for the excitation coils, slip rings, and brushes of the KAF87 reducer. The stator of a PMSM is essentially the same as that of an electrically excited synchronous motor, requiring the input stator current to remain three-phase sinusoidal. To generate constant electromagnetic torque, the system must input a three-phase symmetrical sinusoidal current to the PMSM, and the induced electromotive force in each phase of the PMSM must also be sinusoidal; therefore, PMSMs are also called sinusoidal wave motors.
The greatest advantage of synchronous motors is that the power factor can be changed by adjusting the excitation current. Under a certain active power, this change can produce a U-shaped curve for the synchronous motor. When over-excited, it absorbs leading reactive power from the grid; when under-excited, it absorbs lagging reactive power from the grid.
Changing the excitation current of a synchronous motor can alter its power factor, a capability not found in three-phase asynchronous motors. The changes in the power factor of a synchronous motor when the excitation current is changed can be categorized into three states: normal excitation, under-excitation, and over-excitation. When a synchronous motor is driving a load, it should generally be over-excited, at least operating in normal excitation mode, and should not be allowed to operate in under-excitation mode.
