For a long time, DC motor speed control systems have dominated applications requiring high speed regulation performance. However, DC motors have inherent drawbacks, such as easy wear of brushes and commutators, requiring frequent maintenance. Commutation generates sparks, limiting the motor's maximum speed and restricting its application environment. Furthermore, DC motors are complex in structure, difficult to manufacture, consume large amounts of steel, and have high manufacturing costs. AC motors, especially squirrel-cage induction motors, do not have these drawbacks, and their rotor inertia is smaller than that of DC motors, resulting in better dynamic response. In the same volume, AC motors can have 10% to 70% higher output power than DC motors. In addition, AC motors can be manufactured with larger capacities, achieving higher voltages and speeds. Modern CNC machine tools tend to use AC servo drives, which are increasingly replacing DC servo drives.
Asynchronous Type
Asynchronous AC servo motors refer to AC induction motors. They are available in three-phase and single-phase versions, and in squirrel-cage and wound-rotor types, with squirrel-cage three-phase induction motors being the most common. Its structure is simple, and compared to a DC motor of the same capacity, it is half the weight and only one-third the price. The disadvantage is that it cannot economically achieve smooth speed regulation over a wide range, and must draw lagging excitation current from the power grid. This worsens the power factor of the grid.
This type of squirrel-cage rotor asynchronous AC servo motor is simply called an asynchronous AC servo motor, denoted by IM.
Synchronous Type: Although synchronous AC servo motors are more complex than induction motors, they are simpler than DC motors. Its stator is the same as that of an induction motor, with symmetrical three-phase windings. However, the rotor is different, and according to different rotor structures, it is divided into two main categories: electromagnetic and non-electromagnetic. Non-electromagnetic synchronous motors are further divided into hysteresis, permanent magnet, and reactive types. Hysteresis and reactive synchronous motors have disadvantages such as low efficiency, poor power factor, and limited manufacturing capacity. Permanent magnet synchronous motors are mostly used in CNC machine tools.
Compared to electromagnetic motors, permanent magnet motors have the advantages of simple structure, reliable operation, and higher efficiency; the disadvantages are large size and poor starting characteristics. However, by employing rare-earth magnets with high remanence and coercivity, permanent magnet synchronous motors can be approximately half the size and 60% lighter than DC motors, with rotor inertia reduced to one-fifth that of DC motors. Compared to asynchronous motors, they are more efficient due to the elimination of excitation losses and related stray losses caused by permanent magnet excitation. Furthermore, because they lack the slip rings and brushes required by electromagnetic synchronous motors, their mechanical reliability is the same as that of induction (asynchronous) motors, while their power factor is significantly higher, resulting in a smaller size for permanent magnet synchronous motors. This is because at low speeds, induction (asynchronous) motors, due to their low power factor, have a much larger apparent power for the same output of active power, and the main dimensions of the motor are determined by the apparent power.