The rotor inside the servo motor is a permanent magnet. The U/V/W three-phase electricity controlled by the driver forms an electromagnetic field. The rotor rotates under the action of this magnetic field. At the same time, the encoder of the motor feeds back the signal to the driver, and the driver matches the target according to the feedback value. The values ​​are compared, and the angle of rotation of the rotor is adjusted. The accuracy of the servo motor is determined by the accuracy of the encoder (number of lines)

The structure of AC servo motor can be divided into two parts, namely the stator part and the rotor part. Among them, the structure of the stator is basically the same as that of the resolver, and two-phase windings with a space of 90 degrees of electrical angle are also placed in the stator core. One group is the excitation winding and the other is the control winding. The AC servo motor is a two-phase AC motor. When an AC servo motor is used, a constant excitation voltage Uf is applied to both ends of the excitation winding, and a control voltage Uk is applied to both ends of the control winding. When voltage is applied to the stator winding, the servo motor will quickly rotate.

 

 

The current flowing into the excitation winding and the control winding generates a rotating magnetic field in the motor. The rotation of the rotating magnetic field determines the rotation of the motor. When the voltage applied to any winding is reversed, the direction of the rotating magnetic field changes, and the motor's The direction has also changed. In order to form a circular rotating magnetic field in the motor, it is required that there should be a 90 degree phase difference between the excitation voltage Uf and the control voltage UK. Common methods are as follows:

 

1) Use the phase voltage and line voltage of the three-phase power supply to form a 90-degree phase shift

 

2) Use any line voltage of the three-phase power supply

 

3) Use phase shift network

 

4) Connect capacitors in series in the excitation phase

 

Since the 1980s, with the development of integrated circuits, power electronics technology and AC variable speed drive technology, permanent magnet AC servo drive technology has developed prominently. Famous electrical manufacturers in various countries have successively launched their own AC servo motors and servo drive series. Products are constantly improved and updated. The AC servo system has become the main development direction of the contemporary high-performance servo system, making the original DC servo facing the crisis of being eliminated. Since the 1990s, the AC servo system that has been commercialized in various countries around the world is a sine wave motor servo drive with full digital control. The development of AC servo drives in the field of transmission is changing with each passing day.

 

advantage

 

⑴ There are no brushes and commutators, so it works reliably and has low maintenance and maintenance requirements.

 

⑵It is more convenient to dissipate heat from the stator winding.

 

⑶The inertia is small and it is easy to improve the speed of the system.

 

⑷ Suitable for high-speed and high-torque working conditions.

 

For a long time, in the occasions requiring high speed control performance, the speed control system using DC motor has been dominant. However, DC motors have some inherent shortcomings, such as brushes and commutators are easy to wear and require frequent maintenance. Sparks are generated when the commutator is commutation, which limits the maximum speed of the motor and also restricts the application environment. In addition, the structure of the DC motor is complex, the manufacturing is difficult, the steel materials used are consumed, and the manufacturing cost is high. However, AC motors, especially squirrel-cage induction motors, do not have the above-mentioned shortcomings, and the rotor inertia is smaller than that of DC motors, resulting in better dynamic response. Under the same volume, the output power of an AC motor can be increased by 10% to 70% than that of a DC motor. In addition, the capacity of an AC motor can be made larger than that of a DC motor to achieve a higher voltage and speed. Modern CNC machine tools tend to use AC servo drives, and AC servo drives have replaced DC servo drives.

 

Asynchronous

 

Asynchronous AC servo motors refer to AC induction motors. It is divided into three-phase and single-phase, as well as squirrel-cage type and wire-wound type. Squirrel-cage three-phase induction motors are usually used. Its structure is simple, compared with a DC motor of the same capacity, the weight is 1/2 lighter, and the price is only 1/3 of the DC motor. The disadvantage is that a wide range of smooth speed regulation cannot be realized economically, and the lagging excitation current must be absorbed from the grid. As a result, the power factor of the power grid deteriorates.

 

The asynchronous AC servo motor of this squirrel cage rotor is referred to as an asynchronous AC servo motor for short, denoted by IM.

 

Synchronous

 

Although synchronous AC servo motors are more complex than induction motors, they are simpler than DC motors. Its stator is the same as an induction motor, with symmetrical three-phase windings on the stator. The rotor is different. According to different rotor structures, it is divided into electromagnetic and non-electromagnetic. Non-electromagnetic type is divided into hysteresis type, permanent magnet type and reaction type. Among them, hysteresis and reactive synchronous motors have disadvantages such as low efficiency, poor power factor, and small manufacturing capacity. Permanent magnet synchronous motors are often used in CNC machine tools. Compared with the electromagnetic type, the permanent magnet type has the advantages of simple structure, reliable operation, and higher efficiency; the disadvantage is that it is large in size and poor in starting characteristics. However, the permanent magnet synchronous motor adopts high remanence induction and high coercivity rare earth magnets, which can be about 1/2 smaller than the DC motor, 60% less in mass, and 1/5 of the rotor inertia of the DC motor. Compared with asynchronous motors, it has high efficiency due to the use of permanent magnet excitation, which eliminates excitation loss and related stray losses. Because there are no slip rings and brushes required by electromagnetic synchronous motors, its mechanical reliability is the same as that of induction (asynchronous) motors, but its power factor is much higher than that of asynchronous motors, so that the volume of permanent magnet synchronous motors is larger than that of asynchronous motors. The electric motor is smaller. This is because at low speed, the induction (asynchronous) motor has a much higher apparent power when outputting the same active power due to its low power factor, and the main size of the motor is determined by the apparent power.