What are the five main categories of DC servo motors?
Based on the method of generating the magnetic field, DC motors can be divided into five types: separately excited, permanent magnet, shunt-excited, series-excited, and compound-excited. Permanent magnet motors use soft magnetic materials such as oxides, AlNiCo, and rare earth elements to create the excitation magnetic field.
Structurally, DC servo motors include general armature type, slotless armature type, printed armature type, wound-rotor type, and hollow cup armature type. To avoid contact with the brush commutator, there are also brushless DC servo motors. Based on the control method, DC servo motors can be divided into field control type and armature control type. Clearly, water-magnetic DC servo motors can only use armature control, and most general electric-injection DC servo motors also use armature control.
In CNC machine tools, the commonly used DC servo motors in the feed system mainly include the following types:
(1) Small inertia DC servo motor
Small inertia DC servo motors are named for their small moment of inertia. These motors are generally of the water-magnetic type, with armature windings available in three types: phaseless armature, printed armature, and hollow cup armature. Because small inertia DC motors minimize the armature's moment of inertia, they achieve the fastest response speed. These servo motors were widely used in early CNC machine tools. Small inertia servo motors are still in use on CNC machine tools in some countries, such as France.
(2) DC torque motor
Also known as a high-inertia, wide-range DC servo motor. On one hand, due to its larger rotor diameter and increased number of coil winding turns, it has a larger torque and a larger turning radius compared to other types of motors. It can also operate for extended periods under significant overload torque. Therefore, it can be directly connected to a leadscrew without the need for an intermediate transmission device. On the other hand, because it has no excitation circuit losses, its overall size is smaller than other similar DC servo motors. Another outstanding feature is its ability to achieve smooth operation at low speeds, with the lowest speed reaching 1 r/min, or even 0.1 r/min. Therefore, this type of servo motor is widely used in CNC machine tools.
(3) Brushless DC servo motor
This type of servo motor is also called a commutatorless motor. It has no commutator. It consists of a synchronous motor and an inverter, which is controlled by a rotor position sensor mounted on the rotor. Essentially, it is an AC speed-regulating motor, and its speed regulation performance can reach the level of a DC servo generator. Furthermore, the elimination of the commutator and brush components greatly improves the motor's lifespan.
DC Servo Motor Classification_DC Servo Motor Structure
Structure of DC servo motor
1. The rotor, also known as the armature, is made of stacked silicon steel sheets with coils embedded on its surface. When direct current is applied, it generates an electromagnetic torque that drives the load to rotate under the action of the stator magnetic field.
2. Brushes and commutator segments: In order to keep the generated electromagnetic torque in a constant direction and enable the rotor to rotate continuously and uniformly in a fixed direction, the brushes are connected to the external DC power supply, and the commutator segments are connected to the armature conductors.
2. Stator: The stator magnetic field is generated by the stator's magnetic poles. Based on the method of magnetic field generation, DC servo motors can be divided into permanent magnet type and externally excited type. Permanent magnet type magnetic poles are made of permanent magnet material, while externally excited type magnetic poles are made of stacked stamped silicon steel sheets. A constant magnetic field is generated by passing a direct current through the externally wound coil.
