A DC servo motor is a special-purpose DC motor used in automatic control systems, also known as an actuator motor. It converts input voltage signals into mechanical signals such as angular displacement and angular velocity on the shaft. The working principle, basic structure, and internal electromagnetic relationships of a DC servo motor are the same as those of a general-purpose DC motor.
DC servo motors are controlled by DC voltage and are classified into ordinary DC servo motors, disc armature DC servo motors , coreless DC servo motors, and slotless DC servo motors. Ordinary DC servo motors have two basic structural types: permanent magnet and electromagnetic. Electromagnetic motors are further divided into four types: separately excited, shunt excited, series excited, and compound excited. Permanent magnet motors can be considered as separately excited motors.
Features: Small rotor diameter, large axial dimension; low moment of inertia, resulting in fast response time. However, the rated torque is relatively low, and it generally must be matched with a gear reduction device. Used in small CNC machine tools with high speed and light load.
1. Basic Structure of DC Servo Motor
The diagram shows the structure of a DC servo motor, which mainly consists of three parts: stator, rotor, brushes, and commutator segments.
2. Classification of DC servo motors
(1) Based on the different structures of the electric motor itself, it can be divided into the following categories:
The improved DC servo motor has a smaller moment of inertia, stronger overload capacity, and better commutation performance.
Low-inertia DC motors minimize the rotor's rotational inertia, thus achieving the best fast-speed characteristics.
Permanent magnet DC servo motors can operate for extended periods under high overload torque, have a large moment of inertia, no excitation circuit losses, and can operate at low speeds.
A brushless DC motor consists of a synchronous motor and an inverter, and the inverter is controlled by a rotor position sensor mounted on the rotor.
(2) According to the different excitation methods of DC motors on the excitation winding, they can be divided into four types: separately excited, shunt excited, series excited and compound excited.
Characteristics and types of DC servo motors
The structure of a DC servo motor is the same as that of a regular DC motor, except that it is made longer and thinner to reduce rotational inertia. Its excitation winding and armature are powered by two separate power supplies. There are also permanent magnet types, where the magnetic poles are permanent magnets. Armature control is typically used, meaning the excitation voltage f is constant, and the established magnetic flux Φ is also constant, while the control voltage Uc is applied to the armature. The wiring diagram is shown below.
DC servo motors are typically used in systems with slightly higher power output, typically ranging from 1W to 600W.
DC servo motors can be classified into the following types:
Typical DC servo motor
Slotless armature DC servo motor
Hollow cup-shaped armature DC servo motor
Printed winding DC servo motor
Brushless DC servo motor
The mechanical characteristics (n=f(T)) of a DC servo motor are the same as those of a separately excited DC motor, and are also expressed by the following formula:
n=Uc/KE·Φ-Ra/KE·KT·Φ·T
The figure below shows the mechanical characteristic curves of a DC servo motor under different control voltages (Uc is the rated control voltage).
As shown in the diagram: under a certain load torque, when the magnetic flux remains constant, increasing the armature voltage increases the motor speed; conversely, decreasing the armature voltage decreases the speed; when Uc = 0, the motor stops immediately. To reverse the motor, the polarity of the armature voltage can be changed.
Technical parameters of DC servo motor
1. Rated power refers to the rated output power of the motor shaft, that is, the output power of the motor when it is running under rated conditions. Rated power allows the motor to operate continuously for extended periods without overheating.
2. Rated voltage refers to the rated voltage that should be applied to the excitation winding and armature control winding of the motor when it is running under rated conditions.
3. Rated current refers to the current in the windings of a motor when it is driven by a load at rated power under rated voltage. Rated current is generally the maximum current allowed for long-term continuous operation of the motor.
4. Rated speed, also known as maximum speed, refers to the speed at which the motor outputs rated power under rated voltage. The speed range of a DC servo motor is generally below the rated speed.
5. Rated torque refers to the output torque of the motor when it is running under rated conditions.
6. Maximum torque refers to the maximum torque that a motor can output in a short period of time, reflecting the motor's instantaneous overload capacity. DC servo motors generally have strong instantaneous overload capacity, and their maximum torque can typically reach 5 to 10 times the rated torque.
7. Electromechanical time constant τj and electromagnetic time constant τd
These reflect the duration of the two transient processes of the DC servo motor. τj is usually less than 20ms, and τd is usually less than 5ms. The ratio between the two is usually greater than 3, so the DC servo motor can usually be approximated as a first-order inertial element.
8. The thermal time constant refers to the time required for the motor windings to rise to 63.2% of the rated temperature rise.
9. The damping coefficient, also known as the internal damping coefficient, is the reciprocal of the slope of the mechanical characteristic curve.
Characteristic parameters of DC servo motors
(1) The torque-speed characteristic curve is also called the working curve.
(2) Load-duty cycle curve
