Detailed Analysis of DC Servo Motor

There are two types of commonly used servo motors, those that operate on AC power are called AC servo motors, and those that operate on DC power are called DC servo motors. This article only provides a detailed analysis of DC servo motors.

The function of a servo motor is to convert the input voltage signal into angular displacement or angular velocity output on the shaft. In other words, a servo motor refers to a control motor whose speed and direction change with the size and direction of the input voltage signal. Servo motors can carry a certain amount of load and serve as actuators in automatic control systems, so they are also called executive motors.

 

The performance requirements for servo motors in automatic control systems can be summarized as follows.

 

(1) No self rotation phenomenon. Before the control signal arrives, the servo motor rotor remains stationary; After the control signal arrives, the rotor rotates rapidly; When the control signal disappears, the servo motor rotor should immediately stop rotating. The phenomenon of the motor continuing to rotate when the control signal is zero is called "self rotation", and eliminating self rotation is a necessary condition for the normal operation of the self-control system.

(2) Low no-load starting voltage. When the motor is unloaded, the small control voltage from the stationary state to continuous operation of the rotor, regardless of its position, is called the starting voltage. The smaller the starting voltage, the higher the sensitivity of the motor.

(3) The linearity of mechanical and regulatory characteristics is good, and it can smoothly and stably regulate speed over a wide range.

(4) Fast and responsive. The electromechanical time constant is small, so servo motors require a small moment of inertia.

 

1. Classification And Structure Of Dc Servo Motors

 

DC servo motor is a special purpose DC motor in automatic control systems. Its structure is not fundamentally different from general DC motors, and it is also composed of two parts: stator and rotor.

 

The function of the stator is to establish a constant magnetic field, and the stator poles are equipped with excitation windings. In DC servo systems, electromagnetic and permanent magnet DC servo motors are commonly used. At present, the electromagnetic excitation method is a separate excitation method, and the armature and excitation winding are powered by two independent power sources.

 

The hollow cup armature permanent magnet DC servo motor consists of an outer stator and an inner stator, and the hollow cup armature rotates in the air gap between the inner and outer stators. The outer stator is made of soft magnetic material as the iron core, which is equipped with a concentrated winding (two semi-circular magnetic poles are made of magnets or magnetized on a circular magnetic steel to generate N and S poles). The inner stator is made of cylindrical soft magnetic material, which serves as part of the magnetic circuit and can reduce magnetic resistance. An armature is a hollow cup-shaped cylinder made of non-magnetic materials (such as plastic), directly installed on the motor shaft. A winding formed by arranging hollow cups along the circumferential axis and curing them with epoxy resin. The power supply is applied to the armature winding through brushes and diverters. The armature core length and diameter of a typical DC servo motor are larger than those of a regular DC motor, with the aim of reducing its flywheel torque and improving response speed.

 

In recent years, with the development of technology, new varieties of DC servo motors have emerged, such as brushless DC servo motors.

 

2. Working Principle Of Dc Servo Motor

 

The working principle of DC servo motors is also the same as that of ordinary small DC motors. For a separately excited DC servo motor, if an excitation current is applied to the excitation winding to establish a constant magnetic field, an electromagnetic torque will be generated when the armature winding passes through the current, causing the rotor to rotate. When one of the excitation winding or armature winding loses power, the motor immediately stops rotating. By changing the magnitude and direction of the excitation current, the speed and direction of the motor can be changed to meet the control requirements of the servo motor. When the load torque is constant, keeping the armature power supply voltage constant and controlling the motor speed by changing the excitation current is called magnetic field control; Keeping the excitation current constant and controlling the motor speed by changing the power supply voltage is called armature control. Due to the ideal characteristics and accuracy of the latter, DC servo motors generally adopt armature control, which uses armature voltage as the control signal voltage, while magnetic field control is only used for low-power motors.

 

The basic working principle of a DC servo motor is the same as that of a general DC motor. When the excitation winding is connected to a constant voltage, the armature winding that receives the control signal receives the control voltage signal. When the current flows through the armature winding, the magnetic flux generated by it interacts with the magnetic flux generated by the excitation winding to generate electromagnetic torque, causing the armature to rotate. By changing the size of the control voltage signal, the speed of the motor can be changed to achieve the goal of speed regulation.

 

When using servo DC servo motors, the following precautions should be taken:

 

① When using an electromagnetic armature controlled DC servo motor, the excitation power should be connected first, and then the armature voltage should be applied. During operation, it is advisable to avoid power outage of the excitation winding as much as possible to prevent excessive armature current and motor overspeed.

② When selecting different forms of armature control power supplies, it is important to leave appropriate margin in their capacity.