Theory of DC Servomotors

Basics of Servomotors

Servomotors are the electromechanical energy conversion devices. In other words, a Servomotors is a rotary actuator or linear actuator which is used for precise control of linear or angular position, velocity and acceleration.

A servomotor consists of an electric motor coupled with a sensor for position feedback. Servomotors are used in feedback control systems as the output actuators. Hence, servomotors are also called as control motors.

Servomotors are not used for continuous energy conversion. The power ratings of servomotors vary from a fraction of a watt to a few hundred watts. The servomotors have low rotor inertia and hence they have a high speed of response.

Practically, the rotors of servomotors are designed with long rotor length and smaller diameters. A servomotor has larger size as compared to a conventional motor of same rating. The servomotors generally operate at very low speeds and sometimes zero speed.

Applications of Servomotors

The servomotors are widely used in −

• Radars and communication devices
• Computers
• Robots and other automation systems
• Machine tools
• In-line manufacturing
• Pharmaceutics and food services
• Remote controlled toy cars
• Tracking and guidance systems
• Process controllers, etc.

Types of Servomotors

There are two types of servomotors −

• DC servomotors
• AC servomotors – (two-phase and three-phase)

Theory of DC Servomotors

DC servomotors are either the separately excited DC motors or the permanent magnet DC motors. The schematic diagram of a separately excited DC servomotor is shown in Figure-1. The armature of a DC servomotor has a large resistance so that the torque-speed characteristics are linear and have a large negative slope, i.e., the torque decreases with increasing speed of the motor (see Figure-2).

The negative slope of torque-speed characteristic of the DC servomotor provides viscous damping for the servo-drive system. The speed of a DC servomotor is controlled by changing the armature voltage.

Also, the armature MMF and the excitation field MMF are in quadrature in a DC servomotor (see Figure-3). This provides a fast torque response of the DC servomotor since the torque and the flux become decoupled. Hence, a step change in the armature voltage or current produces a quick change in the position or speed of the rotor. The power rating of DC servomotors can vary from a few watts to several hundred watts.

Important – Most of the high-power servomotors used in industries are DC servomotors.