AC Servo motor – Working Principle, Circuit Diagram
Table of Contents
AC Servo Motor
The term “AC servo motor” refers to a particular class of servomotor that uses AC electrical input to generate mechanical output in the form of precise angular velocity. With some exceptions in terms of designing features, AC servomotors are essentially two-phase induction motors.
The output power of an ac servomotor can be anywhere from a few watts to several hundred watts. In contrast, the usable frequency range is 50 to 400 Hz. As here, the use of a particular type of encoder provides feedback regarding speed and position, it offers closed-loop control to the feedback system.
Servomotors have been discussed in earlier articles. Additionally, we have seen that servomotors can be broadly divided into two types: ac and dc servomotors. As rotary actuators intended to translate electrical input into mechanical acceleration, servomotors are well known to us. It uses a servomechanism, in which the motor’s speed and final position are controlled by position feedback.
Basically, the motor rotates as a result of the applied electrical input, obtaining a certain angle, and then returning to the input where it is compared to see if the achieved position is what was desired or not. In this manner, the precise position is acquired.
Construction of AC Servo Motor
An ac servomotor is regarded as a two-phase induction motor, as we already stated at the beginning. However, ac servomotors have some unique design elements that are absent from standard induction motors; as a result, it is said that the two are built somewhat differently.
It is mainly composed of two major units, stator and rotor.
Stator: First have a look at the figure shown below, representing stator of ac servomotor:
The stator of an ac servo motor is made up of two distinct windings that are evenly spaced apart at a 90° angle. The main or fixed winding is one of the two windings, and the control winding is the other. The stator’s main winding receives a constant ac signal as input. The control winding, however, is given the variable control voltage, as the name implies. The servo amplifier provides this variable control voltage.
It is to be noted here that to have a rotating magnetic field, the voltage applied to the control winding must be 90° out of phase w.r.t the input ac voltage.
Rotor: The rotor is generally of two types; one is squirrel cage type while the other is drag cup type.
The squirrel cage type of rotor is shown below:
This type of rotor is made with aluminum conductors, has a long length and a small diameter, and weighs less. A normal induction motor’s torque-speed characteristics have both positive and negative slope regions, which correspond to unstable and stable regions, respectively. However, because ac servo motors are made to be highly stable, a positive slip region cannot exist in their torque-slip characteristics. Additionally, the motor’s developed torque must linearly decrease with speed.
The rotor circuit resistance should be high and have little inertia in order to accomplish this. This is why it is important to maintain a smaller diameter to length ratio when building the rotor. The squirrel cage motor’s smaller air gaps between its aluminum bars make it possible to use less magnetizing current.
Here is an illustration of a drag cup type rotor: The construction of this type of rotor differs from the squirrel cage type. It consists of a drag cup surrounded by an aluminum laminated core with small air gaps on either side. A driving shaft that is attached to these drag cups makes operation easier. In applications where there is a low power requirement, the two air gaps on either side of the core reduce inertia.
Working Principle of AC Servo Motor
The figure below represents the AC two-phase induction motor that uses the principle of servomechanism:
At first, the main winding of the stator of the ac servomotor receives a constant ac voltage. Through the control winding, the other stator terminal of the servomotor is linked to the control transformer. The shaft of the synchro generator rotates at a specific speed and assumes a specific angular position as a result of the reference voltage being offered. Additionally, the shaft of the control transformer is angled differently from the shaft of the synchro generator by a specific amount. The error signal is further provided by the comparison of two angular positions. The error signal is produced by comparing the voltage levels of the corresponding shaft positions, to be more precise.
The voltage level at the control transformer and this error signal are the same. The servo amplifier receives this signal after which it produces variable control voltage. With this applied voltage, the rotor once more reaches a certain speed, begins to rotate, and continues to do so until the error signal value decreases to zero, causing the motor in the AC servomotors to move to the desired position.
The figure below represents the torque-speed characteristics of the two-phase induction motor:
We have already talked about how the motor needs to be built to have linear torque-speed characteristics, or torque that changes linearly with speed. However, as we can see in the illustration above, the torque-speed characteristics in this case are not linear. This is true because it is based on the reactance to resistance ratio. When the ratio of reactance to resistance is low, it means that the motor has high resistance and low reactance; in this case, the characteristics are more linear than when the ratio is high.
- These are low weight devices.
- There is not much noise generated at the time of operation.
- It offers reliability as well as stability in operation.
- As brushes and slip rings are not present here thus it reduces maintenance cost.
- It offers almost linear torque-speed characteristics.
Applications of AC Servo Motor
Due to the many benefits offered by AC servomotors,. These are primarily used in computers, position-controlling devices, and instruments that use servomechanism. Additionally, these are used in robotics machinery, machine tools, and tracking systems.