What is an AC Servo Motor?

The term "AC servo motor" refers to a particular kind of servo motor that employs AC electrical input to generate mechanical output in the form of accurate angular velocity. With certain exceptions in terms of engineering features, AC servomotors are essentially two-phase induction motors.

The output power of an ac servo motor might be anything from a few watts to several hundred watts. In contrast, the usable frequency range is 50 to 400 Hz. As above, the usage of a certain sort of encoder delivers input about speed and position, it offers closed-loop control to the feedback system.

Construction of an AC Servo Motor

An ac servo motor is considered as a two-phase induction motor, as we just stated at the beginning. However, ac servomotors have some unique design elements that are absent in standard induction motors; as a result, it is believed that the two are built substantially differently.

The stator and rotor are its two key structural components.

• Stator: To begin, look at the following illustration of an ac servomotor's stator:

Two distinct windings that are evenly spaced apart and at 90� angles make up the stator of an ac servo motor. The term "main" or "fixed" winding refers to one of the two windings, while the term "control" refers to the other.

The primary winding of the stator receives an input that is a continuous ac signal. although the control winding receives variable control voltage, as the name indicates. The servo amplifier is used to produce this changing control voltage.

It should be noted that the voltage provided to the control winding must be 90 degrees out of phase with the input ac voltage in order to create a revolving magnetic field.

• Rotor: There are typically two sorts of rotors; one is a drag cup type and the other is a squirrel cage type.

Below is an illustration of a rotor with a squirrel cage:

This sort of rotor has a long, narrow diameter and is made of aluminum conductors, which makes it lighter in weight.

It should be noted that there are regions with both positive and negative slopes in the torque-speed characteristics of a conventional induction motor, which correspond to unstable and stable zones, respectively.

However, because ac servo motors are made to be highly stable, a positive slip zone cannot exist in their torque-slip characteristics. Additionally, the motor's produced torque must linearly decrease with speed.

The rotor circuit resistance should be high and have little inertia in order to do this. This is why it is important to have a reduced diameter to length equal while 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 style rotor:

The construction of this type of rotor differs from the squirrel cage type. It comprises a drag cup surrounded by an aluminum laminated core with small air holes on either side.

A drive shaft that is linked to these drag cups makes operating easier.

In applications where there is a minimal power need, the two air gaps on either side of the core reduce inertia.

Two Phase AC Servo Motor

Servo motors are available for two- and three-phase AC systems. The two dispersed windings in the stator of the two-phase AC servo motor are electrically 90 degrees apart from one another. One winding, referred to as the Reference or Fixed Phase, receives power from a source of constant voltage. The other one is called Control Phase, and it comes with a variable voltage supply.

Below is a graphic showing how the two-phase AC servo motor is connected:

A servo amplifier is often used to supply the control phase. Phase difference between control voltage and reference phase voltage determines how fast and how much torque the rotor spins at. The phase difference can be switched from leading to lagging or vice versa to change the direction of the rotor's spinning.

The graphic below depicts the torque-speed characteristic of a two-phase AC servo motor.

The motor receives positive damping from the negative slope, which indicates a high rotor resistance and improves stability. For practically all control voltages, the curve is linear. Based on information as the Drag Cup Servo motor, as illustrated in the image below, the weight and inertia of the motor are reduced to increase the motor's reaction to a light control input.

A thin cup of conductive non-magnetic substance serves as the rotor of the Drag cup servo motor. The conducting cup has a central iron core that is fixed in situ. The magnetic circuit is finished with this setup. Due to the thin material used to make the motor's rotor, it will have a high resistance, which causes a high starting torque.

Three Phase AC Servo motors

For usage as servo motors in high power servo systems, three-phase induction motors with voltage control are employed. Among linked circuit devices, a three-phase squirrel cage induction motor exhibits considerable nonlinearity. By employing a control technique called vector control or field -oriented control, it is employed as a linear decoupled machine.

In this kind of machine, the current is adjusted such that the torque and flux are separated. Due to the decoupling, there is a rapid increase in speed and torque.

Features

1. These gadgets are light in weight.
2. It provides both operational stability and dependability.
3. At the time of operation, hardly much noise is made.
4. It has torque-speed characteristics that are virtually linear.
5. The expense of maintenance is decreased because there aren't any brushes or slide rings here.

Use of AC Servomotors

Applications for AC servomotors include the following:

Due to the many benefits afforded by AC servomotors, these are primarily used in computers, position-controlling devices, and instruments that use servomechanism. Additionally, they are used in robotics machinery, machine tools, and tracking systems.