AC Servomotors:

It is basically an induction motor with two types of windings coupled on the stator. This two windings are connected with the two voltage sources in the 90 degree phase difference with the same and due to this phase difference some rotating magnetic field will be produced which were producing like induction motors. First winding is known as reference winding or main winding which is connected to the constant AC supply voltage and other winding is connected to the voltage obtained from servomotors. Construction of an AC servomotors is as shown below:

The rotor used in this type of motors would be like cage type rotor with some aluminium bars imposed on in the slots and short circuited by the end rings. In this type of motors air gap is kept at its minimum in order to get the maximum flux linking. A drag Cup is used in some other types of rotor due to that inertia of the rotating system becomes will become low and this will help to reduce the power consumption. These types of motors will be used in the frequency range of the 50 Hz to 400 Hz and from milliwatts power consumption to the few hundred watts.

Torque speed characteristics:

The torque and speed characteristics of servomotors should be linear in nature this is the basic requirement and which are shown below

We can see the effect of the rotor resistance on the linearity of the characteristics. We can improve the linearity of such type of motors with increase in the rotor resistance. The conclusion is that characteristics will become more linear in nature as we are increasing the rotor resistance when we are comparing it with the reactance. Let’s have a look on the effect of the control voltage on the torque speed characteristics and control voltage is the voltage which is applied to the control winding of the motor. When control voltage will be increased, motor starts rotating at higher speed for same torque (N3>N2>N1). If control voltage is increased in equal steps of then characteristics will be parallel with each other as shown below:

Advantages of AC servomotors:

1) Maintenance is not required as there is no use of sliprings and brushes.

2) Light weight in design

3) Simple in construction

4) Can be operated in smoother and quite

5) Controllers used in it are simple

6) Reliable in operation due to linear torque and speed characteristics


1) Robotics

2) Process control equipment’s

3) Machine tools

4) Process controllers

5) AC positions and applications

6) Instrument servos

7) Portable drilling machines

8) Sewing machines