AC ServomotorsYogesh Kumar

HOD,EE.

Overview Servo motors are used for angular

positioning, such as in radio control airplanes.

The output shaft of a servo does not rotate freely, but rather is made to seek a particular angular position under electronic control.

The basic principle of operation is the same as that of other electromagnetic motors.

Types of servo motor

AC servo Motor These are used in low power servo mechanism. It is a two phase induction motor. power varies from fraction watt to few

hundred watts. Operating frequency is 50 Hz to 400 Hz.

Construction (Stator)

The stator carries two winding , uniformly distributed and displaced by 90 degree.

One winding is called main winding or fixed winding or reference winding.

this winding is excited by a constant voltage a.c. supply.

The other winding is control winding which is excited by variable control voltage.

this voltage is 90 degree out of phase with respect to the reference winding.

Rotor (Squirrel Cage )

This has small diameter and large length. Aluminium conductors are used to keep weight small. Its resistance is very high to keep torque speed

characteristics as linear as possible.

Drag Cup Type Rotor

There are two air gap in such a construction which reduce inertia.

These are used in low power application. Aluminum is used for cap construction.

Torque speed characteristics

Features and Application Light in weight. Robust construction Reliable and stable operation. Smooth and noise free operation. large torque to weight ratio. Large R/X Ratio. No brushes or slip rings hence maintenance free.

Use1. Servomechanism2. Remote positioning device3. Tracking system.

T.F. of AC Servo Motor

From torque speed characteristic

If To is stalling torque and Wo is no load speed.

T.F. of servo motor

Block diagram

Block Diagram