A.C. Servomotor

A.C. Servomotor

Most of the servomotors used in low power servomechanisms are a.c. servomotors. The a.c. servomotor is basically two phase induction motor. The output power of a.c. servomotors varies from fraction of watt to few hundred watts. The operating frequency is 50 Hz to 400 Hz.

1. Construction

It is mainly divided into two parts namely stator and rotor.

The stator carries two windings, uniformly distributed and displaced by 90°, in space. One winding is called main winding or fixed winding or reference winding. This is excited by a constant voltage a.c. supply. The other winding is called control winding. 

It is excited by variable control voltage, which is obtained from a servoamplifier. This voltage is 90° out of phase with respect to the voltage applied to the reference winding. This is necessary to obtain rotating magnetic field. The schematic stator is shown in the Fig. 9.8.1.

2. Rotor

The rotor is generally of two types. The one is usual squirrel cage rotor. 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. Air gap is kept very small which reduces magnetising current. This cage type of rotor is shown with skewed bars in the Fig. 9.8.2 (a). The other type of rotor is drag cup type. There are two air gaps in such construction. Such a construction reduces inertia considerably and hence such type of rotor is used in very low power applications. The aluminium is used for the cup construction. The construction is shown in the Fig. 9.8.2 (b).

3. Torque-Speed Characteristics

The torque-speed characteristics of a two phase induction motor, mainly depends on the ratio of reactance to resistance. For small X to R ratio i.e. high resistance low reactance motor, the characteristics is much more linear while it is nonlinear for large X to R ratio as shown in the Fig. 9.8.3.

In practice, design of the motor is so as to get almost linear torque-speed characteristics. The Fig. 9.8.4 shows the torque-speed characteristics for various control voltages. The torque varies almost linearly with speed. All the characteristics are equally spaced for equal increments of control voltage. It is generally operated with low speeds. 

4. Features of A.C. Servomotor

The a.c. servomotor has following features :

i) Light in weight

ii) Robust construction

iii) Reliable and stable operation

iv) Smooth and noise free operation

v) Large torque to weight ratio

vi) Large R to X ratio i.e. small X to R ratio

vii) No brushes or slip rings hence maintenance free

viii) Simple driving circuits.

5. Applications

Due to the above features it is widely used in instrument servomechanisms, remote positioning devices, process control systems, self balancing recorders, computers, tracking and guidance systems, robotics, machine tools etc.

Review Questions

1. Write a note on a.c. servomotors.

2. Explain the construction, working, characteristics and applications of a.c. servomotor.