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. servomotor 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. 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 magnetizing current. This cage type of rotor is shown with skewed bars in the Fig. 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. 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 non-linear for large X to R ratio as shown in the Fig.3.
In practice, design of the motor is so as to get almost linear torque-speed characteristics. The Fig. 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. Servomotors
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.
6. Transfer Function of A.C. Servomotors
The various approximations to derive transfer function are,
i) A servomotor rarely operates at high speeds. Hence for a given value of control voltage, T α N characteristics are perfectly linear.
ii) In order that Tα N characteristics are directly proportional to voltage applied to its control phase, we assume T α N characteristics are straight lines and equally spaced.
Torque at any speed 'N' is,
where dθm/dt is speed of motor.
If load consists inertia Im and friction Bm we can write,
Now Laplace transform of equation (1) is
Equating equations (2) and (3)
Single flow graph of a.c. servomotor is as shown in the Fig. 5.
Hence block diagram of a.c. servomotor is
Note: As slope is negative, in the above equation [ Bm-m] shows that total friction increases due to m. As it adds more friction, the damping improves, improving stability of the motor. This is called internal electric damping of 2 ph a.c. servomotor.