Control System Components Part I – AC Servo Motor

The servosystem is one in which the output is some mechanical variable like position, velocity or acceleration. Such systems are generally automatic controls system which work on the error signals. The error singals are amplified to drive the motors used in such systems. These motors used in servosystems are called as servomotors. These motors are usually coupled to the output shaft i.e. load through gear train for power matching.

These motors are used to convert electrical signal applied, into the angular velocity or movement of shaft.

Requirements of Good Servomotor:

The servomotors which are designed for use in feed back control systems must have following requirements:

i)  Linear relationship between electrical control signal and the rotor speed over a wide range.
ii) Inertia of rotor should be as low as possible. A servomotor must stop running without any time delay, if control signal to it is removed. For low inertia, it is designed with large length to diameter ratio, for rotors. Compared to its frame size, the rotor of a servomotor has very small diameter.
iii) It response should be as fast as possible. For quickly changing error signals, it must react with good response. This is achieved by keeping torque to weight ratio high.
iv)  It should be easily reversible.
v)  It should have linear torque-speed characteristics.
vi) Its operation should be stable without any oscillations or overshoots.

Types of Servomotors:

The servomotors are basically classified depending upon the nature of the electric supply to be used for its opearation.

The types of servomotors are as shown in the following chart:

servo motors_1

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. servomotor varies from fraction of watt to few hundred watts. The operating frequency is 50 Hz to 400 Hz.

Construction:

ac servo motorIt 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 as 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 form 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 14.1.

Rotor

The rotor is generally of two types. The one is usual squirrel cage rotor. This has small diameter and large length. Aluminiu conductors are used to keep weight small. Its resistance is very high to keep torque speed charaterstics 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. 14.2(a). The other types 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 application.

Torque-Speed Characteristics:

The torque-speed characteristics of a two phase induction motor, mainly depends on the ratio of reatance to resistance. For small X to R ratio i,e. high resistance low reactance motor the characteristics is much more while it is nonlinear for large X to R as shown in the Fig.14.3

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In pratice, design of the motor is so as to get almost linear torque speed characteristcs. Fig. 14.4 shows the torque-speed characteristics for various control equally spaced for equal increments of contro voltage. It is generally operated with low speeds.

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