Servomotors:

Before starting the concept of the servomotors firstly we need to know about the concepts related to it and the ease and uses of such motors in the real time so that we can better understand the concept.

This is the very common that now a days all things are getting changed as the days are passing but at the same time technology and existing things will also change as per the modern era so this is applicable in case of servomotors as well so here will discuss the same.

Servomotors plays an important role in automated control system where some comparison should be done with some reference so some errors will be amplified and will be reused to drive the Motors is known an servomotors; hence the signal error will be converted to some angular velocity in order to correct the error

Servomotors are not used to drive the normal loads which are normally driven by the other types of motors but they can be useful in positioning of the radio applications and military applications to correct the position and the velocity so depending on the supply servomotors are classified in to

1. AC servo motors

2. DC servomotors

  • AC Servomotors:

Ø It is basically an induction motor with two types of windings coupled on the stator.

Ø 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.

Ø 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.

Ø Construction of an AC servomotors is as shown below:

Ø 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.

Ø In this type of motors air gap is kept at its minimum in order to get the maximum flux linking.

Ø 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.

Ø These types of motors will be used in the frequency range of the 50 Hz to 400 Hz and from milli watts power consumption to the few hundred watts.

  • Torque speed characteristics:

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

Ø We can see the effect of the rotor resistance on the linearity of the characteristics

Ø We can improve the linearity of such type of motors with increase in the rotor resistance.

Ø 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.

Ø 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.

Ø When control voltage will be increased, motor starts rotating at higher speed for same torque (N3>N2>N1).

Ø 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

  • Applications:
  • Robotics
  • Process control equipment’s
  • Machine tools
  • Process controllers
  • AC positions and applications
  • Instrument servos
  • Portable drilling machines
  • Sewing machines
  • DC Servomotors:

DC servomotors are further classified in two more categories:

1. Field Controlled DC servomotor:

Ø The construction of field Controlled DC motor is shown below:

Ø We are getting and signal from servo amplifier and it is applied to the field winding

Ø In this type of motor armature is connected to the constant current source

Ø The field current will be controlled by the voltage from servo amplifier and this will control the flus produced in the motor.

Ø The voltage produced in the servo amplifier will be in proportional to the error signal produced in the control system in which the servomotor is being used this amplified error signal obtained from servo amplifier is used for controlling the field current.

Ø The response of DC servomotors to any change in the servo amplifier output is explained below:

ü As the error will change, the voltage from servo amplifier will also change

ü Due to this filed current will also change

ü This will change the flus

ü And then motor will respond to the change in flux Φ so as to minimize the error.

  • Disadvantages of field controlled DC servo motors:

Ø The time Constant Lf, Rf is large where Lf is the inductance of the field winding and Rf is its resistance.

Ø Hence the field current will also change slower in response to the any change in Vf.

Ø And due to the motor will respond slowly to any error produced in the system in which it is being used.

Ø This drawback can be overcome by using the armature controlled servo motors.

2. Armature controlled servo motors:

Ø The connections of such type motors are shown below:

Ø In this constant current source is applied to the field winding hence If remains constant.

Ø However the servo amplifier output is connected to the armature so that the armature current is dependant om the error produced in the system in which the servo motor is being used

Ø The response of the armature controlled DC servo motor to any change in the servo amplifier output is explained as:

ü As current changes, The Voltage Va from servo amplifier also change

ü This will change the armature current

ü So torque produced by the motor changes

ü The motor will respond to change in torque in such a way that the error is minimized.

  • Advantages of armature controlled DC motor:

Ø The time constable La/ Ra is small hence motor responds quickly as compared to the field controlled DC servo motor.

Ø Hence this type of motors are practically preferred to the controlled field type motors.

  • Torque speed characteristics of DC servo motor:

Ø The Torque speed characteristics of DC servo motors are shown below:

Ø Its shows that these are linear in nature and shows the effect of the armature voltage Va.

Ø Higher torques for the same speed and higher speed for same torque are obtained as the armature voltage is increased.

  • Applications of DC Servomotors:

1. Position control system

2. Process Controllers

3. Military applications

4. Air craft control systems

5. Servo stabilizers

6. Robotics.