Servo motor is basically a special purpose motor which is employed with control system. Servo motor operation is based on error signal given to it. The basic principle of servo motor is servo mechanism. Servo mechanism is basically a mechanism of control system in which output and input of system is compared and control variable is adjusted according to error signal. The size of servo motor varies from 0.05HP to 1000HP.
In the servo motor, The direction of torque is depends upon the polarity of control voltage given to it. With the change in polarity of control voltage, Direction of motor torque is also changes. The motor torque is directly proportional to applied control voltage. Thus, The control voltage which depends upon the error signal has an important role in proper working of servo motor.
Based on the mode of supply, The servo motors are divided into two types:
- AC Servo Motor
- DC Servo Motor
AC and DC Servomotors has their own advantages. AC servomotors are preferred because these motors are free from commutation problem for example Noise and wearing of brushes. AC servomotors are highly reliable because they do not have commutation problems. DC servomotors are preferred because these motors has high starting torque. These motors are perfect for application where high starting torque is required.
AC SERVO MOTOR
The servo motors which are employed in AC system are known as AC servo motors. These AC servo motors are further divided into two types:
- Two Phase AC Servo Motor
- Shaded Pole AC Servo Motor
1. Two Phase AC Servo Motor
The two phase AC servo motor consists of two stator windings. The two stator windings are placed at a 90° phase difference. One winding is known as Control winding and other one is known as Reference winding. The reference winding is connected with AC supply through a capacitor. The control winding is connected with AC amplifier. The error signal from the feedback control system is given to AC amplifier which amplifies the error signal and sent it to control winding. The squirrel cage rotor is used in two phase AC servo motor. At normal condition, The squirrel cage rotor remains in standstill position but when the error signal applied to control winding, Torque is produced due to interaction of magnetic field of control and reference winding. The direction of torque depends upon the polarity of error signal and the rotation of torque is in such a way that it reduces the error signal.
2. Shaded Pole AC Servo Motor
Shaded pole AC servo motor consists of Shaded pole distributed winding, Field winding and Phase sensitive relay. The shaded pole AC servo motor has squirrel cage rotor. The field winding of servo motor is connected with fixed AC supply and Phase sensitive relay is connected with the AC amplifier. AC amplifier is used to amplify the error signal which is given to it by feedback control system. Phase sensitive relay is used to actuate the contacts which short circuit the shaded pole distributed winding. The distributed winding is used to change the direction of rotor according to error signal. The shaded pole distributed winding is divided into two pairs. The phase sensitive relay actuate the contact and short circuit the one pair of distributed winding when error signal is applied to it through AC amplifier. When the polarity of error signal changes, The relay actuate the contacts to short circuit another pair of distributed winding.
There is one limitation in Shaded pole AC servo motor, which is that the motor will only operate when the amplified error signal has sufficient magnitude to actuate the contacts to short circuit the shaded pole distribution winding.
DC SERVO MOTOR
The DC motors which are employed in control system is known as DC Servo Motors. The DC servo motors are further divided into four types:
- Field Controlled DC Servo Motor
- Armature Controlled DC Servo Motor
- Series Split Field DC Servo Motors
- Permanent Magnet Armature Controlled DC Servo Motor
1. Field Controlled DC Servo Motor
Field controlled DC servo motor as the name suggests, is a type of servo motor in which field flux is controlled by error signal. The field winding is connected with DC amplifier and Armature is connected with constant current source. The error signal from feedback control system is given to DC amplifier which amplifies the signal and sent it to field winding. The torque produced by motor is directly proportional to field flux and field current. The direction of motor can be reversed by reversing the polarity of field.
2. Armature Controlled DC Servo Motor
In the armature controlled DC servo motor as the name suggests, Armature control method is used. The armature circuit is connected to error signal through DC amplifier. The DC amplifier is used to amplify the error signal. The field winding is connected with constant current DC source. The armature control method has advantages over field control method. The armature control method has quick response because the armature circuit is resistive as compared to highly inductive field circuit. When due to error signal, sudden change occur in armature voltage, it will cause changes in torque immediately with respect to armature voltage. The armature voltage control method is used up to 1000HP DC motor.
3. Series Split Field DC Servo Motors
Series split field DC servo motors consists of field winding which is divided into two portions. The field winding is equally divided into two windings which are equal in MMF. One winding is known as Main winding and other one is known as Auxiliary winding. Both main and auxiliary winding are connected to DC amplifier through a common point. Due to these connections, The torque produced by main winding and the torque produced by auxiliary winding is opposite in direction. The DC amplifier is connected with error signal and it amplifies the signal. The armature is connected with constant current DC source. The split field arrangement has improved dynamic response because field is always excited.
4. Permanent Magnet Armature Controlled DC Servo Motor
In this type of DC servo motor, Permanent magnets are used for constant field excitation. The armature is placed inside the magnetic field of circular rings of permanent magnet. The armature circuit is connected with DC amplifier. The error signal is given to DC amplifier by feedback closed loop control system which amplifies the error signal and sent it to armature circuit. The pole pieces in this motor are laminated to reduce the arcing at brushes. The hysteresis and eddy current losses are also negligible. The permanent magnet armature controlled DC servo motor is a popular type of servo motor and manufactured in different sizes.