Servo Motor Construction, Working and Applications

Servo Motor AC or DC? Construction, Working and Applications
Servo Motor Construction, Working and Applications


A servo motor is a basic electrical device that is used to rotate or move the gadgets with great precision and accuracy at different angles and with different velocities. It is a closed-loop feedback-controlled system. The main feature that distinguishes it from other motors is its propensity to work accurately with distance and precise angles. It can rotate the object in both counterclockwise as well as in a clockwise direction with the same capability. Servo motors are usually rated in kg/cm while other motors are usually rated in KVA. It means how much weight does a motor can lift if the load is suspended at a specific distance from the shaft of is of great importance in industrial application where accurate movements are required.

There are practically two types of servo motor depending how it is powered

  • AC servo Motor
  • DC servo motor

AC servo motor is powered with an AC source while DC servo motor is powered with a DC source. AC servo motor has less efficiency, operates smoothly, delivers low power, small weight, and low maintenance is required. Whereas, DC servo motor has high efficiency, delivers high power, large weight, and requires time to time maintenance. AC motor finds its applications in high speed working.


Servo motor is conventionally constructed by using the ordinary motor, position sensors, gear system and a controlled circuit. The controller can be any microcontroller like Arduino, STM, TIVA, etc. While position sensor can be Encoder for AC servo motors used in Industries and it is a potentiometer for DC servo motors.

A DC servo motor is constructed by using DC motor which has armature and field winding coupled with gearbox, controller and Potentiometer, while AC servo motor is constructed using an Induction motor consisting of rotor and stator with gear system and Encoders.

Servo Motor Diagram


Servo motor is basically a closed loop feedback system. A closed loop feedback system controls the output of the system by varying input. The output is compared with a reference signal and error is generated; this error signal is the input signal to controller which generates a PWM according to the error. PWM is the input of the motor, the output of the motor is sensed by position sensor and this is compared with input and again error signal is generated This process continues until the error signal is zero, means there is no difference between output and referenced signal. The amount of rotation is determined by the duty cycle of the pulse. The figure below shows that if ON time of the pulse is less than a specific time period, it rotates below 90 degree and if it is greater, than it rotates till 180 degrees.

In DC servo motors the input signal is applied to dc motors which in turns rotates the shaft and gears, the rotation of gears is basically our output which is fed back to potentiometer whose knobs rotates and changes its resistance. As resistance is changed so voltage is varied which is error signal that is fed into controller and accordingly PWM is generated.

The DC motor has armature and field winding, one of the winding is provided with fixed voltage while the other winding is powered with varying error signal. Let’s assume we are using armature-controlled method in which torque depends upon the armature current which means error signal is provided to armature which controls the torque of the motor. 

In AC servomotors we have two parts rotor and stator. A stator has two windings reference winding and controlled winding which are displaced by 90 degrees. Fixed signal is applied at reference winding while a variable error signal is applied at the controlled winding.

As error signal is applied to the stator both the windings produce a flux which is at a phase difference with each other, so torque is produced, and rotor starts rotating. After that the output is fed back to encoder which sense the speed and accordingly send signal to microcontroller.


This tiny package finds its application in many industries. It is of great importance for many applications.

  • Servo motors are used widely in robotics. In every joint of the robot servo motor is installed so that precise rotation is obtained.
  • These are installed in cameras to adjust the focus for obtaining better quality images
  • These are used to track the direction of the sun in solar panels
  • Servo motors are used in the textile industries in knitting the fabrics
  • It is also used in aircraft for balancing.
  • Servo motors are also installed in automatic food packing machines
  • Servo motor finds itself in wood and metal cutting machine for shaping different devices
  • The movement of dish antennas in the direction of the satellite is also accomplished by using this tiny gadget
  • The toy cars also have servo motors installed in them to turn their wheels
  • Automated doors also work by using servo motors

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