AC,DC servo motor and stepper motor

May 14 • General • 7042 Views • 2 Comments on AC,DC servo motor and stepper motor


A servo motor is defined as an automatic device that uses an error correction routine to correct its motion. It is typically paired with some type of encoder to provide positioning and speed feedback. A servo motor can be ac, dc or brush less dc motor. It is typically the motor which is selected when it is essential that there is a high degree of probability that it will closely track what is asked for it. It consists of motor which is coupled to the sensor for position feedback through a reduction gear box.



The servo motor is used in the feedback control system. It has low rotor inertia and high speed of response. The servo motor is also known as control motor.

Types of servo motor

1.AC servo motor

2. DC servo motor

AC servo motor

These motors have two parts namely stator and rotor. The ac servo motors are the 2 phase induction motor.

The stator has two distributed winding which are displaced from each other by 900. One winding is called as main winding which is excited by constant ac voltage. The other winding called the controlled winding is excited by variable control voltage of same frequency as the main winding but having a phase displacement of 900 electrical. The direction of rotation depends upon the phase relationship of voltages applied to the two windings. The direction of the rotation of the rotor can be reversed by reversing the phase difference between the control voltage and the main voltage.

The rotor of ac servo motor is of two type

a. Squirrel cage rotor

b. Drag cup type rotor  



The  Squirrel cage rotor have large length and small diameter so its resistance is very high. Its air gap is kept small. The resistance of drag type rotor is very high and hence have high starting torque. Generally a cup of non magnetic conducting material is used and stationary core is placed between the conducting cup to complete the magnetic circuit.

 DC servo motor 



The dc servo motors are separately excited or permanent magnet dc servo motors. The armature of dc servo motor has large resistance therefore its torque speed characteristics is linear. The dc sevo motors can be controlled from the armature side or wheel side. in the field controlled dc servo motor L/R ratio is large i.e time constant for the field circuit is large. Due to which the response is low and hence they are not commonly used. The speed of motor can be controlled by adjusting the voltage applied to the armature. In the armature controlled type the time constant is small and hence the response is fast. The efficiency of the armature controlled dc servomotor is better than field controlled motor.

Stepper motor

A stepper motor is electromechanical device. It is brushless, synchronous motor that converts the digital pulses into mechanical shaft rotation. In stepper motors the movement of rotor is in decrease step. Each every revolution of the stepper motor is divided into discrete number of step and the motor is sent a discrete pulse for each step.

There are three type of stepper motor



1. Variable reluctance stepper motors

2. Permanent magnet stepper motors

3. Hybrid type

.Permanent magnet stepper motors

components of the PM type servomotor

components of the PM type steppermotor

The permanent magnet type of the stepper motor  is basically operated on the reaction between the electromagnetic fiela and the rotor of the permanent magnet type.The rotor is mounted at each end such that it has a permanent magnet .The faces of the stator pole and the teeth on the rotor surface are offset such that only limited number of the rotor teeth are aligned to the energized stator pole.The step angle is determined by the number of teeth of the on the rotor and the stator that occurs each time the polarity is reversed.

When the permanent magnet type of the motor is supplied with the dc signal in one of its stator winding,such that the rotor overcomes the residual torque and with the stator field it lines up.. The holding torque  is defined as the maximum static torque that can be applied to the shaft of an excited motor without causing a continuous rotation

. Variable reluctance stepper motors

VR type of servomotor

VR type of stepper motor

The VR stepper motor has no permanent magnet as well as no residual torque to hold the motor when it is turned off at one position when it is turned off.,in this way it differs from the permanent magnet type stepper motorThe rotor teeth aligns with the stator poles when the stator is energized..The stator field changes by changing the windings that are energized in the rotor,such that the rotor moves to the new position

Hybrid type stepper motor

hybrid type of stepper motor

hybrid type of stepper motor

The hybrid type of the stepper motorconsists of the soft iron which are in the two pieces and permanent type of the rotor which is axially magnetized. The motor is operated on the principle of the bothe permanent magnet and the variable reluctance tpe of the stepper motorThe stator is similar to the VR type it differs in the way such that only one of the two coils of the phase are wound on single pole. The two of the coils at one pole are wound in the particular configuration such that known as bifilar connection.every teeth of the hybrid type is uniformly spaced.It is to be noted that the teeth on the two sections of every poles are misaligned by half tooth pitch.The torque is developed by the interaction of the magnetic field of the permanent magnet and the stator produced magnetic field.

Importance of stepper motor

1. Excellent response to starting or reversing or stopping.

2. A quite wide range of the rotational speed can be realized as the speed is directly proportional to frequency of the input.

3. The motor has full torque at standstill condition.

Related question and answer

1. what is servomotor?

Ans: Servo motor is defined as an automatic device that uses an error correction routine to correct its motion.

2Differentiate between ac and dc servo motor.

Ans: The ac servomotor has low power output, efficiency is less, there are no brushes and slip rings hence it is maintenance free, no radio frequency noise and smooth operation.

For dc servomotor, it has high power output, high efficiency, frequent maintenance required, brushes produce radio frequency noise, noisy operation.

3 For the feedback control system what properties servomotor should have?

Ans: The properties are

2. Response should be fast.

3. Relationship between the electrical control system and rotor speed should be linear.

4. It should have linear torque speed characteristics.

4 Define holding torque of the stepper motor.

Ans: It is defined as the maximum static torque that can be applied to the shaft of an excited motor without causing a continuous rotation.

5 What is the usage of stepper motor?

Ans: A stepper motors are used in computer peripheral system such as printer, tape drive, process control system, robotics control system, watches etc.

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2 Responses to AC,DC servo motor and stepper motor

  1. Rachita Mishra says:

    This article is about Ac and DC servomotor that is a rotary actuator that allows for precise control of angular position.It consists of a motor coupled to a sensor for position feedback, through a reduction gearbox. It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servomotors.

  2. Shilpa Ranjan says:

    Motor is used to drive any machine which is consists of stator & rotor. Focused on the AC , DC & Stepper Motor. This post explained the concept of motor in detail.Going through the post can be helpful !!

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