AC Servo Motor : Construction, Working, Transfer function & Its Applications

A servomotor acts like a rotary actuator that is mainly used to change electrical input into mechanical acceleration. This motor works based on servomechanism wherever the position feedback is utilized for controlling the speed & the final location of the motor. Servo motors turn & get a certain angle based on the applied input. Servo motors are small in size but they are very energy efficient. These motors are classified into two types like ac servomotor & dc servomotor but the main difference between these two motors is the source of power used. The performance of a DC servo motor mainly depends on voltage only whereas an AC servo motor depends on both the voltage & frequency. This article discusses one of the types of servo motors – an AC servo motor – working with applications.

What is AC Servo Motor?

A type of servomotor that generates mechanical output by using AC electrical input in the precise angular velocity form is called an AC servo motor. The output power obtained from this servomotor mainly ranges from watts to a few 100 watts. The operating frequency of ac servo motor ranges from 50 to 400 Hz. The AC servo motor diagram is shown below.

AC Servo Motor
            AC Servo Motor

The main features of ac servo motors mainly include; these are fewer weight devices, providing stability and reliability within the operation, noise is not generated while operating, providing linear torque-speed characteristics, and decreased maintenance costs when slip rings & brushes are not present.

Please refer to this link to know more about AC Servo Motor Types

AC Servo Motor Construction

Generally, an AC servo motor is a two-phase induction motor. This motor is constructed by using a stator and a rotor like a normal induction motor. Generally, the stator of this servo motor has a laminated structure. This stator includes two windings that are placed 90 degrees apart in space. Because of this phase variation, a rotary magnetic field is generated.

AC Servo Motor Construction
AC Servo Motor Construction

The first winding is known as the main winding or also known as fixed phase or reference winding. Here, the main winding is activated from the constant voltage supply source whereas the other winding like the control winding or control phase is activated by the variable control voltage. This control voltage is simply supplied from a servo amplifier.


Generally, the rotor is available in two types squirrel cage type & drag cup type. The rotor used in this motor is a normal cage-type rotor including aluminum bars fixed in slots & short-circuited through end rings. The air gap is kept minimum for maximum flux linking. The other type of rotor like a drag cup is mainly used where the inertia of the rotating system turns low. So this helps in decreasing power consumption.

Working Principle of AC Servomotor

The working principle of the ac servo motor is; firstly, a constant ac voltage is given at the startor’s main winding of the servomotor and another stator terminal is connected simply to the control transformer throughout the control winding. Because of the applied reference voltage, the synchronous generator’s shaft will rotate at a specific speed & obtains a certain angular position.

AC Servo Motor Circuit
AC Servo Motor Circuit

In addition, the control transformer’s shaft has a specific angular position that is compared to the angular point of the synchro generator’s shaft. So the two angular positions comparison will provide the error signal. More particularly, the levels of voltage for the equivalent shaft positions are evaluated which produces the error signal. So this error signal communicates with the present voltage level at the control transformer. After that, this signal is given to the servo amplifier so that it generates uneven control voltage.

By this applied voltage, again the rotor achieves a specific speed, starts revolution & maintains until the error signal value reaches zero so attaining the preferred position of the motor within the AC servomotors.

Transfer Function of AC Servo Motor

The transfer function of the ac servo motor can be defined as the ratio of the L.T (Laplace Transform) of the output variable to the L.T (Laplace Transform) of the input variable. So it is the mathematical model that expresses the differential equation that tells the o/p to i/p of the system.

If the T.F. (transfer function) of any system is known, then the output response can be calculated for different types of inputs to recognize the system’s nature. Similarly, if the transfer function (T.F) is not known, then it may be experimentally found by simply applying known inputs to the device & studying the output of the system.

AC servo motor is a two-phase induction motor which means it has two windings like control winding (main field winding) and reference winding (exhilarating winding).

AC Servo Motor for Transfer Function
  AC Servo Motor for Transfer Function

So we need to find out the transfer function of ac servo motor i.e, θ(s)/ec(s). Here ‘θ(s)/’ is the output of the system whereas the ex(s) is the input of the system.

In order to find out the transfer function of the motor, we need to find out the what is torque developed by the motor ‘Tm’ and the torque developed by the load ‘Tl’. If we equate the equilibrium condition like

Tm = Tl, then we can get the transfer function.

Let, Tm = torque developed by the motor.
Tl = torque developed by the load or load torque.
‘θ’ = angular displacement.
‘ω’ = d θ/dt = angular velocity.
‘J’ = moment of inertia of the load.
‘B’ is the dashpot of the load.

Here the two constants to be  considered are K1 & K2.

‘K1’ is the slope of control phase voltage vs torque characteristics.
‘K2’ is the slope of speed torque characteristics.

Here, the torque developed by the motor is simply denoted by

Tm = K1ec- K2 dθ/dt —–(1)

The load torque (TL) can be modeled by considering the torque balance equation.

Applied torque = opposing torque due to J,B

Tl = TJ + TB = J d^2θ/dt^2 + B dθ/dt^2 + B —–(2)

We know that the equilibrium condition Tm = Tl.

K1ec- K2 dθ/dt = J d^2θ/dt^2 + B dθ/dt^2 + B

Apply Laplace transform equation to the above equation

K1Ec(s) – K2 S θ(S) = J S^2θ (S) + B S θ(S)

K1Ec(s) = JS^2θ (S) + BSθ(S)+ K2S θ(S)
K1Ec(s) = θ (S)[J S^2 + BS + K2S]

T.F = θ (S)Ec(s) = K1/ J S^2 + BS + K2S

= K1/ S [B + JS + K2]

= K1/ S [B + K2 + JS]

= K1/ S (B + K2) [1 + (J/ B + K2) *S]

T.F = θ (S)Ec(s) = K1/(B + K2) / S[1 + (J/ B + K2) *S]

T.F = Km / S[1 + (J/ B + K2) *S] => Km / S(1 + STm)] = θ (S)Ec(s)

T.F = Km / S(1 + STm)] = θ (S)Ec(s)

Where, Km = K1/ B + K2 = motor gain constant.

Tm = J/ B + K2 = motor time constant.

AC Servo Motor Speed Control Methods

Generally, servo motors have three control methods like position control, torque control & speed control.

The position control method is used to determine the size of the rotating speed throughout external input frequency signals. The angle of revolution is determined by the no. of pulses. The position and velocity of a servo motor can be directly assigned through communication. As the position of the method can have extremely strict control over the position & speed then it is normally used within the positioning application.

In the torque control method, the output torque of the servo motor is set by analog input at the address. It can change the torque by simply changing the analog in real time. In addition, it can also change the value at the relative address through communication.

In speed control mode, the motor speed can be controlled by analog input & pulse. If there are precision requirements & no concern about so much torque then speed mode is better.

Characteristics of AC Servo Motor

The torque speed characteristics of an ac servo motor are shown below. In the following characteristics, the torque is changing with the speed but not linearly because it mainly depends on the ratio of reactance (X) to resistance (R). The low value of this ratio involves that the motor has high resistance & low reactance, in such cases, the motor characteristics are more linear than the high ratio value for reactance (X) to resistance (R).

Torque Speed Characteristics
Torque Speed Characteristics


The advantages of AC servo motors include the following.

  • The speed control characteristics of this motor are good.
  • They generate less amount of heat.
  • They offer high efficiency, more torque per weight, reliability & reduced RF noise.
  • They need less maintenance.
  • They have a longer life expectancy in the nonexistence of a commutator.
  • These motors are capable of handling higher current surges in industrial machinery.
  • At high speeds, they offer more constant torque.
  • These are highly reliable.
  • They provide high-speed performance.
  • These are well-suitable to unstable load applications.

The disadvantages of AC servo motors include the following.

  • AC servo motor control is more difficult.
  • These motors can be broken by constant overload.
  • Gearboxes are frequently necessary to transmit power at high speeds.


The applications of AC servo motors include the following.

  • AC servo motors are applicable where position regulation is significant & usually found in semiconductor devices, robots, aircraft & machine tools.
  • These motors are used in the instruments which operate on servomechanism like in computers & position control devices.
  • AC servo motor is used in machine tools, robotics machinery & tracking systems.
  • These servo motors are used in a variety of industries because of their efficiency & versatility.
  • The AC servo motor is used in most common machines & appliances like water heaters, ovens, pumps, Off-road vehicles, equipment in gardens, etc.
  • Many of the appliances & tools that are used every day around the house are power-driven by AC servo motors.

Thus, this is an overview of ac servo motors – working with applications. These motors are used in many applications like instruments that operate on servomechanism and also machine tools, tracking systems & robotics. Here is a question for you, what is an induction motor?