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User Manual & Technical Reference Transfer Function of Two Phase Ac Servo Motor


Two phase a.c. servomotor is one of the very important electromechanical actuators having applications in the area of control systems .The study of its operating principle and features form a part of the first course on automatic control systems in electrical engineering curriculum. It’s small size, low inertia and almost noise and frictionless operation makes the a.c. servomotor particularly attractive in aircraft and spacecraft applications. The characteristics of an a.c. motor are usually non-linear. To simplify the analysis a linearism model is developed. The experimental work revolves around determination of the parameters of the motor and thus its transfer function.


The following figure shows the pictorial & Front panel



Voltmeter - Read the control winding voltage. Ammeter - Read the control winding current. LCD Display - Display the motor speed. Variable AC source - Vary the control winding AC voltage from 0 V to 230V.


‘S1' - ON/OFF the control winding voltage ‘S2' - ON/OFF the main winding voltage


Variable AC source - AC 0V - 230V Optical Encoder - MOC 7811

AC Servo Motor

Main winding voltage - AC 230V, 50Hz, 1∅

Control winding voltage - AC 230V, 50Hz, 1∅

No load current per phase - 300 mA(max)

Locked Rotor current per Phase - 350 mA(max)

Input Power - 100 Watts

Power factor - 0.8

No Load speed - 1400 rpm.

Full Load speed - 900 rpm

Full load torque - 1.9 kg cm

Phase splitting capacitor for

Single phase operation - 2.5 ∅ F, 500 VAC

Moment of inertia (J) - g/cm2

Viscous Friction co-efficient (B)



An AC servomotor is basically a two phase induction motor except for certain special design features. A two phase servomotor differs in the following two ways from a normal induction motor.

Speed Torque characteristics of induction motor and AC servomotor
a) Normal induction motor
b) AC servomotor

  • The rotor of the servomotor is built with high resistance, so that its X/R (Inductive reactance / Resistance) ratio is small which results in linear speed torque characteristics.(But conventional induction motors will have high value of X/R which results in high efficiency and non-linear speed-torque characteristics). The Speed-torque characteristics of normal induction motor (Curve-a) and AC servomotor (Curve-b) are shown in figure.
  • The excitation voltage applied of two stator windings should have a phase difference of 90°.


Construction of AC servomotor

The AC servomotor is basically a two phase induction motor with some special design features. The stator consists of two pole-pairs (A-B and C-D) mounted on the inner periphery of the stator, such that their axes are at an angle of 90° in space. Each pole-pair carries a winding. One winding is called reference winding and the other is called a control winding. The exciting current in the winding should have a phase displacement of 90°. The supply used to drive the motor is single phase and so a phase advancing capacitor is connected to one of the phase to produce a phase difference of 90°. The stator constructional features of AC Servo Motor are shown in figure.

Stator Construction of AC servomotor

The rotor construction is usually squirrel cage or drag-cup type. Rotor Construction of AC Servo Motor is shown in figure - 4. The squirrel cage rotor is made of laminations. The rotor bars are placed on the slots and short circuited at both ends by end rings. The diameter of the rotor is kept small in order to reduce inertia and to obtain good accelerating characteristics.

Rotor Construction of AC servomotor

The Drag-cup construction is employed for very low inertia applications. In this type of construction the rotor will be in the form of hollow cylinder made of aluminum. The aluminum cylinder itself acts as short circuited rotor conductors. (Electrically both the types of rotor are identical).

Working Principles of AC servomotor

Working of Servomotor as ordinary induction motor

The stator winding are excited by voltages of equal rms magnitude and 90° phase difference. These results in exciting currents i1 and i2 that are phase displayed by 90° and have equal rms values. These current give rises to a rotating magnetic field of constant magnitude. The direction of rotation depends on the phase relationship of the two currents (or voltages). The exciting current shown in figure -5 produces a clockwise rotating magnetic field and phase shift of 180° in i1 will produce an anticlockwise rotating magnetic field.

Wave form of Stator & Rotor excitation current

The rotating magnetic field sweeps over the rotor conductors. The rotor conductors experience a change in flux and so voltages are induced in rotor conductors. This voltage circulates currents in the short circuited rotor conductors and the currents create rotor flux. Due to the interaction of stator and rotor flux, a mechanical force (or torque) is developed on the rotor and so the rotor starts moving in the same direction as that of rotating magnetic field.

Working Principles of AC servomotor in control system

The symbolic representation of an A.C servomotor with control system component is shown in figure - 6. The reference winding of A.C servomotor is excited by a constant voltage source with frequency of 50Hz. The speed of A.C servomotor is controlled by controlling the control voltage. The error output of error detector is fed to PI controller, due to the error, controller take control action (i.e. to give control voltage) to firing circuit. The firing circuit generates the pulse's to rotate the motor at required speed.

Symbolic representation of AC servomotor in control system.

Characteristics Curves of AC servomotor

The speed Torque characteristics of a servomotor is very important for its performance in a servo system. The shape of these characteristics largely depends on the value of the rotor resistance. The curves showed in figure-7 show typical speed torque characteristics for several values of rotor resistances.

Speed Torque characteristics of Servomotor with several values of resistance

It can be seen that for higher values of rotor resistance the torque speed characteristics is linear. If the rotor resistance is either R4 or R5, the motor is referred to as two phase servo motor. For servo application the motor characteristics should be linear with negative slope (With positive damping). It can be proved that for stable operation positive damping is essential. Therefore ordinary two phase induction motor with low rotor resistance is not suitable for servo applications.

Speed Torque characteristics of AC servomotor

Figure shows the linear torque-speed characteristics of a two phase servo motor for various control voltages. In this trainer, we are obtained only this type of torque speed characteristics only.

In servo applications the voltages applied to the stator windings are not balanced. One of the phase known as reference phase is excited by a constant voltage and the other phase known as the control phase is energized by a voltage which is 90° out of phase with respect to the reference phase. The control phase voltage is of variable magnitude and polarity. The direction of rotation of the motor reverses as the polarity of the control voltage signal changes sign. Fig.9 shows the Torque-speed characteristics of the AC Servo Motor for various control voltage magnitudes.

Figure (9) and (10) show that the torque speed curves is not Linear except at very low speeds. For speeds near zero, all the curves are straight lines parallel to the characteristics at rated input voltage (ec= Er), and are equally spaced for equal increments of the input voltage. Under this assumption the torque Tc generated by the motor is represented by the equation.

Where k1 and K2 are the slopes as defined in figures and are motor parameters.