An AC servomotor uses AC electric input to produce mechanical output in the form of precise angular velocity. For low-power applications, the two-phase squirrel cage induction type AC servo motors are used. The three-phase squirrel cage induction motors have been modified for application in high-power servo systems.
The schematic diagram of a two-phase AC servomotor is shown in the figure.
The stator of a two phase AC servomotor consists of two distributed windings which are displaced from each other by 90° electrical.
One winding is known as reference phase and is supplied from a constant voltage source.
The other winding is known as control phase and is supplied with a variable voltage of the same frequency as the reference phase voltage, but is displaced by 90° electrical.
The control phase is normally supplied from a servo amplifier. The speed and the torque developed in the motor is controlled by the phase difference between the control phase voltage and the reference phase voltage. The direction of the rotation of the rotor can be altered by changing the phase difference from leading to lagging or vice-versa, between the control phase voltage and the reference phase voltage.
The torque-speed characteristics of a two-phase AC servo motor for various control voltages is shown in the figure. From the characteristics curve, it can be seen that the characteristics remain almost constant for various control voltages. The high rotor resistance ensures a negative slope for the torque-speed characteristics over its entire operating range and hence furnishes the servomotor with positive damping for good stability.
The response of a 2-phase servomotor to very small control signals being improved by decreasing the weight and inertia of the motor and this design of the servomotor is known as drag-cup servomotor.
In this motor, a thin cup of non-magnetic material is used as the rotor. A stationary iron core at the middle of the conducting cup completes the magnetic circuit of the motor. As the rotor is thin and hence its resistance is very high. Therefore, the drag-cup servomotor has a high starting torque.
A three-phase squirrel cage induction motor is a highly non-linear coupled circuit machine. By using a control method called vector control or field-oriented control, it can be used as a linear decoupled machine.
In the vector control method, the currents in the machine are controlled in such a way that its torque and flux become decoupled. This results in high-speed response and the high-torque response of the motor. Therefore, the 3-phase induction motor with the vector control method can be used as servomotors for applications in high power servo-systems.
AC servomotors are widely used in the following applications −