Explain construction and working of AC servomotor.

Explain construction and working of AC servomotor.
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Answer :

Construction of AC Servo Motor: AC servo motor is mainly composed of a stator and a rotor. Laminated stator core is usually made of silicon steel. Two phase windings are placed in the stator slots at 90 degree electrically apart from each other. One phase winding is the field winding and the other is the control winding. Field winding and control winding are excited as shown in figure. The rotor is squirrel cage type or drag cup type rotor. 

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Working of AC Servo Motor: These motors are used for position control or in servo mechanisms, hence are termed as servomotors. A voltage Vm is applied to the main or reference winding at zero angle, while Va is the voltage applied to control winding which controls the torque-speed characteristics with 90° angle. The 90° space displacement of the two coils/windings and the 90° phase difference between the voltages applied to them result in production of rotating magnetic field in the air gap. This rotating magnetic field is cut by rotor conductors and emf is induced in them. Since rotor is short-circuited, the rotor currents flow. The interaction between rotor currents and rotating magnetic field results in force (or torque) acting on rotor. Due to the force or torque acting on the rotor, it is set in motion. 

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