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An AC servomotor is basically a tw o phase induction motor except for certain special design features. A two phase servomotor differs in the following two ways from a normal induction mo tor.
1.The rotor of the servomotor is built with high resistance, So that its X/R (Inductiv e reactance / Resistance) ratio is small which results in linear speed torque characteristics.(But conventiona l induction motors will have high value of X/R which results in high efficiency and non-linear speed-torquecharacteristics). The Speed-torque characteristics of norm al induction motor (Curve-a) and AC servomotor (Curve-b) are shown in figure.
2.The excitation voltage applied of t wo 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 spe cial design features. The stator consists of two pole pa irs (A- B and C - D) mounted on the inner peri phery of the stator, such that their axes are at an angle of 90° in space. Each pole - pair carries a w inding. 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 conne cted to one of the phase to produce a phase differen ce of 90°. The stator constructional features of AC servo motor are shown in
The rotor construction is us ually 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 characteristic
The Drag - cup constructio n 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 cyli nder itself acts as short circuited rotor
conductors. (Electrically both the types of rotor are identical).
Working Principles of AC Servomotor
The stator winding are excited by voltages of equal rms magnitude and 90° phase difference. These results in exciting currents i 1 and i2 that are phase displayed by 90° and have equal rms values. These current give rises to a r otating magnetic field of constant magnitude.
The direction of rotation depends on the phase relationship of the two curr ents ( or voltages). The exciting current shown in fig urE5 produces a clockwise rotating magnetic field and phase shift of 180° in will produce an antic lockwise rotating magnetic field
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