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
- 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
(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