From Motors and Drives
AC Servomotors
This type of motor is basically a two-phase induction motor, capable of
reverse operation. To achieve the dynamic requirements of a servo system,
the servomotor must have a small diameter, low inertia, and high-resistance
rotors. The low inertia allows for fast starts, stops, and reverse of
direction. The high-resistance rotor provides for almost linear speed/
torque characteristics and accurate control.
An AC servomotor is designed with two phases set at right angles to each other. A fixed or reference winding is excited by a fixed voltage source. The control winding is excited by a variable voltage source, usually the servo amplifier. Both sets of windings are usually designed with the same voltage per turns ratio (meaning that with equal voltage applied to each winding, the same magnetic flux will be produced). This allows for maximum control of speed, with very little speed drift. In many cases, the design of the AC servomotor offers only reasonable efficiency, at the sacrifice of high starting torque and smooth speed response. Figure 3-45 indicates a typical AC servomotor design.
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