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AC Servo Motors
Timing Electric Motors (cont.) Ac and
dc electric motors can be used as timing motors. Dc electric timing
motors are used for portable applications, or where high acceleration
and low speed variations are required. These electric motors offer
advantages, which include starting torque as high as ten times running
torque, efficiency from 50 to 70%, and relatively easy speed control.
But some form of speed governor, either mechanical or electronic,
is required.
Ac motors use readily available power, are lower in cost, have improved life, and do not generate RFI. However, ac motors cannot be readily adapted to portable applications, have relatively low starting torques, and are much less efficient than dc motors.
AC Servo Motors: Ac servo motors are
used in ac servomechanisms and computers which require rapid and
accurate response characteristics. To obtain these characteristics,
servo motors have small-diameter high-resistance rotors. The small
diameter provides low inertia for fast starts, stops, and reversals,
while the high resistance provides a nearly linear speed-torque
relationship for accurate control.
Servo motors are wound with two phases physically at right angles or in space quadrature. Servo motors feature a fixed or reference winding is excited from a fixed voltage source, while the control winding is excited by an adjustable or variable control voltage, usually from a servoamplifier. The servo motor windings are usually designed with the same voltage-turns ratio, so that power inputs at maximum fixed-phase excitation and at maximum control-phase signal are in balance.
In an ideal servo motor, torque at any speed is directly proportional to the servo motor's control-winding voltage. In practice, however, this relationship exists only at zero speed because of the inherent inability of an induction servo motor to respond to voltage input changes under conditions of light load.
The inherent damping of servo motors decreases as ratings increase,
and the servo motors have a reasonable efficiency at the sacrifice
of speed-torque linearity. Most larger servo motors have integral
auxiliary blowers to maintain temperatures within safe operating
ranges. Servo motors are available in power ratings from less than
1 to 750 W, in sizes ranging from 0.5 to 7-in. OD. Most servo motors
are available with modular or built-in gearheads.
Servo Motors: Basics of AC Servo Motor Design Engineering
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