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AC Motors
Polyphase AC Motors: Polyphase squirrel-cage
ac motors are basically constant-speed machines, but some degree
of flexibility in operating characteristics results from modifying
the rotor slot design. These variations in ac motors produce changes
in torque, current, and full-load speed. Evolution and standardization
have resulted in four fundamental types of ac motors.
AC Motors - Designs A and B:
General-purpose ac motors with normal starting torques and currents,
and low slip. Fractional-horsepower polyphase ac motors are generally
design B. Because of the drooping characteristics of design B, a
polyphase ac motor that produces the same breakdown (maximum) torque
as a single-phase ac motor cannot attain the same speed-torque point
for full-load speed as single-phase ac motors. Therefore, breakdown
torque must be higher (a minimum of 140% of the breakdown torque
of single-phase, general-purpose ac motors) so that full-load speeds
are comparable.
AC Motors - Design C: High
starting torque with normal starting current and low slip. AC motors
are normally used where breakaway loads are high at starting, but
which normally run at rated full load and are not subject to high
overload demands after running speed has been reached.
AC Motors - Design D: High
slip, ac motor starting torque, low starting current, and low full-load
speed. Because of the high slip, speed can drop when fluctuating
loads are encountered. This ac motor design is subdivided into several
groups that vary according to slip or the shape of the speed-torque
curve.
AC Motors - Design F: Low starting
torque, low starting current, and low slip. These AC motors are
built to obtain low locked-rotor current. Both locked-rotor and
breakdown torque are low. Normally these ac motors are used where
starting torque is low and where high overloads are not imposed
after running speed is reached.
AC Motors: Basics of AC Motor Design Engineering
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