Universal AC Motors: Universal ac
motors operate with nearly equivalent performance on direct current
or alternating current up to 60 Hz. AC motors differ from a dc motors
due to the winding ratios and thinner iron laminations. DC motors
runs on ac, but with poor efficiency. Universal ac motors can operate
on dc with essentially equivalent ac motor performance, but with
poorer commutation and brush life than for an equivalent dc motor.
An important characteristic of universal ac motors is that it has the highest horsepower-per-pound ratio of any ac motor because it can operate at speeds many times higher than that of any other 60-Hz electric motor.
When operated without load, universal ac motors tend to run away, speed being limited only by windage, friction, and commutation. Therefore, large universal ac motors are nearly always connected directly to a load to limit speed. On portable tools such as electric saws, the load imposed by the gears, bearings, and cooling fan is sufficient to hold the no-load speed down to a safe value.
With a universal ac motor, speed control is simple, since electric motor speed is sensitive to both voltage and flux changes. With a rheostat or adjustable autotransformer, ac motor speed can be readily varied from top speed to zero.
Synchronous AC Motors: Synchronous ac motors are inherently constant-speed electric motors and they operate in absolute synchronism with line frequency. As with squirrel-cage induction ac motors, speed is determined by the number of pairs of poles and is always a ratio of the line frequency.
Synchronous ac motors are made in sizes ranging from subfractional self-excited units to large-horsepower, direct-current-excited ac motors for industrial drives. In the fractional-horsepower range, synchronous ac motors are used primarily where precise constant speed is required.
In large horsepower sizes applied to industrial loads, synchronous ac motors serve two important functions. First, ac motors provide highly efficient means of converting ac energy to mechanical power. Second, ac motors can operate at leading or unity power factor, thereby providing power-factor correction.
There are two major types of synchronous ac motors: nonexcited
and direct-current excited electric motors.
AC Motors: Basics of AC Motor Design Engineering