Electric Motor Applications Machinery's Handbook, 31st Edition
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speeds with as little variation as 1 ⁄ 2 to 1 percent of full load speed from full load to no load over a line voltage variation of ± 10 percent and over any normal variation in motor tem perature and ambient temperature. Applications: These direct-current, adjustable-voltage drives, as they are sometimes called, have been applied successfully to such machine tools as planers, milling machines, boring mills and lathes, as well as to other industrial machines where wide, stepless speed control, uniform speed under all operating conditions, constant torque acceleration and adaptability to automatic operation are required. Compound-Wound Motors.— In the compound-wound motor, the speed variation due to load changes is much less than in the series-wound motor, but greater than in the shunt- wound motor (ranging up to 25 percent from full load to no load). It has a greater starting torque than the shunt-wound motor, is able to withstand heavier overloads, but has a nar rower adjustable speed range. Standard motors of this type have a cumulative-compound winding, the differential-compound winding being limited to special applications. They are used where the starting load is very heavy or where the load changes suddenly and vio lently as with reciprocating pumps, printing presses and punch presses. Types of Polyphase Alternating-Current Motors.— The most widely used polyphase motors are of the induction type. The “squirrel cage” induction motor consists of a wound stator which is connected to an external source of alternating-current power and a laminated steel core rotor with a number of heavy aluminum or copper conductors set into the core around its periphery and parallel to its axis. These conductors are connected together at each end of the rotor by a heavy ring, which provides closed paths for the cur- rents induced in the rotor to circulate. The rotor bars form, in effect, a “squirrel cage” from which the motor takes its name. Wound-rotor Type of Induction Motor: This type has, in addition to a squirrel cage, a series of coils set into the rotor which are connected through slip-rings to external vari- able resistors. By varying the resistance of the wound-rotor circuits, the amount of current flowing in these circuits and hence the speed of the motor can be controlled. Since the rotor of an induction motor is not connected to the power supply, the motor is said to oper- ate by transfer action and is analogous to a transformer with a short-circuited secondary that is free to rotate. Induction motors are built with a wide range of speed and torque characteristics which are discussed under “Operating Characteristics of Squirrel-cage Induction Motors.” Synchronous Motor: The other type of polyphase alternating-current motor used indus trially is the synchronous motor . In contrast to the induction motor, the rotor of the syn chronous motor is connected to a direct-current supply which provides a field that rotates in step with the alternating-current field in the stator. After having been brought up to syn chronous speed, which is governed by the frequency of the power supply and the number of poles in the rotor, the synchronous motor operates at this constant speed throughout its entire load range. Operating Characteristics of Squirrel-Cage Induction Motors.— In general, squirrel- cage induction motors are simple in design and construction and offer rugged service. They are essentially constant-speed motors, their speed changing very little with load and not being subject to adjustment. They are used for a wide range of industrial applications calling for integral horsepower ratings. According to the NEMA (National Electrical Manufacturers Association) Standards, there are four classes of squirrel-cage induction motors designated respectively as A , B , C , and D . Design A motors are not commonly used since Design B has similar characteristics with the advantage of lower starting current. Design B motors may be designated as a general purpose type suitable for the majority of polyphase alternating-current applications such as blowers, compressors, drill presses, grinders, hammer mills, lathes, planers, polishers, saws, screw machines, shakers,
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