Three Phase Induction Motors - Chapter Four Three Phase...

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Chapter Four Three Phase Induction Machine 4.1 Introduction Three-phase induction motors are the motors most frequently encountered in industry. They are simple, rugged, low-priced, and easy to maintain. They run at essentially constant speed from zero to full-load. The speed is frequency-dependent and, consequently, these motors are not easily adapted to speed control. However, variable frequency electronic drives are being used more and more to control the speed of commercial induction motors. In this chapter we cover the basic principles of the 3-phase induction motor and develop the fundamental equations describing its behavior. We then discuss its general construction and the way the windings are made. Squirrel-cage, wound-rotor ranging from a few horsepower to several thousand horsepower permit the reader to see that they all operate on the same basic principles.
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Three-Phase Induction Machine 217 4.2 Principal components A 3-phase induction motor (Fig.4.1) has two main parts: a stationary stator and a revolving rotor. The rotor is separated from the stator by a small air gap that ranges from 0.4 mm to 4 mm, depending on the power of the motor. Fig.4.1 Three-phase induction motor. The stator (Fig.4.2) consists of a steel frame that supports a hollow, cylindrical core made up of stacked laminations. A number of evenly spaced slots, punched out of the internal circumference of the laminations, provide the space for the stator winding. The rotor is also composed of punched laminations. These are carefully stacked to create a series of rotor slots to provide space for the rotor winding. We use two types of rotor windings: (1) conventional 3-phase windings made of insulated wire and (2)
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218 Chapter Four squirrel-cage windings. The type of winding gives rise to two main classes of motors: squirrel cage induction motors (also called cage motors) and wound-rotor induction motors. Fig.4.2 Exploded view of the cage motor of Fig.4.1, showing the stator, rotor, end-bells, cooling fan, ball bearings, and terminal box. The fan blows air over the stator frame, which is ribbed to improve heat transfer. A squirrel-cage rotor is composed of bare copper bars, slightly longer than the rotor, which are pushed into the slots. The opposite ends are welded to two copper end-rings, so that all the bars are short-circuited together. The entire construction (bars and end-rings) resembles a squirrel cage, from which the name is derived. In small and medium-size motors, the bars and end-rings are made of diecast aluminum, molded to form an integral block.
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Three-Phase Induction Machine 219 A wound rotor has a 3-phase winding, similar to the one on the stator. The winding is uniformly distributed in the slots and is usually connected in 3 wire wye. The terminals are connected to three slip rings, which turn with the rotor. The revolving slip-rings and associated stationary brushes enable us to connect external resistors in series with the rotor winding. The external resistors are mainly used during the start up period; under normal running conditions, the three brushes are short-circuited.
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