Monday, January 12, 2015

Three-Phase Induction Motors: Three-phase, squirrel-cage induction motor and The rotating magnetic field .

Three-Phase Induction Motors
THREE-PHASE, SQUIRREL-CAGE INDUCTION MOTOR

A three-phase, squirrel-cage induction motor is shown in Figure 16–1. This motor is simple in construction and is easy to maintain. For a given horsepower rating, the physical size of this motor is small, when compared with other types of motors. It has very good speed regulation under varying load conditions. This motor is used for many industrial applications because of its low purchase price, rugged construction, and operating characteristics.

Construction

The basic structure of a three-phase, squirrel-cage induction motor consists of a stator, a rotor, and two end shields that house the bearings supporting the rotor shaft.

The stator is a three-phase winding that is placed in the slots of a laminated steel core. The winding itself is made of formed coils that are connected to give three single-phase windings spaced 120 electrical degrees apart. The three separate single-phase windings are connected in wye or delta. Three line leads from the three-phase stator windings are brought out to a terminal box mounted on the frame of the motor.

The rotor has a cylindrical core consisting of steel laminations (Figure 16–2). Aluminum bars are mounted near the surface of the rotor. These bars are brazed or welded to two aluminum end rings. Some types of squirrel-cage induction motors are smaller than others

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and have aluminum end rings that are cast in one piece. The rotor shaft is supported by bearings housed in the end shields.

THE ROTATING MAGNETIC FIELD

The principle of operation for all three-phase motors is the rotating magnetic field. There are three factors that cause the magnetic field to rotate:

1. The fact that the voltages of a three-phase system are 120° out of phase with each other

2. The fact that the three voltages change polarity at regular intervals

3. The arrangement of the stator windings around the inside of the motor

Figure 16–3A shows three ac voltages 120° out of phase with each other, and the stator winding of a three-phase motor. The stator illustrates a two-pole, three-phase motor.

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Two-pole means that there are two poles per phase. AC motors seldom have actual pole pieces as shown in Figure 16–3A, but they will be used here to aid in understanding how the rotating magnetic field is created in a three-phase motor. Notice that pole pieces 1A and 1B are located opposite each other. The same is true for poles 2A and 2B, and 3A and 3B. Pole pieces 1A and 1B are wound with wire that is connected to phase 1 of the three-phase system. Notice also that the pole pieces are wound in such a manner that they will always have opposite magnetic polarities. If pole piece 1A has a north magnetic polarity, pole piece 1B will have a south magnetic polarity at the same time.

The windings of pole pieces 2A and 2B are connected to line 2 of the three- phase system. The windings of pole pieces 3A and 3B are connected to line 3 of the three-phase system. These pole pieces are also wound in such a manner as to have the opposite polarity of magnetism.

To understand how the magnetic field rotates around the inside of the motor, refer to Figure 16–3B. Notice that a line labeled A has been drawn through the three voltages of the system. This line is used to illustrate the condition of the three voltages at this point in time. The arrow drawn inside the motor indicates the greatest strength of the magnetic field at the same point in time. It is to be assumed that the arrow is pointing in the direction of the north magnetic field. Notice in Figure 16–3B that phase 1 is at its maximum positive peak and that phases 2 and 3 are less than maximum. The magnetic field is, therefore, strongest between pole pieces 1A and 1B.

In Figure 16–3C, line B indicates that the voltage of line 3 is zero. The voltage of line 1 is less than maximum positive, and line 2 is less than maximum negative. The magnetic field at this point is concentrated between the pole pieces of phases 1 and 2.

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In Figure 16–3D, line C indicates that line 2 is at its maximum negative peak and that lines 1 and 3 are less than maximum positive. The magnetic field at this point is concentrated between pole pieces 2A and 2B.

In Figure 16–3E, line D indicates that line 1 is zero. Lines 2 and 3 are less than maxi- mum and in opposite directions. At this point in time, the magnetic field is concentrated between the pole pieces of phase 2 and phase 3.

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In Figure 16–3F, line E indicates that phase 3 is at its maximum positive peak, and lines 1 and 2 are less than maximum and in the opposite direction. The magnetic field at this point is concentrated between pole pieces 3A and 3B.

In Figure 16–3G, line F indicates that phase 2 is zero. Line 3 is less than maximum positive, and line 1 is less than maximum negative. The magnetic field at this time is concentrated between the pole pieces of phase 1 and phase 3.

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In Figure 16–3H, line G indicates that phase 1 is at its maximum negative peak, and phases 2 and 3 are less than maximum and in the opposite direction. Notice that the magnetic field is again concentrated between pole pieces 1A and 1B. This time, however, the magnetic polarity is reversed because the current has reversed in the stator winding.

In Figure 16–3I, line H indicates that phase 2 is at its maximum positive peak and phases 1 and 3 are less than maximum and in the negative direction. The magnetic field is concentrated between pole pieces 2A and 2B.

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In Figure 16–3J, line I indicates that phase 3 is maximum negative, and phases 1 and 2 are less than maximum in the positive direction. The magnetic field at this point is concentrated between pole pieces 3A and 3B.

In Figure 16–3K, line J indicates that phase 1 is at its positive peak, and phases 2 and 3 are less than maximum and in the opposite direction. The magnetic field is again concentrated between pole pieces 1A and 1B. Notice that in one complete cycle of the three-phase voltage, the magnetic field has rotated 360° around the inside of the stator winding.

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If any two of the stator leads is connected to a different line, the relationship of the voltages will change and the magnetic field will rotate in the opposite direction. The direction of rotation of a three-phase motor can be reversed by changing any two stator leads.