Three-Phase Induction Motors Essay Example for Free

Three-Phase installation Motors EssayThree-phase induction machines account for the great majority of applications that call for motors with index finger ratings over 5 hp. They are used to power pumps, fans, compressors, and grinders, and in other industrial applications. Rotating Stator Field. The stator of a three-phase induction machine contains a set of windings to which three-phase electrical power is applied. The stator field buttocks be visualized as a set of north and south poles rotating around the circumference of the stator. (North stator poles are where magnetic flux lines leave the stator, and south stator poles are where magnetic flux lines immortalize the stator. Because north and south poles occur in pairs, the total number of poles P is always even. The direction of rotation of the field in a three-phase induction machine can be reversed by interchanging each two of the line connections to the electrical source. We will see that this reverses the directio n of mechanical rotation. You may find the item that interchanging two of the electrical connections to the source reverses the direction of rotation to be useful in working with three-phase motors. Squirrel-Cage Induction Machines. The rotor windings of a three-phase induction machine can take two forms.The simplest, least expensive, and most hard is cognize as a squirrel-cage rotor. It consists simply of bars of aluminum with shorting rings at the ends. The squirrel cage is implant in the laminated iron rotor by casting molten aluminum into slots cut into the rotor. In the squirrel-cage induction machine, there are no external electrical connections to the rotor. The other type of rotor construction is known as a wound rotor. Torque is produced in an induction motor assuming purely repellent impedances for the rotor conductors.However, the impedances of the conductors are not purely resistive. Because the conductors are embedded in iron, there is significant serial inductance associated with each conductor. The rotor slows down from synchronous speed, the stator field moves past the rotor conductors. The magnitudes of the voltages induce in the rotor conductors increase linearly with slip. For small slips, the inductive reactance of the conductors is negligible, and maximum rotor current is aline with maximum stator field, which is the optimum situation for producing torsion.Thus, the torque tends to level out as the motor slows. Because the poles on the rotor tend to become aligned with the stator poles, the torque decreases as the motor slows to a stop. The torque for zero speed is called any the starting torque or the stall torque. The maximum torque is called either the pull-out torque or the breakover torque. The motor designer can modify the shape of the torque speed feature article by variations in the dimensions and geometry of the motor and by materials selection.