Three-phase induction machines account for the great majority of applications that call for motors with power 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 can 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 enter 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 any two of the line connections to the electrical source. We will see that this reverses the direction of mechanical rotation. You may find the fact 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 rugged is known as a squirrel-cage rotor. It consists simply of bars of aluminum with shorting rings at the ends. The squirrel cage is embedded 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 resistive 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 series 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 induced in the rotor conductor’s increase linearly with slip. For small slips, the inductive reactance of the conductors is negligible, and maximum rotor current is aligned with maximum stator field, which is the optimum situation for producing torque.
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 either 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 characteristic by variations in the dimensions and geometry of the motor and by materials selection.