The rotor is a moving component of an electromagnetic system in the electric motor, electric generator, or alternator. Its rotation is due to the interaction between the windings and magnetic fields which produces a torque around the rotor's axis.
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In induction motors, generators and alternators have an electromagnetic system consisting of a stator and rotor. There are two designs for the rotor in an induction motor: squirrel cage and wound. In generators and alternators, the rotor designs are salient pole or cylindrical.
There are mainly four types of rotor.
Squirrel cage rotor:-
The squirrel-cage rotor consists of laminated steel in the core with evenly spaced bars of copper or aluminum placed axially around the periphery, permanently shorted at the ends by the end rings. This simple and rugged construction makes it the favorite for most applications. The assembly has a twist: the bars are slanted, or skewed, to reduce magnetic hum and slot harmonics and to reduce the tendency of locking. Housed in the stator, the rotor and stator teeth can lock when they are in equal number and the magnets position themselves equally apart, opposing rotation in both directions.
Characteristics of Squirrel cage rotor:-
♦ This rotor rotates at a speed less than the stator rotating magnetic field or synchronous speed.
♦ Rotor slip provides necessary induction of rotor currents for motor torque, which is in proportion to slip.
♦ When rotor speed increases, the slip decreases.
♦ Increasing the slip increases induced motor current, which in turn increases rotor current, resulting in a higher torque for increase load demands.
♦ Rotor slip provides necessary induction of rotor currents for motor torque, which is in proportion to slip.
♦ When rotor speed increases, the slip decreases.
♦ Increasing the slip increases induced motor current, which in turn increases rotor current, resulting in a higher torque for increase load demands.
Wound rotor:-
The rotor is a cylindrical core made of steel lamination with slots to hold the wires for its 3-phase windings which are evenly spaced at 120 electrical degrees apart and connected in a star configuration. The rotor winding terminals are brought out and attached to the three slips rings with brushes, on the shaft of the rotor. Brushes on the slip rings allow for external three-phase resistors to be connected in series to the rotor windings for providing speed control.
Characteristics of Wound rotor:-
♦ This rotor operates at constant speed and has lower starting current.
♦ External resistance added to rotor circuit, increases starting torque.
♦ Motor running efficiency improves as external resistance is reduced when motor speed up.
♦ Higher torque and speed control.
Salient pole rotor:-
The rotor is a large magnet with poles constructed of steel lamination projecting out of the rotor’s core. The poles are supplied by direct current or magnetized by permanent magnets. The armature with a three-phase winding is attached to three slip rings with brushes riding on them and mounted on the shaft. The field winding is wound on the rotor which produces the magnetic field and the armature winding is on the stator where voltage is induced. Direct current from an external exciter or from a diode bridge mounted on the rotor shaft, produces a magnetic field and energizes the rotating field windings and alternating current energizes the armature windings simultaneously.
Characteristics of Salient pole rotor:-
♦ This rotor operates at a speed below 1500 revolutions per minute and 40% of its rated torque without excitation.
♦ It has a large diameter and short axial length.
♦ Air gap is non uniform.
♦ Rotor has low mechanical strength.
Cylindrical rotor:-
The cylindrical shaped rotor is made of a solid steel shaft with slots running along the outside length of the cylinder for holding the field windings of the rotor which are laminated copper bars inserted into the slots and is secured by wedges. The slots are insulated from the windings and are held at the end of the rotor by slip rings. An external direct current source is connected to the concentrically mounted slip rings with brushes running along the rings. The brushes make electrical contact with the rotating slip rings. DC current is also supplied through brushless excitation from a rectifier mounted on the machine shaft that converts alternating current to direct current.
Characteristics of Cylindrical rotor:-
♦ The rotor operates at speed between 1500 to 3000 RPM.
♦ It has strong mechanical strength.
♦ Air gap is uniform.
♦ Its diameter is small and has a large axial length and requires a higher torque than salient pole rotor.