1.2.2 Insulated Rotor Windings and 1.2.3 Squirrel Cage Induction Motor Rotor Windings

1.2.2 Insulated Rotor Windings
In many ways, the rotor winding has the same components as the stator, but with important changes. In all cases, copper, copper alloy, or aluminum conductors are present to act as a conduit for current flow. However, the steady-state current flowing through the rotor winding is usually DC (in synchronous machines), or very low frequency AC (a few Hz) in induction machines. This lower frequency makes the need for a laminated stator core less critical.
       The conductors in rotor windings are often embedded in the laminated steel core or surround laminated magnetic steel. However, round rotors in large turbogenerator and highspeed salient pole machines are usually made from forged magnetic steel, since laminated magnetic steel rotors cannot tolerate the high centrifugal forces.
       Synchronous machine rotor windings, as well as wound rotor induction motors, contain electrical insulation to prevent short circuits between adjacent conductors or to the rotor body. As will be discussed in Chapters 3 and 5, the insulating materials used in rotor windings are largely composites of organic and inorganic materials, and thus have poor thermal and mechanical properties compared to copper, aluminum, or steel. The insulation then often determines the expected life of a rotor winding.

1.2.3 Squirrel Cage Induction Motor Rotor Windings

SCI rotor windings are unique in that they usually have no explicit electrical insulation on the rotor conductors. Instead, the copper, copper alloy, or aluminum conductors are directly installed in slots in the laminated steel rotor core. (Smaller SCI rotors may have the aluminum conductors cast in place.) In normal operation, there are only a few volts induced on the rotor conductors, and the conductivity of the conductors is much higher than that of the steel core. Because the current normally only flows in the conductors, electrical insulation is not needed to force the current to flow in the right paths. Reference 1.9 describes the practical aspects of
rotor design and operation in considerable detail.
       The only time that significant voltage can appear on the rotor conductors is during motor starting. This is also the time that extremely heavy currents will flow in the rotor windings. Under some conditions during starting, the conductors make and break contact with the rotor core, leading to sparking. This is normally easily tolerated. However, some SCI motors operate in a flammable environment, and this rotor sparking may ignite an explosion. Therefore, some motor manufacturers do insulate the conductors from the rotor core to prevent the sparking [1.10]. Since such applications are rare, for the purposes of this book, we assume
that the rotor is not insulated.
       Although SCI rotor windings are generally not insulated, for completeness, Section 9.4 does discuss such rotors, and Chapters 12 and 13 present some common tests for SCI rotor winding integrity.