Double-Cage Rotor Circuit Loops: Weak Skin Effect

Abstract

Several windings can be located on the rotor of an induction machine such as a machine with a double-cage rotor. An induction machine with a double-cage rotor combines good starting and operating characteristics through the use of starting and working windings on the rotor. The starting winding has low reactance and relatively high resistance. It corresponds to the squirrel-cage type winding composed of brass or aluminum bars laid at the top of the rotor slot. The working winding corresponds to the squirrel-cage type winding composed of copper bars laid on the bottom part of the rotor slot. Such a winding has relatively low resistance. When the machine starts, the currents basically flow in the starting winding. During operation under load conditions, the currents of the slip frequency are induced in the working winding. The effective operation of such an electric machine is enabled by the use of the slit located between the bars of the starting and working rotor windings. To facilitate this, the starting and working winding bars are laid in place spaced at the height of the rotor slots. In the electromagnetic regard, an induction machine with a double-cage rotor is characterized by the presence in the body of the rotor of a leakage field coupling with the starting and working windings. Therefore, the magnetic field in the double-cage rotor represents a more complex picture of the distribution. Currently, various forms of interpreting the rotor leakage field distribution picture exist. In these conditions, establishing the circuit loops formed by the currents induced in the rotor windings and determining their impedances become complicated. In this chapter, the circuit loops of the double-cage rotor and their impedances are considered under weak skin effect conditions.