Design of Rotating Electrical Machines, 2nd Edition

5

Resistances

Together with inductances, resistances define the characteristics of an electrical machine. From an efficiency point of view, resistances are important electrical machine parameters. In many cases, the resistive losses form a dominant loss component in a machine. In electrical machines, the conductors are usually surrounded by a ferromagnetic material, which encourages flux components to travel through the windings. This, again, may cause large skin effect problems if the windings are not correctly designed.

5.1 DC Resistance

The resistance of a winding can first be defined as a DC resistance. According to Ohm’s law, the resistance RDC depends on the total length of a conductor in a coil lc, the number of parallel paths a in windings without a commutator, per phase, or 2a in windings with a commutator, the cross-sectional area of the conductor Sc and the conductivity of the conductor material σc

(5.1)

Resistance is highly dependent on the running temperature of the machine, and therefore a designer should be well aware of the warming-up characteristics of the machine before defining the resistances. Usually, we may first investigate the resistances of the machine at the design temperature or at the highest allowable temperature for the selected winding type.

Windings are usually made of copper. The specific conductivity of pure copper at room temperature (+20 °C) ...