Minimizing skin effect in random wound high speed machine stator

Usually skin effect is neglected in the design of medium speed machines when using random wound windings. That is because it is thought that parallel strands mix enough so that the skin effect can be neglected. In medium speed machines operating in the frequency range of 100 - 300 Hz the skin effect problem may come up in cases where the slots have only one or a couple of turns despite the fact that the turn has been divided into many parallel sub-conductors. Tests were carried out with 310 kW medium speed machines with nominal frequency of 200 Hz. One of the machines was wound in a normal way and the other machine was wound by twisting the strands in the coil-end areas (not line-end) by 180 degrees. It is shown that taking care of the transpose of parallel wires in a coil, skin effect and losses remain tolerable. Before making final decisions on about what kinds of windings are embedded in a machine, they can be verified by some analytical or finite element methods. FEM gives quite good results compared to measurements done with real rotating machines.