Current sheets of the same space harmonics, which impose flux-density components of different phase velocities in the air-gap, cause oscillations in the electrical torque. These may damage the shaft or coupling. The difference in phase velocity causes a time dependent phase shift in flux density components. The torque, which is given by the product of the radial and tangential flux density components, is shown in fig. 5.1. The case with the same space harmonics in both components is given in fig. 5.1 a), b) and with different space harmonics in fig. 5.1 c), d).