Double speed motors are generally divided into two main groups as 'Double Winding' and 'Dahlander Winding' Although both windings can be made with very different number of poles in split-winding motors, the number of two poles in Dahlander windings should be multiple: 2/4, 4/8, 6/12. In split winding motors, the ratio of winding powers can be very different - within certain limits. The main determinant here is the torque required by the device / machine to be driven by the motor at that speed, and therefore the power. Lathe milling etc. In applications requiring almost constant torque, such as in some machines, forces close to each other at both revolutions may be required. Whereas, for example, in fan exercises, as the torque changes with the changing speed, big differences may be required in the ratios of the powers at both speeds.