Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. The subject is necessarily technical because it involves not only the mechanical factors speed and torque but also the electrical quantities voltage, current and flux. Moreover, any or all of these five fundamental quantities may, in fact usually do, vary and are affected by other-factors or conditions. Hence the problems must be analyzed and solved with thoroughness to obtain results of real value and cannot be properly treated in a popular manner. Nevertheless care has been taken to introduce and explain each step or result as clearly as possible, and to illustrate each case, when feasible, by a specific numerical example based upon standard commercial motors.