Theory and design of NEMA resistors for motor starting and speed control

The National Electrical Manufacturers´ Association classification of resistors for motor starting and speed control specifies the initial current peaks and the duty cycle for which the resistor must be designed. The nature of the load driven by the motor decides the valley current at which succeeding resistor steps must be switched in starting, and consequently the number of steps in the resistor, and points in the controller. The rms current, and the time each step is in the circuit, taken in conjunction with the cyclic current-carrying capacity of individual resistor elements, determines the selection of the elements on the basis of their continuous-current rating. It is usual to assume an rms current of 125 per cent, and switching at equal time intervals. However, in this paper, formulas are developed for more accurately calculating these values, and the formulas and necessary constants are tabulated. Consideration is given to d-c shunt and series-connected motors, and to wound-rotor a-c induction motors. Resistors for regulating duty are also discussed.