Abstract :
The paper describes a method of making calculations required in grading resistors for electric traction equipments. The principal features of the method are the use of geometrical progressions in the determination of the resistance grading and the relating of the geometrical progressions to the shape of the motor characteristic curve. It is thought that the clearest way of presenting the method is to work out examples in full, with explanations accompanying each stage of the working. Although the calculations are given in detail, there are many short cuts available to the experienced user. Expedients such as notching at one current in series and at another in parallel have not been introduced, the examples of the method being confined to basic principles only. It is assumed that the notching relay employed is that which has two series coils, one in each motor circuit. Two examples have been worked out, one for a unit-contactor equipment and the other for a cam-operated equipment. In the second example, allowance has been made for the time lag between the release of the notching relay and the closing of the resistance contactor next in sequence.
