Elementary Dynamo Design; With Numerical Examples (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1908 Excerpt: ...corresponding change of speed unless suitable change is made in the amount of resistance. For many purposes this is a serious disadvantage, and as a matter of fact this method of speed regulation is never used except in cases such as those mentioned above, where an attendant is constantly at hand to regulate the speed at every instant as may be required. 3. Speed Variation by Shunt Resistance.--A second method of speed control for a continuous-current motor is applicable to shunt-wound or compound-wound motors, and consists of introducing a resistance in series with the shunt winding. Since the shunt current is only a small percentage of the current flowing in the motor, the energy lost in a shunt resistance is inconsiderable, and therefore this method is greatly superior to the former one on the score of efficiency. In a shunt-wound motor for any given position of the resistance switch, the speed will be approximately independent of the load. The effect of introducing resistance into the shunt circuit of a motor is of course to decrease he current in this winding, therefore to decrease the number of ampere-turns on the machine and consequently the total magnetic flux. The back E.m.f. generated by the motor is proportional to N and to the speed. On introducing resistance, this back E.m.f. is diminished, since the value of N is decreased. The-e.m.f. applied to the terminals of the motor is, however, constant, and if the back E.m.f. falls, a greater current will flow through the armature. This will cause it to accelerate, and the speed will increase until the back e.m.f. is again brought up to a value corresponding to the E.m.f. applied at the terminals. A numerical example will make this clear. The motor designed in Chapter IV. normally runs at 600 rev...