Page:Electrical Engineering Volume 1.djvu/506

2036 3214. At starting, while the current is increasing in the magnetizing-coil circuit, the inductance of the magnetizing coils increases its apparent resistance, and a part of the energy supplied to the coils is stored up in the magnetic field which is being established. As soon as the current in the magnetizing coils reaches its maximum value, however, and so long as it remains constant at this value, there is no reactance present, and the entire amount of energy delivered to the coils is expended in heating the wire; that is, it requires (directly) no energy to maintain a magnetic field at a constant value, the field depending on the ampere-turns that are acting on the magnetic circuit. It is obvious, however, that in order to force the current through the wire of which the magnetizing coil is composed, energy must be expended, but this energy appears entirely as heat, and, consequently, is wasted as far as any practical application of it is concerned. The number of watts expended in sending the current through the magnetizing coils should, therefore, be made as small as the design of the machine will permit, both to prevent any excessive waste of energy and to prevent possible damage by the heat liberated. In practice, the loss of energy from this cause varies from about 2 per cent, of the total output of the machine in larger sizes, to 5 or more per cent, in the smaller.

3215. In shunt-wound machines the magnetizing coils are exposed to the full difference of potential that exists between the brushes of the machine, and, consequently, should use only a small amount of current in order that the loss in watts should be the required small percentage of the output. From this it follows that the wire used for the magnetizing coils should be of small size and of considerable length, making a large number of turns around the magnets, both to give the necessary resistance to keep the current at its proper value and to allow of this small current furnishing the requisite number of ampere-turns. In series-wound machines, however, as the total current flowing gives the magnetizing force, the magnetizing coils need to have