ELECTRIC ALTERNATING CURRENT MACHINERY).
The next step was to add to the num ber of coils on the armature so that during each portion of a revolution some part of the armature conductors would be doing maximum work. Should an additional coil be added to the ideal generator, at right angles to the first coil, the capacity of the machine will be doubled. This complicates, to some extent, the collector rings and may necessitate the opening of the circuit when current is flowing so as to cause sparking and burning of the brush. A machine built on these lines would, therefore, be better adapted for generating small currents as the sparking at the brushes would be otherwise very destructive to the commutator. Machines of this type are known as open coil.
The next important step was made by Gramme and Pacinotti, which was to close the coils with themselves so as to form a con tinuous circuit in the armature and connect one collector section to each coil at its junction with the next one, the number of sections being the same as the number of coils. In the four coil armature, the current generated can either pass to the collecting brush directly, or when it moves out of position so that the contact is broken and made with the next section, the current can flow through the armature coils to the same brush if necessary, and when that coil passes from one polar position to another and is giving current of opposite polarity this current can flow directly to the other brush, and so continuous current is generated. There is also no point at which the circuit is open. There may be a slight sparking as the section moves from the brushes, but violent sparking is reduced as there is always another path for the current to flow to either brush.
The drum armature is distinct from the ring armature in that the wires are wound on the outside of the core and do not pass through it. This type is frequently called the "Siemens' armature on account of the number of suc cessful machines built by Siemens. Of the whole number of lines of force passing between the poles and through the core, there are very few lines passing in the inside, they being di verted by an iron core so that they pass through the wires on the outside of the core; the con ductors inside of the core are thus of little use, their only function being to complete the cir cuit and carry current between the successive turns on the outside of the core; so that by winding the wires on the outside surface only, the amount of idle wire is reduced, the only material that is not active being the cross-con necting pieces at the ends.
The Gramme ring was used very largely on early machines for the reason that it afforded means for easier mechanical construction, and machines of this type were generally successful, on account of their simplicity. Pacinotti de signed a core having teeth similar to a gear wheel. In this way the air gap between the armature and pole pieces could be reduced somewhat, resulting in an increased number of lines of force. It also afforded an additional support to the coils and added to the mechan ical strength of the machine.
To be considered next are the field magnets: There are a number of constructions which may oe employed. (1) The so-called permanent 3teel magnet which consists of a bar or bars of steel bent to the shape desired, tempered and magnetized. The method of magnetizing these magnets consisted of placing them in contact with other magnets or with an electro-magnet. The present method would be to insert the steel bar into a helix carrying a heavy current and in a short time the bar would be magnetized. The dynamos built with permanent steel mag nets of this type are what is lcnown as magneto lynarnos. The chief objection to this form of magnet is that a steel magnet cannot be made as powerful as an iron magnet which is ener gized or, as it is commonly called, excited from a source of electricity. In the first generators permanent magnets were used, but a great step in advance in dy-namo design was to arrange the magnet poles so as to be self-excited. A portion of the current generated in the arma ture is sent around the coils wound around the cores of these field magnets so as to excite them. At first, however, magnets were substituted consisting of soft iron upon which was wound a coil of copper wire, the current for energizing these pole pieces being first supplied from a small magneto generator or a voltaic battery. Sometimes the machine will not generate on starting up not having sufficient current to ex cite the magnets and it is necessary to excite them f rom some external source so as to give the initial strength to the magnets and allow them to build up from the current gen erated in the armature. It is usually found that there is sufficient residual magnetism left in the iron of the field magnets, aftcr the machine has once been in operation, to start the current in the machine and properly build np the fields.