Commutators.—A commutator consists of an assemblage of small forgings or castings of copper called segments, Fig. 3, which are thor oughly insulated from each other. These seg ments are assembled around a tube. The tube is threaded at both ends to receive nuts to hold the segments together. Mica is usually em ployed to insulate the segments from each other, from the nuts and from the tube. In addition to its high insulating properties, mica possesses the advantage that it wears under the brushes at about the same rate that copper does. This maintains a smooth surface at all times upon the surface of the commutator. The maximum voltage between adjoining segments is seldom allowed to exceed 20 volts. Assuming that the voltage is uniformly distributed, a 240-volt ma chine with a closed winding would therefore have 12 bars between adjoining brushes, or for a two-pole machine 24 commutator segments. In Fig. 3 the perpendicular projection, "tP," of the commutator segment protects the coils on the armature, and is made to receive the ex tremities of the coils of the armature windings. The wires in some cases are fastened to the lugs, "P," with set-screws and then soldered. The commutator complete is keyed to the arma ture shaft.
Brushes.— Carbon and copper are the two materials usually selected for brushes; carbon for high potential machines and copper for low potential machines. When a coil is entering commutation it has a current of electricity flow ing in it. The direction of this current changes when the coil is commutated. The current flow ing in the coil before commutation tends to keep up because of its self-induction. In the time it takes a strip of insulation to pass under the brush, the current flow in the coil must be stopped, and a current of equal value started in the opposite direction. Both operations may be done by counter E. M. F., or the current flow may be stopped by a high resistance and started by fringe flux. The transition resistance of brush to commutator usually causes a fall in the potential of about one volt for every brush. The pressure of the brushes against the com mutator varies, the average value is about 1% pounds to the square inch. The necessary area of rubbing surface of carbon brushes is one square inch for 40 amperes, and copper brushes permit of 200 amperes for the same cross section. See Fig. 5.
Field Magnets and Field Frame.— The field magnets produce the magnetic flux in which the armature rotates. The flux is cre ated by a current of electricity traversing many turns of wire which are wound upon iron cores. A north or a south pole is produced, depending upon the direction in which the current flows through the coil. The coils on the field magnets are connected in series and so arranged that the polarity changes consecutively from pole to pole. Modifications are sometimes made to this form of •winding when it is desirable to produce a machine which will supply a three wire circuit. The machine is then designed as a four-pole machine having two adjoining north poles and two south poles similarly placed.
The laws governing the flux of magnetic lines are similar to the laws of current flow.
The flux which will flow in a magnetic circuit is equal to the magnetizing force .(termed the magnetomotive force) divided by the reluctance of the circuit.
Magnetomotive Force Flux =4= Reluctance The reluctance of a magnetic circuit is a property analogous to the resistance of an elec tric circuit. To obtain a maximum flux through an armature with a given magnetomotive force it is necessary to reduce the reluctance of the magnetic circuit to a minimum.
In the design of a generator it is often con venient to first design the armature and then obtain the magnitude of the flux which must be generated by the field magnets. Having se lected a given material for a field frame, a given air gap, a given length and cross-section of magnetic circuit, the total reluctance of the cir cuit is determined. The reluctance varies di rectly as the length of the circuit, directly as the reluctivity of the material and inversely as the cross-section. Knowing the total flux and the reluctance of the circuit, the magnetomotive force is determined, and this quantity divided by 1.257 gives the 'number of ampere turns necessary to produce the magnetic field.
Field Coil Generators may be shunt wound, series wound or compound wounds. (See Fig. 4.) In the series generator the armature circuit, the field circuit and the ex ternal circuit are all in series. Series motors are used for traction purposes as they give a large starting torque. Series dynamos are used for arc lighting, as they generate a constant current. Shunt wound machines have their field circuit, their armature circuit and their external circuit connected in multiple. A shunt machine is designed to generate a constant potential, or when operated as a motor to run on constant potential. The field coils of a generator may be separately excited, in which case an ex ternal source furnishes the current for exciting the coils. Separately , excited machines are usually employed in large power-houses where it is desirable to have a flexible system, which means the ability to vary the pressure sent out on the line at any instant of time. In the United States where 40,000 horse-power gen erators are now in successful operation, each machine is taken as a unit and the control of the exciting current for the fields is a very important factor. The machines are separately excited; the exciting dynamo being supple mented by a bank of storage batteries as an emergency. A compound winding consists of an additional winding upon the field, this wind ing being connected in series with the line. It may be placed in series with the armature and the field connected in multiple, or it may be con nected in series with the armature alone, its connection depending upon the percentage of compounding required. By compounding a machine the strength of the magnetic field is increased in proportion to the current generated by the machine, the percentage depending upon the number of turns in the compound coil and as to the way in which it is introduced in the circuit. If a shunt machine be not compounded, the potential falls with an increase above nor mal of the generator output.