In regard to field windings, two distinct types are used: (1) the series winding, in which all the current generated in the armature passes around the field poles and thence out to the line or circuit ; and (2) the shunt winding in which a portion only of the current is used in the field, the connection being made across the main terminals of the generator. In the first case the wire on the field windings is necessa rily large so as to carry all the current for which the machine is designed and in the sec ond case it is a small wire of many turns, the product of amperes and turns being about the same in either case. In another design, both a shunt and a series winding of a few turns is employed, constituting a compound winding.
It will be seen that in the first case, that of the series winding, the field strength will de pend upon the resistance of the total circuit, including the resistance of the artnature, the field winding and the external circuit. In a machine of this type the voltage or pressure generated will vary in proportion to the de mands. This is the standard winding for the senes arc machines used for city lighting, such as the Brush and Wood types. In the case of the shunt-wound machine the current flowing m the field coil depends upon the pressure be tween the generator terminals, so that with an Increased output and consequent loss in the armature the voltage will fall off slightly, thus reducing the field strength. This necessitates some means of. varying the field current so as to maintain a uniform pressure at the gener ator terminals. This is usually accomplished by means of an external resistance in the field circuit composed of German-silver or iron wire which can be varied by means of a switch-head so arranged as to cut out certain portions of this resistance step-by-step and so increase the current through the fields, thus preserving a uniform voltage.
In a combination of series and shunt wind ings commonly called the compound type, as the output of the generator is increased, there is a greater flow of current through the series windings and consequent increase of magnetic strength of field so that it is possible to com pensate for the loss due to the resistance of the armature windings and maintain a uniform voltage at the generator terminals. The vol tage as well as the output of the dynamo de pends upon the strength of the field mag-nets, the magnetic permeability of the material and the rate at which the lines of force are cut by the armature conductors, so that the higher the speed the greater the voltage output of the dynamo. In the early machines very high speeds were common, armatures of small diam eters being employed. These were objection able for mechanical reasons so that the design was changed in order to increase the number of pole pieces. Instead of the field being com posed of two poles, it was arranged so that a greater number of poles could be used, this type of machine being known as the multipolar dynamo. As each conductor would pass be tween A number of poles during each revolu tion the speed could be proportionally reduced.
The dynamo, sts previously stated, is a ma chine for converting energy in the form. of mechanical power into electrical power, or vice versa, so that a motor is a machine for convert ing energy in the form of electricity into me chanical power. The early types of motors
were based on the principle that a ffiagnet would attract the opposite pole of another mag net, and if one set of magnets is arranged on a wheel, and the other stationary, the movable magnets will be drawn around. To make this effective it will be necessary to interrupt the forces at what might be called the dead centres so that the wheel would have continuous mo tion. This is accomplished by either introduc ing a screen, or, more satisfactorily, by the use of electro-magnets with a movable contact so that the magnets are energized intermittently, allowing the wheel to revolve in accordance with impulses received from the magnetic poles.
When we consider the dynamo as a motor, the current supplied to the terminals may take two paths, one through the armature and the other through the field coils. The field current energizes the pole pieces, and the current trav eling in the armature is similar to another mag net inasmuch as a coil carrying the current will be attracted or repelled by a magnet according to the direction of the current through the coif, so that the wire will be forced around by at traction and repulsion. By considering the effect of the commtitator the motion is seen to be continuous. When the armature starts to revolve the conditions then existing will be similar to the armature in action as a dynamo, and an electromotive force will be generated in the armature wires, which will ,be in the oppo site direction to the incoming current. This is what is called the counter electromotive force of the motor, and will tend to reduce the amount of current which will flow through the armature conductors. It is, therefore, evident that when a motor is started there will be a rush of current through the armature, as the resistance is very small, and as there is no counter electromotive force while the machine is not in motion to check the flow. For this reason, in the direct current motor it is neces sary to introduce an external resistance into the armature circuit to hold back the current which would flow, until the machine approaches full speed. The resistance is then gradually re duced until full speed is obtained. The effect of this counter electromotive force when the resistance is cut out entirely is materially to assist the self-regulating qualities of the ma chine. Any load applied to the motor would tend slightly to reduce the speed, which effect, by also reducing the counter electromotive force and allowing more current to flow through the armature, tends to keep the speed from falling much below normal in the shunt motor. Motors can be built either with a plain shunt field winding or with a series and shunt winding, depending on their requirements. The direction of rotation depends on the direction of the current through the armature. To re verse the rotation, therefore, it is only neces sary to reverse the current in the armature, leaving field connections as they are. If the current is changed in both field and armature, the result would naturally be that the machine will continue to revolve in the same direction as before.