A dynamo transforms mechanical into electrical energy, and a motor transforms electrical into mechanical energy. The two operations are reversible, and may be effected in the same machine; a dynamo may be used as a motor, or a motor may become a dynamo.
_A dynamo is a motor when it is driven by a current of electricity, and it is a dynamo when it is driven by mechanical power and produces an electric current._ If a motor be driven by an engine, it can deliver a current of electricity which is able to operate other motors or electrical apparatus or lights. A simple form of electric machine is shown in Fig. 236, which is a general form of the electric motor. In this there are two projections of steel, _H_ and _G_, which are made electro-magnets by the current flowing through the wires wound around them from any source of electricity, such as a battery at _I_ and _J_. These magnets have poles facing toward an armature, _K_, on a shaft. The poles _G_ and _H_ are called the “salient” poles; the poles _M_ and _P_ are called the “consequent” poles. The magnetic flow or field is shown by the dotted lines. On the periphery of the armature are wires in the slots shown. As this armature revolves, there will be a tendency for electricity to flow through the wires.
In order to distribute a current of electricity through these wires it is necessary to make a complete circuit. As each of the wires in the slots passes in front of a pole, a pressure or electro-motive force will be generated, and its direction will depend upon whether the pole is a North or a South pole, _i. e._, + or -.
NOTE.—In the above illustrations I and J represent the ordinary electric battery; in electrical literature such marks always indicate a battery.
The pressure or electro-motive force generated in the wires moving in front of the North, or positive field poles, will be in one direction, while that of those in front of the South, or negative field poles, will be in the opposite direction. Therefore, if two such wires be connected together at one end of the armature, the free terminals of the wires at the other end of the armature will have the sum of the electro-motive forces generated in the two wires. The wires so connected can be considered as a turn of a single wire instead of two separate wires, and this turn may be connected in series with other turns, so that the resulting electro-motive force is the sum of that in all the turns and all the wires so connected. It is customary to connect the coils of an armature so that the electro motive force given is that obtained from half the coils in series. The other half of the coils is connected in parallel with the first half, so that the currents flowing in the two halves will unite to give a current in the external circuit equal to twice the current in the two armature circuits or paths.