Methods of dynamo electric machine may be separately excited or self excited. When self-excited the machine builds up slowly to its normal potential and when dis connected from the main circuit the field mag nets gradually discharge themselves as the speed of the machine decreases. The polarity of a self-excited machine may become reversed or the machine may fail to generate sufficient pres sure to magnetize the field magnets. Both of these conditions are serious. When the field coils of the separately excited machine are con nected to an external circuit they definitely as sume their proper polarity and there is no pos sibility of this polarity becoming reversed. In addition to this the field magnets of the sep arately excited machine rapidly build up to the point where the machine generates its normal potential. The disadvantage of this machine is that when disconnected from its external excit ing source its fields must be slowly discharged as there is danger of the fields discharging through the insulation of the machine, if it is opened suddenly.
Speed Consider a motor oper ating under normal conditions from an external supply of E. M. F. The armature of this motor is rotating in a magnetic field and therefore has an E. M. F. induced in its armature wind ings. The direction of this E. M. F. is such as to tend to send a current of electricity in the opposite direction to that passing through the armature under the influence of the external supply. This E. M. F. is termed the counter electromotive force. The pressure of the ex ternal source remains approximately constant and the counter E. M. F. varies with the arma ture speed. As the motor is loaded its speed tends to decrease. This decrease diminshes the counter E. M. F., and a larger current is permitted to pass through the machine. The difference between the impressed and the counter E. M. F.'s divided by the resistance of the armature gives the magnitude of the armature current. It is obvious that a very large current would flow through a station ary armature of low resistance, if the arma ture were connected directly to an ordinary commercial lighting or power circuit. To pre vent this large current flow, devices called start ing boxes are employed for starting motors. The function of a starting box is to first com plete the exciting circuit of the field magnets — and then gradually to cause a rise of the im pressed potential upon the armature terminals, from zero to that of the supply circuit. The latter is accomplished by gradually diminishing resistance which is in series with the armature until the speed of the machine is almost normal, when all the resistance is entirely removed.
Nikola Tesla was the first to see the pos sibilities of generating alternating currents. This invention is called Tesla's rotating mag netic field, and it made possible the present long-distance transmission of power by elec tricity. This condition has led to expansion of territory by the users of alternating-current dynamos, until practically all companies en gaged in the distribution of electricity have settled down to the same practice, generating current with alternators of the polyphase type at large central stations and transmitting it at high pressure to substations, where it can be sent out under lower pressure, as either direct or alternating current, or used to charge storage batteries.
Induction Motors.— In the year 1888, Mr. Tesla introduced his dynamo electric machine, which is now termed an induction motor. This motor had two great advantages over the direct current motor, in that it possessed no com mutator or brushes to be cared for, and it could be operated by alternating currents. It possessed the disadvantage that there was no ready means of regulating its speed. There is considerable difference between the construction of an induction motor and that of a direct-cur rent motor. Induction motors have a revolving element whose laminations are somewhat simi lar to those of the armature of a direct current motor, as they fulfil the same function. The movable part of an induction motor is called the rotor and the stationary element surrounding the rotor is called the stator. In a common form of rotor, called the squirrel cage, the in ductors are insulated and embedded diagonally in slots around the periphery, and at each end of the rotor these inductors are connected to gether to a common ring. The coils of the stator which create the magnetic field are so wound that several out-of-phase alternating currents have their magnetic actions superim posed upon each other and thus produce a field which continually shifts. This field is termed a rotating field. The stator field induces cur rents in the rotor which react upon the field and cause the rotor to revolve. .The motion of the rotor is not synchronous with respect to the field of the stator, but slower. The ratio of the deficit of the rotor speed to the speed of the stator field. is termed the slip of the induc tion motor.