The Montgomery advertiser., 1901-08-30
The ADVERTISERS
 ~ HOME STUDY CIRCLE

I tional to the speed and to the strength of the field.
 The energy of any generator of power Is measured by the product of its "torque," or rotative effort and speed. In an electric motor the speed is proportional to the generated c. e. m. f.: if the field magnet is of con- stant strength the torque will be pro- portional to the current in the arma- ture wires. Some of the e. m. that Is impressed upon the motor is not use- full for producing speed. Just as in an engine, some of the steam pressure | Is wasted in overcoming the friction of the piston; so in the electric motor. some volts are used in overcoming the ''ohmic" resistance of the wires. This wasted voltage will be directly propor- | tional to the current used. The lower | the resistance of the armature wires 1 the less will be this loss, just as in the steam engines, the better the fit of | the piston the less the wasted pres | sure. If an armature could be wound | of wire having no resistance there ! would be no volts wasted, but all would | be available for producing speed; that | is, the armature would revolve at such | a speed that its c. e. m. f. would just equal the applied e. m. f. It Is fal- lacy, both of speech and of principle | to state that for high potential the armature must be wound with high l resistance; the resistance is just as much of defect in a high potential I armature as in a low one To adapt the armature for higher potential there must be proportional increase in the number of conductors: as the i space is limited smaller wires of higher resistance must be used. but this in- l ;- unavoidable.
 | For stationary purposes motors are usually provided with shunt- wound field magnets. identical in construc
 ton with plain shunt-dynamos. Is the supply is at constant potential, as from an incandescent light circuit, the l field magnets will be of constant strength and the armature. indepen- dently of the load, will reyolye at almost uniform speed Whatever variation of speed takes place is the fault of the armature resistance If an armature has a resistance of one ohm and is connected to 110-volt circuit the speed will be, as In the case of any prime mover. highest when run ming free-no belt on; the armatura requires only current enough to estab- fish torque, sufficient to overcome friction of bearings and brushes. eddy currents in armature core and few other smaller losses. Suppose the cur rent to be one-half amp, the e. m. f. to drive this current through the arma- ture wires will be i multiplied by D, equal to 5 volt--that is. the speed would be such as to generate c. e. m. f. of 1895 volts. which. added to the 5 volt. would equal the applied 110 volts. Suppose now fun load Is ap- xHli amp. is required; the lost volts will be multiplied by 10, equal to 10; the speed will have lessened until the c. e. m. f. equals only 100, the other ten be- ing wasted in overcoming ohmic resis- tances. The speed will be about 10 per cent less than before. If the arma- ture had only five-tenths ohm resis-
 tance the reduction in speed would have been only per cent. Converse ly. if the same machine was to be used as generator some volts additional to those needed in the exterior circuit would be required for overcoming the same resistance; to supply 110 volts at the brushes and a ten-amp. current the armature must revolve fast enough to generate 120 volts, for ten would be wasted in forcing the current through the armature wires. As the armature resistance is decreased its speed as a
 motor and generator approach each other. In given case a 10 h. D. motor at full load revolves at 800 revolutions, as k. W. gen. it requires to be driven at $50 revolutions. One method of determining the proper speed for a generator is to let it run as a motor and then provide for driving 1t a little faster; if driven slower 1t will either generate very feebly or not at all The direction of rotation is also learned i; ordinarily the winding Is so covered with canvas that the routes either in armature or field cannot be followed
 By trying the shunt machine as a motor adjusting the brushes to the position of least sparkling, speed and current, the adjustments are nearly } right for generator and the armature should be driven in the same direction.
 A motor that is to provide varia ble speed, as one attached to an elevator or street car, should have it's field magnets connected in series with the armature; if sufficient iron is used in the field cores the torque will vary not directly as the current, but as the square of the current; for doubled current means also twice the strength of field magnet Such motors given a powerful starting torque. As will be soon explained, they know no limit to speed and their use must be confined
 to cases in which the load Is inseper-
 ately connected; otherwise the arma- ture would immediately attain such a speed as to burst itself. Originally series motors were the only kind known. and were operated on arclight circuits. To protect the armature from excessive speed centrifugal gov ernors were attached, which moved arms over contacts in such a way as to cut out successive portions of the field windings or to rock the brushes into less favorable positions
 As the amount of current was fixed by virtue of the regulator on the dynamo,
 the torque of the motor would be 1, properly reduced, Such motors were shut down by short circuting them. no rheostat needed in starting. For use as a generator a series machine must be rotated in the opposite direction
 It will generate at any speed if the exterior resistance is low enough.
 On a constant potential system the current Is unlimited, 'and starting from a position of rest, when an armature Is not generating any (. e. m. f. re- sistance must be inserted in the arma- ture circuit and gradually removed as the speed increases. When fully under
 way the C. e. m. f. Is the controlling , element. Suppose the motor just illustrated. was started without a rheostat; armature resistance is one | ohm then by Ohms law the current