Transmission: Polyphase Induction Motors

Friday, July 1, 1898
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THE _IOURNAL the current in such large motors “ lags " behind the electromotive force, and hence the useful effect is less than would be the case if the current and electromotive force were coincident in phase. The current, however, exerts its full heating effect upon the con- ductors, and the excess current causes a corresponding drop of potential in the circuit. It is therefore desirable to reduce the lag as much as possible; i. r., to bring the real and apparent energies -e at. \f- __ - _» » » 0 . .4 ,. ' ` '- nf ,f -5 .~., tai ......, V _ ‘ OF ELECTRICITY. 35 motor, but when the switch is thrown in the opposite direction the anto»converters are cut out and the motor is connected directly to the circuit. Three or four wires are connected directly to the auto-starter, and the same number of wires connect the anto- starter to the motor. The anto~converters are arranged with loops so that one of several voltages may be applied for starting, thus adjusting the torque for the work the motor has to do. Thus a motor operating a machine having great inertia may be made to give a correspondingly strong starting torque, or one driving a light device may be adjusted for ex- tremely light starting torque with a corresponding re- duction of current. On two-phase circuits in which there are four wires, different electromotive forces exist between the differ- ent pairs of wires, and in some cases this affords a simple way of obtaining a low electroniotive force for starting. A simple double throw switch is required, by which the low electromotive force for starting is obtained by one -, -. connection, and the full electromotive force for running 1), by throwing the switch to the opposite position. - When it is necessary to install motors in a grain eleva- tor, a woolen mill, a mine, or in any place exposed to indammahle gases or floating particles, or not easily ac- cessible, the starting devices may be located at a con- venient point more or less remote from the motor, thus eliminating all danger from Ere due to possible sparks. 1 n 'N r P4 \ ' t t Y t is argl’ ,,, ,_ “pc `” fa x .- . 1 af"/' Q gtk / f ~ we v'.f/’.‘- \ \' v if -,, J \ ,, , ~ 3 ‘ sf 4 ’ ~ , . 1 l It l < l / _ | . . J ‘ .~ l L '1- lfm. 1.-PRIMARY, WITH WINDING CUt\1I*I.F2'l`E, to coincide as near as we can. If it were possible to entirely elim- inate the lag, the real and apparent horse-power would be equal, and the power factor would be too. Special attention has been given to this matter in the designing of these machines, with the result that their power factors are very high. The designs and principles governing the construction of the type “C” motor are fortunately favorable to a particularly at- tractive feature; i. zz, the maintaining of n high and almost con- stant ediciency from full load to one-half load. Under the conditions which prevail in a very large proportion of electric motor service, the_inotors are operated much of the time at con- siderably less than their full rated capacities. With the type “C” motor, therefore, by maintaining a constant efhciency, there can be secured to the user an all day or average efficiency very much above that heretofore possible, with either direct or alterna- ting current motors. The variation of speed between no load and full load is small, beingless than that found in direct current motor practice. A polyphase induction motor maybe started by connecting it directly to the circuit with an ordinary switch, and small motors are so started in practice. The larger motors, however, are started on a reduced voltage, the full electromotive force of the circuit not being applied \\ntil the motors have reached a considerable speed. In the Westinghouse system, the low electromotive force is or- dinarily obtained by the auto-starter, shown on page 37, and which consists of a double throw switch mounted on a cast-iron box in which are t\vo auto~converters. When the switch is closed in the position shown in the c\\t, the auto-converters are connected across the» circuits and deliver a low electromotive force to the __. ,Z-5 ,aaa yuh#-**’ ;‘§'»»<:e-»¢¢"“‘*"3 4° Ag; =v.\-\?t-383| ,M » a’3¢"iU ¢:,1 a - ,,_:“‘..Ll 1? in if 1 ,bf PEL Eli , . _-_:.*"` t,`T:_- _ ,,.; ,._:=,;§§g3;:;.~;f '2=~‘__..“ fe':.__ - -I -- _ » _ .__.-. ~ _ gg; fx., ~, . \ ~.\ M-f ~» ;-.___,s_ - .t-ef (_: __ f_.. f~ ,¢e»»...'-M1 .s r nz ’ ua ":‘=_f,' ,, --f _ f":! l il, .e ._ , 1 ..= . sf " 1f“*"@5'» ~ * s;“é~5" ~~~ff;4 `“` “9;°`~5° ` f ,_ __ _ Y- 3, I, . _;é:,.;L , , 'ig ,Tig ,.,,,. , ,_ wt ttf :< ;,“j§ _ 2 » = . t ` wr' ‘ ii; _ ~ ,M s . if t it r the ,_ ~ ~~,; ~ 3 ‘ . , , I E ' ~ L' l _ 1 1` ~' t , N "~», » L - .4 _ $55 ""“t (V 1 \ . . *I ».,, t vm, 3.-snconnam' comummra, snownm corfr-an sans commcrian To man amos.

34 THE JOURNAL OF ELECTRICITY. .lulyl898 » » ’ 9 TQUSHTI SSI O11 POLYPHASE INDUCTION MOTORS. l.'1`l€RNA'I`lNG current motors, although developed and per- fected much later than direct current motors, were soon recognized to possess such great practical advantages that they have hy rapid strides largely supplanted the latter in mauv important applications. They have, in fact, dune more; being advantageously \\sed in places where electric po\\‘cr could not he previously employed by reason of some of the inherent characteristics of direct current apparatus. Alternating current apparatus, owing to the held which it origi- nally occupied, was for a long time looked upon by many as ap- plicable only to the transmission of power over long distances. The extreme simplicity and Hexibility of the alternating system, hon-ever, added to the attractions of the polyphasc motor, have possible clcnicuts; i. r._ :\ stationary part. permanently connected to the nmin circuits, and a rotating part having no electrical con- nection with any other portion, and absolutely no electrical contacts or adjustments; in fact, no sliding or working friction except that of the shaft in the journals. It will therefore be ob- scrverl that as a piece of moving machinery this type of motor is designed and built to operate for long periods of time with but a very small amount of cleaning and an occasional renewal of oil. The hollow cylindrical frame of cast iron shown in Hgnre 2, in which the primary is mounted, forms a base for the machine and also supports the two brackets carrying the self-oiling be:\rings. Ordinarily, perforated iron plates fitted into these brackets protect the rotating element and permit excellent ventilation, but they may be replaced by solid plates when it is desired to make the motor absolutely dust-proof. It will be noted that the housing completely encloses the primary and secondary elements, and protects them from any external damage. The primary element, also shown in Hgnre 2, is built up of lami- led to the use of polyphase apparatus for _ the distribution of power in factories, V_ ` ww __ llafell Sheet iffm flllgsv Sloued U" the mills, mines, and other industrial estab- '§,V':f"l rg, » Hi V, V lllslde 1° "i°€“`5 the °°“d“"[°f5- These lishments where the distances are com- H ‘~=. ', " NY* ii" fmgs 3’¢_nl:l1dl}f Sl-‘PP°f'~Ed l’Y Ute Cast paratively short. A large factor in the V gm V - V iron hous1ng,wlnch encloses the primary. Present p0p,,1a,,ity of Rue, ‘ V ~ 1 The conductors are machine ,,a,,,,g ,,,m.e,,t power is due He. Z jk," " V "VV wound coils, which are thor- tothe high state ofperfection ,~¥ » ~ I _ _ ;“§lll}’ fllsgaledtll’ Eff’ Ye to which the improved poly- tiiyffi ` ‘ , ~` _ ~ Clllg P ECG! 011 10 C0l'P~ ,’,~_ f_, V; » ,