Document BG36MwVEvjoE6gNbVjXRokOj

* mi::4;' 15 Oldgrem of rovenibla mognetfc drive fhof Includox a fynchronovt affernatora/id on Induction motor, ai shown In 16, The motor has two distributed field windings REVERSIBLE DRIVES A cross-section of a reversible mag netic drive described in Engineering (London) Oct. 17, 1941, is shown in Fig. 16. It consists of an alternator and a magnetic coupling, the equivalent of an induction motor. The alternator's field poles P and the rotor R of the induc tion motor are fixed to the input shaft. The alternator's stator is supported in a frame bolted to the unit's bedplate. Field core and windings IV for the in duction motor are mounted on a spider keyed to the output shaft. This core is slotted on its outer periphery and has ACKNOWLEDGMENTS two 3-phase star-connected windings. For one direction of rotation one wind ing is excited by dc, and for the other direction the other winding is excited by 3-phase ac, as indicated in Fig. 15. For the output shaft to operate in the same direction as the input one, and at practically the same speed, the power unit is started and brought up to about half speed. Then, excitation is applied to the magnetic drive's dc winding, which produces magnetic poles in core IV as with a salient-pole winding. As the squirrel-cage winding in core R rotates through the magnetic field, current is generated in it that reacts on the latter to produce torque as in an induction motor. This torque accelerates the out put shaft in the same direction and up to about the same speed as the input shaft. In this direction of rotation, oper ation is the same as for the other 2element magnetic drives described previ ously, the alternator remaining inactm For reverse direction of the drive, I' dc winding of the induction motorJ disconnected, the alternator's field ej|| cited and its windings connected to'lgl ac winding on the induction motoitl rotor. Current from the alternator'?! generated at a frequency that produij a rotating magnetic field in the indiiej tion motor's rotor of twice the speed;"* the ifiput shaft and in the opposite rection. This is necessary for revi rotation, since mechanically the oul member of the magnetic coupling off tinues to rotate at input speed. A reveh magnetic field of twice input speed in posed on this motion then results inp . reversed field of normal speed so thj the output shaft runs at normal spc& in reverse direction. | If the outer member connected to thj input shaft has ten poles and is driiio at 300 rpm when excited with dc, it proj duces a 25-cycle forward-rotating mat netic field. To generate a correspondini field in reverse direction it is necessarj to apply a 50-cycle rotating magntdj field in relation to the outer memKu However, owing to the forward ratafia of the outer member, the over-all effeg relative to the rotor is a 25-cycle revere field and a corresponding speed. Mechanically the coupling elemuitr may be arranged in different ways. For simplicity the alternator has been shown, separate from the magnetic coupling but a commoner arrangement is a com bination of the two in a single unit. This1, drive may be considered as a normal? magnetic coupling in which an alternator; on the input shaft supplies an induction motor on the output shaft. However,; it has the advantage that the alternator; is seldom used, and is idle during lor-j| ward rotation of the propeller. There-! fore it can be rated on a short-time bash: and be considerably smaller than for the; usual continuous-duty electric drive, In preparing material on mag netic clutches Power Is Indebted to Cutler-Hammer Inc, pings Magnetic Separator Co, Stearns Magnetic Mlg Co, all of Milwau kee, WIs.; and to Twin-Disc Clutch Co, Racine, WIs. Material from which tho elec tric-coupling and magnetic-drive articles wore written was sup plied by Dynamatlc Carp, Keno sha, . WIs.) Electric Machinery Mfg Co, Minneapolis, Minn.; Elliott Co, Rldgway, Pa.; Gen eral Electric Co, Schenectady, N. Y.; and Westlnghouse .Electric ' Corp, East Pittsburgh, Pa. 'A/ternaloti sjo>Sr,\ ,Three^phcse>. "'"'jy.Maqmhc driver^ * 86 1961 TOWER Febrvory lWifi