Evolution of the Squirrel Cage Armature.—In the early experiments with rotating magnetic fields, copper discs were used; in fact, it was then discovered that a mass of copper or any conducting metal, if placed in a rotating magnetic field, will be urged in the direction of rotation of the field.
Ferraris used a copper cylinder as in figs. 1,710 and 1,738, which was the first step in the evolution of the squirrel cage armature. The trouble with an armature of this kind is that there is no definite path provided for the induced currents.
SO CALLED SQUIRREL CAGE
Figs. 1,738 to 1,744.—Evolution of the squirrel cage armature. The early experiments of Arago, Herschel, Babbage and Baily demonstrated that a mass of copper or any conducting metal, if placed in a revolving magnetic field, will be urged to revolve in the direction of the revolving field. They used discs, but Ferraris used a copper cylinder as shown in figs. 1,710 and 1,738; this was the first squirrel cage armature. Figs. 1,739 to 1,744 show the gradual development of the primitive device shown in fig. 1,738; fig. 1,739, Ferraris' cylinder with slots restricting the path of induced currents; fig. 1,740, Dobrowolsky's so called squirrel cage which he embedded in a solid iron core, as in fig. 1,741; fig. 1,742, design with insulated bars and laminated core to prevent eddy currents in the core; fig. 1,743, laminated core with ventilating ducts; fig. 1,744, modern squirrel cage armature representing the latest practice as built by Mechanical Appliance Co. The core is built up of discs punched from No. 29 gauge electrical sheet, insulated from each other and firmly clamped between end plates locked on the shaft. The slots in the discs are of the same general form as those in the core. Heavy fibre end pieces, punched to match the discs are placed at each end of the core, to prevent the bars coming in contact with the sharp edges of the teeth. The winding is made up of rectangular copper bars, passing through slots in the core, and short circuited on each other by means of copper end rings of special design. The bars are pressed into holes punched in the end rings, and the contact is then protected from corrosion by being dipped in a solder bath. The bars are insulated from the iron of the core by fibre cell projecting beyond the end of the slot. To secure ventilation the short circuiting rings are set some distance from the end of the core. In this way the bars between the core and the ring act as the vanes of a pressure blower, forcing a large volume of air through the field coils and ventilating openings.
