A squirrel-cage rotor is the rotating part of the common squirrel-cage induction motor. It consists of a cylinder of steel laminations, with aluminum or copper conductors embedded in its surface. In operation, the non-rotating stator winding is connected to an alternating current power source; the alternating current in the stator produces a rotating magnetic field. The rotor winding has current induced in it by the stator field, like a transformer except that the current in the rotor is varying at the stator field rotation rate minus the physical rotation rate. The interaction of the magnetic fields of currents in the stator and rotor produce a torque on the rotor. By adjusting the shape of the bars in the rotor, the speed-torque characteristics of the motor can be changed, to minimize starting current or to maximize low-speed torque, for example. Squirrel-cage induction motors are very prevalent in industry, in sizes from below up to tens of megawatts (tens-of-thousand horsepower). They are simple, rugged, and self-starting, and maintain a reasonably constant speed from light load to full load, set by the frequency of the power supply and the number of poles of the stator winding. Commonly used motors in industry are usually IEC or NEMA standard frame sizes, which are interchangeable between manufacturers. This simplifies application and replacement of these motors. History Galileo Ferraris described an induction machine with a two-phase stator winding and a solid copper cylindrical armature in 1885. In 1888, Nikola Tesla received a patent on a two-phase induction motor with a short-circuited copper rotor winding and a two-phase stator winding. Developments of this design became commercially important. In 1889, Mikhail Dolivo-Dobrovolsky developed a wound-rotor induction motor, and shortly afterwards a cage-type rotor winding. By the end of the 19th century induction motors were widely applied on the growing alternating-current electrical distributions systems. Structure The motor rotor shape is a cylinder mounted on a shaft. Internally it contains longitudinal conductive bars (usually made of aluminium or copper) set into grooves and connected at both ends by shorting rings forming a cage-like shape. The name is derived from the similarity between this rings-and-bars winding and a squirrel cage. The solid core of the rotor is built with stacks of electrical steel laminations. Figure 3 shows one of many laminations used. The rotor has a larger number of slots than the stator and should be a non-integer multiple of the number