![]() ![]() Generally, thick bars have good torque and are efficient at low slip, since they present lower resistance to the EMF. Rotors for three-phase will have variations in the depth and shape of bars to suit the design classification. The same basic design is used for both single-phase and three-phase motors over a wide range of sizes. The material is a low carbon but high- silicon iron with several times the resistivity of pure iron, further reducing eddy-current loss, and low coercivity to reduce hysteresis loss. It is made of thin laminations, separated by varnish insulation, to reduce eddy currents circulating in the core. Because the magnetic field in the rotor is alternating with time, the core uses construction similar to a transformer core to reduce core energy losses. The iron core serves to carry the magnetic field through the rotor conductors. The constructions that offer the least feedback use prime numbers of rotor bars. ![]() The number of bars in the rotor determines to what extent the induced currents are fed back to the stator coils and hence the current through them. The greatest common divisor of 36 and 40 is 4, with the result that no more than 4 bars of the stator and rotor can be aligned at any one time, which also reduces torque fluctuations. The laminations shown in the photo have 36 bars in the stator and 40 bars in the rotor. If rotor bars were parallel to the stator poles, the motor would experience a drop and then recovery in torque as each bar passes the gap in the stator. The conductors are often skewed slightly along the length of the rotor to reduce noise and smooth out torque fluctuations that might result at some speeds due to interactions with the pole pieces of the stator, by ensuring that at any time the same fraction of a rotor bar is under each stator slot. The difference in speed is called slip and increases with load. In effect the rotor is carried around with the magnetic field but at a slightly slower rate of rotation. In turn these currents lengthwise in the conductors react with the magnetic field of the motor to produce force acting at a tangent orthogonal to the rotor, resulting in torque to turn the shaft. The relative motion between this field and the rotor induces electric current in the conductive bars. The field windings in the stator of an induction motor set up a rotating magnetic field through the rotor. Since the voltage developed in the squirrel cage winding is very low, and the current very high, no intentional insulation layer is present between the bars and the rotor steel. Larger motors have aluminium or copper bars which are welded or brazed to end-rings. A very common structure for smaller motors uses die cast aluminium poured into the rotor after the laminations are stacked. The rotor bars may be made of either copper or aluminium. Laminations, with 36 slots for the stator and 40 slots for the rotor By the end of the 19th century induction motors were widely applied on the growing alternating-current electrical distributions systems. In 1889, Mikhail Dolivo-Dobrovolsky developed a wound-rotor induction motor, and shortly afterwards a cage-type rotor winding. Developments of this design became commercially important. 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. Galileo Ferraris described an induction machine with a two-phase stator winding and a solid copper cylindrical armature in 1885. This simplifies application and replacement of these motors. ![]() Commonly used motors in industry are usually IEC or NEMA standard frame sizes, which are interchangeable between manufacturers. 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. Squirrel-cage induction motors are very prevalent in industry, in sizes from below 1 kilowatt (1.3 hp) up to tens of megawatts (tens-of-thousand horsepower). The interaction of the magnetic fields of currents in the stator and rotor produce a torque on the rotor.īy 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. 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. 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. It consists of a cylinder of steel laminations, with aluminum or copper conductors embedded in its surface. A squirrel-cage rotor is the rotating part of the common squirrel-cage induction motor. ![]()
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