squirrel-cage motor

A squirrel-cage motor is an asynchronous induction motor in which the rotor consists of a cylindrical iron core with aluminum or copper bars embedded parallel to its axis, connected at both ends by conducting rings. Current is induced directly into these bars by the rotating magnetic field created by the stator windings, eliminating the need for brushes, slip rings, or external excitation. The name derives from the resemblance of the rotor assembly to a hamster wheel.

The rotor bars are typically cast aluminum or high-conductivity copper, with bar-to-ring connections allowing current to flow in closed loops. When three-phase AC current energizes the stator windings, it generates a rotating magnetic field that rotates at synchronous speed, determined by the supply frequency and number of poles. This field induces current in the rotor bars, which interact with the magnetic field to produce torque. The rotor speed lags slightly behind synchronous speed, a difference called slip, typically 3 to 5 percent at full load.

Squirrel-cage motors dominate industrial applications due to their simplicity, ruggedness, and low maintenance. They require no brush replacement or commutator cleaning. Starting torque varies by rotor design: deep-bar or double-cage rotors increase starting torque and reduce starting current, while standard designs prioritize efficiency at running speed. Frame sizes range from fractional horsepower units in household appliances to several thousand horsepower in large industrial drives.

Slip determines motor performance. At no load, slip is nearly zero. As load increases, slip increases roughly proportionally, raising rotor current and losses. Stator copper losses remain relatively constant with load, but rotor losses vary with slip squared. Motor efficiency peaks around 75 percent load on properly designed machines. Torque-slip relationships differ by rotor design: general-purpose motors provide moderate starting torque, while high-starting-torque variants accept higher starting currents to deliver torque needed for loaded starts such as in compressors or conveyors.

Three-phase squirrel-cage motors are universal in industrial settings because they operate directly on standard industrial supplies, generate smooth constant torque, and require minimal control complexity. Single-phase versions exist but are less efficient and require run capacitors or centrifugal switches. Problems typically arise from overheating due to inadequate cooling or overloading, contamination of air vents reducing heat dissipation, and rotor bar breakage from thermal cycling or mechanical shock during starting. Proper sizing, ventilation, and soft-start methods extend motor life significantly.