Squirrel Cage Rotor Bar Construction

A squirrel cage rotor is a key component of an induction motor. It consists of conductive bars (usually aluminum or copper) running axially along the periphery of the rotor core. These bars are not connected to any external circuit. Instead, they are permanently connected (short-circuited) at both ends by heavy rings made of the same conducting material. These rings are called end rings.

The name "squirrel cage" comes from the resemblance of this bar and end-ring structure to a rotating cage used for pet squirrels.

Rotor Bar Short-Circuiting and Induced Current

When the stator of the induction motor creates a rotating magnetic field, this field cuts the rotor bars. According to Faraday's law of electromagnetic induction, a voltage is induced in these rotor bars. Since the bars are short-circuited by the end rings, the induced voltage causes current to flow through the bars and the end rings. This induced current flows in a closed path within the rotor itself.

The interaction between the stator's rotating magnetic field and the magnetic field produced by the induced rotor current creates a torque, causing the rotor to rotate.

Squirrel Cage vs. Slip Ring Rotor

It is useful to compare the squirrel cage rotor with the slip ring rotor. A slip ring rotor has a wound winding connected to slip rings on the shaft. Brushes ride on these slip rings, allowing external resistance to be connected to the rotor circuit. Adding external resistance is commonly used to improve starting torque and control speed in slip ring induction motors.

However, the squirrel cage rotor's design, with its permanently short-circuited bars and end rings, inherently prevents any external connection to the rotor bars. There are no slip rings or brushes to facilitate such a connection.

Preventing External Resistance Insertion

Because the rotor bars in a squirrel cage rotor are permanently short-circuited by the end rings as part of their fundamental construction, there is no possibility to break this internal circuit to insert external resistance. The entire conductive structure of bars and end rings forms a self-contained, fixed resistance circuit.

Therefore, the permanent short-circuiting of the rotor bars at the ends by the end rings directly ensures that insertion of external resistance into the rotor circuit is not possible in a squirrel cage induction motor.