Electric motor

Squirrel cage rotor

Squirrel cage rotor

A squirrel cage rotor is the rotating part of the common squirrel cage induction motor. The squirrel-cage motor is a type of AC motor.

This type of rotor 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 a current induced by the stator field, and produces its own magnetic field. The interaction of the two magnetic field sources produces torque in the rotor.

By adjusting the shape of the bars in the rotor, you can change the torque characteristics of the motor, to minimize the starting current or maximize the low speed torque, for example.

Squirrel cage induction motors are very common in the industry, in sizes from less than a kilowatt to tens of megawatts. These AC motors are simple, robust and self-starting, and maintain a reasonably constant speed from light load to full load, established by the frequency of the power supply and the number of poles of the stator winding.

Operation of the squirrel-cage rotor

An induction motor behaves like a transformer: the cage forms the short-circuited secondary coil. Under the influence of the alternating magnetic rotating field generated by the stator coils, the induction currents will start to run in the secondary windings. Due to the Lorentz force that is integrated between the magnetic field and the current carrying cage, the rotor will start spinning.

However, the rotation speed of the rotor will be slightly less than the rotation speed of the magnetic rotating field generated by the stator coils. If the rotor were to operate synchronously with the stator field, there would be no difference in flow or generation of energy and, consequently, there would be no force. That's why machines with a cage rotor are called asynchronous machines or induction machines.

One characteristic of the squirrel-cage rotor is the low starting or starting torque in combination with the relatively large starting current (approximately 5 to 7 times the rated current). Over the course of time, therefore, various alternative constructions have been devised to improve the approach behavior of the cage anchor.

Resistance rotor

By producing the rotor cage of a material with a higher specific resistance (for example, brass instead of copper), the resistance of the rotor will increase. As a result of this increase, the starting torque of the electric motor will increase and the starting currents will decrease.

The disadvantage is that during normal operation the copper losses in the motor are much higher than in the normal box motor (low efficiency). This type of rotor is used mainly for motors that are not continuously in operation but that require a high starting torque.

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Last review: May 10, 2018