Squirrel Cage Induction Motors

We have reviewed DC motors in previous posts. Squirrel-cage induction motors are very prevalent in industry, in sizes from fractional horse powers up to tens of megawatts. 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. This simplifies application and replacement of these motors. They are also called induction motors or asynchronous motors. These motors are used widely in different industries. They are less expensive than other motors with low maintenance needs, efficient, and generally inexpensive.

How does AC motors work?

As mentioned before, to run an electric motor, we need a magnetic field. Here, magnetic field is generated by an electric current in both rotors and stators. According to Lorentz law, when a conductive material is embedded in a magnetic field, a force in induced to it. This force makes the rotor to rotate.

AC Motor Parts

As seen in the below section view, each induction motor has a rotor and a stator which are separated by an air gap.

Stator

Stator is the stationary part of AC motors. An electric field moves through the coils in the stator and generates a magnetic field. Some slots are designed in stator for coil winding. In three phase induction motors, there are three sets of windings with 120 degrees phase difference. This outside part, produces a rotating magnetic field.

Rotor

Rotor is the rotating part of motor. This part is mounted on the solid shaft of the motor. Rotor consists of a cylinder of steel laminations with aluminum or copper conductors embedded in its surface. The most common design for rotors is squirrel-cage design.

To have a rotation, we need an interaction between two magnetic fields. The rotor magnetic field may be produced by permanent magnets or AC electrical winding.

There is an air gap between these two parts which plays an important role if motor efficiency. If the air gap is too large, magnetic field loss would decrease motor efficiency and if too small, mechanical problems and noise may occur.

Why asynchronous?

For rotor currents to be induced, the speed of the physical rotor must be lower than that of the stator’s rotating magnetic field; otherwise the magnetic field would not be moving relative to the rotor conductors and no currents would be induced. As the speed of the rotor drops below synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the windings and creating more torque. The ratio between the rotation rate of the magnetic field induced in the rotor and the rotation rate of the stator’s rotating field is called “slip”. Under load, the speed drops and the slip increases enough to create sufficient torque to turn the load. For this reason, induction motors are sometimes referred to as “asynchronous motors“.

Why AC Motors?

AC motors are:

  • Inexpensive
  • Low maintenance needs
  • Easy to operate
  • Constant speed
  • High torque start
  • Durability

Kratos Controls Inc supplies one of the best brands in squirrel cage induction motors. You may visit different types here and call us to talk about different solutions we have for your needs.


In the next post, we will talk about types of AC motors.