What is a squirrel cage motor?

The squirrel cage motor is an induction electric motor, whose rotating part or rotor is constituted by a set of conductive bars parallel to the axial direction and arranged in a cylindrical shape around the axis.

This shape is reminiscent of a cage like those used to catch squirrels in the old North American west, hence the name. They are also the most economical, durable and of less maintenance, because they lack carbons, brushes or collectors in the rotor, which do not need to be electrically connected to any external current source. squirrel cage motor

The first rotary field motors were independently devised between 1885 and 1886 by two great electrical geniuses: Galileo Ferraris and Nikola Tesla. These engines were the predecessors to today’s squirrel cage engines.

The squirrel cage motor is of alternating current, which can be three-phase, two-phase or single-phase. Depending on the type of power supply, the design may vary a bit, but the operating principle is always the same. squirrel cage motor

Squirrel cage motor operation

The operating principle is based on the generation of a rotating magnetic field in the center of the motor, by a static winding on its periphery, which is fed with alternating current.

Said rotating magnetic field induces currents in the bars that make up the rotor cage, and these currents in turn produce a secondary magnetic field that interacts with the primary field, producing a torque or moment on the rotor.

The key to operation is in the production of a rotating magnetic field perpendicular to the axis of rotation. This rotating field exerts a torsional magnetic force on the longitudinal bars of the cage when current flows. squirrel cage motor

To generate the current in the conductive bars parallel to the cage’s axis of rotation, an external current source is not required, since the rotating field itself, by magnetic induction, is capable of inducing a current on the cage bars.

That is as long as there is a difference between the speed of rotation of the magnetic field and the speed of rotation of the rotor. squirrel cage motor

Rotating magnetic field in a three-phase motor

Squirrel cage motors can be either three phase or single phase. In the case of the three-phase motor, that is, the one that works with three-phase alternating current, each phase leads the previous one by 120º, that is, one third of a period.

In every electric motor there are two parts:

• Stator , the peripheral part of the motor that is fixed with respect to its casing.
• Rotor , rotating central part of the engine.

In the stator there is a package of grooved and enameled sheets (to avoid eddy currents or eddy currents) and high magnetic permeability.

The cables covered with insulating varnish pass through the grooving, forming at least three windings or coils, out of phase by 120º. The three coils are fed with three-phase alternating current and each phase also advanced by 120º with respect to the previous one. squirrel cage motor

At each instant the superposition of the magnetic fields gives a resulting field perpendicular to the axis of rotation of the motor. As time progresses, the combined magnetic field of the three coils maintains its amplitude, but its direction always perpendicular to the axis of rotation, rotates with a frequency equal to that of alternating current, generally between 50 and 60 Hz.

Squirrel cage rotor

It consists of two conductive rings connected by eight or more longitudinal conductive bars, parallel to the axis of rotation.

Torque over rotor

To understand how the rotating field produces torque on the cage, a minimum cage can be imagined, made up of two diametrically opposed longitudinal bars.

When this cage is originally at rest and thanks to the electromotive force, the rotating field that passes through it induces a charge movement in each bar. However, since the bars are short-circuited at their ends by a conductive ring, a current flow is established between the opposing bars.

On the other hand, as the bars have movement relative to the stator field, a force of magnetic origin appears on them, known as the Lorentz force, which is perpendicular to the radial field of the stator and to the direction of the current in each bar.

For there to be current and torque on the bars, it is necessary that they have relative movement with respect to the radial magnetic field produced in the stator. squirrel cage motor

Therefore, the rotation speed of the cage is always less than that of the magnetic field. Due to this lack of synchrony between the rotor and the field, this is an asynchronous motor.

Therefore, a pair of opposite forces is produced in each bar, which produces a torque on the simplified cage, and in the same way with cages with more than two bars.

Iron core rotor

An improvement consists of placing the embedded cage in a set of laminated and enameled discs, made of material with high magnetic permeability, such as iron. squirrel cage motor

The purpose is to multiply the intensity of the magnetic fields produced by both the stator and the rotor itself. It is thanks to the interaction between these two fields that the torque is produced on the rotor.

Experience has shown that if the cage bars are skewed to the axis of rotation, the motor runs smoother with less vibration.

At a higher load on the rotor, the speed of the rotor’s sliding relative to the rotational speed of the stator’s magnetic field also increases. Therefore, the maximum currents and the maximum torques are produced when the rotor is locked, which is why the overloaded motor can suffer overheating and therefore damage to the insulating varnishes and enamels of the coils and of the plates that make up the cores of the stator and rotor.

Squirrel cage motor applications

Three phase squirrel cage motors are preferred for industrial applications. They are less recommended for domestic uses in which the asynchronous single-phase motor is preferred, since the three-phase current generally does not reach the residences.

Centrifugal pumps

Squirrel cage motors are preferred for centrifugal pumps.

Lathes and milling machines

They are also ideal in large lathes and mills, as well as industries where conveyor belts and blowers are required.

Sheet cutting and die-cutting 

These types of motors are suitable for the heavy industry of dies and sheet metal shearing. squirrel cage motor

Advantage

Squirrel cage motors have numerous advantages over other types of electric motors:

1. For equal power, squirrel cage motors are more compact and lighter than synchronous motors.
2. They are completely scalable, that is, they can be built from small to very large. squirrel cage motor
3. The torque of squirrel cage motors is generally higher than that of other types of motors, very suitable for heavy use.
4. The efficiency of three-phase squirrel cage motors is over 70%. Asynchronous single-phase motors have a lower performance, but always higher than direct current motors.
5. Due to the development of power electronics, it is possible to control the speed of said motors electronically, by varying the frequency of the current.

Disadvantages

Among the main disadvantages can be mentioned:

1. Induction motors have a high current demand at the moment of starting, which is why they are not suitable for applications in which the motor must constantly start and stop, since it would imply an overload in the electrical system.
2. Even though power electronics have advanced, their rotational speed is not as controllable as that of stepper motors.