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Shaded-pole motor

Among the different types of motors that exist, the shaded-pole motor can be defined as a single-phase, short-circuited motor. The name derives from the shading provided to the poles. A shaded-pole motor is a type of as squirrel-cage induction motor. It may also be referred to as a Frager’s loop motor. More about these and how they work will be explained below.

What is a shading coil?

A shading coil is a strip of wire or copper that is closed in a short circuit. The function of the shading coil is to replace the traditional starter or secondary winding . To complement the definition of the shading coil, it should be said that the motor to which it gives its name works with salient poles; when the pole of the motor is energized, there is a winding on its salient core, which is formed by these wire coils, which are the only part of the motor that receives electricity.

Motor operation with a shading coil

The function of the shading coils is to cause the magnetic field in the fixed part of the machine or stator to have a sweeping motion in the path of the pole faces. This movement actually goes from the region where the coil is not present to the place where the coil is present. This generates a series of impulses on the rotor which, when repeated, can drag the rotor and cause it to move.

In this way, when the coil is supplied with mains voltage, there is a variable magnetic flux throughout the motor area. This magnetic flux, when it passes through the coils, generates voltage, and as they are short-circuited, a large current is produced.

How the power to start the motor is generated

When the polar coil is powered, current 1 appears, which does in turn generate flux 1. It is this flux 1 that produces current l2, along the short-circuited coil and the rotor bars, generating in turn flux 2 and 3. Fluxes 2 and 3 are opposite to flux 1, and are generated according to Lenz’s Law. As soon as flux 1 is turned off, fluxes 2 and 3 become the maximum points, and the north of 3 is therefore attracted by the south of 2. This initiates the rotation of the rotor due to the presence of a sliding field.

The importance of the shading coil

In a shaded-pole motor, this shading coil becomes its essential component: it generates the magnetic field that causes a sweeping motion in the stator along the polar faces. This is what produces the impulses which, because they are highly repetitive, drive the rotor and make it work.



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