Brushless Mortor|Basic

Structure of 3-Phase Full-Wave Brushless Motors


Points of this article

・The coils of a 3-phase brushless motor are fixed to a substrate, and are wound about an iron core.

・The coils are fixed in place; a permanent magnet or magnets (rotor) rotates on the outside.

・In general, Hall elements are used to detect the position of the rotor (magnet).

First, the structure of 3-phase brushless motors is explained.

Outer Appearance and Structure of 3-Phase Full-Wave Brushless Motors

The photos below are examples of the outer appearance and structure of brushless motors.

On the left is an example of a spindle motor for rotating the disc of an optical disc device. There are a total of nine coils installed, three for each of three phases. On the right is an example of a spindle motor for a FDD device; there are 4 coils for each of 3 phases, for a total of 12 coils. The coils are fixed on a substrate, wound around an iron core.

The disc-shaped objects placed on the right sides of the coil sections are permanent magnet rotors. The peripheral parts are permanent magnets; the shaft of the rotor is inserted into the center of the coil section to install covering it, so that the permanent magnet surrounds the outside of the coils.

Diagram of the Internal Structure of a 3-Phase Full-Wave Brushless Motor, and Coil Connection Equivalent Circuit

Next, a summary diagram of the internal structure and an equivalent circuit of coil connections are presented.

The summary diagram of the internal structure takes as an example a motor with a simple structure, having 2 poles (2 magnets) and 3 slots (3 coils). It is similar to the structure of a brushed motor with the same numbers of poles and slots, but here the coils are fixed and the magnets can rotate. Of course, there are no brushes.

In this case the coils are Y-connected, and current is supplied to the coils by semiconductor elements or the like, to control current inflow and outflow according to the position of the rotating magnets. In this example, Hall elements are used to detect the magnet position. Hall elements are positioned between coils, and the voltages that they generate according to the strength of the magnetic field are detected and used as position information. In the photo of the FDD spindle motor shown above, one can see Hall elements for position detection between the coils (on the upper side of the coil).

Hall elements are well-known magnetic sensors. They convert the magnitude of a magnetic field into a voltage of corresponding magnitude, and the sign of the voltage indicates the orientation of the magnetic field. Below is a schematic diagram illustrating the Hall effect.

Hall elements utilize the phenomenon in which, when a current IH flows in a semiconductor and magnetic flux B crosses the current at a right angle, a voltage VH occurs in a direction perpendicular to both the current and the magnetic flux. This phenomenon was discovered by the American physicist Edwin Herbert Hall, and so is called the Hall effect. The equation for the generated voltage VH is as follows.

 VH = (KH / d)・IH・B  where KH: Hall coefficient, d: thickness of material traversed by the magnetic flux

As the equation indicates, when the current is increased, the voltage also increases. This is used to detect the position of the rotor (magnets).

The next article will explain the principle of operation.

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Photo P60_2  Brushless direct current motors

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