2-2-6. Detecting the Rotor
When switching the current that flows through the coils of a brushless DC motor, you must first detect the rotor position. For that purpose, a "semiconductor sensor" made of a Hall element is generally used. The voltage output from a Hall element varies depending on the magnetic force applied to it.
Since the size of a Hall element is several millimeters as shown in Fig. 2.30, you can install it inside a motor.
As shown in Fig. 2.31, if magnetic flux B transmits at right angles to current I flowing through the semiconductor, this element generates voltage e at right angles to current I and magnetic flux B. This phenomenon characteristic of the Hall element is used for the design.
The above phenomenon occurs when, according to Fleming's left-hand rule, electrons move along the side face of the semiconductor by the force applied to electrons. This phenomenon was discovered by an American scientist called E. H. Hall, and thus called the Hall effect.
Outputs of a Hall element are reciprocally switched when the N- or S-pole of a motor comes close.
Fig. 2.32 illustrates the simplest principle of switching the current that flows through a brushless motor by use of a Hall element.
As an aside, be careful not to mistake Hall [hc:l] of Hall element for hole [houl] of, for instance, hole carrier.
<Short column> Hall IC
Since the output from a Hall element is low, it is, in actual use, amplified using an electronic circuit or converted into a switching output using an electronic circuit called a comparator.
A Hall IC is an IC on which a Hall element and an electronic circuit are integrated into a single chip.
Hall ICs are classified into the linear type and switching type. Both are used selectively depending on the control method.