A switched reluctance or brushless motor is an electrical motor that includes a rotor and a stator. Torque in a reluctance motor is produced by the tendency of the rotor to move to a position relative to the stator in which the reluctance of a magnetic circuit is minimized, i.e., a position in which the inductance of an energized stator winding is maximized. In a switched reluctance motor, circuitry detects the angular position of the rotor and sequentially energizes phases of the stator windings as a function of rotor position.
A common circuitry device used for rotor orientation/position detection is a photosensor, or transmissive, sensor system. A photosensor system includes an infrared radiation (IR) emitting component aligned across a gap with an infrared radiation (IR) detecting component. The IR emitting component may employ LED technology, and the IR detecting component may be a phototransistor or a photodiode. The photosensor system is positioned inside the motor so that as the rotor moves, an encoder crosses through the gap between the IR components. Radiation is thus blocked by the encoder over a short time interval. The photosensor system electronically communicates this information to the control system of the motor. The control system of the motor may then use the data for determining the rotor speed and position and energize phases of stator windings accordingly. Apertures on one or both of the IR emitting and IR detecting components may be used to focus the radiation beam to obtain precise measurements. Multiple photosensor systems may be used for rotor-sensing to provide detailed feedback to the control system of the motor.
In a switched reluctance motor, a typical photosensor system may include an “all-in-one” unit for both of the IR components. These units contain the IR emitting or IR detecting element (LED, phototransistor, photodiode, etc.) surrounded by an injection molded assembly. The injection molded assembly is manufactured with an aperture slot configured at an appropriate position to allow the infrared radiation to pass through. The assembly has a mounting mechanism to attach the unit to the motor housing as well as electrical connections to provide a communication path to the control system of the motor. One example of an “all-in-one” unit of a photosensor system used in switched reluctance motors is the Honeywell™ P/N HOA1887 transmissive sensor.
Companies that manufacture switched reluctance motors are constantly searching for ways to decrease costs and increase profitability. Finding a more cost effective way to provide the photosensor system function in a brushless motor may enhance the bottom line of the company.