motors can be single-phase, two-phase or three-phase and use Hall effect sensors mounted on their windings for rotor position sensing and low cost closed-loop commutator control. BLDC motors are commonly used where precise speed control is necessary, as in computer disk drives or video cassette recorders. The spindles within CD, CD-ROM (etc.) drives, and mechanisms within office products, such as fans, laser printers and photocopiers. They have several advantages over conventional motors: They are more efficient than AC fans using shaded-pole motors, running much cooler than the AC equivalents. This cool operation leads to much-improved life of the fan's bearings. Without a commutator, the life of a BLDC motor can be significantly longer compared to a brushed DC motor with a commutator. Commutation tends to cause electrical and RF noise; without a commutator or brushes, a BLDC motor may be used in electrically sensitive devices like audio equipment or computers. The same Hall effect sensors that provide the commutation can provide a convenient tachometer signal for closed-loop control (servo-controlled) applications. In fans, the tachometer signal can be used to derive a "fan OK" signal as well as provide running speed feedback. The motor can be synchronized to an internal or external clock, providing precise speed control. BLDC motors do not spark, making them better suited to environments with volatile chemicals and fuels. Sparking also generates ozone, which can accumulate in poorly ventilated buildings. BLDC motors are usually used in small equipment such as computers and are generally used in fans to remove heat. They make little noise, which is an advantage in equipment that is affected by vibrations. Modern BLDC motors range in power from a fraction of a watt to many kilowatts. Larger BLDC motors rated up to about 100 kW are used in electric vehicles. They also find use in electric model aircraft. Switched reluctance motor The switched reluctance motor (SRM) has no brushes or permanent magnets, and the rotor has no electric currents. Torque comes from a slight misalignment of poles on the rotor with poles on the stator. The rotor aligns itself with the magnetic field of the stator, while the stator field windings are sequentially energized to rotate the stator field. The magnetic flux created by the field windings follows the path of least magnetic sending the flux through rotor poles that are closest to the energized poles of the stator, thereby magnetizing those poles