With the development of technology, motor control has been from a brief open, close, start, stop, to clamp, voltage, frequency conversion, vector, torque control, for the motor better control is actually to make the motor more accurate positioning, more stable speed and more detailed torque changes.

　　Induction Motor (AC Induction Motor) method has the following two: a test voltage control method: change the voltage of the stator coil, you can change the speed. For example the electric fan that the family USES, use toggle switch to change electric fan rotate speed. Servo motor; B. Frequency control method: the frequency of power is 60Hz, if the use of microcontroller or digital signal processor, operation rectifier components (MOSFET, IGBT), the power into different frequencies, that can change the communication motor speed. Stepper motor communication induction motor frequency, variable voltage control, advantage is miniaturization, lightweight, low price, but the defect is power conversion (easy to heat), electromagnetic interference (easy to produce noise), noise and oscillation (high harmonic and tissue resonance).

　　In order to solve the above problems, the academia and industry are active to dc motor, dc brushless motor to overcome. Dc motor has excellent stability, quick response, its input and output almost linear characteristics, so that it can have outstanding control function, so widely used in high-precision control occasions. However, because the brush and commutator of dc motor are a kind of mechanical friction touch, it is easy to occur abrasion, spark and electrical noise, resulting in a reduced reliability. Brushless dc motor is the dc (brush) motor rectifier section (brush and commutator) to electronic circuit replacement, and retains the advantages of dc motor, can quickly start and stop; The speed is proportional to the voltage. Torque is proportional to the current advantage, suitable for high accuracy, high stability and high speed occasions. Its drawback is that due to the material and volume of the permanent magnet, it is relatively difficult to produce more than the horsepower output.