【正文】 however, the windings are distributed in a different manner. Most BLDC motors have three stator windings connected in star fashion. Each of these windings are constructed with numerous coils interconnected to form a winding. One or more coils areplaced in the slots and they are interconnected to make a winding. Each of these windings are distributed over the stator periphery to form an even numbers of poles. There are two types of stator windings variants: trapezoidal and sinusoidal motors. This differentiation is made on the basis of the interconnection of coils in the stator windings to give the different types of back Electromotive Force (EMF). Refer to the “What Is Back EMF” section for more information.As their names indicate, the trapezoidal motor gives a back EMF in trapezoidal fashion and the sinusoidal motor!ˉs back EMF is sinusoidal, as shown in Figure and Figure2. In addition to the back EMF, the phase current also has trapezoidal and sinusoidal variations in the respective types of motor. This makes the torque output by a sinusoidal motor smoother than that of a trapezoidal motor. However, this es with an extra cost, as the sinusoidal motors take extra winding interconnections because of the coils distribution on the stator periphery, thereby increasing the copper intake by the stator windings.Depending upon the control power supply capability, the motor with the correct voltage rating of the stator can be chosen. Fortyeight volts, or less voltage rated motors are used in automotive, robotics, small arm movements and so on. Motors with 100 volts, or higher ratings, are used in appliances, automation and in industrial applications.RotorThe rotor is made of permanent magnet and can vary from two to eight pole pairs with alternate North (N) and South (S) poles.Based on the required magnetic field density in the rotor, the proper magnetic material is chosen to make the rotor. Ferrite magnets are traditionally used to make permanent magnets. As the technology advances, rare earth alloy magnets are gaining popularity. The ferrite magnets are less expensive but they have the disad vantage of low flux density for a given volume. In contrast, the alloy material has high magnetic density per volume and enables the rotor to press further for the same torque. Also, these alloy magnets improve the sizetoweight ratio and give higher torque for the same size motor using ferrite magnets.Neodymium (Nd), Samarium Cobalt (SmCo) and the alloy of Neodymium, Ferrite and Boron (NdFeB) are some examples of rare earth alloy magnets. Continuous research is going on to improve the flux density to press the rotor further.Figure4 shows cross sections of different arrangements of magnets in a rotor.Hall SensorsUnlike a brushed DC motor, the mutation of a BLDC motor is controlled electronically. To rotate the BLDC motor, the stator windings should be energized in a sequence. It is important to know the rotor position in order to understand which winding will be energized following the energizing sequence. Rotor position is sensed using Hall effect sensors embedded into thestator.Note: Hall Effect Theory: If an electric current carrying conductor is kept in a magnetic field, the magnetic field exerts a transverse force on the moving charge carriers which tends to push them to one side of the conductor. This is most evident in a thin flat conductor. A buildup of charge at the sides of the conductors will balance this magnetic influence, producing a measurable voltage between the two sides of the conductor. The presence of this measurable transverse voltage is called the Hall effect after E. H. Hall who discovered it in 1879.Figure5 shows a transverse section of a BLDC motor with a rotor that has alternate N and S permanent magnets. Hall sensors are embedded into the stationary part of the motor. Embedding the Hall sensors into the stator is a plex process because any misalignment in these Hall sensors, with respect to the rotor magnets, will generate an error in determination of the rotor position. To simplify the process of mounting the Hall sensors onto the stator, some motors may have the Hall sensor magnets on the rotor, in addition to the main rotor magnets. These are a scaled down replica version of the rotor. Therefore, whenever the rotor rotates, the Hall sensor magnets give the same effect as the main mag nets. The Hall sensors are normally mounted on a PC board and fixed to the enclosure cap on the nondriving end. This enables users to adjust the plete assembly of Hall sensors, to align with the rotor magnets, inorder to achieve the best performance.Based on the physical position of the Hall sensors, there are two versions of output. The Hall sensors may be at 60!a or 120!a phase shift to each other. Based this, the motor manufacturer defines the mutation sequence, which should be followed when controlling the motor.Note: The Hall sensors require a power supply. The voltage may range from 4 volts to 24volts. Required current can range from 5 to 15mAmps. While designing the controller, please refer to the respective motor technical specification for exact voltage and current ratings of the Hall sensors used. The Hall sensor output is normally an opencollector type. A pullup resistor may be required on the controller side.COMPARING BLDC MOTORS TO OTHER MOTOR TYPESCompared to brushed DC motors and induction motors, BLDC motors have many advantages and few disadvantages. Brushless motors require less maintenance, so they have a longer life pared with brushed DC motors. BLDC motors produce more output power per frame size than brushed DC motors and induction motors. Because the rotor is made of permanent magnets, the rotor inertia is less, pared with other types of motors. This improves acceleration and deceleration characteristics, shortening operating cycles. Their linear speed/torque characteristics produce predictable speed r