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Structure of Brushless DC Motor

By Cayla | Published on Sep 14,2015

1. Stator

Stator of a brushless DC motor is determined by the stator punching sheet (steel superposition) and windings placed in each slot (Figure 1). The structure of general brushless DC motor stator is the same as common power asynchronous motor, the difference is just the way windings distribute. Most of the three-phase brushless DC motor windings are wound into a star-shaped. Each phase winding is made of a plurality of coils. The number of windings per pole are equal.


Brushless DC motor can be divided into the connection type of trapezoid and sinusoidal nowhere, thus will produce different types of counter electromotive force depending on the stator winding different drive current, from Figures 2 and 3, we learn that the counter electromotive force of trapezoidal wave motor is trapezoidal, the counter electromotive force of sine wave motor is sinusoidal. Similarly, their phase current are also trapezoidal and sinusoidal. But the output torque of sine wave motor is more smooth than the output torque of trapezoidal wave motor. According to the rated power, appropriate input voltage should be selected. Voltage of motor in automotive, robotics, and other products is 48V or lower than 48V, the voltage of the motor in the automation equipment, household appliances and other industrial applications, is greater than or equal to 100V.



2. Rotor

The rotor is composed of the permanent magnets, magnet poles N and S are placed alternately. Select the appropriate permanent magnet according to the required magnetic field density. Ferrite permanent magnets is very common, as the technology advances, applications of rare earth permanent magnets are more and more extensive. Compare ferrite permanent magnetic materials with rare earth permanent magnets, its relatively inexpensive, but low magnetic flux density, and the rare earth permanent magnets is expensive, but it was a large maximum energy product with high remanence. Under the same size, rare earth permanent magnet can obtain higher torque than ferrite permanent magnets. Shirt cobalt permanent magnet and NdFeB rare earth permanent magnet are the representative of rare earth permanent magnets. Figure 4 is the pole structure of several different rotors..


3. Hall Sensor

Different from brush DC motor, the commutation of brushless DC motor can be electronically controlled. While brushless DC motor is running, the stator windings must be energized according to a certain order. If we know the position of the rotor, we can add appropriate signal in the stator windings. Rotor position can be determined by a Hall sensor. Most of the brushless DC motor has three Hall sensors embedded.

When the rotor permanent magnet pole go through the Hall sensor, the sensor will give a high or low electrical level, indicating that the N pole or S pole go through. We can accurately determine the motor commutation based on the signal Hall sensor obtained.


Figure 1-5 is a cross-section of the brushless DC motor, it can be seen from the figure that the Hall sensor is attached to the motor. The process of Hall sensors being embedded in the stator is very complicated, because if the position of relative poles were slightly misaligned, will cause errors in determining rotor position. To simplify the process of installing the Hall sensor on the stator, not only main rotor magnets are installed on the motor, but also the Hall sensor magnets are installed on the rotor, their size is smaller than the rotor magnet. Whenever the rotor rotates, the Hall sensor will produce the same effect as the main magnet. Hall sensor magnet is usually mounted on the PCB board, fixed in a non-drive unit housing cover, which allows the user to adjust all of Hall sensors as a whole, in order to align with the rotor magnet to get the best performance.

According to the position of Hall sensor, there are two outputs. Hall sensor phase shift between the output signal may be an electrical angle of 60 ° or 120 °.


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