Why is the brushless motor powered by three phases? Why not use two phases and four phases?

The brushless DC motor is composed of a motor body and a driver, which is a typical mechatronics product. Since the brushless DC motor is operated in an automatic control mode, it will not add a starting winding on the rotor like the synchronous motor that starts under heavy load under frequency conversion speed regulation, and will not produce oscillation and out-of-step when the load suddenly changes. The permanent magnets of small and medium-capacity brushless DC motors are now mostly made of rare earth NdFeB (Nd-Fe-B) materials with high magnetic energy levels. Therefore, the volume of rare earth permanent magnet brushless motor is one frame size smaller than that of three-phase asynchronous motor of the same capacity

The reason why brushless motors are three-phase is because three-phase power supply can provide higher power and efficiency than single-phase power supply.

  • Part 1: Brushless Motor Basics
    A brushless motor is an electric motor that does not use fossil fuels and rotates by changing the direction of the current and the electromagnetic field. Compared to traditional brushed motors, brushless motors have higher efficiency and reliability, while also eliminating the need for maintenance and replacement of carbon brushes.
  • Part 2: Advantages of three-phase power supply
    Three-phase power supply has the following advantages over single-phase power supply:
    • High power output: Three-phase power supply can provide higher power output than single-phase power supply. This is because in three-phase electricity, the spacing of the three current waveforms is 120 degrees apart, which continuously provides the electrical energy required to drive the motor. This well-balanced current distribution reduces power fluctuations and increases the efficiency and output capacity of the motor.
    • Simplified motor control: Three-phase power supply can simplify the motor control system. By staggering the transmission of three current waveforms to the motor, the rotation of the motor can be easily realized. This staggered design makes the motor run more smoothly and can reduce harmonic interference.
    • Reduced cable size and cost: Compared to single-phase power supply, three-phase power supply can achieve the same power output by reducing the current. Due to the lower current, cable size and cost can be reduced, and energy consumption can be reduced.
    • Reduced energy consumption: In a three-phase alternating current system, the three current waveforms can be balanced against each other to cancel out eddy current losses. This reduces the energy consumption of the motor and increases the energy efficiency of the overall system.
  • Part III: Why not use two-phase or four-phase
    Why don't brushless motors use two-phase or four-phase power supply instead of three-phase power supply? This is due to the fact that the three-phase power supply has the best balance and efficiency characteristics.
    • Two-phase power supply: Although two-phase power supply can achieve rotary motion, the lack of a third phase will cause the motor to run unbalanced. In practice, such a motor may generate vibration and noise, and its control system is more complex. In addition, two-phase power supply is less efficient compared to three-phase power supply.
    • Four-phase power supply: Although four-phase power supply can achieve smooth rotation of the motor, its control system is more complex. Since the motor needs to process four current waveforms, more sensors and control algorithms are required to ensure precise control. In contrast, three-phase power requires fewer sensors, the control system is more simplified, and has high reliability and efficiency. As a result, four-phase power supply is not common in brushless motors.
  • Part IV: Summary
    In summary, the reason why brushless motors are three-phase is because of the advantages of three-phase power supply, such as high power output, simplified motor control, reduced cable size and cost, and reduced energy consumption. Compared to two- or four-phase power supplies, three-phase power supply offers the best balance and efficiency characteristics. Therefore, three-phase power supply is the first choice for brushless motors, which are widely used in various fields, including industrial, transportation, and household appliances.

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