An Overview of Commonly Used Control Techniques for Brushless DC Motors
3 phase brushless DC motors are popular miniature motors that power a variety of applications, including medical devices, surgical tools, and fans. And while almost all the world’s BLDC motors are powered by either batteries or DC power supplies, BLDC motors actually fall within the AC motor category. This is due to the windings of a brushless DC motor, which carries alternating current (AC).
Keep in mind that the driver (also called the inverter) of BLDC motors accomplishes the DC to AC conversion. The motor driver plays a critical role in the optimal operation of BLDC motors, as these are used to control the direction and timing of the current into the motor coils. Now let’s explore the performance of two commonly utilized BLDC motor control techniques: trapezoidal commutation and sinusoidal commutation.
For further details regarding BLDC drivers, please reference our related blog, General Overview of Drivers for BLDC Motors.
BLDC Control Techniques: Trapezoidal Commutation
In trapezoidal control, only two motor phases are excited at a time while the third phase is disconnected. The switching sequence of trapezoidal commutation has been summarized in Table 1, which shows details of each switch conduction interval for a trapezoidal commutation; e.g. in 0 to 60-degree switches SW1 and SW6 conduct with positive and negative polarity respectively. The current flow direction for one time instant (300 to 360-degree interval) is shown in Figure 1.
Table 1- Switching sequence of trapezoidal commutation
Figure 1. Current in conduction interval of trapezoidal commutation.
BLDC Control Techniques: Sinusoidal Commutation
In sinusoidal control, all three phases of the motors are excited at the same time, which is sinusoidally synchronized by the control. Each sinewave must be 120 electrical degrees from the other. The current flow direction is shown in Figure 2.
Figure 2. Current in conduction interval sinusoidal commutation.
Table 2. Qualitative comparison between trapezoidal and sinusoidal commutation control performance parameters.
Both types of BLDC motor control present advantages and drawbacks, which means that selecting the ideal control technique depends largely on your application’s unique requirements. Trapezoidal control provides smooth operation at high speed, but it suffers from higher torque ripples at low speed; meanwhile, sinusoidal commutation provides smooth operation at low speed. Sinusoidal control algorithms are more complex to implement, which means this control technique should be used in applications that prioritize maximum torque and smooth motion, including some surgical hand tools and electric vehicles. In applications where higher torque ripple is not an issue, such as industrial drives, drone motors, and HVAC blowers, driving the motor with a simple control algorithm like trapezoidal control is a better idea.
Trying to determine the ideal BLDC motor control technique for your application? Reach out to our engineers today for assistance!