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Understanding BLDC Motor Control Algorithms

Chapter 3

Exploring Motor Behavior via Simulation

The model introduced in the previous section logs signals such as speed, voltage, current, and torque. To explore the behavior of the motor and controller, the model is now run. The top plot shows the desired speed in green together with the measured speed in red. As you see here, the speed ramps up from 100 to 500 rpm in 100-rpm increments. In the bottom plot, you can see how the controller adjusts voltage to make the motor rotate at the desired speed.

Simulation results of the closed-loop B L D C motor control with plots showing how the voltage is adjusted to keep the measured speed at its desired value.

RPM ramp-up with corresponding voltage control.

One thing that immediately catches the eye is the ripples in the measured speed signal. The enlarged plot on the bottom shows when the rotor transitions between sectors. Looking at the sector and the speed plots together makes it clear that the observed speed ripples are somehow connected to commutation because the rippled pattern aligns with the beginning of each commutation cycle.

Plots showing the ripples in the measured speed and changing sectors at the beginning of each commutation cycle.
Plots revealing the connection between the rippled pattern in the measured speed and the commutation.

Remember what happens during commutation. One of the phases is pulled high or low while the other one remains in its current state, and the third phase is opened. If the three-phase currents changed instantly as seen in the below plot, the observed rippling pattern would disappear.

Illustration of three-phase currents changing instantaneously at the beginning of each commutation cycle.

But in reality, when driving a phase, the current does not change instantaneously and requires a certain rise time.

Illustration of ripples in the measured speed and torque due to rising three-phase currents during commutation.

Rippling pattern in the measured rpm signal (red, top plot) with observed three-phase currents (middle plot) and torque (bottom plot).

When you look at the three-phase currents in the middle plot above, you can see how they rise over time, which in turn leads to ripples in the speed. But speed is not the only affected signal; there are ripples in the torque response as well, shown in the bottom plot, as current and torque are proportionally related. This torque ripple is one of the key drawbacks of trapezoidal control of BLDC motors. Some of the shortcomings of the trapezoidal control are overcome by a more advanced technique called field-oriented control, typically used for driving PMSM motors.

Check out the following overview for more information on field-oriented control: Field-Oriented Control with MATLAB and Simulink