Brushless DC Motors--Part II: Control Principles
Having understood the construction and basic operating principle of BLDC motor in the first part of this article, it becomes important to understand the motor control options available for the reliable operation and protection of motors. Based on the functions served, motor control can be classified into following categories:
- Speed control
- Torque control
- Motor protection
Implementation of these control functions requires monitoring of one or more motor parameters and then taking corresponding action to achieve the required functionality. Before getting into the details of these control function implementations, it is important to understand the implementation of logic and hardware required to build up the rotation of the motor or to establish commutation.
Commutation implementationAs discussed in the previous part of this article, based on the position of the motor (identified using feedback sensors), two of the three electrical windings are energized at a time. To be able to energize the windings, external circuitry is required to be able to meet the current requirements of the motor. A typical control circuit with a 3-phase winding connection is shown in Figure 1. V1, V3, V5 and V2, V4, V6 make a 3-phase voltage source inverter connected across the power supply. V1 and V4 form one bridge. V1 is high side, which is connected to the high voltage DC source while V4 is low side, which is connected to ground.
Following the same procedure, the 6-step driving sequence for a BLDC motor can be generated. Table 1 provides the switching sequence for power circuitry based on a Hall sensor output.
However, if the rotation has to be reversed, then the sequence needs to be reversed as well. Figure 3 shows the excitation waveform, including phase current, phase voltage, Hall sensor, and sector value. The top half of the figure shows the 3-phase winding excitation current and voltage in which black lines are phase current, while green, red, and blue lines are the phase voltage. As the phase current is trapezoidal, we call 6-step BLDC control trapezoidal control.
The Hall sensor and the excitation have a fixed relationship. Typically, there are two types of Hall sensors. For the first type, for each HALL phase, their waveforms have a 60-degree time-lapse. For the second type, the waveform time-lapse is 120 degrees.
With a basic understanding of commutation, let us now switch to the implementation of control functions, which are critical for any motor design.
Speed controlFollowing the commutation sequence in a given order helps in ensuring the proper rotation of the motor. Motor speed, then, depends upon the amplitude of the applied voltage. The amplitude of the applied signal is adjusted by using pulse width modulation (PWM). Figure 4 shows the switching signals for various power devices.
It can be noted from the above diagram that the higher side transistors are driven using PWM. By controlling the duty cycle of the PWM signal, the amplitude of the applied voltage can be controlled, which in turn will control the speed of the motor. To be able to achieve the required speed smoothly, the PI control loop is implemented as shown in Figure 5.