A brushless DC motor (BLDC), also known as an electronically commutated motor (ECM, EC motor) or synchronous DC motor, is a synchronous motor powered by DC power through an inverter or switching power supply, which generates AC current to drive the motor phases through a closed-loop controller. The controller provides current pulses to the motor windings that control the motor speed and torque.
The advantages of brushless motors over brushed motors are high power-to-weight ratio, high speed, and electronic control. Brushless motors are used in computer peripherals, handheld power tools, and vehicles.
We will describe how to program a DC motor control in the following sections, but if you just want to try out the results, download the GreenPAK Designer software, plug the GreenPAK Development Kit into your computer, and click through the program to design a device.
Construction and operating principle
The construction and operation of brushless motors are very similar to AC motors and brushed DC motors. Like all other motors, BLDC motors consist of a rotor and a stator (Figure 1).
The BLDC motor stator is made of laminated steel to carry the windings. The windings in the stator can be arranged in two patterns – star (Y) or triangle (Δ). The main difference between the two patterns is that the Y pattern provides high torque at low RPM, while the Δ pattern provides low torque at low RPM. This is because in the Δ configuration, half of the voltage is applied to the undriven winding, which increases losses and, in turn, increases efficiency and torque. The BLDC motor is controlled using an electrical cycle. There are 6 states in one electrical cycle. The motor commutation sequence based on Hall sensors is shown in Figure 2.
The basic principle of BLDC motor operation is the same as that of brushed DC motors. For brushed DC motors, feedback is achieved using a mechanical commutator and brushes. In BLDC motors, feedback is achieved using multiple feedback sensors. The most commonly used sensors are Hall sensors and optical encoders.
In a three-phase BLDC, the number of poles is a multiple of 3 and the number of magnets is a multiple of 2. Depending on the number of magnets and teeth, each motor has a different number of steps per revolution. To calculate the number of steps (N), we need to know how many poles and magnets are used in the motor. The motor used in this article has 12 teeth (poles) and 16 magnets.
Therefore, to make 1 turn, we need to generate 48 steps.
design
The main block diagram and typical application circuit are shown in Figure 3 and Figure 4 respectively.
This design uses 2 inputs to control the motor speed and direction. PIN#8 controls the direction; a high on Pin#8 means the motor is rotating clockwise, and a low means it is rotating counterclockwise. PIN#2 is used to control the speed by inputting a frequency. No frequency signal on this pin will turn off the driver and the motor will stop. Applying a frequency to this pin within the first 500 milliseconds will start the motor. Using the input frequency allows us to control the motor speed very precisely. To calculate the RPM, we need to know how many electrical steps the motor consists of:
The motor in this article has 48 steps, so at a frequency of 5kHz the motor will run at 6250 RPM.
The design can be divided into four parts (Figure 5): the Hall sensor processing block, the gate driver block, the PWM control or speed control block, and the protection block.
The processing module of the Hall sensor includes ACMP (ACMP0, ACMP3, ACMP4), de-glitch filter (DLY1, DLY5, DLY6) and DFF (DFF6, DFF7, DFF8). The Hall sensor used in this project has 4 pins; VDD, GND and 2 differential outputs connected to the IN+ and IN- inputs of ACMP. The internal Vref component is set to 1.2 V and used as VDD for the Hall sensor. The filtered signal from the ACMP goes to the D input of the DFF. The input frequency clocks these DFFs and sets the rotation speed. The signal from these DFFs goes to the gate driver and the 3-bit LUT14 configured with XNOR. The result is that the output level alternates every time any Hall sensor changes its polarity. Both edge detectors generate the actual speed frequency (Hall frequency), which is compared with the input frequency to generate a PWM signal to control the rotation speed.
The gate driver module includes 12 3-bit LUTs to commutate the external transistors based on the feedback from the Hall sensors. Six of the LUTs (3-bit LUT8 – 3-bit LUT13) are used for the CW direction, and the other six (3-bit LUT1 – 3-bit LUT6) are used to switch the CCW direction. The module also includes three 2-bit LUTs (2-bit LUT4, 2-bit LUT5, and 2-bit LUT6) to mix the signals of the PMOS transistors of each phase with PWM to ensure that the speed is independent of the load.
The PWM control consists of the PWM2 component, counter CNT8, finite state machine FSM1, 3-bit LUT15, 2 DFFs (DFF0 and DFF1), rising edge detector PDLY0, and inverter INV0. DFF0 and DFF1 are used together as a frequency comparator; when the input frequency is higher than the Hall frequency, the DFF0 nQ output goes low; when the input frequency is lower than the Hall frequency, the DFF1 nQ output goes low. When the “+” input is low, the PWM2 OUT+ output generates a PWM signal with a duty cycle ranging from 256/256 to 1/256. When the “-” input is low, the PWM2 OUT+ generates a PWM with a duty cycle varying from 1/256 to 256/256. The PWM frequency is about 100 kHz, and the duty cycle of the IC is set to 0% at startup. The motor stops until the input frequency applied to PIN2. Once the frequency is applied to PIN2, the DFF0 nQ output will go low and the PWM will increase the duty cycle from 0 to 99.6%. The motor will continue to rotate until the Hall sensor exceeds the input frequency. At this point, the DFF0 nQ output will go high and the DFF1 nQ output will go low. This reversal causes the PWM duty cycle to decrease to an acceptable value at the instantaneous VDD and load seen by the motor. The system will constantly work to balance the PWM duty cycle. The functionality of FSM1, CNT8, 3-bit LUT15, and PWM2 is described in more detail in Application Note AN-1052 .
The protection block consists of 2 relays (DLY2 and DLY9), counter CNT0, and 2-bit LUT0 configured as an XOR gate. This part is designed to protect the motor and external FET from burning out. If the motor is stuck or cannot be started, the Hall sensor will not provide the feedback needed to turn off the motor. If no feedback is received after 100 milliseconds, the DLY2 output will go low and the 2-bit LUT0 will turn off the motor. If this happens, CNT0 and DLY9 will try to start the motor every 500 milliseconds for 8 milliseconds. This period of time is enough to start the motor, but not enough to cause damage to the motor.
Summarize
This article describes how to control a three-phase brushless DC motor using the SLG46620 GreenPAK IC and Hall Effect sensors. The SLG46620 also contains other features that can be used for this project. For example, the ADC in the GreenPAK can interpret the input DC voltage and generate PWM pulses based on that value instead of using the input frequency.
Dialog GreenPAK reverses this design process by putting configurability back into the hands of the designer. By using this GreenPAK application as a universal three-phase BLDC motor control solution, designers can select the pinout and external FETs that meet the unique electrical specifications of their project. In addition, even taking into account the external FETs, the Dialog GreenPAK solution cost is still low and small enough that the system design and BOM cost are very competitive compared to dedicated ICs.
