Design BLDC Motor Speed Controller in Simulink 

Introduction

In this blog, we will be going through what a BLDC or Brush Less DC motor is, how is it different from the Permanent Magnet Synchronous Machines, and how to design a Speed Controller for BLDC in Simulink.

Required Add-Ons

• Simscape
• Simscape Electrical

BLDC Motors

            Everyday devices to the more complex machines use brushless dc motors that convert electrical energy into rotational mechanical energy. Brushless dc motors, also referred to as BLDCs, have many advantages over the brushed dc motor. BLDC motors require less maintenance and provide higher efficiency. That’s why these motors replaced brushed dc motors in many applications.

The way these motors are controlled is very different. These motors require a complex controller to convert DC power to the three-phase voltages. To better understand this, let’s look at the BLDC motor structure.

Here the rotor is a Permanent magnet, and the stator is a three-phase Coil winding. This is a DC motor turned inside out. There are various BLDC motors with different magnet arrangements. The stator may have a different number of windings, and the rotor may have multiple pole pairs.

            A permanent magnet (PM) AC motor, which has a trapezoidal back-EMF, is referred to as a BLDC motor. The BLDC motor drive system is based on the feedback of the rotor system at fixed points for communication of the phase currents.

The BLDC motor requires quasi-rectangular shaped currents fed into stator windings of the machine. Alternatively, the voltage may be applied to the motor every 120⁰, with a current limit to hold the current within motor capabilities. Because the phase currents are excited in synchronism with the constant part of the back-EMF, constant torque is generated.

            The electromagnetic torque developed by the BLDC motor is a function of PM flux and current. Back-EMF in each phase is trapezoidal and is 120⁰ displaced (electrical) to each other. A rectangular current pulse is injected into each phase. The phase current coincides with the flat portion (120⁰) of the back-EMF waveform. Hence, the motor develops almost constant torque.

Design BLDC Controller in Simulink

Note: Before designing the BLDC controller in Simulink, ensure that Simscape and Simscape electrical are correctly installed.

Select the permanent magnet synchronous machine, change its back EMF to trapezoidal in its block parameters, and set the number of phases to 3. To view the output of this block, we need to use the bus selector. In the bus selector, you can view the parameters that the PMSM (Permanent Magnet Synchronous Machine) can give as output. Select the required parameters that you wish to view and close the block. The following image is the switching logic block,

The switching block takes the three hall sensor digital signals from the motor as input. It generates the three logic signals for the PWM drive circuit. The following image is the PWM drive circuit,

The PWM drive circuit develops six device driving signals of the three-phase inverter supplying the BLDC motor. The following image is the block diagram o the three-phase inverter,

Take the rotor speed as the feedback and feed the error signal to the PID block. Now connect the PID block output to the PWM drive circuit to generate the six device driving signals of the three-phase inverter supplying the BLDC motor accordingly. The following image is the scope data before zooming,

The first channel shows the rotor speed of the motor, the speed of the motor is gradually reached to the set value. The remaining three channels are the three hall sensor digital signals. The following image is the scope data after zooming.

The following image is the scope data showing the back EMF generated by the BLDC motor,

As we discussed earlier, the back EMF generated by the BLDC motor is trapezoidal.

Conclusion

            Nowadays, DC motors are replaced by these BLDC motors, which require complex controllers, unlike regular DC motors. So, understanding the working of the BLDC motor and its controller will give better intuition on the working latest gadgets. “Simulink” and “Simscape” have made simulating these motors and verifying the control system for these motors easy.