WORKING AND CONSTRUCTION OF BLDC MOTOR

 

ABSTRACT

Electric vehicles are the best solution for green transportation due to their high efficiency and zero greenhouse gas emissions. Various electric motors have been used as the propulsion system of electric vehicles. Performance of brushed Direct Current (DC) motors, induction motors, switched reluctance motors, and permanent magnet Brushless DC (BLDC) motors are compared according to the in-wheel motor technology requirements under normal and critical conditions through simulation. This study shows that BLDC motors are the most suitable electric motor for the high-performance electric vehicles. An accurate model of a BLDC motor is needed to investigate the motor performance for different control algorithms. Therefore a BLDC motor with an ideal back-Electro Motive Force (EMF) voltage and its control drive are modelled in Simulink. Correct performance of the BLDC motor drive model is validated through experimental data. Direct torque control technique is a type of flux linkage based sensorless control methods in the BLDC motors. In this thesis, direct torque control switching technique of the BLDC motor is discussed. Results of this study show effective torque control, reduction of torque ripples and improved performance of the BLDC motor compared to the conventional switching control techniques. An optimized back-EMF zero crossing detection based sensorless technique of the BLDC motor is presented in this thesis. The proposed sensorless algorithm generates commutation signals of the BLDC motor according to back-EMF zero crossing detection points of only one phase of the motor. This algorithm is simple and remarkably reduces sensing circuitry, noise susceptibility and cost of the sensorless BLDC motor drives. A digital pulse width modulation (PWM) switching technique is implemented to control the speed of the BLDC motor. Stability of the proposed sensorless BLDC motor drive using a digital PWM speed controller is analysed by Lyapunov’s second method. A novel condition for duty cycle of the PWM speed controller is introduced for stability analysis of the BLDC motor drive.  Brushless DC Motor overcomes many problems of the brushed DC Motor and has been widely applied in various fields. The development of BLDCM control system requires reliable operation, excellent performance of control algorithm, low cost and short development cycle. This paper proposes the speed control of BLDC motor for an electric vehicle. The flexibility of the drive system is increased using digital controller. The 3-phase inverter is implemented using Smart Power Module for feeding BLDC motor. The proposed system accepts Hall sensor signals from the motor and is programmed for desired speed. Experimental results verify the effective developed drive operation.

 

1.    Introduction

2.    Hall Sensor Sequence Calibration of BLDC Motor.

3.    Motor Control Unit of BLDC Motor

4.    Methods of Programming for vehicular use

5.    Conclusion

6.    Reference.

 

Introduction

Permanent magnet synchronous motors are mainly divided into two various types based on their back-EMF waveform; the one with a sinusoidal-wave back-EMF that is called Permanent Magnet Synchronous AC Motor (PMSM) and the other with a trapezoidal-wave back-EMF that is called Permanent Magnet Brushless DC (BLDC) Motors. A BLDC motor with the trapezoidal back-EMF produces larger torque compared to a PMSM with the sinusoidal back-EMF. The focus of this thesis is on the three-phase star connected BLDC motors BLDC motors are a novel type of the conventional DC motors where commutation is done electronically, not by brushes. Therefore, a BLDC motor needs less maintenance, has lower noise susceptibility and lesser power dissipation in the air gap compared to a brushed DC motor due to absence of the brushes. Permanent magnet rotors can vary from two pole pairs to eight pole pairs BLDC motor needs a complex control algorithm due to the electronic commutation that is done according to the exact position of the permanent magnet rotor. There are two algorithms for rotor detection; one method that uses sensors and the other does not that is called sensorless. Hall Effect sensors are normally mounted on the non-rotating end inside the BLDC motor with 120 electrical degree phase difference at the constant position to detect rotor angle.

 

Motor Control Unit of BLDC Motor

BLDC motors are in the category of synchronous motors. Principle of their operation is similar to the brushed DC motors, however BLDC motors are commutated electronically and have a permanent magnet rotor. Electronic commutation increases the control drive complexity of the BLDC motor. Control techniques of the BLDC motors are divided into two categories; control drives using sensors and sensorless drives.

This system is essentially an Electronic Control Unit (ECU) like all other ECUs in a vehicle. It comprises of a Microcontroller ported with the Motor Drive Logic (software) and integrated with some other Motor-Control peripherals (hardware). The Motor Drive Logic is the software algorithm that drives the motor in the intended manner. The peripherals like gate driver IC, MOSFETS etc. are part of the control system that performs auxiliary tasks like error feedback handling, signal conditioning, current and voltage amplification. We were also introduced to how a Motor Control System works. But before we get started with this, let’s first learn about the components of a Motor control system:

Fig. Block Diagram of Control Unit of BLDC Motor

Microcontroller  

Microcontroller Unit (MCU) is essentially the component where the Motor Drive Logic is stored. For instance, in an electronic power steering application, a steering control algorithm will be stored in a microcontroller platform. The algorithm is used to vary the speed of the motor using Pulse Width Modulation (PWM) signals. Input and Output buttons can be interfaced with this MCU. PWM signal which is generated as the output and is interfaced with the next core component of the Motor Control System.

PWM Module

The PMW module of the board is configured to operate in symmetric and independent mode. 3 PWM signals are required to switch the high side switches of the inverter used. Since, in the switching sequence, at any instant of time no two switches are on, hence PWM can only be applied to high side switches. The low side switches can provide high output whenever required through any GPIO output port. Thus, there is no requirement of providing any deadtime for the PWM pulses. PWM is operated in unison with the hall sensors. The status of the hall sensors are taken as input through GPIO ports. The corresponding PWM to the input status of hall sensors is enabled at that instant. The table below shows the PWM and logic high requirement of the switches based on the hall sensor values.

 

Gate Driver IC:

Gate Driver IC also known as a pre-driver is that next core component! It, receives the PWM signal from the MCU and amplifies it to make it suitable for the MOSFET. This was a little confusing for us because knowing that MOSFETs are generally used to amplify the voltage, then what is the purpose of having a Gate Driver IC in a motor control system. We were informed that there is a certain threshold voltage value that is required by a MOSFET to be able to function; Gate driver IC provides that threshold voltage. Also, there are some signal conditioning algorithms in the Gate driver IC that removes the noise from the signal and smoothens it.

MOSFETs

These are field effect transistors that can amplify a few milliamps current to ten times and small voltage value to several hundred volts. As electric motors require minimal switching time in varying the current flow to the motor, MOSFETs are an indispensable part of the Motor Control System.

Voltage Source Inverter (VIS)

To control the phases during six-step commutation, a three-phase inverter is used to direct the DC power to the three phases, switching between positive (red) and negative (blue) current. To supply positive current to one of the phases, the switch connected to that phase at the high side needs to be turned on, while for negative current the low side switch needs to be on. Doing this in the pattern described above while the rotor is at an angle between 60 and 120 degrees to the stator magnetic field, the three-phase inverter keeps the motor rotating at a constant speed. Varying motor speed can be achieved by adjusting the applied voltage. Another way of controlling the motor speed without varying the source voltage is pulse-width modulation, or PWM.

Hall Effect Sensors

The role of Hall Effect sensors in automotive motor control application is to determine the position of the rotor. It is essentially a transducer that will induce a voltage when the flowing charge comes under the effect of a permanent magnet. Hall sensors are fitted to the stator so that its position is known before the motor coils are energized. The signal to start or stop an electric motor comes from the microcontroller. Unlike a brushed DC motor, where there are brushes to complete the motor circuit, BLDC motors use Hall-effect sensors to energize the motor coils. The microcontroller uses the input from Hall-effect sensor to determine the position of the rotor. Based on the position, it will energize the motor coils thus creating a rotating electrical field. This field will move the rotor along with it.

 

How Motor Control Unit works?

Consider a motor with three coil windings in the stator and a single pole-pair in the rotor. This type of BLDC motor is driven by six-step commutation or trapezoidal control by a three-phase inverter where the correct phases are commutated every 60 degrees for continuous rotation of the motor. To learn more about six-step commutation and how it works, see the 

A DC voltage source provides a constant voltage to the three-phase inverter, which converts the DC power to three-phase currents to energize different coil pairs in turn. You can see in the image below that when the applied voltage is constant (plot on the left), the motor turns at a constant speed (plot on the right) due to the proportional relationship between voltage and speed.

However, if you want to enable the motor to run at different speeds, you need a controller that will adjust the magnitude of the applied voltage. The following steps outline how to build this algorithm.

Using Hall Sensors for Sector Detection

First, to control the rotor you have to measure its angular position and speed by using sensors like Hall effect sensors. The Hall sensor doesn’t provide information on precisely where the rotor is within a sector. Instead, it detects when the rotor transitions from one sector to another (see the animation), which is the only input needed to determine when to commutate the motor phases.

What is still missing, however, is the information about which two out of the three phases have to be commutated with each sector crossing. The correct phases are specified by a commutation logic circuit as discussed in the next section.

Commutating Phases with a Commutation Logic Circuit

The following block diagram shows how different components of the motor control algorithm interact with each other. The commutation logic circuit computes the switching pattern for the three-phase inverter. In the commutation logic table, the letters A, B, and C stand for the three phases of the motor. The high side of the three-phase inverter is labeled with H and the low side with L. If the rotor is within the first sector, the commutation logic selects the top switching pattern, which dictates that the high side switch of phase A and the low side switch of phase C are turned on. 

As the rotor transitions to other sectors, the next switching pattern is selected accordingly and sent to the three-phase inverter. In summary, the sensor tells us when to commutate the rotor, and the commutation logic determines the correct phases to energize during each commutation. This way we’re able to spin our motor. The next goal will be to enable the motor to spin at different speeds.

 

A BLDC motor, a three phase voltage source inverter and a closed loop control algorithm are the main sections of the BLDC motor drives. A BLDC motor includes two separate electrical and mechanical parts. Three Hall Effect sensors (with 120 electrical degree phase difference) detect the rotor position of the motor. Hall Effect signals are decoded in the controller and the appropriate voltage space vectors are chosen to supply the motor. Corresponding switching signals are fed to the three phase VSI to supply voltages to the windings of BLDC motor. In this section speed of the BLDC motor is adjusted by a digital PWM speed controller in a closed loop scheme.

 

Program for BLDC Motor Speed Control

We have to create a PWM signal with varying duty cycle from 0% to 100% with a frequency of 50Hz. The duty cycle should be controlled by using a potentiometer so that we can control the speed of the motor.

Hall Sensor Sequence Calibration of BLDC Motor

The goal of calibration is to minimise any measurement uncertainty by ensuring the accuracy of test equipment. ... Calibration quantifies and controls errors or uncertainties within measurement processes to an acceptable level. Here our topic is Hall Sensor Sequence Calibration of BLDC Motor.

A Hall effect sensor varies its output voltage based on the strength of the applied magnetic field. According to the standard configuration, a brushless DC (BLDC) consists of three Hall sensors located electrically 120 degrees apart. A BLDC motor with the standard Hall placement (where the sensors are placed electrically 120 degrees apart) can provide six valid combinations of binary states: for example, 001,010,011,100,101, and 110. The sensor provides the angular position of the rotor in degrees in the multiples of 60, which the controller uses to determine the 60-degree sector where the rotor is present.

The target model runs the motor at a low speed (10 RPM) in open loop and performs V/f control on the motor. At this speed, the d-axis of the rotor closely aligns with the rotating magnetic field of the stator.

When the rotor reaches the open-loop position zero, it aligns with the phase a-axis of the stator. At this position (corresponding to a Hall state), the six-step commutation algorithm energizes the next two phases of the stator winding, so that the rotor always maintains a torque angle (angle between rotor d-axis and stator magnetic field) of 90 degrees with a deviation of 30 degrees.

The Hall sequence calibration algorithm drives the motor over a full mechanical revolution and computes the Hall sensor sequence with respect to position zero of the rotor in open-loop control.

Generate Code and Run Model on Target Hardware

1. Complete the hardware connections.

2. Open the target model for the hardware configuration that you want to use.

3. Update these motor parameters in the Configuration panel of the target model.

·        Number of pole pairs

·        PWM frequency [Hz]

·        Data type for control algorithm

·        Motor base speed

·        Vd Ref in per-unit voltage

4. Load a sample program to CPU2 of LAUNCHXL-F28379D.

5. Click Build, Deploy & Start on the Hardware tab to deploy the target model to the hardware.

6. Click the host model hyperlink in the target model to open the associated host model. You can also use the open_system command to open the host model. Use this command for a F28379D based controller:

You can use the Scope in the host model to monitor the open-loop rotor position and Hall sequence values.

7. In the Host Serial Setup block mask in the host model, select a Port name.

8. Click Run on the Simulation tab to run the host model and start Hall sequence calibration for six-step commutation control. The motor runs and calibration begins when you start simulation. After the calibration process is complete, simulation ends and the motor stops automatically.

Note: If the motor does not start or rotate smoothly, increase the value of the Vd Ref in Per Unit voltage field (maximum value is 1) in the Configuration panel. However, if the motor draws high current, reduce this value.

As a convention, six-step commutation control uses a forward direction of rotation that is identical to the direction of rotation used during Hall sequence calibration. To change the forward direction convention, interchange the motor phase wires, perform Hall sequence calibration again, and then run the motor by using six-step commutation control.

9. See these LEDs on the host model to know the status of calibration process:

·        The Calibration in progress LED turns orange when the motor starts running. Notice the rotor position and the variation in the Hall sequence value in the Scope (the position signal indicates a ramp signal with an amplitude between 0 and 1). After the calibration process is complete, this LED turns grey.

·        The Calibration complete LED turns green when the calibration process is complete. Then the Calibration Output field displays the computed Hall sequence value.

Note: This example does not support simulation.

To immediately stop the motor during an emergency, click the Emergency Motor Stop button.

For examples that use six-step commutation using a Hall sensor, update the computed Hall sequence value in the bldc. Hall sequence variable in the model initialization script linked to the example.

Conclusion

We know that Brushed DC motor has more losses due to its mechanical commutator, that losses overcome in BLDC motor as BLDC motor has electronic commutation and control system it gives precise control over operation and less losses as compared to brushed motors. The combination of mechanics and electronics has led to various advancements in automotive motor control system. A simple brushless DC motor is now being deployed to make some smart moves as it is driven by motor drive logic. If you drive a modern car, you must have noticed how smooth the power steering, power window and seat adjustment etc. has become. All this has been possible with advanced motor control system that not only drive a motor in different speeds and directions, but also in an efficient and smooth manner.