Week-7 Challenge: DC Motor Control

•  TITLE = DC Motor control

Aim=

1.A)  RUN MATLAB demo model 'speed control of a DC motor BJT H-bridge '.

   .B) Comment on the armature current shoot-up from the scope results.

    .C) Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.

2)  Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results. (Refer to article - Multiquadrant operation of motor )

 

3) write a breif about operation of BLDC Motor.

PART 1 ) A AND B)  Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

   Comment on the armature current shoot-up from the scope results.

What is BJT

A Bipolar Junction Transistor, or BJT, is a solid-state device in which the current flow between two terminals (the collector and the emitter) is controlled by the amount of current that flows through a third terminal (the base).

How does BJT works-

A BJT is a type of transistor that uses both electrons and holes as charge carriers. A signal of small amplitude if applied to the base is available in the amplified form at the collector of the transistor. This is the amplification provided by the BJT.

H-Bridge:-

An H-bridge is an electronic circuit that switches the polarity of a voltage applied to a load. These circuits are often used in robotics and other applications to allow DC motors to run forwards or backwards.

The term H-bridge is derived from the typical graphical representation of such a circuit. An H-bridge is built with four switches (solid-state or mechanical). When the switches S1 and S4 (according to the first figure) are closed (and S2 and S3 are open) a positive voltage is applied across the motor. By opening S1 and S4 switches and closing S2 and S3 switches, this voltage is reversed, allowing reverse operation of the motor.

Using the nomenclature above, the switches S1 and S2 should never be closed at the same time, as this would cause a short circuit on the input voltage source. The same applies to the switches S3 and S4. This condition is known as shoot-through.

For running the matlab demo of ‘Speed control of a DC motor using BJT H-bridge’ open the matlab software and in Help section type the ‘Speed control of a DC motor using BJT H-bridge’ the matlab model will get open.

In help type the ‘Speed control of a DC motor using BJT H-bridge’ and the model information will get open , after that open that mode in matlab window.

The model will look like follows after opening:-

The Bipolar Junction Transistor (BJT) when used for power switching applications, operates as an IGBT. When it is conducting (BJT operating in the saturated region), a forward voltage Vf is developed between collector and emitter (in the range of 1 V). Therefore, the IGBT block can be used to model the BJT device.

The IGBT block does not simulate the gate current controlling the BJT or IGBT. The switch is controlled by a Simulink® signal (1/0). The DC motor uses the preset model (5 HP 24V 1750 rpm). It simulates a fan type load (where Load torque is proportional to square of speed). The armature mean voltage can be varied from 0 to 240 V when the duty cycle (specified in the Pulse Generator block) is varied from 0 to 100%.

The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models). Two transistors are switched simultaneously: Q1 and Q4 or Q2 and Q3. When Q1 and Q4 are fired, a positive voltage is applied to the motor and diodes D2-D3 operate as free-wheeling diodes when Q1 and Q4 are switched off. When Q2 and Q3 are fired, a negative voltage is applied to the motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.

The motor block as parameters set as follows-

The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.

'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.

The duty cycles are set  in pulse generator block with 75 % duty cycle-

When the simulation is run for the first time , without changing any parameters , keeping all parameters same the results obtained are discussed below-

The max speed which can be achieved is about 951 rpm

The maximum torque which is obtained is 6.04Nm and minimum torque is -6.04 Nm

Results :-

B ) Comment on the armature current shoot-up from the scope results.

The armature current  shoot up when motor changes direction from forward to reverse  is about 69.90A , which is too large we have to overcome this and reduce the current shoot up we need to modify the mode , this can be done by changing the Duty cycles.

We can change the Duty cycles by opening the pulse generator block-

Open the pulse generator block and change the duty cycles and observe the results-

 

Here the duty cycle  is set to 50%

The results obtained after running this simulation are as follows-

The max speed which can be achieved is about 443 rpm which is been reduced from 951 rpm

The maximum torque which is obtained is 1.3Nm and minimum torque is -1.003 Nm

The current shooting is been reduced to 17A.

 

 Now again the duty cycle is lowered and the results are been checked-

Here the duty cycle  is set to 30%

The results obtained after running this simulation are as follows-

The max speed which can be achieved is about 165 rpm which is been reduced from 951 rpm

The maximum torque which is obtained is 0.18Nm and minimum torque is -0.009 Nm

The current shooting is been reduced to -5.42A.

 

The second method by which we can reduce the current shooting of motor when changing the direction from positive to negative can be achieved by increasing the resistance of D3 diode.

 Conclusion :-

For lowering the current shooting we have to decrease the duty cycles , after lowering the duty cycle percentage then the current shooting is been reduced.

1.C) Refer to help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with H-bridge model.

THEORY OF FOUR QUADRANT CHOPPER DC DRIVE=

A four quadrant chopper is a chopper which can operated in all the four quadrants.In this chopper The power can flow either from source to load or load to source . In first quadrant, a Class-E chopper acts as a Step-down chopper whereas in second quadrant it behaves as a Step-up chopper. This type of chopper is also known as Class-E or Type-E chopper. This article describes the working principle and operation of Class-E chopper with the help of circuit diagram.

Working Principle / Operation of Class-E Chopper:

The circuit of a four quadrant chopper or class-E chopper basically consists of four semiconductor switches CH1 to CH4 and four diodes D1 to D4. The four diodes are connected in anti-parallel. The circuit diagram of this type of chopper is shown below.

In the above circuit diagram, the choppers are numbered CH1 to CH4. For first quadrant operation CH1 is made ON, for second quadrant operation CH2 is made ON and so on.

First Quadrant Operation:

For first quadrant operation, CH4 is kept ON, CH3 is kept OFF and CH1 is operated. When both CH1 & CH4 are ON simultaneously, the load gets directly connected to the source and hence the output voltage becomes equal to the source voltage. This essentially means that vo = vs. It may be noted that the load current flows from source to load as shown by the direction of io.

When CH1 is switched OFF, the load current free wheels through CH4 and D2. During this period, the load voltage and current remains positive.

Thus, both the output voltage vs and load current io are positive and hence, the operation of chopper is in first quadrant. It may be noted that, Class-E chopper operates as a step-down chopper in this case.

Second Quadrant Operation:

To obtain second quadrant operation, CH2 is operated while keeping the CH1, CH3 & CH4 OFF. When CH2 is ON, the DC source in the load drives current through CH2, D4, E and L. Inductor L stores energy during the On period of CH2.

When CH2 is turned OFF, current is fed back to the source through D1, D4. It should be noted at this point that (E+Ldi/dt) is more than the source voltage Vs. As load voltage Vo is positive and Io is negative, it is second quadrant operation of chopper. Since, the current is fed back to the source, this simply means that load is transferring power to the source

For second quadrant operation, load must contain emf E as shown in the circuit diagram. In second quadrant, configuration operates as a step-up chopper.

Third Quadrant Operation:

To obtain third quadrant operation, both the load voltage and load current should be negative.

Polarity of Load emf E reversed

For third quadrant operation, CH1 is kept off, CH2 is kept ON and CH3 is operated. When CH3 is ON, load gets connected to source and hence load voltage is equal to source voltage. But carefully observe that the polarity of load voltage vo is opposite to what shown in the circuit diagram. Hence, vo is assumed negative.

Now, when CH3 is turned OFF, the negative load current free wheels through the CH2 and D4. In this manner, vo and io both are negative. Hence, the chopper operates in third quadrant.

Fourth Quadrant Operation:

To obtain fourth quadrant operation, CH4 is operated while keeping CH1, CH2 and CH3 OFF. The polarity of load emf E needs to be reversed in this case too like third quadrant operation

When CH4 is turned ON, positive current flows through CH4, D2, L and E. Inductance L stores energy during the time CH4 is ON. When CH4 is made OFF, current is fed back to the source through diodes D2, D3. Here load voltage is negative but the load current is always positive. This leads to chopper operation in fourth quadrant. Here, power is fed back to the source from load and chopper acts as a step-up chopper.

The operation of a four quadrant chopper or Class-E chopper :-

DC7 MODEL

To open the DC 7 model open the help window in matlab and type the DC7 , after that the Dc7 model will appear in the window open that file in matlab.The model looks like as shown below.

The 'DC7' motor drive in Simscape™ Power Systems™ software, referred to as the four-quadrant chopper DC drive block in MATLAB/Simulink. It represents a four-quadrant, DC-supplied chopper drive for DC motors, featuring closed-loop speed control with the four-quadrant operation. This closed-loop speed control gives the armature current of the machine as an output. By using a PI current controller and adjusting the chopper duty cycle, we get the commanded armature current as per our needs. The duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper. 

Compared to other DC drives that are available, the main advantage of using the four-quadrant chopper DC drive is that it can operate in all 4 quadrants. In each quadrant, its characteristics represent a different mode of operation.

The first quadrant is forward motoring, the second is reverse braking, the third is reverse motoring and the fourth is forward braking. Another advantage is that it comes with a high switching frequency DC-DC converters, resulting in a lower armature current ripple compared to thyristor-based DC drives.

The Simulink block of DC7 and its circuit diagram is given below:

Here the power supply is not dc. It is a 380v 50Hz AC power supply. It means that the rotor of the generator turns 50 cycles per second, the current changes 50 times per second back and forth, direction changes 100 times. That means the 380V changes from positive to negative, and from negative to positive voltage, this process converts 50 times/second.

The model contains AC to DC Converter (i.e Rectifier Bridge + Braking Chopper), Four-Quadrant Chopper DC Drive, bus model subsystem. The applied AC 380V is converted to a 515V DC supply by an AC to DC converter. This converter includes a rectifier and a braking chopper. The braking chopper is used to convert the voltage from the motor while regenerative braking, exceeding a threshold voltage into thermal energy. This is done by the brake resistor connected in series with the diode and a switching device inside the braking chopper. The braking chopper activates when the voltage reaches 560V and it switches OFF when the voltage is 520V. Though the AC source is converted to 515V, we must include losses. Hence the voltage is kept at 520V. 

The motor runs based on the speed reference block provided.

Four quadrant operation of DC Motor - A motor operates in two modes – Motoring and Braking. A motor drive capable of operating in both directions of rotation and of producing both motoring and regeneration is called a Four Quadrant variable speed drive. In motoring mode, the machine works as a motor and converts the electrical energy into mechanical energy, supporting its motion. In braking mode, the machine works as a generator and converts mechanical energy into electrical energy and as a result, it opposes the motion. The Motor can work in both, forward and reverse directions.

Function	Quadrant	Speed	Torque	Power Output
Forward Motoring	I	+	+	+
Forward Braking	II	+	-	-
Reverse Motoring	III	-	-	+
Reverse Braking	IV	-	+	-

                     

Applications of 4 - Quadrant Operation 

• Compressor, pump and fan type load requires operation in the I quadrant only. As their operation is unidirectional, they are called one quadrant drive systems.
• Transportation drives require operation in both directions.
• If regeneration is necessary, application in all four quadrants may be required. If not, then the operation is restricted to quadrants I and III, and thus dynamic braking or mechanical braking may be required.
• In hoist drives, a four-quadrant operation is needed.

The Simulink DC7 four-quadrant drive contains a speed and current controller, regulator switch, chopper, dc motor

Speed controller block - The inputs are speed reference and motor speed, and outputs a current reference. This output current reference is in turn used by the current controller block to obtain the electromagnetic torque needed to reach the desired speed.

Regulation Switch block - The Regulation Switch block consists of a DC machine (wound-field type) torque controller model and a Simulink Switch model. The latter is used to select the regulation mode, i.e. torque or speed regulation of the DC drive. The torque controller provides a DC current reference. A second reference DC current is obtained from the speed controller. Depending on the regulation mode, the Simulink switch selects between the two DC current references.

Current controller block - The Current Controller (DC) block models a PI current controller for use with a DC machine. The block has multiple instances, depending on the DC drive type (chopper DC drive or thyristor-based DC drive) and operating quadrants (one-, two-, or four-quadrant). It has two operating modes to represent a detailed or an average-value current controller. The DC current controller block inputs are the current reference (in pu) and the armature current flowing through the motor. The outputs are current reference, duty cycles, PWM pulses, and reference current error. From here the PWM pulses are fed to the chopper block where the switches are present.

Chopper block - The Chopper block models a standard half-bridge (one leg, two IGBTs) or full-bridge (two legs, four IGBTs) converter model. It has two operating modes to represent a detailed or an average-value chopper. Each operating mode has multiple instances depending on the DC motor drive type (two- or four-quadrant operation). The chopper block outputs the desired armature current to the DC motor. To avoid the sudden surge in the current fluctuations, a smoothing inductance is connected in series with the chopper.

DC Machine - Here the motor is coupled to a linear load, which means that the mechanical torque of the load is proportional to the speed.

OUTPUT OF DC7 MODEL SCOPE=

The scope shows the motor armature voltage and current, the four IGBT pulses and the motor speed on the scope. The current and speed references are also shown. The motor is coupled to a linear load, which means that the mechanical torque of the load is proportional to the speed.

Advantages:

1. 4 quadrant choppers have more control over the deceleration of the motor and can more accurately brake the motor. This will make the deceleration smoother.
2. The second benefit is that it reduces the risk of current spikes, which can damage the motor and the controller itself because this can occur when the motor is made to brake while running at a high speed. So the controller life is generally longer with 4 quadrant choppers, by mitigating those sudden surges or spikes from the output.
3. A third advantage is that 4 quadrant choppers are more efficient because lesser energy is wasted as heat and lesser current spikes occur which means lesser power dissipation.
4. In the case of a vehicle, the 4 quadrant chopper makes regenerative braking possible so that when the motor is made to brake, the kinetic energy of the rotor moving is transferred back as electrical energy which can then be used to charge a battery. 

Difference between 4 Quadrant chopper and H-Bridge:-

Four Quadrant Chopper	BJT H-Bridge
it is used for industrial and commercial applications	its commonly used for Robotics
1.       Forward motoring 2.       Forward braking 3.       Reverse motoring 4.       Reverse breaking	1.       Forward motoring 2.       Reverse motoring
To change the rotation of motor like forward and reverse , Signals are interrupted with one another.	It has low switching frequency while comparing it with 4 quadrant chopper
Armature current varies a little while switching from one quadrant to another due to high switching frequency.	The Armature current shoots up in the graph indicates the variation while switching from forward to reverse operation.

Part 2= Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results. (Refer to article - Multiquadrant operation of motor )

therory of 2 quadrant chopper=

Two quadrant chopper known as C-type chopper. Type-C chopper is obtaining from type-A and type-B chopper in parallel. We will always on voltage is positive v0 and free  whiling diode FD is presented across load.

In two quadrant chopper voltage is always positive and may be positive or negative current. In first quadrant voltage and current both are positive so motor rotate positive direction so called forward motoring.

 second quadrant voltage remains positive and current is negative direction so that called as regenerative braking.

Now we talking about the model of two quadrant DC chopper:-

 

Model Construction :

Click on Simulink Browser and open Library to Search for Component required to Build C-Class chopper as follows: 

• Power GUI
• Pulse generator
• Current measurement
• Voltage measurement
• Diode
• MOSFET
• Logical operator (NOT)
• Series RLC
• Scope

1. Arrange the Blocks as per the circuit connection shown above in the Model.
2. Double click on MOSFET and DIODE Block to Disable (uptick show measurement port), and rotate the diode in upward direction as per our requirement (source should be facing in Downward direction for MOSFET and for DIODE source should be in upward direction)
3. Open series RLC block and Change the branch type to (L) with default value 1 mill H, once done arrange according to circuit diagram.
4. Open logical operator and select NOT in Operation options, connects its Input to output of PULSE Generator block. The reason we use NOT gate to ensure both the Switches does not conduct simultaneously if they do so supply will be short-circuited.
5. We will be giving the tapping for the PULSE gate terminal for 1 switch MOSFET1 and from NOT gate we are giving it to other switch MOSFET2. So if MOSFET1 is conducting MOSFET2 will not conduct because we are using the NOT gate.
6. Once done we will be measuring the voltage by connecting its 1^st port at staring of inductor and other to DC2 supply which is connected adjacent.
7. for scope we connect three input, first one connected pulse generator,second one connecting current and for third connected voltage .
8. Double click on main DC Voltage supply and set amplitude (V) to 24 V and for adjacent DC2 supply set amplitude as 12v.
9. Open Pulse generator block set amplitude to 10 for better viewing, time period set to 1 sec, pulse width is 50 (because we 50% to be ON and 50% to be OFF) .i.e. NOT gate is ON when 50% OFF

The working of the Model is given below:

Type C chopper is obtained by connecting type –A  and type –B choppers in parallel.  We will always get a positive output voltage V₀  as the freewheeling diode FD is present across the load.  When the chopper is on the freewheeling diode starts conducting and the output voltage v₀ will be equal to V_s. The direction of the load current i₀ will be reversed. The current i₀  will be flowing towards the source and it will be positive regardless of the chopper is on or the FD conducts.  The load current will be negative if the chopper is or the diode D2 conducts.  We can say the chopper and FD  operate together as a type-A chopper in the first quadrant. In the second quadrant, the chopper and D2 will operate together as type –B chopper.

The average voltage will be always positive but the average load current might be positive or negative. The power flow may be like the first quadrant operation i.e from source to load or from load to source as the second quadrant operation.  The two choppers should not be turned on simultaneously as the combined action may cause a short circuit in supply lines. For regenerative braking and motoring, this type of chopper configuration is used.

Let the voltage of the DC voltage source be 24 volts.

Let us consider an series RLC branch block and select inductor L . Thus block works as a load.

To consider LE load i.e motor load , and dc voltage source of 12 volt is also connected in series with inductor.

The pulse generator block is used to create gate pulse.

The not gate is used in the model so that the two MOSFET will not get closed at the same time otherwise short circuit will occur. Due to use of not gate, when one mosfet is ON then the other mosfet will get OFF and when other mosfet is ON then the first mosfet will get OFF .

The amplitude of pulse generator is kept 10 so that we can clearly see the pulses at scope and let period be 1 seconds and let pulse width be 50%.

OUTPUT=

The voltage waveform is not going less than zero, the minimum value of voltage is zero and maximum value is always in the positive direction.

However in current waveform, the current increases and decreases, current may be positive as well as negative.

The pulse amplitude is 10 thus from pulse waveform we can see that the pulse amplitude is 10.

PART 3) operation of BLDC Motor.

A brushless DC motor (also known as a BLDC motor) is an electronically commuted DC motor that does not have brushes. The controller provides pulses of current to the motor windings that control the synchronous motor's speed and torque.

These types of motors are highly efficient in producing a large amount of torque over a vast speed range. In brushless motors, permanent magnets rotate around a fixed armature and overcome the problem of connecting current to the armature. Commutation with electronics has a large scope of capabilities and flexibility. They are known for smooth operation and holding torque when stationary. In conventional Brushed DC Motors, the brushes are used to transmit the power to the rotor as they turn in a fixed magnetic field. A BLDC motor used electronic commutation and thus eliminates the mechanically torn brushes.

Before explaining the working of a brushless DC motor, it is better to understand the function of a brushed motor. In brushed motors, there are permanent magnets on the outside and a spinning armature that contains an electromagnet inside. These electromagnets create a magnetic field in the armature when the power is switched on and help to rotate the armature.

The brushes change the polarity of the pole to keep the rotation on of the armature. The basic working principle for the brushed DC motor and brushless DC motor is the same i.e. internal shaft position feedback.

A Brushless DC motor has only two basic parts: the rotor and the stator. The rotor is the rotating part and has rotor magnets whereas the stator is the stationary part and contains stator windings. In BLDC permanent magnets are attached to the rotor and move the electromagnets to the stator. The high-power transistors are used to activate electromagnets for the shaft turns. The controller performs power distribution by using a solid-state circuit.

Construction of BLDC motors:

The main design difference between brushed and brushless motors is the replacement of a mechanical commutator with an electric switch circuit. Keeping that in mind, a BLDC motor is a type of synchronous motor in the sense that the magnetic field generated by the stator and the rotor revolve at the same frequency.

Brushless Motors are available in three configurations: single-phase, two-phase, and three-phase. Out of these, the three-phase BLDC is the most common one.

Types of Brushless DC Motors

Basically, BLDC are of two types, one is outer rotor motor and other is inner rotor motor. The basic difference between the two is only in designing, their working principles are same.

Inner Rotor Design

In an inner rotor design, the rotor is located in the centre of the motor and the stator winding surround the rotor. As the rotor is located in the core, rotor magnets do not insulate heat inside and heat get dissipated easily. Due to this reason, inner rotor designed motor produces a large amount of torque and validly used.

Outer Rotor Design

In outer rotor design, the rotor surrounds the winding which is located in the core of the motor. The magnets in the rotor trap the heat of the motor inside and do not allow to dissipate from the motor. Such type of designed motor operates at lower rated current and has low cogging torque.

The following image shows the cross-section of a BLDC Motor.

As we can see in the image, a BLDC motor consists of two main parts: a stator and a rotor.

Stator: 

The structure of the stator of a BLDC motor is similar to that of an induction motor. It is made up of stacked steel laminations with axially cut slots for winding. The winding in BLDC is slightly different from that of the traditional induction motor.

Generally, most BLDC motors consist of three stator windings that are connected in star or ‘Y’ fashion (without a neutral point). Additionally, based on the coil interconnections, the stator windings are further divided into Trapezoidal and Sinusoidal Motors.

In a trapezoidal motor, both the drive current and the back EMF are in the shape of a trapezoid (sinusoidal shape in case of sinusoidal motors). Usually, 48 V (or less) rated motors are used in automotive and robotics (hybrid cars and robotic arms).

Rotor: 

The rotor part of the BLDC Motor is made up of permanent magnets (usually, rare earth alloy magnets like Neodymium (Nd), Samarium Cobalt (SmCo), an alloy of Neodymium, Ferrite, and Boron (NdFeB)).

Based on the application, the number of poles can vary between two and eight with North (N) and South (S) poles placed alternately. The following image shows three different arrangements of the poles. In the first case, the magnets are placed on the outer periphery of the rotor.

The second configuration is called a magnetic-embedded rotor, where rectangular permanent magnets are embedded into the core of the rotor. In the third case, the magnets are inserted into the iron core of the rotor. 

Now, some crucial things are a must to have in the case of BLDC motors i.e. its sensors.

The BLDC motor has no Brushes, the commutation is controlled electrically. To rotate the motor, the windings of the stator must be energized in a sequence and the position of the rotor which the North poles and South poles of the rotor must be known to precisely energize a particular set of stator windings.

A Position Sensor, which is usually a Hall Sensor is generally used to detect the position of the rotor and transform it into an electrical signal.

HALL-EFFECT SENSOR:

A Hall-effect sensor (or simply Hall sensor) is a device to measure the magnitude of a magnetic field. Its output voltage is directly proportional to the Magnetic field strength through it. The Hall-sensor is used for, Proximity Sensing, Positioning Sensor, Speed Detection, and Current Sensing.

Most BLDC Motors use three Hall Sensors that are embedded into the stator to sense the rotor’s position. The output of the Hall Sensor will be either Low or High. Depending on whether the North or South pole of the rotor passes near it. By combining the results from the three sensors, the exact sequence of energizing can be determined.

THE WORKING PRINCIPLE OF BLDC MOTOR:

• We know that when a current is applied through a coil, a magnetic field is generated and the orientation of the field lines i.e. the poles of the generated magnet will depend on the direction of the current flowing through the coil.
• Now, by supply current to coil A, it will generate a magnetic field and attract the rotor magnet. The position of the rotor magnet will shift slightly clockwise and will align with coil A. similarly for coil B and coil C the magnet rotates in the clockwise direction

• Here, the Three-phase BLDC motor is used and the phases are A, B, &C.
• These phases are connected to an Electronically commutator which is a power electronic controller which acts as an Inverter.
• The Hall sensor is placed between poles to sense the position of the rotor.
• Here three Hall sensors are placed in Stator.
• Each phase signal is combined with High, Low, and OFF.
• The hall signal is taken as the frequency of 0 to 360 degrees.When The Hall-1 is OFF to ON, Hall-2 is OFF, and Hall-3 is OFF to ON. So Hall-1 and Hall-3 are present.
• Then phase A is in the OFF signal, phase B is OFF to High, and phase C is Low. 
• Similarly, in three phases if one phase is of the other two phases are acts as positive and negative.

To understand it more easily a gif of the same above circuit is given below:

Advantages of BLDC Motor: 

• Brushless motors are more efficient as their velocity is determined by the frequency at which current is supplied, not the voltage.
• As brushes are absent, the mechanical energy loss due to friction is less which enhanced efficiency.
• BLDC motor can operate at high speed under any condition.
• There is no sparking and much less noise during operation.
• More electromagnets could be used on the stator for more precise control.
• BLDC motors accelerate and decelerate easily as they are having low rotor inertia.
• It is a high-performance motor that provides large torque per cubic inch over a vast speed range.
• BLDC motors do not have brushes which make them more reliable, high life expectancies, and maintenance-free operation.
• There are no ionizing sparks from the commutator, and electromagnetic interference is also get reduced.
• Such motors cooled by conduction and no airflow are required for inside cooling.

Limitations of BLDC Motors: 

• These motors are costly compared to brushed motors
• The need for additional sensors
• The electronic controller required to control this motor is expensive
• Complex Driver-Circuit is required
• The limited high power could be supplied to the BLDC motor, otherwise, too much heat weakens the magnets and the insulation of winding may get damaged.

Application of BLDC Motors:  

• Electric vehicles, hybrid vehicles, and electric bicycles can be constructed.
• Industrial robots, CNC machine tools, and simple belt-driven systems
• Fans, pumps, and blowers with a low noise-inducing environment.

CONCLUSION:

• MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge and the armature current shoot-up is studied.
• The Four-Quadrant Chopper DC Drive (DC7) block is also studied and compared it with the H-bridge model.
• 2-quadrant chopper is developed using simulink model and obtained the results.
• Operation of BLDC motor is studied.