Control Speed of DC Motor Using Power Electronics Devices

AIM : To Control Speed of DC Motor Using Power Electronics Devices.

POWER ELECTRONICS DEVICES :

• Power electronics is the application of solid-state electronics to the control and conversion of electric power.
• The first high power electronic devices were mercury-arc valves. In modern systems, the conversion is performed with semiconductor switching devices such as diodes, thyristors, and power transistors such as the power MOSFET and IGBT.
• Power electronic devices may be used as switches, or as amplifiers. An ideal switch is either open or closed and so dissipates no power; it withstands an applied voltage and passes no current, or passes any amount of current with no voltage drop.
• Semiconductor devices used as switches can approximate this ideal property and so most power electronic applications rely on switching devices on and off, which makes systems very efficient as very little power is wasted in the switch.

• Basic types of Power Semiconductor Switches :

• BJT ( Bipolar Junction Transistor )
• MOSFET ( Metal-Oxide Semiconductor Field-Effect Transistor )
• IGBT ( Insulated-Gate Bipolar Transistor )

BJT ( Bipolar Junction Transistor ) :

• A Bipolar Junction Transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipolar transistor allows a small current injected at one of its terminals to control a much larger current flowing between two other terminals, making the device capable of amplification or switching.
• BJTs use two junctions between two semiconductor types, n-type and p-type, which are regions in a single crystal of material. The junctions can be made in several different ways, such as changing the doping of the semiconductor material as it is grown, by depositing metal pellets to form alloy junctions, or by such methods as diffusion of n-type and p-type doping substances into the crystal.
• Bipolar transistor integrated circuits were the main active devices of a generation of mainframe and mini computers, but most computer systems now use integrated circuits relying on field effect transistors. Bipolar transistors are still used for amplification of signals, switching, and in digital circuits. Specialized types are used for high voltage switches, for radio-frequency amplifiers, or for switching heavy currents.

MOSFET ( Metal-Oxide Semiconductor Field-Effect Transistor ) :

• A MOSFET is a specific type of metal–oxide–semiconductor field-effect transistor (MOSFET) designed to handle significant power levels. Compared to the other power semiconductor devices, such as an insulated-gate bipolar transistor (IGBT) or a thyristor, its main advantages are high switching speed and good efficiency at low voltages. It shares with the IGBT an isolated gate that makes it easy to drive. They can be subject to low gain, sometimes to a degree that the gate voltage needs to be higher than the voltage under control.
• The design of power MOSFETs was made possible by the evolution of MOSFET and CMOS technology, used for manufacturing integrated circuits since the 1960s. The power MOSFET shares its operating principle with its low-power counterpart, the lateral MOSFET. The power MOSFET, which is commonly used in power electronics, was adapted from the standard MOSFET and commercially introduced in the 1970s.
• The power MOSFET is the most common power semiconductor device in the world, due to its low gate drive power, fast switching speed, easy advanced paralleling capability, wide bandwidth, ruggedness, easy drive, simple biasing, ease of application, and ease of repair. In particular, it is the most widely used low-voltage (less than 200 V) switch. It can be found in a wide range of applications, such as most power supplies, DC-to-DC converters, low-voltage motor controllers, and many other applications.

IGBT ( Insulated-Gate Bipolar Transistor ) :

• An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch, which, as it was developed, came to combine high efficiency and fast switching. It consists of four alternating layers (P–N–P–N) that are controlled by a metal–oxide–semiconductor (MOS) gate structure.
• Although the structure of the IGBT is topologically the same as a thyristor with a "MOS" gate (MOS-gate thyristor), the thyristor action is completely suppressed, and only the transistor action is permitted in the entire device operation range. It is used in switching power supplies in high-power applications such as variable-frequency drives (VFDs), electric cars, trains, variable-speed refrigerators, lamp ballasts, arc-welding machines, and air conditioners.
• Since it is designed to turn on and off rapidly, the IGBT can synthesize complex waveforms with pulse-width modulation and low-pass filters, so it is also used in switching amplifiers in sound systems and industrial control systems. In switching applications modern devices feature pulse repetition rates well into the ultrasonic-range frequencies, which are at least ten times higher than audio frequencies handled by the device when used as an analog audio amplifier. As of 2010, the IGBT is the second most widely used power transistor, after the power MOSFET.

OBJECTIVES :

1. To Run MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’. Modify the model such that armature current doesn’t shoot up when motor changes direction from forward to reverse.

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

3. To Make a suitable EV model using DC7 block and make result report.

4. To Explain in a brief about operation of BLDC motor.

STUDY - 1 :

SIMULINK MODEL FOR SPEED CONTROL OF A DC MOTOR USING BJT H-BRIDGE :

SIMULINK INPUT BLOCK :

SIMULINK BLOCK EXPLANATION :

• Matlab has a inbuilt block to simulate the function of "Speed control of a DC motor using BJT H-bridge".
• When the Bipolar Junction Transistor (BJT) is used for power switching applications, then its 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). 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).
• The armature voltage is controlled with the input of duty ratio at 500 Hz frequency and is stored as P1,P2,P3,P4 control block and it is sent as the control signal to all the IGBT switches respectively.
• The direction of the rotating motor changes with the change in the polarities of the current and it is achieved by the following, Two transistors are switched simultaneously: Q1 and Q4 or Q2 and Q3.
• When Q1 and Q4 switches are closed, 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 switches are closed, a negative voltage is applied to the motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.
• When the direction of the rotating motor changes there will be a lot of stress developed in the IGBT Transistors.

Simulation - 1 :

At Duty Cycle 75%

• One simple and easy way to control the speed of a motor is to regulate the amount of voltage across its terminals and this can be achieved using “Pulse Width Modulation”.
• For experimental section we will set the pulse width (that is refer as the duty cycle) of pulse generator block as 75% at the same time period.
• The input of the P1,P2,P3 and P4 are also changed according to the drive cycle.
• When Q1 and Q4 switch is ON the motor rotates in forward direction that state Q3 switch and Q2 switch is in OFF.
• When Q2 and Q3 switch is ON the motor rotates in reverse direction that state Q1 and Q4 switch is in OFF condition.

INPUT PULSE GENERATOR BLOCK :

OUTPUT CURRENT PLOT :

• In this current output the amount of current is acted is shown as the graphical representation during the period of duty cycle.
• This scope shows IGBT current and Diode current passed in the quadrant Q3 and D3. where Q3 is represented as IGBT current and D3 is represented as Diode current in scope.
• The switch Q3 is OFF on the starting stage of current flow and Diode current is ON on that stage.
• During the 0.5sec of time D3 switch is ON that are given according to pulse generator and after the time period of 0.5sec D3 switch is OFF. An IGBT current is acted after 0.5sec and Q3 switch remains ON.

OUTPUT RESULT :

• The speed and armature current is obtained by the DC motor and load torque is calculated by Load Torque proportional to square of speed,
  T=k*w^2
• In starting point, there is a sudden shootup is acted.
• In this scope shows that speed is increased in certain period of time and that time armature current is in positive terminal whereas load torque also moves forward.
• After the time period of 0.5sec which is given by pulse generator sudden shoot-up is acted and speed gets decreased in that time armature current is in negative terminal whereas torque act as reverse action.
• Here the sudden shoot up is acted while the armature current is processed at a positive and negative terminal.

Simulation - 2 :

BY CHANGING THE PULSE GENERATOR INPUT VALUE (TO REDUCE THE SHOOT UP OF ARMATURE CURRENT) : At Duty Cycle 50%

• One simple and easy way to control the speed of a motor is to regulate the amount of voltage across its terminals and this can be achieved using “Pulse Width Modulation”.
• For experimental section we will reduce the pulse width (that is refer as the duty cycle) of pulse generator block as 50% at the same time period.
• The input of the P1,P2,P3 and P4 are also changed according to the drive cycle.
• When Q1 and Q4 switch is ON the motor rotates in forward direction that state Q3 switch and Q2 switch is in OFF.
• When Q2 and Q3 switch is ON the motor rotates in reverse direction that state Q1 and Q4 switch is in OFF condition.

INPUT PULSE GENERATOR BLOCK :

OUTPUT CURRENT PLOT :

• Here we see that the sudden shoot up is not acted by comparing with 75 of pulse width. Same cycle time is followed in this experiment also.
• In this observation a starting current is generated as diode current for time period of 0.5sec then IGBT current is acted while the diode current is being stoped.
• where Q3 is represented as IGBT current and D3 is represented as Diode current in scope. This graph shows when the switch Q3 is OFF on the starting stage of current flow and Diode current is ON on that stage.
• During the 0.5sec of time D3 switch is ON that is given according to the pulse generator and after the time period of 0.5sec D3 switch is OFF. An IGBT current is acted after 0.5sec and Q3 switch remains ON.
• There will be a little variation are shown in the graph at the starting point of current but the complete reduction of shoot up are not held here.

OUTPUT RESULT :

• In this above scope, a flow of armature current is displayed and speed, load torque is also displayed.
• The changes are made in pulse width as 50% from the pulse generator and results are shown that voltage fluctuation is held in starting point of forward state and reverse state.
• That sudden fluctuation is reduced in this experiment by reducing the pulse width(duty cycle of motor) of the input supply. An changes are also held in speed and load torque where the constant level of speed and torque is acted in above scope compare with 75% of pulse width.
• Time period of forward and reverse action is maintained as same as the previous duty cycle.
• In this observation the shoot up is reduced when motor changes direction from forward to reverse compared to 75 of pulse
  width.

Simulation - 3 :

At Duty Cycle 40%

• In previous experiment there will be an reduction of shoot up(fluctuation) is observed, But we want to modify the model such that armature current doesn’t shoot up when motor changes direction from forward to reverse.
• For the experimental section, we will reduce the pulse width (that is refer as the duty cycle) of pulse generator block as 40% at the same time period.
• The input of the P1,P2,P3 and P4 are also changed according to the drive cycle.
• When Q1 and Q4 switch is ON the motor rotates in forward direction that state Q3 switch and Q2 switch is in OFF.
• When Q2 and Q3 switch is ON the motor rotates in reverse direction that state Q1 and Q4 switch is in OFF condition.

INPUT PULSE GENERATOR BLOCK :

OUTPUT CURRENT PLOT :

• This graph shows that IGBT current and diode current flow from transistor Q3 and D3 after changing the pulse width(duty cycle) as 40. where Q3 is represented as IGBT current and D3 is represented as Diode current in scope.
• This graph shows when the switch Q3 is OFF on the starting stage of current flow and Diode current is ON on that stage.
• During the 0.5sec of time D3 switch is ON that are given according to pulse generator and after the time period of 0.5sec D3 switch is OFF. An IGBT current is acted after 0.5sec and Q3 switch remains ON.
• The flow of current is not much varied in IGBT current and Diode current when we compared it with 75 of pulse width. The shoot up(fluctuation) is less in this duty cycle when motor changes direction from forward to reverse

OUTPUT RESULT :

• In this above scope a flow of armature current is displayed and speed, load torque are also displayed.
• From the above experiments shows that shoot up (voltage fluctuation) doesn't appear while we reducing the pulse width (duty cycle) in pulse generator. But raise in fluctuation of armature current is not completely acted during motor changes direction from forward to reverse. Because starting point of current is raised during forward and reverse direction of motor. voltage fluctuation is acted on that point.
• Time period of forward and reverse action is maintained as same as the previous duty cycle.
• By the reduction of duty cycle(pulse width)as 40 we observed that armature current doesn’t shoot up when motor changes direction from forward to reverse.
• The constant flow of armature current is observed in this experiment during motor changes direction forward to reverse.
• Therefore the shoot up is completely minimized by this Pulse Width Modulation method.

STUDY - 2 :

FOUR QUADRANT CHOPPER CIRCUIT OPERATION :

• Four quadrant of any drives or DC Motor means that the machine operates in four different ways. They are Forward Braking, Forward motoring, Reverse motoring and Reverse 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.
• I quadrant (Forward Motoring) - power developed is positive and the machine is working as a motor supplying mechanical energy.
• II quadrant (Forward Braking) - In this quadrant the direction of rotation is positive, and the torque is negative, and thus, the machine operates as a generator developing a negative torque, which opposes the motion.
• III quadrant (Reverse Motoring) - The motor works, in the reverse direction. Both the speed and the torque have negative values while the power is positive.
• IV quadrant (Reverse Braking) - The torque is positive, and the speed is negative. This quadrant corresponds to braking in the reverse.

SIMULINK MODEL OF DC7 - Four-Quadrant Chopper 200HP DC Drive :

• The 200 HP DC motor is separately excited with a constant 150 V DC field voltage source.
• The armature voltage is provided by an IGBT converter controlled by two PI regulators. The converter is fed by a 515 V DC bus obtained by rectification of a 380 V AC 50 Hz voltage source.
• In order to limit the DC bus voltage during dynamic braking mode, a braking chopper has been added between the diode rectifier and the DC7 block.

• There are Two Controller present,

• Speed Controller
• Current Controller

Speed Controller Model :

• The speed regulator gives the Armature current level required in order to obtain the electromagnetic torque needed to reach the desired speed.
• To avoid sudden over-current that destabilize the system while following the reference speed changes there is a acceleration and deceleration Ramps.

Current Controller Mode :

• The current regulator controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the four IGBT devices.
• For proper system behaviour, the instantaneous pulse values of IGBT devices 1 and 4 are opposite to those of IGBT devices 2 and 3. This generates the average armature voltage needed to obtain the desired armature current.
• In order to limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.

OUTPUT RESULT :

• Here starting the simulation we can observe 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.
• The speed reference is set at 500 rpm at t = 0 s. Observe that the motor speed follows the reference ramp accurately (+400 rpm/s) and reaches steady state around t = 1.3 s.
• The armature current follows the current reference very well, with fast response time and small ripples. Notice that the current ripple frequency is 5 kHz.
• At t = 2 s, speed reference drops to -1184 rpm. The current reference decreases to reduce the electromagnetic torque and causes the motor to decelerate with the help of the load torque.
• At t = 2.2 s, the current reverses in order to produce a braking electromagnetic torque (dynamic braking mode). This causes the DC bus voltage to increase.
• At t = 3.25 s, the motor reaches 0 rpm and the load torque reverses and becomes negative. The negative current now produces an accelerating electromagnetic torque to allow the motor to follow the negative speed ramp (-400 rpm/s). At t = 6.3 s, the speed reaches -1184 rpm and stabilizes around its reference.

Comparison Between Four-Quadrant Chopper DC Drive (DC7) Model and H-bridge Model :

Four-Quadrant Chopper DC Drive (DC7) Model :

• It Operates in Four-Quadrant.
• PI controller used for speed and current control.
• To change the rotation of motor like forward and reverse, Signals are interrupted with one another.
• It has high switching frequency while comparing BJT H-bridge.
• Armature current here varies a little while switching from one quadrent to another due to high switching frequence.

H-bridge Model :

• It Operates in Two-Quadrant.
• PI controller not used for speed and current control.
• To change the rotation of motor the polarity of the curent is changed.
• It has low switching frequency while comparing Four-Quadrant Chopper.
• Here the armature current shoot up in the graph indicates the variation while switching from forward to reverse operation.

STUDY - 3 :

Electric Vehicle Modelling using DC7 block :

• The DC7 example block in simulink allows us to analyse the Four quadrant operated DC Motor by proving the input voltage supply and speed reference.
• An Electric vehicle uses a DC Motor which needs to perform all the four operations of Forward and reverse motoring and braking.
• So this DC7 model can be modified a little bit to model and analyse a Electric vehicle and the results can be observed.

Modifications :

• Drive cycle
• Linear Load Torque
• Power source

Drive cycle :

• In the DC7 block initially there is the motor speed reference block which gives the motor speed requirements in Rpm.
• For modelling a vehicle we need the vehicle speed with respect to time changes and so, instead of speed reference block, we are adding a drive cycle block using the simulink library.
• There an excel file containing the drive cycle inputs is imported to the model.

Drive cycle (20 sec)

• The drive cycle shows the time on the first column and the speed values in kmph in the second column.

Linear Load Torque :

• For modelling an Electric vehicle we need the load characteristics of the car consisting of all the external forces acting on the vehicle.
• This includes all type of forces but in here for simple modelling we are deriving an equation considering the Rolling Resistance force and the Air drag forces that are involved.
• The Vehicle Load Torque equation is given by,

T = 24.7 + 0.0051 * (w)^2

Subsystem of vehicle load characteristics

• This equation is inserted in the model using a subsystem which takes (w) as the input from the DC7 block and calculates the load characteristics equation.
• The linear load block is replaced with the above subsystem and it is connected back to the DC7 block.

Power source :

• Finally the AC power source is replaced to a DC Voltage source with similar voltage level as that of a real car's battery voltage.

• From the simulink library the DC Voltage source is selected and a voltage level of 300 volts is assigned to the source.
• Then a current measurement block is connected to the voltage source and a scope is added to plot the current output from the power source to the Motor.
• Atlast the power source is connected to the motor.

SIMULINK MODEL OF EV USING DC7 - Four-Quadrant Chopper 200HP DC Drive :

OUTPUT RESULT :

• The above figure shows the simulation results for a basic Electric vehicle that uses a DC motor.
• Here starting the simulation we can observe the motor armature voltage and current, the four IGBT pulses and the motor speed on the scope.
• The speed reference is based on the drive cycle data input with respect to time.
• Here we can see the motor's current and voltages behaviour with respect to the driving conditions. The armature current follows the current reference very well, with fast response time and small ripples. Notice that the current ripple frequency is 5 kHz.
• From the fourth plot we see the current values (i.e) the supplied armature current and the motor terminal current values with respect to the drive cycle's speed condition througt the time band.
• The motor speed also perfectly matches the drive cycle reference speed.
• Thus Electric vehicle is modelled by modifying the Four quardrant chopper DC7 block.

STUDY - 4 :

Brushless DC Motor (BLDC) :

A Brushless DC electric motor (BLDC motor or BL motor), also known as an electronically commutated motor (ECM or EC motor) or synchronous DC motor, is a synchronous motor using a direct current (DC) electric power supply. It uses an electronic closed loop controller to switch DC currents to the motor windings producing magnetic fields which effectively rotate in space and which the permanent magnet rotor follows. The controller adjusts the phase and amplitude of the dc current pulses to control the speed and torque of the motor. This control system is an alternative to the mechanical commutator (brushes) used in many conventional electric motors.

What is a Brushless DC motor (BLDC)?

• A brushless DC motor (known as BLDC) is a permanent magnet synchronous electric motor which is driven by direct current (DC) electricity and it accomplishes electronically controlled commutation system (commutation is the process of producing rotational torque in the motor by changing phase currents through it at appropriate times) instead of a mechanically commutation system.

• Unlike conventional brushed type DC motor, wherein the brushes make the mechanical contact with commutator on the rotor so as to form an electric path between a DC electric source and rotor armature windings, BLDC motor employs electrical commutation with permanent magnet rotor and a stator with a sequence of coils. In this motor, permanent magnet (or field poles) rotates and current carrying conductors are fixed.
• The armature coils are switched electronically by transistors or silicon controlled recitifiers at the correct rotor position in such a way that armature field is in space quadrature with the rotor field poles. Hence the force acting on the rotor causes it to rotate. Hall sensors or rotary encoders are most commonly used to sense the position of the rotor and are positioned around the stator.
• The rotor position feedback from the sensor helps to determine when to switch the armature current. This electronic commutation arrangement eliminates the commutator arrangement and brushes in a DC motor and hence more reliable and less noisy operation is achieved. Due to the absence of brushes BLDC motors are capable to run at high speeds.
• The efficiency of BLDC motors is typically 85 to 90 percent, whereas as brushed type DC motors are 75 to 80 percent efficient. There are wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large power ranges.

Construction of BLDC Motor :

• The construction of a brushless motor system is typically similar to a permanent magnet synchronous motor (PMSM), but can also be a switched reluctance motor, or an induction (asynchronous) motor. They may also use neodymium magnets and be outrunners (the stator is surrounded by the rotor), inrunners (the rotor is surrounded by the stator), or axial (the rotor and stator are flat and parallel).

• BLDC motors can be constructed in different physical configurations. Depending on the stator windings, these can be configured as single-phase, two-phase, or three-phase motors. However, three-phase BLDC motors with permanent magnet rotor are most commonly used.
• The construction of this motor has many similarities of three phase induction motor as well as conventional DC motor. This motor has stator and rotor parts as like all other.
• Stator of a BLDC motor made up of stacked steel laminations to carry the windings. These windings are placed in slots which are axially cut along the inner periphery of the stator.
• These windings can be arranged in either star and delta However, most BLDC motors have three phase star connected stator. Each winding is constructed with numerous interconnected coils, where one or more coils are placed in each slot. In order to form an even number of poles, each of these windings is distributed over the stator periphery.
• The stator must be chosen with the correct rating of the voltage depending on the power supply capability. For robotics, automotive and small actuating applications, 48 V or less voltage BLDC motors are preferred. For industrial applications and automation systems, 100 V or higher rating motors are used.

Types of Brushless DC Motor :

Basically, BLDC are of two types, one is outer rotor motor and other is inner rotor motor. The basic difference between the two are 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.

Hall Sensors :

• Hall sensor provides the information to synchronize stator armature excitation with rotor position. Since the commutation of BLDC motor is controlled electronically, the stator windings should be energized in sequence in order to rotate the motor. Before energizing a particular stator winding, acknowledgment of rotor position is necessary. So the Hall Effect sensor embedded in stator senses the rotor position.
• Most BLDC motors incorporate three Hall sensors which are embedded into the stator. Each sensor generates Low and High signals whenever the rotor poles pass near to it. The exact commutation sequence to the stator winding can be determined based on the combination of these three sensor’s responses.

Working Principle and Operation of BLDC Motor :

• BLDC motor works on the principle similar to that of a conventional DC motor, i.e., the Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force. As a consequence of reaction force, the magnet will experience an equal and opposite force. In case BLDC motor, the current carrying conductor is stationary while the permanent magnet moves.

• When the stator coils are electrically switched by a supply source, it becomes electromagnet and starts producing the uniform field in the air gap. Though the source of supply is DC, switching makes to generate an AC voltage waveform with trapezoidal shape. Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate.
• Consider the figure below in which motor stator is excited based on different switching states. With the switching of windings as High and Low signals, corresponding winding energized as North and South poles.
• The permanent magnet rotor with North and South poles align with stator poles causing motor to rotate. Observe that motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment). By this way motor moves in a clockwise direction.
• Here, one might get a question that how we know which stator coil should be energized and when to do. This is because; the motor continuous rotation depends on the switching sequence around the coils. As discussed above that Hall sensors give shaft position feedback to the electronic controller unit.
• Based on this signal from sensor, the controller decides particular coils to energize. Hall-effect sensors generate Low- and Highlevel signals whenever rotor poles pass near to it. These signals determine the position of the shaft.

Brushless DC Motor Drive :

• As described above that the electronic controller circuit energizes appropriate motor winding by turning transistor or other solid-state switches to rotate the motor continuously. The figure below shows the simple BLDC motor drive circuit which consists of MOSFET bridge (also called as inverter bridge), electronic controller, hall effect sensor and BLDC motor.

• Here, Hall-effect sensors are used for position and speed feedback. The electronic controller can be a microcontroller unit or microprocessor or DSP processor or FPGA unit or any other controller. This controller receives these signals, processes them and sends the control signals to the MOSFET driver circuit.
• In addition to the switching for a rated speed of the motor, additional electronic circuitry changes the motor speed based on required application. These speed control units are generally implemented with PID controllers to have precise control. It is also possible to produce four-quadrant operation from the motor whilst maintaining good efficiency throughout the speed variations using modern drives.
  

Advantages of Brushless DC Motors (BLDC) :

• It has no mechanical commutator and associated problems.
• High efficiency due to the use of permanent magnet rotor.
• High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed.
• Smaller motor geometry and lighter in weight than both brushed type DC and induction AC motors.
• Long life as no inspection and maintenance is required for commutator system.
• Higher dynamic response due to low inertia and carrying windings in the stator.
• Less electromagnetic interference.
• Quite operation (or low noise) due to absence of brushes.

Disadvantages of Brushless DC Motors (BLDC)r :

• These motors are costly.
• Electronic controller required to control this motor is expensive.
• Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors.
• Requires complex drive circuitry.
• Need of additional sensors.

Applications of Brushless DC Motors (BLDC) :

BLDC motors fulfill many requirements that a brushed DC motor does. But as they require a complicated control circuit and due to cost considerations, they haven’t yet completely replaced brushed DC motors, especially in low-cost applications. Despite this, there are many applications where BLDC motors dominate –

• Consumer electronics – computer hard drives, small cooling fans, cd/dvd players, etc. and also in modern appliances where quiet operation is desired – such as washing machines, air conditioners, etc.
• Electric Vehicles – many electric vehicles including electric and hybrid cars, electric bikes use BLDC motors.
• They have a wide range of applications in many other areas including robotics, industrial, motion control systems, etc.

CONCLUSION :

• MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’ is simulated And the model is modified in such a way that armature current doesn’t shoot up when motor changes direction from forward to reverse.
• The Four-Quadrant Chopper DC Drive (DC7) block’ is referred in the Matlab website And compared it with H-bridge model.
• Succesfully done a modification of EV model using DC7 block and make result report.
  

REFERENCES :

https://en.wikipedia.org/wiki/Power_electronics#Devices

https://skill-lync.freshdesk.com/support/solutions/articles/43000571899-how-to-model-dc7-four-quadrant-chopper-200-hp-dc-drive

https://en.wikipedia.org/wiki/Brushless_DC_electric_motor