Week-7 Challenge: DC Motor Control

AIM:- DC Motor Control:-

OBJECTIVE:-

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

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

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

4) Explain in a brief the operation of the BLDC motor.

OBJECTIVE:-

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

In the MATLAB/SIMULINK documentation, we can search for the simulation model of the 'Speed control of a DC Motor using BJT H-bridge'.

It is used to generate a chopped voltage and to control the speed of the DC motor.

This model can be opened in SIMULINK from the documentation itself.

As the name suggests, it uses a BJT (Bipolar Junction Transistor which is a type of transistor that uses both electrons and holes as charge carriers, hence the name bipolar.

Other transistors such as FETs (Field Effect Transistors) are unipolar transistors meaning that they only use one type of charge carrier.

BJTs are used for switching and amplifying applications. 

BJTs have 3 terminals which are: the emitter (E), the base (B), and the collector (C). Current flows in only the emitter and the collector and the amount of current that flows through them is controlled by the base.

BJTs use semiconductors in their working, and so there are 2 types of BJT:

NPN type and PNP type. Their circuit symbols are shown below:

PNP Transistors:-

PNP transistor is a bipolar junction transistor constructed by sandwiching an N-type semiconductor between two P-type semiconductors.

A PNP transistor has three terminals – a Collector (C), Emitter (E), and Base (B). The PNP transistor behaves like two PN junctions diodes connected back to back.

These back-to-back PN junction diodes are known as the collector-base junction and base-emitter junction.

Regarding the three terminals of the PNP transistor, the Emitter is a region is used to supply charge carriers to the Collector via the Base region.

The Collector region collects most of all charge carriers emitted from the Emitter.

The Base region triggers and controls the number of current flows through the Emitter to the Collector.

PNP Transistor Symbol and Construction

The construction of a PNP transistor is very similar to the construction of an NPN transistor.

In an NPN transistor, one P-type semiconductor sandwiched by two P-type semiconductors. And in the PNP transistor, one N-type semiconductor sandwiched by two P-type semiconductors.

NPN TRANSISTOR:-

Definition: The transistor in which one p-type material is placed between two n-type materials is known as an NPN transistor.

The NPN transistor amplifies the weak signal enters into the base and produces strong amplify signals at the collector end.

In an NPN transistor, the direction of movement of an electron is from the emitter to the collector region due to which the current constitutes in the transistor.

Such type of transistor is mostly used in the circuit because their majority charge carriers are electrons which have high mobility as compared to holes.

Construction of NPN Transistor:-

The NPN transistor has two diodes connected back to back.

The diode on the left side is called an emitter-base diode, and the diodes on the left side are called a collector-base diode. These names are given as per the name of the terminals.

BJT H-Bridge:-

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 the square of speed). The armature means 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.

This is the SIMULINK model for the BJT H-Bridge used in the speed control of a DC motor:-

Here, we can see the 'Pulse Generator 500Hz' block. If we open it, we will get a dialog box with its settings which are shown below:-

 initially, we run MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’ with all its default values and different pulse width (duty cycle) 75%, 55%, and 50%

From this, we get the following three plot results.

With 75% pulse width                              With 55% pulse width                              With 50% pulse width 

• From the 1^st plot, we can see that the current plot has initially a very high value and then becomes steady and after that shows the same path in the -ve direction.
• In these three plots, the value of this current shoot-up is decreasing and the 3^rd plot has an almost smooth current flow.
• To get a steadier and smoother current flow we need to decrease and control the duty cycle
• This shot-up happens because of the switching in the circuit that causes heat losses during switching.
• To avoid these losses, we use HEAT SINK material to dissipate the heat.
• From this, the motor can change its direction of rotation easily and very smoothly.

OBJECTIVE:-

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

The One-Quadrant Chopper DC Drive (DC5) block represents a one-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors.

This drive features closed-loop speed control with a one-quadrant operation.

The speed control loop outputs the reference armature current of the machine.

Using a PI current controller, the chopper duty cycle corresponding to the commanded armature current is derived.

This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.

The main advantage of this drive, compared with other DC drives, is its implementation simplicity.

In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained.

However, this drive only operates in one quadrant (forward motoring), which limits its presence in applications where two/four operating quadrants are required.

The detailed circuit diagram for the DC7 block is shown below:

dc7_example 

 Operation

• Forward Motoring: motor rotates clockwise and the controller applies a clockwise drive.
• Reverse Motoring: motor rotates anti-clockwise and the controller applies an anti-clockwise drive.
• Forward Braking: motor rotates clockwise and the controller applies an anti-clockwise drive.
• Reverse Braking: motor rotates anti-clockwise and the controller applies a clockwise drive.

Advantages

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.

The second benefit is that it reduces the risk of current spikes (and possibly damage the motor and the controller) which 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. 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.

Finally, 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. 

Comparison to H-Bridge

Both BJT H-bridges and 4 quadrant choppers have their advantages and disadvantages. H-Bridges like the 4 quadrant choppers are also capable of the braking operation without using air resistance or friction or physical brakes to bring the motor to rest.

4 quadrant DC choppers are more commonly used in automobiles whereas BJT H-bridges are commonly used in robotics and electromechanical devices.

Unlike 4 quadrant DC choppers, BJT H-bridges are not capable of performing regenerative braking but they can perform dynamic braking.

It is this difference that makes 4 quadrant DC choppers useful in hybrid and electric vehicles.

In 4 quadrant DC choppers, the current signals are interrupted with one another to change the direction of rotation of the motor.

This is done by a speed reference or a drive cycle from which the speed values are converted to current signals which interrupt one another to change the rotation of the direction of the motor.

However, in BJT H-bridges, the polarity of the current is reversed (depending on the duty cycle) to change the direction of rotation of the motor.

Also, the current signals in 4 quadrant DC choppers are chopped meaning that they are not continuous, they are discrete and so take less time for simulations.

But in BJT H-bridges, the current values are not chopped, meaning that they are continuous and so take more time for simulations. 

Finally, in 4 quadrant DC choppers, a PI controller is used to control the speed and the current, and in the H-bridge, this is not implemented. 

 OBJECTIVE:-

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

On the left-hand side, we can see that I have replaced the default blocks of the DC7 model with new blocks of my making.

The first block to be replaced is the linear load torque block. For this model of the EV, we will be using a new equation that characterizes the relation between the torque and the speed which is shown below:

$T_m=0.0051{\omega }^2+24.7$

where:

The angular speed of the motor (radians per second) - $\omega$

The torque of the motor (Nm) - Nm

To model this relation in SIMULINK, we need to first insert the math function block which will perform power functions.

In our case, we need to square the angular speed of the motor.

So a connection is made between the DC chopper drive block to the math function block.

Math Function Block:

Next, the math function block is connected to a 'gain' block to multiply the squared angular speeds with a coefficient, 0.0051 as needed by the torque-speed equation.

The 'gain' block is modified with the gain value of 0.0051.

Gain Block: 

A new block called a 'constant' block is created with a constant value of 24.7 as needed by the torque-speed equation.

Constant Block: 

The gain block and the constant block are separate and both connect to the 'add' block so that the terms from the 2 blocks are added together which gives the RHS of the torque-speed equation.

The add block is then connected to the 'Tm' terminal of the DC chopper drive block. 

Add Block:

Next, a DC voltage source block is inserted from the library and the negative terminal is connected to the 'Gnd' i.e. ground terminal of the DC chopper drive block.

The DC voltage source is modified with a voltage amplitude of 200V.

DC Voltage Block:

The positive terminal of the DC voltage source is connected to a 'current measurement' block of which the negative terminal is connected to the 'Vcc' terminal of the DC chopper drive block.

Current Measurement Block:

The 'i' terminal of the current measurement block is connected to a newly made 'scope' block.

Scope Block:

Finally, a drive cycle is inserted from the workspace of my own making.

This drive cycle has a duration of 7seconds and it is just a numeric matrix of time and velocity in the first and second columns respectively.

This drive cycle block is connected to the 'SP' terminal of the DC chopper drive block. The drive cycle block and the cycle itself is shown below:

Then, the 'stop time' of the simulation is set to 7 seconds because the drive cycle is also 7 seconds in duration.

After this, the simulation is run and the output of the simulation is seen from the scope block connected to the selector block. 

The screenshot of the scope output is shown below:

4) Explain in a brief the operation of the BLDC motor.

This document describes the design of a 3-phase BLDC(Brushless DC) motor drive based on Freescale’s 56800/Dedicated motor control devices.

BLDC motors are very popular in a wide variety of applications.

Compared with a DC motor, the BLDC motor uses an electric commutator rather than a mechanical commutator, so it is more
reliable than the DC motor.

In a BLDC motor, rotor magnets generate the rotor’s magnetic flux, so BLDC motors achieve higher efficiency.

Therefore, BLDC motors may be used in high-end white goods (refrigerators, washing machines, dishwashers, etc.), high-end pumps, fans, and other appliances which require high reliability and efficiency

A brushless DC (BLDC) motor is a rotating electric machine, where the stator is a classic 3-phase stator like that of an induction motor, and the rotor has surface-mounted permanent magnets.

In this respect, the BLDC motor is equivalent to a reversed DC commutator motor, in which the magnet rotates while the conductors remain stationary.

In the DC commutator motor, the current polarity is altered by the commutator and brushes.

However, in the brushless DC motor, polarity reversal is performed by power transistors switching in synchronization with the rotor position.

Therefore, BLDC motors often incorporate either internal or external position sensors to sense the actual rotor position, or the position can be detected without sensors.

The BLDC motor is driven by rectangular voltage strokes coupled with the given rotor position;

 The generated stator flux interacts with the rotor flux, which is generated by a rotor magnet, defines the torque and thus the speed of the motor.

The voltage strokes must be properly applied to the two phases of the 3-phase winding system so that the angle between the stator flux and the rotor flux is kept close to90° for the maximum generated torque.

This means the motor requires electronic control for proper operation.

During complementary switching, two transistors are switched on when the phase of the BLDC motor is connected to the power supply.

One primary difference occurs during freewheeling.

During independent switching, all transistors are switched off.

The current continues to flow in the same direction through freewheeling diodes until it falls to zero.

In complementary switching, the complementary transistors are switched on during freewheeling, so the current may be able to flow in the opposite direction

IGBT 1&4 Duty Cycle:-

Initially, there is a spike in the duty of IGBT switches 1&4 because the system is adjusting itself to provide the correct voltage to operate the motor.

Throughout the operation, the duty cycle is seen in pulses as it steadily increases to change the output voltage that is provided to the motor load.

The duty cycle increases to reach a peak value of 0.535 at t = 6 seconds which is when the motor angular speed is a maximum of 30 radians per second.

After this, the duty cycle decreases as the motor angular speed decreases and eventually reaches the duty cycle level at which it started.

The increase and decrease in the duty cycle corresponding to the power requirement by the motor to achieve a particular angular speed.

When the motor angular speed increases, the duty cycle will also increase so that the average voltage that is supplied to the motor will be more.

When the motor angular speed decreases, the duty cycle will also decrease so that the average voltage that is supplied to the motor will be lesser. 

IGBT 2&3 Duty Cycle

Similar to IGBT 1&4, there is initially a spike in the duty cycle as the system adjusts itself to provide the correct voltage to operate the motor.

The duty cycle of IGBT switches 2 and 3 decreases in pulses as the motor angular speed increases.

The decrease in this duty cycle corresponds to the increase in the duty cycle of IGBT 1&4.

In other words, the first 2 graphs are mirror images of each other by form shape.

The duty cycle reaches a minimum of 0.467 at t = 6 seconds when the motor angular speed is at a maximum level of 30 radians per second.

From here, the duty cycle increases for the rest of the drive cycle as the motor speed decreases.

The increase and decrease in the duty cycle corresponding to the power requirement by the motor to achieve a particular angular speed.

When the motor angular speed increases, the duty cycle will also increase so that the average voltage that is supplied to the motor will be more.

When the motor angular speed decreases, the duty cycle will also decrease so that the average voltage that is supplied to the motor will be lesser.