Challenge

Week-7 Challenge: DC Motor Control

SSyed Faizan23 Aug 2022

Aim :--

1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using 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. Explain in a brief about operation of BLDC motor.

Question.1, A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’?

Answer:--

Before Going To Model Of Speed Control Of A DC Motor Using BJT H-Bridge We Will Learn What is BJT

BJT:--

A bipolar junction transistor is a three-terminal "semiconductor" device that consists of two p-n junctions which are able to amplify or magnify a signal. It is a current controlled device. The three terminals of the BJT are the "Base(B), the Collector(C), and the Emitter(E)." A signal of a small amplitude applied to the base is available in the amplified form at the collector of the transistor. This is the amplification provided by the BJT. Note that it does require an external source of DC power supply to carry out the amplification process And It is Oprated Through The Current.

BJTs are of two types namely NPN and PNP based on doping types of the three main terminals. An NPN transistor consists of two semiconductor junctions that have a thin p-doped anode region and PNP transistor also consists of two semiconductor junctions that have a thin n- doped cathode region.

The flow of charge in a Bipolar transistor is due to the diffusion of charge carriers between the two regions belonging to different charge concentrations. Regions of BJT are known as the base, collector, and emitter.

The emitter region is highly doped when compared to other layers. Both collector and base layers have the same charge carrier concentrations. Among these junctions, the base-emitter junction is forward biased, and the base-collector junction is reverse biased. Forward biased means p-doped region has more potential than the n-doped side.

There are still "Two More Power Switches" That are used in EV's And HEV's

MOSFET
IGBT

MOSFET:--

Metal Oxide Silicon Field Effect Transistors commonly known as MOSFETs are electronic devices used to switch or amplify voltages in circuits. It is a voltage controlled device and is constructed by three terminals And Is Oprated By The Field Effect. The terminals of MOSFET are named as follows:

Source
Gate
Drain
Body

IGBT:--

The Insulated Gate Bipolar Transistor (IGBT) is a power switching transistor which combines the advantages of MOSFETs and BJTs for use in power supply and motor control circuits

The EV And HEV Uses Either "MOSFET" OR "IGBT" Transistors,And The MOSFET Runs/Works Faster Than That of The IGBT But The IGBT Can Carry More Load Than MOSFET

The Above Figure Shows Side By Side Comparission Of Transistors

~~MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge:--

An H-bridge is a simple circuit that lets us to control a DC motor to go backward or forward.

In general an H-bridge is a rather simple circuit, containing four switching element, with the load at the center, in an H-like configuration:

The switching elements (Q1..Q4) are usually bi-polar or FET transistors, in some high-voltage applications IGBTs. Integrated solutions also exist but whether the switching elements are integrated with their control circuits or not is not relevant for the most part for this discussion. The diodes (D1..D4) are called catch diodes and are usually of a Schottky type.

The top-end of the bridge is connected to a power supply (battery for example) and the bottom-end is grounded.

In general all four switching elements can be turned on and off independently, though there are some obvious restrictions.

Speed control of a DC motor using 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 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.

~~Simulation

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 Pulse Generator is used to generate pulses which are fed to the four IGBTs to create Pulse Width Modulation. Here in this model, the simulation is such that the motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). 'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.

Initial the Block Parameters for Pulse Generator are as follows;

Now Lets Run And See The Model,

At t = 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction. So in the first half cycle, we can see the motor attains a speed of 1000 rpm a load torque of 6 N.m whereas the second half cycle the same values in the negative direction. Here we can see that the armature current of DC motor does get shoot up. To avoid this, we will have to reduce the pulse width from the pulse generator.

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

Answer:--

Here In The Above Case We can see that the armature current of DC motor does get shoot up. To avoid this, we will have to reduce the pulse width from the pulse generator.And We Are Changing The PULSE Width From 75 to 45 and see

Now if we Run The simulation for 40Pulse Width

As We Can see that the Armature current is almost 0 A, that means the armature current doesn’t shoot up at 45% pulse width. Also the motor speed and torque is reduced to 300 rpm and 0.8 N.m respectively due to the reduction in Pulse width. This indicates that, as we reduce the pulse width, the motor speed and torque reduces and can attain zero at a certain value.

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

Answer:--

Description

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.

The first regulator is a speed regulator, followed by a current regulator. The speed regulator outputs the armature current reference (in p.u.) used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed. The speed reference change rate follows acceleration and deceleration ramps in order to avoid sudden reference changes that could cause armature over-current and destabilize the system. The current regulator controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the four IGBT devices (Pulse Width Modulation). 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.

The Four-Quadrant Chopper DC Drive block uses these blocks from the Electric Drives/Fundamental Drive Blocks library:

Speed Controller (DC)
Regulation Switch
Current Controller (DC)
Chopper

The main advantage of this drive, compared with other DC drives, is that it can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward regeneration). 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, four switching devices are required, which increases thecomplexity of the drive system.

~Simulation

Now Lets Run And See The Model

Based on the above simulation results, we can see that,

The plot of duty cycles IGBT 1&4 are exactly opposite to that of duty cycles IGBT 2&3 as the pulse values of IGBT devices 1 and 4 are opposite to those of IGBT devices 2 and 3.
The desired armature current is obtained from the average armature voltage.
The armature current follows the current reference very well, with fast response time and small ripples.
The Motor speed have reached 500rpm

~Comparing the Four-Quadrant Chopper DC Drive (DC7) block with BJT H-bridge:–-

In Four Quadrant Chopper DC Drive Block, the speed reference input is used to change the direction of the motor rotation whereas in BJT H-bridge model, the polarity is reversed to change the direction of motor rotation.
In BJT H-bridge model, the motor can be controlled by changing the pulse width of the pulse generator whereas in Four Quadrant Chopper DC Drive Block, the speed reference values can be changed to control the motor.
The Four Quadrant Chopper DC Drive operates in four modes wiz. forward motoring the power is positive and it is fisrt Quadrant, forward regenerative braking,power is negative and it is second Quadrant, reversing motoring,power is positive and is third quadrant and reverse regenerative braking mode and in that the power is negative and it is fourth qudrant and is shown in below figure whereas BJT H-Bridge model operates only on two modes wiz. forward and reverse.
PI (Proportional Integral) Controllers are used for speed and current control in DC7 block and it is absent in BJT H-bridge model.
The Four Quadrant Chopper DC Drive Block uses ramps to avoid current shoot up and smooth working stability.
The disturbances and current shoot up is high in H-Bridge model because of switching losses.
This is why the overall performance and stability is good in Four Quadrant Chopper DC Drive comparing BJT H-Bridge model.

Question.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 )

Answer:--

What is a chopper?

A dc‐to‐dc converter, also known as dc chopper, is a static device which is used to obtain a variable dc voltage from a constant dc voltage source. Choppers are widely used in trolley cars, battery-operated vehicles, traction motor control, control of large numbers of dc motors, etc. They are also used as dc voltage regulators. Choppers are of two types: (1) Step‐down choppers, and (2) Step‐up choppers. In step‐down choppers, the output voltage will be less than the input voltage, whereas in step‐up choppers output voltage will be more than the input voltage

Two Quadrant Chopper

In this type of Chopper, the average voltage will be always positive but the average load current might be positive or negative. The two Switches CH1 and CH2 should not be turned on simultaneously as the combined action may cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.

We will always get a positive output voltage V₀ as the freewheeling diode FD is present across the load. When the chopper CH1 is on the current flows from source to load. The inductor stores energy. When Ch1 turn OFF the freewheeling diode starts conducting and the output voltage v₀ will be equal to V_s. The direction of the load current i₀will be reversed. The current i₀ will be flowing towards the source. The load current will be negative when the CH2 is on. The current completes its path through diode D2.

~Simulink Model:--

WORKING

Blocks use for the model

Power GUI
Voltage measurement Block
Current measurement block
DC voltage source
MOSFET
Pulse generator
Diode
Series RLC band
Logical Operator

The Two - quadrant chopper DC drive block represents a two-quadrant, DC - supplied, chopper drive for DC motor.
This drive features closed-loop speed control with a two-quadrant operation. The speed control loop output thereference 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 advantages of this drive, compared with other DC drives, are its implementation simplicity and the that it can operate in two quadrants. In addition due to the use of a high switching frequency DC-DC converter, a lower armature current ripple is obtained. However, for all two-quadrant DC drives reversible and regenerative operations, which are required in most DC drives cannot be obtained.

let us take two mosfet blocks and diodes blocks and connect it as shown in circuit diagram.

the dc voltage source is connected as shown.

let the voltage of 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 ie 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 the other mosfet will get off and when other mosfet is ON 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 0.02 seconds and let pulse weidth be 50%

SCOPE RESULT:--

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. the current may be positive as well as negative.

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

Link for Simulink Model:--

https://drive.google.com/drive/folders/1N8Gzb5qKjuU3wDtffMihGRWF4BUiFp2B?usp=sharing

Question,3. Explain in a brief about operation of BLDC motor?

Answer:--

BLDC MOTOR

Brushless DC electric motors also known as electronically commutated motors .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. BLDC motors are also referred as trapezoidal permanent magnet motors.

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 SCR 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.

In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature winding are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy.

The main design difference between a brushed and brushless motors is the replacement of mechanical commutator with an electric switch circuit. 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 motor does not have any current carrying commutators .The field inside a brushless motor is switched through an amplifier which is triggered by the commutating device like an optical encoder.

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 motors.

The layout of a DC brushless motor can vary depending on whether it is in “Out runner” style or “Inrunner” style.

Outrunner – The field magnet is a drum rotor which rotates around the stator. This style is preferred for applications that require high torque and where high rpm isn’t a requirement.
In runner – The stator is a fixed drum in which the field magnet rotates. This motor is known for producing less torque than the out runner style, but is capable of spinning at very high rpm.

stator

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 or 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.

Rotor

BLDC motor incorporates a permanent magnet in the rotor. The number of poles in the rotor can vary from 2 to 8 pole pairs with alternate south and north poles depending on the application requirement. In order to achieve maximum torque in the motor, the flux density of the material should be high. A proper magnetic material for the rotor is needed to produce required magnetic field density.

ferrite magnets are inexpensive, however they have a low flux density for a given volume. Rare earth alloy magnets are commonly used for new designs. Some of these alloys are Samarium Cobalt (SmCo), Neodymium (Nd), and Ferrite and Boron (NdFeB). The rotor can be constructed with different core configurations such as the circular core with permanent magnet on the periphery, circular core with rectangular magnets, etc.

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 response.

WORKING OF BLDC MOTOR:

The underlying principles for the working of a BLDC motor are the same as for a brushed DC motor; i.e., internal shaft position feedback. In case of a brushed DC motor, feedback is implemented using a mechanical commutator and brushes. With a in BLDC motor, it is achieved using multiple feedback sensors. The most commonly used sensors are hall sensors and optical encoders. Hall sensors work on the hall-effect principle that when a current-carrying conductor is exposed to the magnetic field, charge carriers experience a force based on the voltage developed across the two sides of the conductor.

If the direction of the magnetic field is reversed, the voltage developed will reverse as well. For Hall-effect sensors used in BLDC motors, whenever rotor magnetic poles (N or S) pass near the hall sensor, they generate a HIGH or LOW level signal, which can be used to determine the position of the shaft.

In a commutation system – one that is based on the position of the motor identified using feedback sensors – two of the three electrical windings are energized at a time as shown in figure 4.

In figure 4 (A), the GREEN winding labeled “001” is energized as the NORTH pole and the BLUE winding labeled as “010” is energized as the SOUTH pole. Because of this excitation, the SOUTH pole of the rotor aligns with the GREEN winding and the NORTH pole aligns with the RED winding labeled “100”. In order to move the rotor, the “RED” and “BLUE” windings are energized in the direction shown in figure 4(B). This causes the RED winding to become the NORTH pole and the BLUE winding to become the SOUTH pole. This shifting of the magnetic field in the stator produces torque because of the development of repulsion (Red winding – NORTH-NORTH alignment) and attraction forces (BLUE winding – NORTH-SOUTH alignment), which moves the rotor in the clockwise direction.

This torque is at its maximum when the rotor starts to move, but it reduces as the two fields align to each other. Thus, to preserve the torque or to build up the rotation, the magnetic field generated by stator should keep switching. To catch up with the field generated by the stator, the rotor will keep rotating. Since the magnetic field of the stator and rotor both rotate at the same frequency, they come under the category of synchronous motor.

This switching of the stator to build up the rotation is known as commutation. For 3-phase windings, there are 6 steps in the commutation; i.e., 6 unique combinations in which motor windings will be energized.

ADVANTAGES OF BLDC MOTOR

Less overall maintenance due to absence of brushes
Reduced size with far superior thermal characteristics
Higher speed range and lower electric noise generation.
It has no mechanical commutator and associated problems
High efficiency and high output power to size ratio 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
Low noise due to absence of brushes

DISADVANTAGES OF BLDC MOTOR
These motors are costly
Electronic controller required control this motor is expensive
Requires complex drive circuitry
Need of additional sensors

APPLICATION OF BLDC MOTOR

Brushless DC motors (BLDC) use for a wide variety of application requirements such as varying loads, constant loads and positioning applications in the fields of industrial control, automotive, aviation, automation systems, health care equipments etc.

Computer hard drives and DVD/CD players
Electric vehicles, hybrid vehicles, and electric bicycles
Industrial robots, CNC machine tools, and simple belt driven systems
Washing machines, compressors and dryers
Fans, pumps and blowers.

CONCLUSION:--

1) MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge is studied and the the armature current shoot-up is studied and ‘The Four-Quadrant Chopper DC Drive (DC7) block is also studied and Compare it with the H-bridge model

2) 2-quadrant chopper using simulink model is made and the various results obatined is studied.

3)Operation of BLDC motor is studied.

Reference:--