WEEK-7 CHALLENGE DC MOTOR CONTROL

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.

SOL) 

A)

In matlab/simulink go to help section and search for speed control of DC motor using BJT         H-bridge a model figure opens as shown as above figure It is used to generate chopped voltage and control the speed of the DC motor

It consists of BJT (Bi-polar junction transistor)

DC motor source

Diodes (transistors connected in H-form)

A bipolar junction transistor is used for power switching applications operates as in IGBT.  

(BJT) is a type of transistor that uses both electrons and holes as charge carriers. A bipolar junction transistor also called bipolar transistor is a three-terminal device that can function as electronic switches or signal amplifiers.

BJT has three 3 terminals Base (B) emitter (E) and collector(C) it is solid state device that flows current in two terminals, i.e., collector and emitter and controlled by third device known as terminal or base terminal.

There are two basic types of bipolar junction transistor construction, PNP and NPN, which describes the physical arrangement of the P-type and N-type semiconductor materials from which they are made.

 PNP TRANSISTOR:

It uses one N type semiconductor and 2 P type semiconductors. The N type semiconductor will be the base of the transistor. The collector is connected to positive supply and emitter is connected to negative supply. Emitter is more positive in relation to the base and collector. Terminals. Base terminal supplies voltage and operates the transistor by switching ON and OFF states.  When base voltage is same as emitter voltage transistor turns OFF. When base voltage is less than emitter voltage the transistor is ON state.

NPN TRANSISTOR:

It uses one P type semiconductor and 2 N type semiconductors.  Construction is same as PNP type having 3 terminals. in this type collector is connected to the positive supply and emitter is connected to the negative supply. base terminal supplies the voltage and transistor by switching ON and OFF states. When base voltage is same as emitter voltage transistor turns OFF. When base voltage is higher than emitter voltage the transistor is ON state.

Other type of transistor FET (field effect transistors) and uni – polar transistors they use only one type of charge carriers   

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.

 B) Simulation

Scope after running the default model to see the Shoot up in the armature current when poles are reversed

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 'Current' shows currents flowing in BJT Q3 and diode D3.

OBSERVATIONS: 

We can see that speed of the motor is increased to its top position and maintained upto 0.5 seconds after that it got reduced and goes back negative position

Armature current shoots up after 0.5 seconds due to high duty cycle

We can observe that load torque also increased to the top and maintains upto 0.5 seconds and again decreased to its negative position

OBSERVATIONS FROM CURRENTS SCOPE:

We can clearly observe that only after 0.5 seconds the IGBT currents start flowing.

The diode current stops flowing after 0.5 seconds no current is flowed after 0.5 seconds

The current May be flowing in reverse direction which indicates there is no passage of diode current.

Change the duty cycle so that the motor direction changes from forward direction to reverse direction so that the armature current does not shoot up

Change the pulse width from 75% to 50%

RESULTS AFTER CHANGING THE PULSE WIDTH:

There are less variations compared to first duty cycle

we can see that speed of the motor is increased to its top position in 0.5 seconds after that it slowly got reduced and does not went back to its negative position

The armature current does not shoots up high as much as when compared to first duty cycle

There is no sudden decrease of load torque after 0.5 seconds because we changed duty cycle from 75 % to 50 %

We can clearly observe that only after 0.5 seconds the IGBT currents start flowing.

The diode current stops flowing after 0.5 seconds no current is flowed after 0.5 seconds

C)

CHOPPER: it is a Static device that converts fixed DC input to variable DC output directly. Chopper is an electronic switch that is used to interrupt one signal under the control of another

Due to current and voltage spike occurs to avoid this snubber RC circuit used so chopper is used in motor control and in regenerative braking

The Four-Quadrant Chopper DC Drive (DC7) block in MATLAB/SIMULINK represents a four-quadrant. This drive features closed-loop speed control with four-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 saw tooth carrier signal to obtain the required PWM signals for the chopper.

ADVANTAGE

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).

 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 the complexity of the drive system.

Four-Quadrant Chopper DC Drive block uses 4 blocks from the Electric Drives/Fundamental Drive Blocks library:

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

Enter the dc7 - Four-Quadrant Chopper 200 HP DC Drive

Command in the help section of MATLAB/SIMULINK below interface opens

The above model consists of 3-phase supply source,

AC to DC converter

4 quadrant chopper

Dc motor drive

Quadrant selector

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.

Comparison between 4-quadrant choppers with H-Bridge

H – Bridge operates in 2 quadrants

4 Quadrant choppers are used in automobile field and BJT H-Bridge are used electro mechanical and Robotics devices.

4 quadrant DC choppers can perform regenerative braking BJT Bridge can perform dynamic braking so 4 quadrant DC choppers useful in hybrid and electric vehicles

4 quadrant DC chopper consists of 4 operations Forward motoring, reverse regeneration, reverse motoring, and forward regeneration H-Bridge consists of 2 operations forward motoring and reverse motoring

PIC controllers are used in 4 quadrant DC choppers where as in H-Bridge PID controllers are not used

Current signals are interrupted with one another to change the direction of rotation of motor it is done by speed reference in which speed values are converted to current signals which changes the direction of motor where as  BJT H-Bridge the current changes depending upon the drive  cycle when change in polarity of switches

2) Develop a 2-quadrant chopper using simulink & explain the working

 SOL):

Type C chopper also referred as two-quadrant type A chopper because it operates in 2 quadrants

Class C chopper is two-quadrant chopper that is It operates in 1^st and 2^nd Quadrants

It is obtained by connecting class A and class B choppers in parallel 

In class C chopper output voltage is always +ve and output current can be +ve or –ve

Working

1^st quadrant working is there with CH1 or D2 ON

2^nd quadrant working is there with CH2 or D1 ON

Here we cannot turn on CH1 and CH2 simultaneously, if we turn on DC supply will get short circuit of supply voltage

SIMULINK MODEL OF TWO-QUADRANT CHOPPER:

BLOCK PARAMETERS OF PULSE GENERATOR:

SCOPE

OBSERVATIONS:  In the above waveform the voltage is not going below ZERO the minimum value is zero and maximum value is always in the positive direction

Currents go to positive direction as well as to negative direction based upon the switching actions (based upon the freewheeling diode takes place)

Here we have given amplitude equal to 10 in scope we scope we can see amplitude is 10V

3) Explain in a brief about operation of BLDC motor.

Construction, Working & Applications of BLDC. BLDC motors are superior to brushed DC motors in many ways, such as ability to operate at high speeds, high efficiency, and better heat dissipation.

They are an indispensable part of modern drive technology, most commonly employed for actuating drives, machine tools, electric propulsion, and robotics and for electrical power generation. With the development of sensor less technology besides digital control, these motors become so effective in terms of total system cost, size and reliability.

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. 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 rotates and current carrying conductorsarefixed.

The armature coils are switched electronically by transistors or silicon controlled rectifiers 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.

Working Principle and Operation of BLDC Motor

BLDC motor works on the principle similar to that of Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force. Because 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 on, 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. 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 High level 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 BLDC Motor

BLDC motor has several advantages over conventional DC motors and some of these are

• 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 Motor

• These motors are costly
• Electronic controller required 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

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Applications

Brushless DC Motors (BLDC) are used 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. Some specific applications of BLDC motors are

• 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