Week-7 Challenge: DC Motor Control
1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’
Sln..
BJT-H Bridge:
* The Biplor Junction transistor when used for power switching applications, Operates as an IGBT, When it is conducting BJT operating in the saturated ragion, a forward voltage Vf is developed between collector and emitter (in the range of 1V) 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 used the preset model
(5Hp 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 240V 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 simultaenously, 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 aa 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 operated as free wheeling diodes when Q2 Q3 are swithed off
Chopper:
A Dc chopper is also known ad Dc-Dc Converter, It is a static device which is used to obtain a variable DC voltage form a constant Dc Voltage Source. A Dc-Dc converter is an electronic ckt. or electomechanical device that converts Dc from one voltage level to another. it is a type of electric power converter, Power levels range from very low to very high.
* Choppers are of two types
1) Step Down chopper 2) Step-Up Chopper
MODEL:-
* The model is opened in Simulink Workspace
* Run the model
* Observe the graphs of the armature current.
* A the bottom part of modelm There is a pulse generator block, click on it and run the model
* Check the outputs
* Follow the same for 75% to 50% just to check if the armature voltage is decreased Sharply after half of the simulation time.
RESULT: As Pulse width decreases the step value of the armature current decreases
C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
Sln. Four Quadrant Chopper Dc Drive block:
The 200 HP Dc motor is seprately Excited with a constant 150V DC field voltage source The armature voltage is provided by an IGBT COnverter controlled by two PI regulators, The converter is fed by a 515V DC bus obtained by rectification of 380V AC 50 HZ Volatge source, In order to limit the DC bus volatge during dynamic breaking mode, a braking chopper has been added between the diode rectifier and the DC7 block
* The first regulator is a speed regulator by a current regulator The speed regulator outputs the armature current refernce used by the current controlled in order to obtain the electromagnetic torque needed to reach the desired speed, The speed reference change rate follows acceleration and decceleration ramps in ordder to avoid suddent refrence changes that could cause armature over current and desatbilize the system. The current regulator controls the armature current by computing the appropriate duty ratio of the 5KHz pulses of the four IGBT devices for proper system behaviour the instantaneous pulse values of IGBT devices 1 and 4 are opp to those of IGBT devices 2 & 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 Stability of the voltage and current supply is higher in the four quadrant compared to BJT as the four quadrant uses the step by step ramping statergy for the change in the motor direction whereas the stability is lost.
* Regenerative braking is possible in four quadrant but not in BJT.
* It is widely used in automobile sector to control DC motor Speed and direction in four quadrant Whereas the BJT is used in robotics and electromechanical devices to control the Dc motor speed and direction
* The current values are contionous in BJT and In Four Quadrants it is chopped so that the simulation takes less timw.
* To change the direction of motor current signals are interuupted in four quadrants whereas in BJT the polarity of the current needs to be changed
* The main advantage of four quadrant is that it operates in all the four quadrants but BJT can move only in two directions
* The four Quadrant provides better efficiency and operates at high power levles when compared to BJT that Operates at the moderate power levels.
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 )
Two Quadrant Chopper Dc Drive block:
The two quadrant chopper DC Drive block represents a two qudarnt Dc Supplied chopper drive for Dc motors, This drive features closed loop speed control with two quadrant operation. The speed control loop outputs the reference armature current of the machine. Using a PI Current Controlled 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 fot the chopper.
* The main advantages of this drive compared with the other DC Drives are its implemenatation simplicity and that it can operate in two quadrants (forward motoring and reverse regeneration), In addition due to to the use of high switching frequency Dc-Dc converters, a lower armature current ripple (compared with thyristor based Dc Drives) is obtained However for all two quadrant Dc drives reversible and regenarative operations which are required in most DC drives cannot be obtained.
The two Quadrant chopper Dc drive block uses the blocks from the Electric Drives/ Fundamenal drive blocks libarary.
* Speed Controller (DC)
* Regulation Switch
* Chopper
3. Explain in a brief about operation of BLDC motor.
Sln. BLDC Motor:.
A Brushless Dc electric motor (BLDC motor) also known as an electronically commutated motor or synchronous DC motor is a synchronous motor using a direct current (DC) electric power supply it uses and electtonic closed looped controller to switch DC currents to the motor windings producing magnetic fields which effectivley rortate 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 commutators (brushes) used in many conventional electric motors.
* The construction of a brushless motor system is typically similar to a permanent magnet, synchronous motor but can also be switched reluctance mootr or an induction motor They may also use neodumium magnets and be outrunners, inrunners or axail
* The advantages of a brushless motor over brushed motors are high power wieght ratio, high speed nearly instantaneous control of speed and torque high efficiency and low maintenanance Brushless motors find application in such places as computer peripherals, hand held power tools and vehicles ranging from model aircraft to automobiles in modern washing machines, brushless DC motors have allowed replacement of rubber belts and gearboxes by a direct drive design.
RESULT:
All the three questions are solved with results..