Week-7 Challenge: DC Motor Control
CONTROL OF A DC MOTOR:
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.
1. 
THE ABOVE MODEL IS A SPEED CONTROL OF A DC MOTOR USING BJT H-BRIDGE.
THEORY:
1. Power Electronics: Electronics accompanied for the control and conversion of electric power. This control and conversion is basically done by power converters and these power converters are based upon the circuits which consists various types of transistors and for EV/HEV, mainly IGBT and MOSFET transistors are used. These transistors are made up of semiconductor material with three terminal Collector, Base and Emitter. Power signal flows through collector and emitter whereas control signal act through base and emitter. Also, amplification of signal can be done, which means, controlled power output can be higher than input power.
In the above model the DC motor is connected to the Fan type load with the help of H- bridge. In the H-bridge, there are four power switches, which are used and acts as main junction in deciding forward and reverse motoring.When P1 and P4 signals are on , the other two are in off condition and the motor works in the forward motoring condition where as when the P2 and P3 are on, the other two are in off condition helping the motor to work in reverse motoring condition. Also, the fan type load used the torque speed relation.
The Bipolar junction Transistor when used in power switching applications, it operates as an IGBT. When a transistor is in conducting mode, a forward voltage is developed between collector and emitter.
BIPOLAR JUNCTION TRANSISTOR:
B-BASE, C-COLLECTOR,E-EMITTER
IGBT-INSULATE GATE BIPOLAR TRANSISTOR
DIODE: It is used in circuit to allow the current to pass in one direction and block in the reverse.
WORKING OF H-BRIDGE: It consist of four transistors and in the center dc motor is connected. So, when Q1 and Q4 are fired, a positive voltage is applied to the motor and forward motoring take place and diode D2-D3 operates as freewheel diodes when Q1 and Q4 are switched off. When Q2 and Q3 are fired, a negative voltage is applied to the motor and diode D1-D4 operates as freewheel diode when Q2 and Q3 are switched off.
DC motor spins backwards or forward based upon the connection type as shown in the below figure:
Due to motor inertia , an electrical energy is also generated by the motor.SO, when transistors are in disabled mode to stop the motor, this energy is to be released in someway. That is why, diodes are used in the reverse direction of transistor. Without the presence of diodes, the electrical energy produced by motor can damage the transistor.
SIMULATION RESULTS OF THE MODEL:
OBSERVATIONS:
At 0.5 seconds there is change between forward and reverse mode where duty cyle used for the above simulation was 70%. Also, during the constant speed in both foreward and reverse direction, the armature current also remains constant in both the phases. However, during the exponetial rise and depreciation of speed and torque, the armature current shoot up quickly.
Armature currents shootup can be controlled by the duty cycle. So, if we reduce the duty cycle from 75% to 30%, these current shootups can be minimized.
Following are the simulation results:
C. THE FOUR QUADRANT CHOPPER:
Firstly, a DC-DC power converter is known as chopper. When it operates in first quadrant, it will be called first quadrant chopper. In each quadrant, the function of motor changes. In first quadrant, both torque and speed are positive, so, power remains positive which means, forward motoring whereas in second quadrant, torque becomes negative, which gives us forward braking. Similarly, in third quadrant, it will be reverse motoring due to power positive and in fourth quadrant, it will be reverse braking due to negative power output.
BELOW IS THE FOUR QUADRANT CHOPPER DC DRIVE BLOCK:
The advantage of using this drive is that it operates in all four quadrants. Also, the armature current ripple will be lower due to high switching frequencies. However, complexity of the drive will also increase as the four switches are required for the drive system.
COMPARISON WITH H-BRIDGE:
In H-bridge, by changing the polarity, we can spin the motor in forward or backward direction but here in four quadrant chopper, signals needed to be interupt. Forward braking is possible in H-bridge but reverse motoring and reverse braking is not possible. Speed regulators are not needed in H-bridge whereas it is required in four quadrant chopper.
2. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.
In the two quadrant chopper operation, basically the chopper works in two quadrant, 1st and 2nd. Here the main point comes out is that, Voltage always remain positive and current will be negative and positive simultaneously with respect to the quadrant operation. Also both the switches cannot work at the same time because the circuit can be short circuited. So the two quadrant operation is explained below:
FORWARD MOTORING WORKING:
When the chopper Switch CH1 is ON, the current passes from the voltage source across CH1 towards inductor 'L' and energy is started storing. When chopper switch CH1 is OFF, the stored energy started flowing as current towards FD that is free diode. So in forward motoring, CH1 and FD acts through.
REVERSE BRAKING WORKING:
When the chopper switch CH2 is ON , the stored energy started flowing in the reverse direction towards E or armature from inductor. When the CH2 switch is off, the current flows in negative direction towards the voltage source indicating the regenerative mode.
The above image shows the two quadrant chopper in the simulink environment. The MOSFET power switches are used for the above simulation and connected with the NOT gate so that both the power switches can work one after the other because if they work together at the same time, there will be short circuit. The diodes operation also become important because it prevents the semiconductor to get damaged and also the unidirectional flow of current and voltage.
SIMULATION RESULTS:
The above results shows pulse width with the amplitude of 10 and duty cycle of 50%. Also, the voltage results shows that voltage will always remain positive in both the quadrant and NOT gate works with respect to the duty cycle , which is fed to the power switches. As far as current is concerned, it becomes positive as well as negative showing the two quadrant operation of the chopper.
3. Explain in a brief about operation of BLDC motor.
BLDC stands for brushless dc motor but it can be called as brushed dc motor with electronic commutator. Electronic commutator are power converters(AC to DC, DC to DC, DC to AC). So, in power converters, the arrangements of power switches are in such a way that it will excite the different phases turn by turn. Usually, BLDC motors are 3 phase motors and each phase motors has to be excite synchronously turn by turn. If we talk about brushed type DC motors, it can be tested at NO LOAD speed with the help of a battery but in the case of BLDC, the testing is not possible as it consist of three terminals.
In BLDC motor, rotor magnets are fixed permanent magnets, that is why, it is called Permanent Magnet BLDC motor. If we talk about the stator part, it consist of windings such that current excitation is given on the stator winding. Also the direction of the current is positive in one phase, zero in one phase and negative in one phase. To rotate the the motor in clockwise direction, excitation of current should be given phase by phase.