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1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’. Answer: H bridge: An H-bridge consist of 4 BJT-Diode pairs. It helps us to control the DC motor forward and backward, by switching the two transistors simultaneously. The configuration of H-bridge is in 'H'-Shape,…
Jiji M
updated on 04 Aug 2022
1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.
Answer:
H bridge:
An H-bridge consist of 4 BJT-Diode pairs. It helps us to control the DC motor forward and backward, by switching the two transistors simultaneously. The configuration of H-bridge is in 'H'-Shape, where the 4 BJT-Diode pairs are on either sides of the load, with load at the center. When Q1 and Q4 are triggered, positive voltage is applied to the motor and the diodes D2 and D3 will act as freewheeling diodes when Q1 and Q4 are switched off. When Q2 and Q3 are triggered, then negative voltage is applied to the motor and the diodes D1 and D4 will act as freewheeling diodes, when Q2 and Q3 are switched off.
This is a simple demonstration of H-bridge.
When the transistors Q1 and Q4 are triggered with positive signal, a positive voltage is applied to the motor and motor rotates in the forward direction. At the instant of change in direction, Q1 and Q4 is turned off and D1 and D4 will be conducting. When the transistors Q2 and Q4 are triggered with negative signal, a negative voltage is applied to the motor and the motor rotates in the reverse direction. Once Q2 and Q3 are turned off D2 and D3 will be in on condition. Transistors on the same side cannot be triggered at the same time, since it will create a low-resistance path lead to damage of them.
Speed control of a DC motor using BJT H-bridge explanation:
The model shows simulation of H-Bridge used to generate chopped voltage and to control the speed of motor.
When the BJT is used for power switching apllications, it will act as an IGBT(Insulated Gate Bipolar transistor). When the BJT is operating in the saturated region, a forward voltage will be produced across collector and emitter. It will be in the range of 1V.
There are simulink signals provided to control and simulate the gate current to the IGBT. There is a pulse generator and armature voltage control block, which helps to vary the armature mean voltage from 0 to 240V and to create the simulink signal to control the switch.
The DC motor used in the model is a preset model with specification: 5 HP, 240 V, 1750 RMP, 300v field. It used fan type load where load torque is proportinal to square of speed.
There are 4 BJT-Diode pairs on either sides of the motor. When Q1 and Q4 are triggered, positive voltage is applied to the motor and the diodes D2 and D3 will act as freewheeling diodes when Q1 and Q4 are switched off. When Q2 and Q3 are triggered, then negative voltage is applied to the motor and the diodes D1 and D4 will act as freewheeling diodes, when Q2 and Q3 are switched off.
The 2 scopes, 'Scope' and 'Currents' shows different parameters. 'Scope' shows motor speed, armature current and load torque. 'Currents' shows IGBT(Q3) current and Diode(D3) current.
Simulation Results:
The first plot shows the IGBT current and diode current, while the second plot shows the speed, armature current and load torque. The motor starts with duty cycle of 75% in the positive direction. At t=0.5 sec, the armature voltage will be in reverse direction, and motor runs in the negative direction. From 0 to 0.5 sec, the current flows through the diode D3 and after 0.5 sec, the cirrent flows through IGBT Q3.
From the graph of armature current, it is visible that there is an increase in the armature current at 0 sec and 0.5 sec. Also, when there is change in voltage at 0.5 sec, the speed of the motor also changes.
1.B. Comment on the armature current shoot-up from the scope results.
This is the Scope result showing the armature current, speed and load torque.
From the graph of armature current, it is visible that there is an increase in the armature current at 0 sec and 0.5 sec. The sudden surge in current values at 0 sec and 0.5 secs is due to the transient switching occuring in the circuit, as a result of which the direction of motor changes from positive to negative after 0.5 secs.
Since there is already current flowing through the circuit in between 0 to 0.5 secs, when there is a change in the switching, the current will be added to the already flowing current which has changed its direction, which results in the spike or surge at 0.5 seconds.
However, it is possible to reduce the surge in current, by reducing the duty cycle from 75% to any other duty cycle less than 75%.
Changing the duty cycle of pulse generator signal from 75% to 50%:
The result is as follows:
In the above graph, the duty cycle was changed from 75% to 50%, and we can see that the surge in current has been decreased. However, as a result of change in duty cycle, the load torque and speed of motor also gets changed.
1. C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
The four-quadrant chopper DC drive uses speed controller, current controller, Regulation switch and chopper. The four quadrant chopper is a four-quadrant DC-DC converter or PWM (Pulse Width Modulation) converter, where the supply is a DC supply. This chopper has closed-loop control with four-quadrant operation. This chopper can work in all four-quadrants. The four-quadrant operations are: forward motoring, reverse regeneration, reverse motoring and forward regeneration. It allows the motor to rotate in clockwise and counter-clockwise direction and to perform braking.
Four Quadrant operation:
1. Forward motoring - Motor rotates in clockwise direction and controller applies the drive in clockwise direction.
2. Reverse motoring - Motor rotates in counter-clockwise direction and controller applies the drive in counter-clockwise direction.
3. Forward regeneration - Motor rotates in clockwise direction and controller applies the drive in counter-closewise direction.
4. Reverse regeneration - Motor rotates in counter-clockwise direction and controller applies the drive in clock-wise direction.
The above figure shows the four quadrant operation of the chopper. In the first quadrant operation, CH1 will be operating and CH4 will be in ON condition. Once the CH1 is turned OFF, D2 and CH4 will be conducting, and the load current freewheels through them. Here, load voltage and load current will be positive. In the second quadrant operation, CH2 is operated, keeping, CH1, CH3 and CH4 in OFF condition. Negative current will start flowing and when it is turned OFF, the current flows through D1 and D4. Here load voltage will be positive and load current will be negative. In the third quadrant operation, CH1 is kept OFF, CH2 is in ON condition, CH3 is operated and negative current will be flowing in the circuit. When CH3 is turned OFF, negative load current freewheels through CH2 and D4. Here, both load current and load voltage will be negative. In the fourth quadrant, CH4 will be operating, keeping CH1, CH2 and CH3 in OFF condition. When CH4 is turned ON positive current flows through the circuit. Once CH4 is turned OFF the current freewheels through D2 and D3. Here load voltage will be negative, while load current will be positive.
Comparison of Four quadrant chopper and H-bridge:
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:
A DC-DC converter or chopper is a static device used to obtain variable dc voltage from constant dc voltage source. For a 2-quadrant chopper, the average voltage will be always positive, but the average load current might be positive or negative. For a 2-quadrant chopper, there will be 2 switching devices and 2 diodes. Two quadrant chopper are of two types : Class-C and Class-D
The above figure shows 2-quadrant class-C chopper and its 2-quadrant operation. In Class-C chopper, it operates in 1st and 2nd Quadrant. In this type of chopper, output voltage will be always positive, while the load current will be either in positive or negative direction.There are two switching devices and two diodes connected in the circuit. The switching devices are not simutaneoulsy turned ON. In the first quadrant operation, the switch CH1 and Freewheeling diode FD will be turned ON. In the second quadrant operation, the switch CH2 and diode D2 will be ON. The output voltage will be always positive. since there is a free wheeling diode connected parallel to the load. The current will be reversed in the second quadrant operation and supply voltage will be obtained across the output.
The above figure shows 2-quadrant class-D chopper and its 2-quadrant operation. In Class-D chopper, it operates in 1st and 4th Quadrant. Here the switching devices will operate simultaneously. In the first quadrant operation, CH1 and CH2 will be turned ON and both load current and voltage across load will be positive. In the fourth quadrant operation, D1 and D2 will be working and the current across the load will be positive, however the output voltage will be negative. In both quadrants, the current will be always positive, since the diode and switching device conduct only in one direction.
Simulink Model created for Two Quadrant Class-D Chopper:
The elements used in the Simulink Model are:
The 2 MOSFETs and 2 Diodes are connected across the load. The input DC voltage is connected across the MOSFET-Diode configuration. The current measurement block and voltage measurement blocks are connected across the load, inorder to get the load current and load voltage and they are connected to the scope to get the plot of voltage and current. Pulse generator block is connected to give the trigger pulse for te Gate terminal of both the MOSFETs. A powergui block is given to simulate the model, since we are using Simscape™ Electrical™ Specialized Power Systems blocks. It stores the equivalent Simulink circuit that represents the state-space equations of the model.
Here we are changing the parameters of pulse generator, the duty cycle is changed to 50%, and the amplitude is changed to 10 and period is changed to 0.02 seconds to get the waveform with stop time of 1 seconds.
Using the DC voltage source we are giving an input DC voltage of 12V.
We need to connect a load across the configuration, to represent the motor. Here we are connecting an inductive load. For that we are considering a series RLC branch and then select the Branch type, L, so that the load connected will be inductive type. We are changing the inductance value to 4e-3 H.
Running the model, we got the below outputs:
The output waveform clearly shows that the current on the load is positive in direction, whereas the output voltage will be both in positive and negative direction. This shows that the class-D chopper has output current in positive direction and the output voltage in both positive and negative half cycle.
3. Explain in a brief about operation of BLDC motor.
BLDC motor or Brushless DC motor, are the type of motors with higher efficiency and without any brushes in the rotor. It is also known as synchronous motor using dc electric power supply. It uses an electronic closed loop controller to switch the dc current to the motor winding which in turn induces the magnetic field and starts to rotate. The permanent magnets on the rotor follows the magnetic field. Normally in other motors, there will be brushes on rotor inorder to transfer the current to the rotor circuit. In BLDC motor, there are no brushes, instead they have permanent magnets implanted on the rotor. BLDC motors have electronic commutators, hence they are also known as Electronically commutated motor or ECM.
Construction:
In BLDC permanent magnets are attached to the rotor. The current carrying conductors (armature windings are wounded on the stator poles. Armature windings are switched electronically using transistors or SCR (Silicon Controlled Rectifier) at the correct rotor position. The stator coils(armature winding) are stationary, produces variable DC voltage for rotating the permanent magnet by changing the magnetic field of the stator poles continuously. There can be hallsensors placed inorder to detect the rotation of speed of bldc motor. There are 2 types of rotor construction: Outrunner type, where the rotor will be outside the wheel of vehicle, and the hub-runner type, where the rotor will be within the wheel of vehicle.
Working:
There are 3 windings on the stator poles creating 3 phases, electricaly 120 degrees apart from each other. The coils are wound on the stator poles in such a way that the current will enter in to the circuit completely and go back once completed. When the stator coils are excited, or when the current flows through them, it becomes an electromagnet and produces a magnetic field in air gap. Due to the force of interaction between the stator and permanent magents on the rotor, the rotor starts to rotate. The permanent magnets on the rotor will have South and North poles which arranges according to the magnetc field which causes the rotor to rotate.
Advantages of BLDC motor:
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