Control of DC Motor using Power Electronics
Control of DC Motor using Power Electronics
Aim:
* To Run the MATLAB demo of Speed control of a DC motor using BJT H-bridge and to study and modify the model so that the armature current does not shoot up when the rotating direction of the motor changes.
* To compare the Four-Quadrant Chopper DC Drive (DC7) block with the BJT H - bridge model.
* To Make a suitable EV model using DC7 block and to make result report.
Power Switches:
* In an electric or Hybrid electric vehicles, the vehicle speed is controlled as desiered by varying the level of voltage supplies at the motor terminals.
* The control of the battery voltages is done by the Swiching operation like The control is achieved by the changes in Time axis (i.e) the switch ON time and the OFF time is the reason for the power control.
* These power switches are Semi Conductor Switches and they are capable of hadling high voltage and current inputs.
* Basic types of power semiconductor switches:
1. Power BJT Switches
2. MOSFET Switches
3. IGBT Switches
1. Power BJT Switches:
B - Base ; E - Emitter ; C - Collector
(i) Power BJT means Bipolar Junction Transistor, and they are called \"power\" because they are not ordinary low power signal transistors but can handle high current and voltage levels during switching operations.
(ii) They have two sides, one is the control side which has the Base and Emitter and the other side is the Power side which has a Collector and Emitter.
(iii) If There is no base signal the switch will be open (I=0) and does not allow any current to flow.
(iv) If the there is base signal then it means that switch is closed (V=0) and allows current to flow.
(v) The Power BJT is a current operated semiconductor switch.
2. MOSFET Switches:
(i) MOSFET means Metal Oxide Semiconductor Field Effect Transistors and it can also handle high power and voltage conditions.
(ii) MOSFET also has two sides, one is the control side which has the Gate and Sorce terminal and the other side is the Power side which has a Drain terminal and Source terminal.
(iii) If There is no Gate signal the switch will be open (I=0) and does not allow any current to flow.
(iv) If the there is Gate signal then it means that switch is closed (V=0) and allows current to flow.
(v) All are same like the BJT but only difference is, this one operates based on the field effect (i.e) it is voltage operated
(vi) The MOSFET is the fastest in switching operation compared to all other swiches. Being able to switch fast, they can also handle a reasonable heavy load of current and voltages.
3. IGBT Switches:
G - Gate ; E - Emitter ; C - Collector
(i) IGBT means Insulated Gate Bipolar Transistor
(ii) The IGBT switches has a Gate with insulation and so it can handle high voltage and current conditions.
(iii) The only difference from the previous switches is it\'s operation, From the Control side it functions like a MOSFET & From the Power side it functions like a Tyristor
(iv) The IGBT switches can handle very high current and voltage conditions compared to others but not as fast like a MOSFET.
(v) They find applications in all heavy duty scenarios.
* These are the types of the semiconducting switches.
* In electric and Hybrid electric vehicles mostly MOSGET & IGBT switches are used.
QUESTIONS:
1. Run MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’. Modify the model such that armature current doesn’t shoot up when motor changes direction from forward to reverse.
* Matlab has a inbuilt block to simulate the function of \"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 Input Block
* 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).
* The armature voltage is controlled with the input of duty ratio at 500 Hz frequency and is stored as P1,P2,P3,P4 control block and it is sent as the control signal to all the IGBT switches respectively.
* The direction of the rotating motor changes with the change in the polarities of the current and it is achieved by the following,
* Two transistors are switched simultaneously: Q1 and Q4 or Q2 and Q3.
* When Q1 and Q4 switches are closed, 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 switches are closed, a negative voltage is applied to the motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.
* When the direction of the rotating motor changes there will be a lot of stress developed in the IGBT Transistors.
Simulation 1:
* The simulation is done for DC motor speed control using BJT H - bridge
The Total cycle time = 1 seconds
Forward motoring time = 0.5 seconds
Reverse motoring time = 0.5 seconds
The Battery voltage = 240 volts
The Duty cycle = 75%
The supply voltage = 180 volts
Results:
* The above figure shows the results for the simulation and there are totally three plots in them.
* Each plot shows a particular result and the plots are made in such a way that ,there is a cursor measurement and if we change the cursor on one plot the cursor moves on the otherplots too which paves way to study the result in a better way.
* The cursor enables us to study the behaviour of each parameters with others respectively
plot 1 :
* Shows the angular velocity of the DC motor (w) with the Time scale of (0 to 1 seconds) on the x axis.
The maximum Forward speed of motor = 953 Rpm (0 - 0.5 sec)
The maximum Reverse speed of motor = - 953 Rpm (0.5 - 1 sec)
plot 2:
* Shows the changes in Armature current (Ia) in Amperes (A) with respect to the motor direction of rotation.
The current in forward rotation = 7.83 A (0 - 0.5 sec)
The shoot up current in forward rotation = 37.5 A
The current in Reverse rotation = - 3.4 A (0.5 - 1 sec)
The current Dip in Reverse rotation = - 65.1 A
plot 3:
* Shows the Torque values and its changes of the DC Motor with respect to the motor direction of rotation in (Nm)
The maximum Forward Torque of motor = 6.013 Nm (0 - 0.5 sec)
The maximum Reverse Torque of motor = - 6.011 Nm (0.5 - 1 sec)
* From the result of the above simulation we can observe a current shoot up while the motor increasing in speed (w) and also a negative current shoot up while the motor changes the direction from forward to reverse direction.
* While the motor starts to rotate in forward direction there is a current shoot up because usually in DC Motor the starting current will be high.
Motor equation is , V= E(b) + I(a)R(a).
When we start the motor, there is no back emf to oppose the armature current because the conductors have not yet started rotating. Back emf is produced only when there is relative motion between conductors and magnetic field. When starting the motor, there is no relative motion so no back emf.
* so E(b) = 0 ; I(a) = V / R(a), and so there is a huge rush in current while starting.
* When the polarity is reversed by switching operations the forward rotating motor now start rotating in reverse direction and we observe a negative current dip.
* This is because of the Switching loss that is happening while changing the direction of rotation of the Motor
* This can be controlled by using soft switching techniques (or) by having a little compromise of the motor\'s efficiency.
* The duty ratio can be altered slightly to reduce the disturbance happening in the current.
Simulation 2:
* The simulation is done for DC motor speed control using BJT H - bridge
The Total cycle time = 1 seconds
Forward motoring time = 0.5 seconds
Reverse motoring time = 0.5 seconds
The Battery voltage = 240 volts
The Duty cycle = 45%
The supply voltage = 108 volts
Results:
plot 1 :
* Shows the angular velocity of the DC motor (w) with the Time scale of (0 to 1 seconds) on the x axis.
The maximum Forward speed of motor = 375 Rpm (0 - 0.5 sec)
The maximum Reverse speed of motor = - 287 Rpm (0.5 - 1 sec)
plot 2:
* Shows the changes in Armature current (Ia) in Amperes (A) with respect to the motor direction of rotation.
The current in forward rotation = 4.99 A (0 - 0.5 sec)
The shoot up current in forward rotation = 7.32 A
The current in Reverse rotation = - 5.66 A (0.5 - 1 sec)
The current Dip in Reverse rotation = - 8.69 A
plot 3:
* Shows the Torque values and its changes of the DC Motor with respect to the motor direction of rotation in (Nm)
The maximum Forward Torque of motor = 0.94 Nm (0 - 0.5 sec)
The maximum Reverse Torque of motor = - 0.54 Nm (0.5 - 1 sec)
* From the result of the above simulation we can observe when the duty ratio is brought down to 45% the current dip is reduced from - 65 A to -8.6 A.
* There is a little compromise we have made by reducing the motor\'s efficiency but considering the reduction in current disturbance and also the motor is relatively spinning with considerable speed and Torque we can accept this process.
2. Refer to help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with H-bridge model.
Four Quadrant Operation:
* Four quadrant of any drives or DC Motor means that the machine operates in four quadrants.
* They are Forward Braking, Forward motoring, Reverse motoring and Reverse braking.
* A motor operates in two modes – Motoring and Braking. A motor drive capable of operating in both directions of rotation and of producing both motoring and regeneration is called a Four Quadrant variable speed drive.
* I quadrant - power developed is positive and the machine is working as a motor supplying mechanical energy. The I (first) quadrant operation is called Forward Motoring.
* II quadrant - In this quadrant the direction of rotation is positive, and the torque is negative, and thus, the machine operates as a generator developing a negative torque, which opposes the motion and it is called Forward Braking.
* III quadrant - The motor works, in the reverse direction. Both the speed and the torque have negative values while the power is positive and it is called Reverse Motoring.
* IV quadrant - The torque is positive, and the speed is negative. This quadrant corresponds to braking in the reverse and it is called Reverse Braking.
DC7 four-quadrant chopper DC drive:
* 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.
* There are Two Regulators present,
(i) Speed Regulator
(ii) Current Regulator
(i) Speed Regulator:
* The speed regulator gives the Armature current level required in order to obtain the electromagnetic torque needed to reach the desired speed
* To avoid sudden over-current that destabilize the system while following the reference speed changes there is a acceleration and deceleration Ramps.
(ii) Current Regulator:
* The current regulator controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the four IGBT devices
* 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.
Simulation:
* Before starting the simulation, set the initial bus voltage to 515 V via the GUI block (\'Initial States Setting\' button and \'Cbus\' variable).
* Here starting the simulation we can observe the motor armature voltage and current, the four IGBT pulses and the motor speed on the scope. The current and speed references are also shown.
* The motor is coupled to a linear load, which means that the mechanical torque of the load is proportional to the speed.
* The speed reference is set at 500 rpm at t = 0 s. Observe that the motor speed follows the reference ramp accurately (+400 rpm/s) and reaches steady state around t = 1.3 s.
* The armature current follows the current reference very well, with fast response time and small ripples. Notice that the current ripple frequency is 5 kHz.
* At t = 2 s, speed reference drops to -1184 rpm. The current reference decreases to reduce the electromagnetic torque and causes the motor to decelerate with the help of the load torque.
* At t = 2.2 s, the current reverses in order to produce a braking electromagnetic torque (dynamic braking mode). This causes the DC bus voltage to increase.
* At t = 3.25 s, the motor reaches 0 rpm and the load torque reverses and becomes negative. The negative current now produces an accelerating electromagnetic torque to allow the motor to follow the negative speed ramp (-400 rpm/s). At t = 6.3 s, the speed reaches -1184 rpm and stabilizes around its reference.
Comparision between H_bridge and Four Quadrant DC chopper:
Control method:
* In H-bridge we need to reverse the polarity to change the direction but in Four quadrant DC drive it uses reference speed to change the direction of motor rotation.
* PI Controllers are used for speed and current control in Four quadrant DC drive and H-bridge does not use that.
Stability:
* The current shootup and disturbances are high in H-Bridge because of the switching losses.
* The fourquadrant chopper uses ramps to smoothen the current rush and thus the stabilty is good in the four quadrant DC drives.
Operations:
* Forward and Reverse motoring are the only two operations that the H-Bridge can do it cannot be operated for regenarative braking
* The Four quadrant DC drive operates in four modes forward motoring, reverse motoring,forward braking and reverse braking. Also it can perform regerative braking as well.
* Overall the four quadrant DC drives are better than the H-bridge model.
3. Make a suitable EV model using DC7 block and make result report.
Electric Vehicle Modelling using DC7 block:
* The DC7 example block in simulink allows us to analyse the Four quadrant operated DC Motor by proving the input voltage supply and speed reference.
* An Electric vehicle uses a DC Motor which needs to perform all the four operations of Forward and reverse motoring and braking.
* So this DC7 model can be modified a little bit to model and analyse a Electric vehicle and the results can be observed.
Modifications:
1. Drive cycle
2. Linear Load Torque
3. Power source
1. Drive cycle:
* In the DC7 block initially there is the motor speed reference block which gives the motor speed requirements in Rpm.
* For modelling a vehicle we need the vehicle speed with respect to time changes and so, instead of speed reference block, we are adding a drive cycle block using the simulink library.
* There an excel file containing the drive cycle inputs is imported to the model.
Drive cycle (20 seconds)
* The drive cycle shows the time on the first column and the speed values in kmph in the second column.
2. Linear Load Torque
* For modelling an Electric vehicle we need the load characteristics of the car consisting of all the external forces acting on the vehicle.
* This includes all type of forces but in here for simple modelling we are deriving an equation considering the Rolling Resistance force and the Air drag forces that are involved.
* The Vehicle Load Torque equation is given by,
T = 24.7 + 0.0051 * (w)^2
Subsystem of vehicle load characteristics
* This equation is inserted in the model using a subsystem which takes (w) as the input from the DC7 block and calculates the load characteristics equation.
* The linear load block is replaced with the above subsystem and it is connected back to the DC7 block.
3. Power source
* Finally the AC power source is replaced to a DC Voltage source with similar voltage level as that of a real car\'s battery voltage.
* From the simulink library the DC Voltage source is selected and a voltage level of 300 volts is assigned to the source.
* Then a current measurement block is connected to the voltage source and a scope is added to plot the current output from the power source to the Motor.
* Atlast the power source is connected to the motor.
Electric Vehicle Modelling
Simulation:
* The above figure shows the simulation results for a basic Electric vehicle that uses a DC motor.
* Here starting the simulation we can observe the motor armature voltage and current, the four IGBT pulses and the motor speed on the scope.
* The speed reference is based on the drive cycle data input with respect to time.
* Here we can see the motor\'s current and voltages behaviour with respect to the driving conditions.
* The armature current follows the current reference very well, with fast response time and small ripples. Notice that the current ripple frequency is 5 kHz.
At t = 10 s, the current reverses in order to produce a braking electromagnetic torque (dynamic braking mode). This causes the DC bus voltage to increase.
* From the fourth plot we see the current values (i.e) the supplied armature current and the motor terminal current values with respect to the drive cycle\'s speed condition througt the time band.
* The motor speed also perfectly matches the drive cycle reference speed.
* Thus Electric vehicle is modelled by modifying the Four quardrant chopper DC7 block.