Week-7 Challenge: DC Motor Control

AIM :

1. To 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. 
2. To refer to help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with H-bridge model.
3. To make a suitable EV model using DC7 block and make result report.

DESCRIPTION :

      1. To 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. 

 

BJT:

      A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, use only one kind of charge carrier. A bipolar transistor allows a small current injected at one of its terminals to control a much larger current flowing between two other terminals, making the device capable of amplification or switching.

      BJTs use two junctions between two semiconductor types, n-type and p-type, which are regions in a single crystal of material. The junctions can be made in several different ways, such as changing the doping of the semiconductor material as it is grown, by depositing metal pellets to form alloy junctions, or by such methods as diffusion of n -type and p-type doping substances into the crystal.

H Bridge:

      An H-bridge is a simple circuit that lets you control a DC motor to go backward or forward. 

      A DC motor spins either backward or forward, depending on how you connect the plus and the minus.

      

      If you close switch 1 and 4, you have plus connected to the left side of the motor and minus to the other side. And the motor will start spinning in one direction.

      

      If you instead close switch 2 and 3, you have plus connected to the right side and minus to the left side. And the motor spins in the opposite direction.

MATLAB DEMO MODEL :

      To get the Speed control of a DC motor using BJT H-bridge model, Run the power_Hbridge command by entering it in the MATLAB Command Window.

 

This shows the simulation of an H-bridge used to generate a chopped voltage and to control the speed of a DC motor.

MODEL DESCRIPTION:

      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.

SIMULATION:

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

      From the graph, it is clearly observed that the motor has the maximum rpm of 946.9rpm as omega in Forward motoring of 7.89A of current at its 0.5th sec and after that the curve goes beyond zero which is negative indicating the Reverse motoring of -950.8rpm with -5.12A current. As in the Switch block the threshold time is given as 0.5, So after 0.5th sec the current flow to the motor is changed in opposite direction thus reverse motoring happens. 

     The armature current shoot up when motor changes direction from forward to reverse, this has to be overcomed. 

For this duty cycle which is 75% (Pulse Width) now is reduced and observe the changes in the results.

DUTY CYCLE: 50%

Here the speed of the motor is reduced to 442.4rpm in forward motoring with 6.493A current. And the armature current shoot up has been reduced to a short level.

DUTY CYCLE: 25%

Now the pulse width is reduced to 25, and the results are compared. the motor speed is reduced to 90.92rpm and the armature current and the torque also reduced and the shoot up has been reduced completely.

Now the armature current does'nt shoot up when motor changes direction from forward to reverse.

      2. To refer to help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with H-bridge model.

FOUR QUADRANT CHOPPER:

      A four quadrant chopper is a chopper which can operated in all the four quadrants. The power can flow either from source to load or load to source in this chopper. In first quadrant, a Class-E chopper acts as a Step-down chopper whereas in second quadrant it behaves as a Step-up chopper. This type of chopper is also known as Class-E or Type-E chopper.

First - Quadrant Operation:

      For first quadrant operation, CH4 is kept ON, CH3 is kept OFF and CH1 is operated. When both CH1 & CH4 are ON simultaneously, the load gets directly connected to the source and hence the output voltage becomes equal to the source voltage. This essentially means that v_o = v_s. It may be noted that the load current flows from source to load as shown by the direction of i_o. When CH1 is switched OFF, the load current free wheels through CH4 and D2. During this period, the load voltage and current remains positive. Hence the Power also stays positive and this denotes the Forward Motoring operation.

Second - Quadrant Operation:

      CH2 is operated while keeping the CH1, CH3 & CH4 OFF. When CH2 is ON, the DC source in the load drives current through CH2, D4, E and L. Inductor L stores energy during the On period of CH2. When CH2 is ON, the DC source in the load drives current through CH2, D4, E and L. Inductor L stores energy during the On period of CH2. When CH2 is turned OFF, current is fed back to the source through D1, D4. It should be noted at this point that (E+Ldi/dt) is more than the source voltage Vs. As load voltage Vo is positive and Io is negative, it is second quadrant operation of chopper. Since, the current is fed back to the source, this simply means that load is transferring power to the source.

Third - Quadrant Operation:

      For third quadrant operation, CH1 is kept off, CH2 is kept ON and CH3 is operated. When CH3 is ON, load gets connected to source and hence load voltage is equal to source voltage. But carefully observe that the polarity of load voltage vo is opposite to what shown in the circuit diagram. Hence, v_o is assumed negative. Let us now see what is the status of load current i_o. It may be seen that i_o is flowing in the direction opposite to shown in the circuit diagram and hence negative.

Now, when CH3 is turned OFF, the negative load current free wheels through the CH2 and D4. In this manner, v_o and i_o both are negative. Hence, the chopper operates in third quadrant.

Fourth - Quadrant Operation:

      To obtain fourth quadrant operation, CH4 is operated while keeping CH1, CH2 and CH3 OFF. The polarity of load emf E needs to be reversed in this case too like third quadrant operation. When CH4 is turned ON, positive current flows through CH4, D2, L and E. Inductance L stores energy during the time CH4 is ON. When CH4 is made OFF, current is fed back to the source through diodes D2, D3. Here load voltage is negative but the load current is always positive. This leads to chopper operation in fourth quadrant. Here, power is fed back to the source from load and chopper acts as a step-up chopper.

• When the chopper works in 1st and 3rd quadrant, the average output voltage is less than the input voltage, which means it it Step-down Chopper.
• When the chopper works in 2nd and 4th quadrant, the average output voltage is more than the input voltage, which means it it Step-up Chopper.

MATLAB EXAMPLE MODEL:

      To get ‘The Four-Quadrant Chopper DC Drive (DC7) block’, Run the dc7_example command by entering it in the MATLAB Command Window.

Description:

• 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.
• The first regulator is a speed regulator, followed by a current regulator. The speed regulator outputs the armature current reference (in p.u.) used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed.
• The speed reference change rate follows acceleration and deceleration ramps in order to avoid sudden reference changes that could cause armature over-current and destabilize the system.
• The current regulator controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the four IGBT devices (Pulse Width Modulation).
• 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.

Scope:

Simulation:

• Before starting the simulation, set the initial bus voltage to 515 V via the GUI block ('Initial States Setting' button and 'Cbus' variable).

• Start the simulation. You 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.

Comparison:

      3. To make a suitable EV model using DC7 block and make result report.

       The energy source of an EV is a battery or fuel cell. Let’s take the case of a battery. So the model can be modified with an input of a battery from the Simulink Library Browser.

Modify the energy source at the input side:

      Here a simple DC source is used as a energy source and 500V were given as voltage value for an EV.

Reference Speed:

      Drive cycle were prepared on my own and imported in the matlab which is in the same directory of the DC7 model. And using From Workspace block, the drive cycle has been imported to the model.

Vehicle load characteristics:    

      Linear load torque block can be replaced with a subsystem model according to the load characteristics given by 

 

  

This is equation is made into a subsystem in the model. The speed is squared using Square block, and then mulitplied with the constant value 0.0051 and then sum with the 24.7 to get the torque and then it is directed to the Four quadrant DC chopper block.

It is simulated for 30sec and the scope is attached below,

Scope:

      The Duty cycles IGBT 1&4, Duty cycles IGBT 2&3 are plotted in the graph. And Armature current, Armature voltage were also plotted and can observe the changes in the armature current due to the imported speed data. And motor speed is also plotted. Thus the changed parameters gave a good results and objective is achieved.

CONCLUSION:

• MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’ is simulated. And the model is modified in such a way that armature current doesn’t shoot up when motor changes direction from forward to reverse.
• ‘The Four-Quadrant Chopper DC Drive (DC7) block’ is referred in the Matlab website. And compared it with H-bridge model.
• EV model using DC7 block and results were reported.