Week-7 Challenge: DC Motor Control
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:
Ans:
Objective: To run the demo of a given MATLAB model and modify it according to the given condition.
A. Model Description:
Fundamental Blocks:
1. DC Motor
2. Pulse Generator
3. Gain Blocks
4. IGBT Blocks
5. Scope Blocks
6. DC Voltage Source Blocks
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.
B. Simulation Performance:
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 a negative direction.
'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.
C. Original Model Representation:
D. Original Responses:
1. The first response we get from the IGBT Current and Diode Current as an input to the scope named 'Currents' which is highlighted in Red as below:
2. The second response we get from the Speed, Armature Current and Load Torque as an input to the scope which is highlighted in Red as below:
E. Modified Responses:
As we have seen above that the armature current was shooting up while starting the positive cycle as well as in the negative cycle.
That was happening due to the high duty cycle. So then we have reduced the duty cycle to further extend to get the below results.
The below results shows that after a reduced duty cycle we got the armature current responses which were normal and not shooting up in the positive as well as in the negative direction.
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2. Refer to help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with H-bridge model:
Ans:
Objective: To refer and compare the 'The Four-Quadrant Chopper DC Drive (DC7) block' with the H-bridge model.
A. Model Description:
Fundamental Blocks:
1. Speed Controller (DC)
2. Regulation Switch
3. Current Controller (DC)
4. Chopper
The Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors. This drive features closed-loop speed control with four-quadrant operation. The speed control loop outputs the reference armature current of the machine. Using a PI current controller, 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 for the chopper.
The main advantage of this drive, compared with other DC drives, is that it can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward regeneration). In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained. However, four switching devices are required, which increases the complexity of the drive system.
B. Simulink Model:
Below mentioned is the Simulink Model for the Four-Quadrant Chopper DC Drive (DC7):
c. Responses:
Below mentioned is the input for the response having various inputs as below:
1. Duty cycles for 4 IGBTs
2. Armature Voltage
3. Armature Current
4. Motor Speed
The response in the scope with the inputs highlighted in Red color as highlighted above is as below:
D. Comparison:
| Features | 4-Quadrant Chopper | BJT H-Bridge |
| Application | Automobiles | Robotics |
| Regenerative Braking | Yes | No |
| Dynamic Braking | No | Yes |
| Current Signals | Discrete | Continuous |
| Simulation Time | Less | More |
| PI Controller Implementation | Yes | No |
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3. Make a suitable EV model using DC7 block and make result report:
Ans:
Objective: To make an EV Model using DC7 block and record the results.
A. EV Model Simulink using DC7 Block:
Fundamental Blocks:
1. Drive Cycle Source
2. DC Voltage Source
3. Four-Quadrant Chopper DC Drive
4. Vehicle Torque-Speed System Block
5. Scopes
B. Sub-systems:
A. DC Voltage Source:
If we double-click on the above highlighted block in Red Color; we will get it as follows:
It includes the fundamental blocks as:
1. DC Voltage Source
2. Braking Chopper
3. Resistance
4. Output Ports
B. Vehicle Torque-Speed:
The sub-system named of the block ' Vehicle Torque-Speed' is as follows:
If we double-click on the above highlighted block in Red Color; we will get it as follows:
It is based on the form of a fundamental torque equation which is as:
T = 24.7+0.0051w^2
C. Drive Cycle Source:
Below is the matrix for the Drive Cycle which we import into the Drive Cycle Source Block.
D. Results:
Key Takeaways from results are as follows:
1. Voltage is proportional to the Current.
2. Motor speed follows the reference speed and reaches at peak with the value of 1000rpm.
3. For more Motor speed; more Voltage is also necessary.
4. When the Motor Speed increases, the duty cycle of IGBTs 1&4 also increases.
5. When the Motor Speed decreases, the duty cycle of IGBTs 2&3 also decreases.
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