Week-7 Challenge: DC Motor Control
Q1)
Matlab Model of 'Speed control of a DC motor using BJT H-bridge’:
IGBT 1& 4 along with Diodes 1&4 are involved in forward motoring.
IGBT 2& 3 along with Diodes 2&3 are involved in reverse motoring.
This model is run with the default inputs present and the results are obtained as below:
Graph 1 :
From the graph, we can observe that the armature current is shooting to a hig value during the reverse operation of the motor.
Graph 2 :
This graph shows the working period of the IGBT 3 and Diode3 respectively based on the values of current consumed by it.
So, here the"On time"of IGBT is much higher than the diode, which means the motor is running in the negative direction for a long time.
we can see that (Graph 1) the armature current shoots up during the transition of the motor from clockwise to counter clockwise direction.
This is not the best condition for the motor to function. So, its is important to reduce the shootup of the armature current during the transition period.
Now in order to reduce the armature current shootup, the pulse width of the pulse generator is modified in an iterative manner and
the best result is shown below:
Graph 3:
By varying the pulse width of the PWM, It is clearly visible that the shootup in the current during reversal is reduced to its minimum level.
A magnified view of the above graph:
Q2) Four Quadrant DC Chopper Drive :
This is also known as DC7 block. This is power converter circuit which is generate variable voltage at the output from a fixed voltage input.
Construction:
This consists of 4 power switches (IGBT, MOSFETS, etc.) paired with a Diode each, respectively.
The transistors are supplied with base voltage and controlled using pulse width modulators.
It is considered having 2 legs. 1st leg consists of the transistors(Switches) and Diodes on the left side [(IGBT1, D1); (IGBT2, D2)] and the 2nd leg consists of [(IGBT4, D4); (IGBT3, D3)].
This can be connected to the motor as shown in the figure below,
Working:
As the name suggests, it can operate in 4 quadrants i.e. 4 modes of operation namely,
- Forward Motoring. (Quadrant 1)
- Forward Breaking. (Quadrant 2)
- Backward Motoring. (Quadrant 3)
- Backward Breaking. (Quadrant 4)
X-axis = Current
Y-axis = Voltage
Pulse width modulator is used to generate pulses to the gate inputs of the transistors.
This is done to vary the values of the current and voltage which is fed to the motor to control it.
For making the motor to work in desired motion, these 4 power switches and 4 diodes are used.
Q1 operation:
During forward motoring, both the output current and output voltage are positive. So, both the speed and torque will be in positive direction and the vehicle moves in forward direction. This can be achived by keeping IGBT 1 & 4 (or) IGBT 4 & Diode 2 in "ON state"
Q2 operation: Here, the current is negative, but the voltage remains positive.
When, IGBT 2 and diode 4 is ON forward breaking occurs,which ctores energy in the inductor. and when D1 and D4 are in ON state, which makes the motor to break leading to charging of the battery.
Q3 operation: Both the current and voltage are negative in this operation. This is known as reverse motoring.
Here, IGBT 3 & 2 are in ON state,leading to reverse action of the motor. This can be continued even by keeping IGBT 2 and Diode 4 in On state.
Q4 operation: In this operation, when IGBT 4 and diode 2 is made on, assuming the motor is already in reverse direction, now transiting it into positive direction makes current positive, so the indcutor gets charged by the back Emf of the motor . Again, when making diode 2 & 4 ON, the energy is drained, by redirecting it to the source.which is known as reverse braking.
Advantages :
This method is most suitable for regenerative braking in electric and hybrid electric vehicles.
Comparison with H-bridge:
H-bridge can operate only in 2 quadrants i.e. forward motoring and reverse motoring.
H-bridge is widely used in stepper motor. Ex:Robotic Arms for door fitting in car assembly unit.
DC7 is used to control the speed of DC motor.
The upper hand of DC7 chopper over H-bridge is its 4-quadrant operation.
H-bridges are simple circuits(switches alone w/o diodes,etc.),whereas DC7 is a bit complex in nature.
H-bridges can't create a pure sine wave, instead can generate a Modified sine wave.
power loss in H-bridge will be less compared to DC7 as its contains more Components comparatively.
Flexibility of control is more in DC7.
References :
1) https://www.build-electronic-circuits.com/h-bridge/
2)https://electricalworkbook.com/four-quadrant-chopper-type-e-class-e/
3) https://blog.digilentinc.com/what-is-an-h-bridge/
Q3)
A simple EV motor controller model is configured using the "DC7 block -Four-Quadrant Chopper 200hp DC drive".
The inputs required to the DC7 block is driving cycle, torque of the motor and the DC supply.
The work of the DC7 block is the chop the constant DC voltage to generate the variable DC Voltage, which is requred to run the motor at varying speed as required in the driving cycle.
The chopping of the DC supply voltage is achievd using pwoer switches like IGBT, MOSFET, etc.
Torque Equation : Input to this subsystem is the angular speed of the motor.
Output for this model is as below:
The supply voltage is set to 12.8 voltage. The On period of IGBT's 1&4 is alternative with 2&3, which is clearly shown in this graph.
This switching of the power switches allows the armature current to flow in the opposite direction, which helps in reducing the speed or increase the speed in the negative direction.
When the input voltage is increased to 440volts, the results are different as shown below:
Based on the drive cycle, the duty ratio of all the 4 IGBT is configured to get the required output.
For the same model,the input velocity can also be given using a slider gain block(as below), (or) default drive cycle block in simulink,etc.
output :
The duty ratio of the IGBT's is fluctuating, Since the slider gain was continuously increased to the maximum speed and reduced to its minimum speed.
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