Week-7 Challenge: DC Motor Control

DC MOTOR CONTROL

AIM :

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

QUESTION 1:

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?

PROCEDURE :

• The very first thing is to import the model of Speed control of a DC motor using BJT H-Bridge.
• The imported model of this BJT H-Bridge mainly consists of the blocks DC motor, pulse generator, armature voltage control, IGBT transistor, diodes, scope for plotted results and other constant block and gain blocks to calculate the torque output of the motor.
• So in this model it mainly describes or output the armature current, load torque, speed and also currents of IGBT and diode.
• The working of this model is that a motor rotates in the forward direction and then in the reverse direction.
• So to decrease the peak change in the current we need to change the input signal generator's duty cycle to decrease the sudden shoot in the current.
• The model image is shown below :

The Armature current when the duty cycle % is set to 75% the graphs will be :

• So after changing the input signal generator to 45% of total supply voltage the peak shoot in the armature current will be normalized.
• If we need to switch the motor operations like forward braking, reverse braking we need to change the supply to switches and diodes according to multi quadrant operation of a DC motor.

OUTPUT :

• The change in the pulse signal is given in the below image :

• The output of the Armature current, speed and torque results were given below :
• So from changing the duty cycle to about 45% we can see the reduce in the Armature current shoot peak.
• If the duty cycle is go on increasing the output voltage coming from the bridge circuit to the motor will be getting equal to the input voltage of motor.
• And also the motor duty cycle % cannot be less than 30% as it decreases the ability of motor to have rotations.
  

QUESTION 2 :

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

EXPLAINATION :

CHOPPER DRIVE :

A chopper drive-based speed control is widely preferred as far as speed control is concerned. Choppers are mostly applied in electric cars, conversion of wind and solar energy, and DC motor regulators. A chopper uses the high speed to connect and disconnect from a source load. A fixed DC voltage is applied intermittently to the source load by continuously triggering the power switch ON/OFF. The period for which the power switch stays ON or OFF is referred to as the chopper’s ON and OFF state times, respectively.

A dc‐to‐dc converter, also known as dc chopper, is a static device which is used to obtain a variable dc voltage from a constant dc voltage source. Choppers are widely used in trolley cars, battery-operated vehicles, traction motor control, control of large numbers of dc motors, etc. They are also used as dc voltage regulators. Choppers are of two types: (1) Step‐down choppers, and (2) Step‐up choppers. In step‐down choppers, the output voltage will be less than the input voltage, whereas in step‐up choppers output voltage will be more than the input voltage.

The four-quadrant chopper is a chopper which can operate in all the four quadrants. The power can flow either from source to load or load to source in this chopper. In the first quadrant, a Class-E chopper acts as a Step-down chopper whereas in the second quadrant it behaves as a Step-up chopper. This type of chopper is also known as a Class-E or Type-E chopper. This article describes the working principle and operation of a Class-E chopper with the help of a circuit diagram.

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 vo = vs. It may be noted that the load current flows from source to load as shown by the direction of io.

When CH1 is switched OFF, the load current freewheels through CH4 and D2. During this period, the load voltage and current remain positive. Thus, both the output voltage vs and load current io are positive and hence, the operation of the chopper is in the first quadrant. It may be noted that Class-E chopper operates as a step-down chopper in this case.

SECOND QUADRANT OPERATION :

To obtain 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 turned OFF, the 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 the second quadrant operation of the chopper. Since the current is fed back to the source, this simply means that load is transferring power to the source.

THIRD QUADRANT OPERATION :

To obtain the third quadrant operation, both the load voltage and load current should be negative. The current and voltage are assumed positive if their direction matches with what shown in the circuit diagram. If the direction is opposite to what shown in the circuit diagram, it is considered negative. One important thing to notice is that the polarity of emf E in load must be reversed to have third quadrant operation. Circuit diagram of Class-E chopper for third quadrant operation is shown below.

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, the 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 the fourth quadrant. Here, power is fed back to the source from the load and the chopper acts as a step-up chopper.

The four-quadrant chopper drive is block diagram is given below :

Comparision of H-bridge model and Four-Quadrant Chopper DC Drive (DC7).

Sno	Four-Quadrant Chopper DC Drive (DC7)	H-bridge model
1	In this type of drive there are all possible ways of swithcing like forward motoring, reverse motoring, forward regenerative braking, reverse braking switch.	This can only be operates in forward motoring and reverse motoring only.
2	This cannot be operated by the signal generator.	This cab be operated with the signal generated.
3	The chopper switchings were mostly used in the industrial applications and other large equipmented, huge power drawn applications.	These are used in the recent electric vehicle applications.
4	Maintainance for the chopper drives will be more as these are controlled by many switches individually.	The maintainance for the H-bridge circuit is less than the chopper circuit as this is run by a signal modulation.
5	These will be used in the small machines such as robotic applications.	These needs more space to adjust but have a light weight considering in the whole.
6	This work more efficiently as the change in the frequency is more here.	Efficiency of H-Bridge is less than the Chopper circuit.
7	Managing in changing the signals were not as ease as in the H-bridge.	Ease of operation is better than the Four_Quadrant Chopper DC Drive.

QUESTION 3:

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

The model of an Electric vehicle is built as shown in the below image :

EXPLAINATION :

• In the above model of creating the Electric Vehicle model using DC7 or Four quadrant chopper DC drive we are using a direct DC voltage source.
• To measure the current flowing from the voltage source a current measurement sensor is attached to the voltage source.
• The reference for the speed is given as input to the chopper block.
• The chopper block is then connected to the IGTB transistor.
• This IGTB is connected to the scope and all the results will be plotted in the scope for the reference.

Inside the chopper drive there will be inputs and outputs which are DC motor, Converter, Controller.

OUTPUT :

• According to the given inputs as drive cycle, angular speed data and a DC voltage the plots were displayed in the scope.

• According to the given input of angular speed input upto 5 sec the motor speed is near to 0 and after 5 sec the motor speed is started to increase gradually where the simulation time is set to 20 seconds.
• The maximum duty cycle for IGBT 1&4, 2&3 is about 51%.
• The Armature Voltage is upto 150 v and the armature current is varyed -5 to 5.

CONCLUSION :

1. Sucessfully reduced the shoot up of armature current in the model of speed control of DC motor using BJT H-bridge.
2. Compared the variation between the H-bridge model and Four-Quadrant Chopper DC Drive (DC7).
3. Sucessfully changed the drive cycle and the motor torque characteristics in the DC7 model.

GOOGLE DRIVE LINK :

H-BRIDGE

DC7

USER-DRIVE-CYCLE

ANGULAR SPEED DATA