Week-7 Challenge: DC Motor Control
1.A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.
B. Comment on the armature current shoot-up from the scope results.
Working
In the above circuit, the input voltage is generated from the pulse generator where the amount of supply and duty cycle is assigned in the pulse generator.
Then the switch block is connected to pulse generator through an armature voltage control block.The H - bridge consists of 4 BJT/ Diode pairs. Two transistors are switched simultaneously: Q1 and Q4 or Q2 and Q3.
When Q1 and Q4 are fired a positive voltage is applied to 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 motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.
A switch block at bottom of circuit splits the power supply into 4 parts namely are p1, p2, p3, and p4.
SWITCHES
Q1 and Q4 - FORWARD MOTORING
Q2 and Q3 - REVERSE MOTORING
The DC motor is connected to a battery, and the DC motor's mechanical input is Initiated as torque. Torque for mechanical input of DC motor also switched by goto and from.
The output of the DC motor is connected with the BUS sector whereas the speed is plotted in the scope. A conversion of speed for red/sec to rpm is between the BUS sector and the scope using the converter block
The pulse width was set at 75 in the pulse generator
In the current, the amount of circuit acted is shown as the graphical representation during the period of duty cycle. The scope shows the IGBT current and diode current passed in the quadrant Q3 and D3.
Where Q3 represent as IGBT current and D# is represented as Diode Current in scope
The graph shows that when the switch Q3 is OFF on the starting stage of current flow and diode current is on that stage.
The speed and armature current is obtained by the DC motor and Load torque is calculated by torque proportional to the square of speed.In the starting point, there is a sudden shoot-up in act.
This scope shows that speed is increased in a certain period of time and at that time armature current is positive whereas Load torque also moves forward.
After the time period of 0.5 sec which is given in pulse generator, sudden shoot up acted and speed gets decreased in that time armature current is in negative terminal whereas torque act as reverse action.Here the sudden shoot-up is acted while armature current is processed at a positive and negative terminal.
By opening the pulse generator we can change the pulse width i.e the input value to whatever desire. So here we change the value to 50%.
Here we see that the sudden shoot-up is not acted by comparing with 75 of the pulse width. same cycle time is followed in this experiment also.In this observation starting current is generated as diode current for time period of 0.5 sec, then IGBT current is acted while diode current is being stopped.
This graph shows when the switch Q3 is OFF on the starting stage of current flow and Diode current is ON at that stage.During the 0.5 sec of time D3 switch is ON that is given according to the pulse generator and after the time period of 0.5 sec D3 switch is OFF. An IGBT current is acted after 0.5 sec and Q3 switch remains ON.
There will be little variation as shown in graph at the starting point of current but the complete reduction of shoot up is not held here.
In this above scope, a flow of armature current is displayed and speed load torque is also displayed.The change is made in pulse width as 50% from the pulse generator and results are shown that voltage fluctuation is held in starting point of forwarding reverse state.
The sudden fluctuation is reduced in this experiment by reducing the pulse width of the input supply.
A change is also held speed and load torque where the constant level of speed and torque is acted in above scope compare with the previous 75% of the pulse width.A time period of forwarding and reverse action is maintained as same as the previous Duty cycle.In this observation, the shot up is reduced when motor changes direction from forward to reverse compared to 75% of pulse width.
C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
Four Quadrant Operation of DC motor means that the machine operates in four quadrants. They are forward motoring, Forward braking, reverse motoring and reverse braking.
QUADRANT 1
When the motor draws power from supply, the voltage and current are in same direction.Now the motor is said to be in motoring operation, its operation corresponds to the 1st quadrant.
In the 1st Quadrant power developed in positive and machine is working as motor supplying mechanical energy. The 1st quadrant operation is called Forward motoring.
QUADRANT 2
If there is a possibility of reversing the current direction keeping the voltage polarity same, the motor is said to be operating in 2nd quadrant.This happens when regenerative braking is applied to motor.
During regenerative braking operation, reverse power flow will happen i.e power will be feedback to supply system. @nd quadrant operation is known as braking.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 that opposes the motion.
The kinematic energy of the rotating part is available as electric energy which may be supplied back to mains. In dynamic braking energy may be supplied back to mains. In dynamic braking energy is dissipated in the resistance.
QUADRANT 3
If the voltage polarity and load current are reversed simultaneously, the motor operation will be in the opposite direction, but the power will be consumed by the load.The operation is corresponding the 3rd Quadrant and even it is said to be in motoring operation. The 3rd quadrant operation is known as reverse motoring.The motor works, in the reverse direction. Both the speed and torque have negative values while the power is positive.
QUADRANT 4
If the voltage polarity is reversed maintain the current direction source to load the power is reversed and the operation corresponding to 4th quadrant.Now the motor is said to be in reverse Regenerative operation.In the 4th quadrant, the torque is positive and speed is negative. This quadrant corresponds to braking in the reverse regenerative braking mode.
The 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 PL regulators.The converter is fed by a 515 V DC bus obtained by rectification of a 380V AC 50Hz voltage source.
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 output the armature current reference used by the current controller to obtain the electromagnetic torque needed to reach the desired speed.
The speed reference change rate follows acceleration and deceleration ramps to avoid sudden reference changes that could cause armature over current and destabilize the system.The current regulatory control the armature current by computing the appropriate duty ratio of the 5 Hz pulse of the four IGBT devices.
For proper system behavior, the instantaneous pulse values of IGBT devices 1 and 4 are opposite to those of IGBT 2 and 3.This generates the average armature voltage needed to obtain the desired armature current. To limit the amplitude of the current oscillation, a smoothing inductance is placed in series with the armature circuit.
FOUR QUADRANT CHOPPER DC DRIVE
1.Here the motor can be operated in 4 different modes.
2.Regenerative braking can be achieved.
3.The source is an AC supply which can be rectified to DC varying supply
4.The armature current doesn't shoot up to the value of BJT H-bridge but for low voltage value, it's negligible.
5.Better control of the vehicle
6.Signals are not continuous and they are discrete.
BJT H- BRIDGE DRIVE
1.It is used for changing the direction of the Motor.
2.Regenerative Braking cannot be available
3.The source is of fixed DC supply.
4.The armature current shoots up during the phase transition.
5.Less control of the Vehicle
6.Signals are continuous.
2.Q)Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.
1.PowerGUI
2.Pulse Generator3.DC Voltage Source
3.NOT Operator
4.Mosfet
5.Diode
6.Current Measurement
7.Series RLC Branch with only inductor
8.Voltage Measurement
9.Scope
From the result, we can see that voltage is never going to 0 and always lying on positive side of the axis
Current is increasing and decreasing and also lying in both positive and negative axes.
3.Explain in a brief about operation of BLDC motor.
DC motor which does not have brushes and controller provides current pulses to the motor winding which controls the torque and speed of the motor is BLDC.
In brushless motors, the problem of connecting current to the armature is overcome by permanent magnets rotating around a fixed armature. Over a vast speed range, these type of motors is very efficient in producing large amounts of torque
WORKING PRINCIPLE
There are only two basic parts in the BLDC motor they are stator and rotor. The stator is the resting part and contains windings the rotor is the rotating part and contains magnets. In BLDC permanent magnets are attached to the rotor and move the electromagnets to the stator.
To activate electromagnets for the shaft turns high power transistors are used. Using a solid-state circuit the controller performs power distribution.
Advantages of BLDC
1.More efficient because the velocity of BLDC is determined by the frequency at which the current is applied and does not depend on voltage.
2.Mechanical energy losses because of friction are less due to the absence of brushes.
3.Can operate at high speed under any condition.
4.Less noise and no sparking under any condition
5.For much precise control, more electromagnets can be used
Disadvantages of BLDC
1.Cost is higher compared to brushed dc motor
2.Only limited power could be supplied to the motor because the high amount of heat weakens the magnets and insulation may get damaged.