Question 2

Basic two quadrant chopper

In this we will always get a positive output voltage V0 as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage v0 will be equal to Vs . The direction of the load current i0 will be reversed. The current i0 will be flowing towards the source and it will be positive regardless the chopper is on or the FD conducts. The load current will be negative if the chopper is or the diode D2 conducts. We can say the chopper and FD operate together (as type-A chopper) in first quadrant. In the second quadrant, the chopper and D2 will operate together (as type –B chopper).

The average voltage will be always positive but the average load current might be positive or negative. The power flow may be life the first quadrant operation that is from source to load or from load to source like the second quadrant operation. The two choppers should not be turned on simultaneously as the combined action my cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.

Now we developed a Simulink model similar to this. Simulation time is taken as 1 second.

A pulse generator is taken as input voltage (with duty cycle)

Configuration of input pulse is

MOSFET are used as switches in the model. MOSFET has high switching frequency. The main advantage of this drive is. The design of this drive is simple and it can operate in 2 quadrants. By using MOSFET switching frequency is also increases.

The results are

Question 3

BLDC Motor

• BLDC stands for brushless DC motor.
• BLDC motor is also known as electronically committed or a DC synchronous motor.
• The motor is designed to produce high torque and speed.
• This motor offers long operating life due to the lack of brushes and commutator.

Types

1. Inner rotor type BLDC motor

In this motor rotating part is present in centre

2. Inner rotor type BLDC motor

In this motor stationary part is present in centre, by the help of magnetic pieces rotating part is designed.

Construction

• This is a inner rotor type BLDC motor
• stator is stationary and copper winding placed in BLDC.
• Connections of each phase are separate to one another. As you can see 3 separate phases (A,B,C).
• Rotor is made of permanent magnet. We don’t use brush and commutator in BLDC. Without brush and commutator we cannot give supply. That’s why permanent magnet is used as rotor in BLDC.

Working and operating principle

• One by one phase will energise for continuous rotation.
• First of all we energise the coil A Since the rotor is permanent magnets its polarity is constant.
• Here stator N pole will attract the rotor S pole and vice versa.
• Then rotor will align in such direction as shown in the figure.
• Then phase A is de-energised and we energize phase B.
• Here also starter poles will attract rotor opposite poles.
• And then after Phase B is de-energised and phase C will energise, here also starter poles will attract rotor opposite poles

• Now after completing half rotation we change the terminals to the opposite polarity. And we repeat the process again.
• If we don’t change the polarity after 180 degrees rotation then rotor will not rotate in same direction.
• Because phase A’ has South Pole and also South Pole is present at the portion of rotor. So instead of attraction repulsion will takes place.
• Thus, BLDC completes one revolution in this way.
• But it is not possible to change the polarity manually and we don’t know which phase to energise and when, while the rotor is in rotation.
• To overcome this, an external device in is introduced that is hall sensors. It’s sensors the rotor position and energizes the stator winding separately.

Final diagram of BLDC with hall sensors

Question 1 A

H Bridge is a circuit used to apply voltage to a load in either direction. In our case the load is DC motor.

The simulation is developed based on this circuit.

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 Q2 and Q3 are fired, Q1 and Q4 are switched off and a negative voltage is applied to the motor and diodes D1-D4 operate as free-wheeling diodes

And again the cycle repeats.

Modes :

Q1 and Q4 are Forward motering mode Q2 and Q3 are Reverse motering mode

In the simulation, it shows the simulation of H-bridge used to generate a chopped voltage and to control the speed of a DC motor. DC motor is connected to battery and the DC Motors mechanical input is initialised as torque. The output of the DC motor is connected with a bus sector and the speed is plotted in the scope.

1B

Armature current :

in the given model pulse width is 75%

At first there is a sudden rise in armature current, speed is increased and torque is also increased and moved forward.

After 0.5 sec there is a sudden down in armature current which is followed by speed and torque that is speed is reduced and torque is also reduced.

Now run thesimulation with pulse width with 50%

In this result we observe that there is no sudden rise in armature current at the beginning and comparatively the speed and Torque is also more stable now.

1C

Four quadrant operation of chopper DC Drive (DC7)

This machine operates in four quadrants.

• Forward motoring mode.
• Forward breaking mode.
• Reverse motoring mode.
• Reverse breaking mode.

Quadrant 1

In first quadrant power developed in positive and machine is working as motor. Here torque, speed and power all are in positive direction. This is known as forward motering mode

Quadrant 2

This mode of operation is done only when regenerative breaking is applied to the motor. Here speed is positive but both torque and power is negative.

Quadrant 3

If the voltage polarity and load current is reversed then the motor operates in opposite direction but the power will be consumed by the load. Here speed is negative and the torque is negative, but the power is positive. This is known as reverse motoring mode.

Quadrant 4

If we operate the motor by reversing the voltage polarity then the motor is said to be in reverse regenerative operation. This is known as reverse breaking mode.

s.no	H- bridge	DC 7
1	4 modes of operation cannot be achieved	4 modes of operation can be achieved
2	Regenerative breaking cannot be achieved	Regenerative breaking can be achieved
3	The armature current rises suddenly at the beginning	Armature current rises low compared to H bridge
4	Source is of dc supply	Source is of ac supply and then it is rectified to dc supply by using ac to dc converter