1.)

A.) Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

Answer:-

Matlab command:- power_Hbridge

Discription of above model:-

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.

Motor model selected:

Motor Parameter settings:

Settings of pulse generator for H- Bridge:

Now run this model:-

Simulation results for above model:-

1)IGBT and Diode Current:-

2) Omega, Armature Current and Torque

  B) Comment on the armature current shoot-up from the scope results.

Answer:-

In above result we can see that the armature current shhots up when the motor is changing the direction from froward to reverse. Torque is also becoming negative.

Now the armature current of DC motor can be given by 

Ia=(Va-Eb)/Ra          (=> Ohm's law I=V/R)

where Ia=armature current

          Va= Applied voltage

          Eb= Back emf

          Ra= Armature resistance

As we start the motor, full applied voltage will appear across the armature terminals and back emf will start increasing, hence applied voltage and back emf are opposite in nature(Va-(+Eb)). now when the motor is coming from forward to reverse then the applied voltage and back emf are same in nature(Va-(-Eb)). Now current is directly propotional to this hence While changing the direction the armature current increases because of change in the direction of back emf. Thats why the armature current shoots up while changinf the direction of motor.

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

Answer:-

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 PI regulators. The converter is fed by a 515 V DC bus obtained by rectification of a 380 V AC 50 Hz voltage source. In order 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 outputs the armature current reference (in p.u.) used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed. The speed reference change rate follows acceleration and deceleration ramps in order to avoid sudden reference changes that could cause armature over-current and destabilize the system. The current regulator controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the four IGBT devices (Pulse Width Modulation). For proper system behaviour, the instantaneous pulse values of IGBT devices 1 and 4 are opposite to those of IGBT devices 2 and 3. This generates the average armature voltage needed to obtain the desired armature current. In order to limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.

Comparision of Four quadrant chopper drive and H bridge :

Four Quadrant                                                                      H Bridge

1)Here the source is AC and that                                           1) Here the fixed DDC source.

is converted to DC.

2)Four different modes availble,                                             2) forward or reverse, changing the direction.

forward motoring, reverse motoring,

forward braking, reverse braking

3) Negligible armature current shoot up.                                 3) Armature current shoots up when changing the direction.

4) Batter control for vehicle                                                    4) Poor control for vehicle.

2)Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results. (Refer to article - multiquadrant operation of motor).

Answer:-

Working of two quadrant chopper(type c chopper):-

This is a two quadrant chopper whose operation is bounded between first and second quadrant of V_o – I_o plane. This type of chopper obtained by connecting type-A and type-B chopper in parallel

When CH₁ is ON current flows through abcdefa and inductor L will be charged. Hence output voltage V_o and current I_o both will be positive. When CH₁ is OFF, induction will discharge through D₁ and current I_o will flow through same direction with zero output voltage. So, we can see the operation of CH₁ is nothing but the operation of type-A chopper by which we can operate a chopper in the first quadrant.
When CH₂ is ON, output voltage V_o will be zero but output current I_o will flow in opposite direction of current shown in the figure and inductor will be charged up. When CH₂ is OFF Output voltage.

Which exceeds the value of source voltage V_S. So current flows through diode D₂ and treated as negative. Hence output voltage V_o is always positive and output current I_o is always negative here. We can see operation of CH₂ is nothing but operation of type-B chopper by which we can operate the chopper in the second quadrant.
We can conclude that the operation of type-C chopper is the combined operation of type-A and type-B chopper. This type of chopper is suitable for both forward motoring and forward braking operation.

Matlab Model:-

DC voltage sources:-

Pulse Generator:-

Result:-

3)Explain in a brief about operation of BLDC motor.

Answer:-

What is a BLDC Motor?

A Brushless DC Motor is similar to a Brushed DC Motor but as the name suggests, a BLDC doesn’t use brushes for commutation but rather they are electronically commutated. In conventional Brushed DC Motors, the brushes are used to transmit the power to the rotor as they turn in a fixed magnetic field. As mentioned earlier; a BLDC motor used electronic commutation and thus eliminates the mechanically torn brushes.

Construction of BLDC Motor

The main design difference between a brushed and brushless motors is the replacement of mechanical commutator with an electric switch circuit. Keeping that in mind, a BLDC Motor is a type of synchronous motor in the sense that the magnetic field generated by the stator and the rotor revolve at the same frequency.

Brushless Motors are available in three configurations: single phase, two phase and three phase. Out of these, the three phase BLDC is the most common one.The following image shows the cross-section of a BLDC Motor.

 

Stator

The structure of the stator of a BLDC Motor is similar to that of an induction motor. It is made up of stacked steel laminations with axially cut slots for winding. The winding in BLDC are slightly different than that of the traditional induction motor.

Generally, most BLDC motors consist of three stator windings that are connected in star or ‘Y’ fashion (without a neutral point). Additionally, based on the coil interconnections, the stator windings are further divided into Trapezoidal and Sinusoidal Motors.

In a trapezoidal motor, both the drive current and the back EMF are in the shape of a trapezoid (sinusoidal shape in case of sinusoidal motors). Usually, 48 V (or less) rated motors are used in automotive and robotics (hybrid cars and robotic arms).

Rotor

The rotor part of the BLDC Motor is made up of permanent magnets (usually, rare earth alloy magnets like Neodymium (Nd), Samarium Cobalt (SmCo) and alloy of Neodymium, Ferrite and Boron (NdFeB). Based on the application, the number of poles can vary between two and eight with North (N) and South (S) poles placed alternately. The following image shows three different arrangements of the poles. In the first case, the magnets are placed on the outer periphery of the rotor.

The second configuration is called magnetic-embedded rotor, where rectangular permanent magnets are embedded into the core of the rotor. In the third case, the magnets are inserted into the iron core of the rotor.

Position Sensors (Hall Sensors)

Since there are no brushes in a BLDC Motor, the commutation is controlled electronically. In order to rotate the motor, the windings of the stator must be energized in a sequence and the position of the rotor (i.e. the North and South poles of the rotor) must be known to precisely energize a particular set of stator windings.

A Position Sensor, which is usually a Hall Sensor (that works on the principle of Hall Effect) is generally used to detect the position of the rotor and transform it into an electrical signal. Most BLDC Motors use three Hall Sensors that are embedded into the stator to sense the rotor’s position.

The output of the Hall Sensor will be either HIGH or LOW depending on whether the North or South pole of the rotor passes near it. By combining the results from the three sensors, the exact sequence of energizing can be determined.

Working Principle

Consider the following setup of three windings in the stator designated A, B and C. For the sake of understanding, let us replace the rotor with a single magnet.

 

We know that when a current is applied through a coil, a magnetic field is generated and the orientation of the field lines i.e. the poles of the generated magnet will depend on the direction of the current flowing through the coil.

Using this principle, if we supply current to the coil A so that it will generate a magnetic field and attract the rotor magnet. The position of the rotor magnet will shift slightly clockwise and will align with A.

If we now pass current through coils B and C one after the other (in that order), the rotor magnet will rotate in clock wise direction.

To increase efficiency, we can wind the opposite coils using a single coil so that we get double attraction. Further increasing the efficiency, we can energize two coils at the same time so that one coil will attract the magnet and the other coil will repel it. During this time, the third will be idle. For a complete 360⁰ rotation of the rotor magnet, six possible combinations of the coils A, B and C are applicable and are shown in the following timing diagram. To increase efficiency, we can wind the opposite coils using a single coil so that we get double attraction. Further increasing the efficiency, we can energize two coils at the same time so that one coil will attract the magnet and the other coil will repel it. During this time, the third will be idle. For a complete 360⁰ rotation of the rotor magnet, six possible combinations of the coils A, B and C are applicable and are shown in the following timing diagram.

 

Advantages of BLDC Motors

Since BLDC Motors are electronically commutated, there are several advantages over traditional Brushed DC Motors. Some of them are:

• No wear and tear (due to absence of brushes)
• High efficiency
• Better speed vs torque characteristics
• Long life
• Less noise or noiseless operation
• Significantly higher RPM

Applications of Brushless DC Motors

Some of the areas of applications of BLDC Motors are mentioned below:

• Single speed applications
• Adjustable speed applications
• Position control
• Low noise applications
• High speed applications