Power Electronics Experiments No 6-11

 Experiment No. 6

Objective

To Study the single phase Cyclo converter

Objectives

1. To study about the Cycloconverter

2. To plot the output responses of Cycloconverter

Apparatus Required

Sl.no	Name of components	Rating /Ranging	Quantity
01	1- Ф fully Cycloconverter  (kit)		01
02	DC motor	1HP, 1-Ф ,230V,10A	01
03	Tachometer	0-6000rpm	01
04	Connecting wires	0-5/10A	10
05	Multimeter	0-300/600V	01

Theory

Cycloconverters are basically frequency conversion devices and transform higher frequencies to lower frequencies. The alternating voltage at supply frequency is converted directly to lower frequency without any DC intermediate stage. The advent of thyristors and sophisticated transistorized control circuitry have made them very popular for industrial applications. The cyclo converter consists of a number of phase controlled rectifier circuits connected to an AC supply system, which provides the voltage necessary for delayed phase commutation. The individual circuits (peak based) are controlled so that a low frequency output voltage waveform is from segment of i/p voltages. The power circuit of a simple single phase cycloconverter is shown in fig 1. SCR1 & SCR2 comprise the positive going converter and SCR3 & SCR4 comprise the negative going converter. Thus if SCR1 and SCR2 are activated for one cycle of input voltage and SCR3 & SCR4 for next cycle, a 2/1 frequency reduction can be realized as shown in fig 2 waveforms. In a similar fashion it is possible to obtain an output which is 3/1, 4/1 of the  i/p frequency. This is necessary with more complex schemes involving modulation of triggering angles. With large frequency ratios the advantage of more complex schemes is that less filtering is required for given harmonic distortion in the output waveforms. The logic necessary 1to produce the correct pulse sequence to each SCR gate for each operating frequency is explained as follows.

All the gate signal trains are exactly synchronized to the input signal and also that each gate signal should appear at approximately the zero crossing of the i/p signal. The natural starting point for the logic is the o/p of the zero voltage switch. A block diagram is shown in fig 1. The necessary waveforms of the gate signals for 2/1 frequency reduction are also shown in fig 2. All the fate signals are repeated after 4 inputs half cycle. (I.e. after 4 pulses from the zero voltage switch). The logic for frequency should be driven by a synchronous divide by four converters using zero voltage switch as the clock signal.

Procedure

1)  Switch on the mains supply but keep Power circuit supply in ‘off’ condition. It placed below SW3. Then You can observe the waveforms of clock. It should be 100 Hz square wave signal.

2)  Then select Sw2 and Sw1 as 0 and 1 respectively. So we have selected Sequence for 2/1. Then observe the waveforms at TP1 to TP4.It should be square wave pulse.

3) Thus you can change the selection of SW2 and Sw1 for different sequences and watch the waveforms at Tp1 to Tp4.

4) Now you switch ‘on’ the Power supply for Power side. Then watch the waveform at test point marked as output. Also a light of 24V, 30 W will start glowing. You will get correct frequency division when clock circuit will synchronize correctly with incoming 50Hz A.C. signal.

5) In case if you do not correct waveform at output ,press Sw3 push button Till you get correct synchronized waveform.

6) Observe output waveform and compare it with incoming 50Hz A.C. signal.

Circuit Diagram

Waveforms

Results

Viva Questions

1.    What is Cyclo-converter?

2.    What are the applications of cycloconverter?

3.    What are basic components of cycloconveter?

…

Experiment No. 7

Objective

To conduct the performance for Jone’s Chopper and Morgan’s Chopper

Objectives

1. To study about the Jone’s Chopper and Morgan’s Chopper

2. To plot the output responses for Jone’s Chopper and Morgan’s Chopper

Apparatus Required

Sl.No	Name of components	Quantity
01	Jone’s /Morgan’s Kit	01

Theory

JONE’S CHOPPER

INTRODUCTION:

A chopper can be considered as a switch used to obtain a variable DC voltage from a source of constant DC voltage. With the advancement in semiconductor technology, transistors s used as active devices have been replaced by the static switches and the thyristorised chopper circuits have gained importance in industrial applications. Choppers are also terms as DC. to DC controllers or time ratio controllers. T.R.Cs. are used to obtain, from a fixed DC input voltage, a variable DC voltage.

OPERATION PRINCIPLE:

The Jone’s Chopper circuit is based on class (D) commutation method. The auxillary thyristor is used to switch a charged capacitor across the conducting thyristor. With this additional thyristor T (ON) and T(OFF) can be independently controlled. Refer to circuit diagram (figure No. 2). The thyristor TH2 may be triggered first to charge the commutating capacitor C to the polarity marked in the figure. WhenTH1 is triggered , the charged capacitor C discharges around the path C ,TH1,L1, D1 and gets charged in the opposite direction. The diode D1 prevents the opposite current flow in the series circuit. When TH2 triggered, reverse charged capacitor will be connected across the thyristor TH1 and turn it off. If TH1 is triggered before the capacitor C is charged , then the load current in L2 will induce sufficient voltage of proper (i.e. opposite ) polarity , to charge C negatively. So it becomes immaterial which thyristor is triggered first.

FREQUENCY CONTROL:

It may be noted that UJ 2N2646 marked as T1 in the circuit diagram works as a relaxation oscillator. The potentiometer marked as frequency control varies the R-C time constant and generates a variable frequency source. The pulse transformer is used to couple the firing pulses to the main thyristor TH1.

ON TIME CONTROL:

The UJT marked “T2” is used to control the ON time of the main thyristor TH1, through the relaxation oscillator formed by potentiometer (ON TIME CONTROL) and capacitor C2. The circuit is supplied the DC voltage only when the main thyristor TH1 is turned ON. As soon as the DC supply is available, the relaxation oscillator starts functioning. It must be noted that if ON time is longer than the period for frequency control circuit , the main thyristor will be always ON and proper control is not possible. Likewise , if the ON time is very small , the time required for the capacitor C to get oppositely charged (upper plate –ve) is very small and hence this prevents firing of TH2. Thus the main thyristor TH1 will remain continuously ON.

Procedure

1. Study the set-up. Locate the circuit blocks and try to identify the various circuit elements , with the help of circuit diagram (Fig.2).

2. Ensure the potentiometers marked as “FREQUENCY CONTROL” and “ON TIME CONTROL” are fully in the anti clockwise position (i.e. minimum position).

3. Turn on the main supply. The LED should glow. Ensure 24 volt, 20 watt lamp is in its right place.

4. Place a patch cord across the terminals marked “LINK”.

5. By controlling the frequency control knob, observe that the lamp intensity can be controlled.

6. Keep frequency control in minimum position and operate ON TIME CONTROL knob. We can again observe the intensity variation of lamp i.e. load voltage variation.

7. Observe the waveforms at TP1, TP2 and across the load for these operations.

8. If frequency is increased for fixed on time control, observe that there can be commutation failure for the main thyristor. The lamp should glow with full intensity and no control is possible . Observe the same for increased on time. Under such conditions, remove the LINK momentarily and adjust the controls so that the normal operation is obtained.

JONES CHOPPER WITH R-L LOAD.

If Inductance is to be connected ,connect the same across points A & B with the help of patch cords .Observe waveforms across terminals B and ground.If inductance is not required ,short terminals A & B. Lamp load is permanently connected internally across terminals B and ground.

Circuit Diagram

Waveform

MORGAN’S CHOPPER

INTRODUCTION:

In class B turn off , a reverse voltage is applied to the thyristor by the over swinging of an under damped LC circuit connected across the thyristor. The turn off time is decided by the period of the LC circuit, the load impedance being resistive. The circuit impedance between the load and the DC supply is low. Condenser C charges up to the supply voltage before the trigger is pulse is applied to the gate of the thyristors . When the thyristor is triggered, two currents flow, a load current through the LC circuit and the thyristor in the opposite direction. This resonant current tends to turn off the thyristor. Morgan’s circuit is an example of class B turn off When the thyristor is to be triggered, a capacitor C is charged to the d.c. voltage Edc with upper plate positive. As soon as the thyristor starts to conduct, C discharges through it. Thyristor therefore conducts two components of current, one the load current and the other current of the

resonant circuit, without practically an damping resistance.

The circuit equation for the resonant circuit is given below:

The thyristor turns off at the instant (practically) when I(t) = -iL

The thyristor will fail to commutate if iL > = Edc √ C/ L

Refer fig. (A) For explanation.

When the thyristor turns off, capacitor has a negative charge. The capacitor then charges to a positive value through the load R,L and C forming damped resonant circuit.

INFORMATION OF THE SET UP:

The thyristor TH1 is triggered with the help of a relaxation oscillator output made up of UJT and potentiometer P1 and pulse transformer PT. The series resonant circuit is formed with the help of a variable capacitance and variable inductance L. The capacitance can be varied by making use of the jumpers as shown in figure (1). Appropriate test points are provided across TP1,TP2 and TP3.The CRO should be connected across TP1,TP2 or TP3 for observation of the waveforms.

Procedure

1. Initially study the set up and identify for various components and their locations.

2. Ensure that frequency control potentiometer P1 is in the most anti clockwise direction i.e. minimum position. Keep P2 in minimum position.

3. Place a link across the terminals marked “LINK” so that d.c. supply is made available to the Morgan’s chopper circuit.

4. Connect the CRO terminals across the output test points TP3 and observe the waveforms. Plot the waveforms on the graph paper.

5. Connect a d.c. ammeter (500 mA) across the terminals marked “LINK” taking into account appropriate polarities.

6. Keep the value of C as 10mfd Measure the value of inductance that is being used. Keep the frequency control constant. By varying the load current, verify that thyristor will fail to commutate if I L > Edc √ C/ L Verify this for various values of C and L.

7. Study the waveform of voltages across capacitance (TP1) and across inductance (TP2) and across the load.

NOTE THAT IN MORGAN’S CIRCUIT THERE IS NO POSSIBILITY OF CONTROLLING THE ON TIME OF SCR INDEPENDENTLY

Circuit Diagram

Waveform

Results

Viva Questions

1. What are choppers?

A dc chopper converts directly from dc to dc and is also known as dc-dc converter.

2. What does a chopper consist of?

It can be a power transistor, SCR, GTO, power MOSFET, IGBT or a switching

device.

3. On what basis choppers are classified in quadrant configurations?

The choppers are classified depending upon the directions of current and voltage flows.

These choppers operate in different quadrants of V-I plane. There are broadly following

types of choppers: class a chopper (first quadrant); class B (second quadrant)

Class C and class D (two quadrant choppers), class C in II quadrant and I whereas class D

in IV quadrants, and I class E is four quadrant operator.

4.What are different control strategies found in choppers?

The different control strategies are pulse width modulation, frequency modulation and

current limit control, variable pulse width and frequency.

5.Explain the principle of operation of a chopper?

A chopper acts as a switch, which connects and disconnects the load, hence producing

variable voltage.

6.What are the advantages of DC choppers?

* High ripple frequency, so small filters are required.

*Power factor is better.

*Efficiency is better.

*Small and compact.

*The dynamic response of choppers is fast due to switching nature of the device.

Dept of EC 46 Dr Ambedkar Institute of Technology

Power Electronics Lab Manual VII Sem EC

7.Define duty cycle.

The duty cycle of chopper controls its output voltage. The value of duty cycle lies

between 0 and 1 and is given by Ton/(Ton+Toff).

8.How can ripple current be controlled?

Ripple current is inversely proportional to the frequency and hence can be controlled by

having higher frequency.

9.What is step up chopper?

If the output average voltage is greater than the supply voltage, then the chopper is called

step up chopper.

10.On what does the commutating capacitor value depend on?

It depends on the load current.

11.What are the disadvantages of choppers?

*They can operate only at low frequencies.

*The commutation time depends on the load current.

*The output voltage is limited to a minimum and maximum value beyond which we

cannot get the output voltage.

12.How do they have high efficiency?

DC choppers uses switching principle, hence they have high efficiency.

13.What are the applications of dc choppers?

Battery operated vehicles, switched mode power supplies, traction devices, lighting and

lamp controls, trolley cars, marine hoists, and forklift trucks. Mine haulers etc.

…

Experiment No. 08

Objective

Single phase / three phase Inverter with Resistive/Induction Motor load.

Objectives

1. To study the characteristics of single phase fully controlled converter

2. To plot the responses

3. To study about IGBT and PWM inverter

4. To plot the output waveforms for IGBT based PWM inverter.

5. To study the operation principle of IGBT based PWM inverter

6. To plot the response of three phase IGBT based PWM inverter

7. To compare the results for single phase and three phase IGBT based PWM inverters

Apparatus Required

S.NO	ITEM	RANGE	TYPE	QUANTITY
1	Fully controlled Converter Power circuit kit	1f, 230V,10A	-	1
3	SCR firing circuit kit	1F ,230V,5A	-	1
4	Isolation Transformer	230V/115-55-0-55-115	-	1
5	Auto-transformer	230V/0-270V, 4A	-	1
6	Loading Rheostat	100W / 2A	-	1
7	CRO	20MHz	-	1
8	Patch chords	-	-	15

Theory:

The single phase fully controlled rectifier allows conversion of single phase AC into DC. Normally this is used in various applications such as battery charging, speed control of DC motors and front end of UPS (Uninterruptible Power Supply) and SMPS (Switched Mode Power Supply).

All 4 devices used are thyristors. The turn-on instants of these devices are dependent on the firing signals that are given. Turn-off happens when the current through the device reaches zero and it is reverse biased at least for a duration equal to the turn-off time of the device specified in the data sheet.

            When an uncontrolled (diode) converter is to be simulated, all 4 devices should be fired at a delay angle of 0⁰. When a semi-converter is to be simulated, the lower two devices can be fired at 0⁰ and 180⁰ respectively and the upper two devices are fired at α and 180⁰+α.

         Normally in a fully controlled converter, the current transfer takes place from T1 to T2 instantaneously without  any time delay. But when a source inductance is present , the stored energy in Ls has to be expended before the current transfer or commutation takes place from T1 to T2. Because of this, T1 and  to T2 will conduct simultaneously from α to α+u (where u is the overlap angle) causing short circuiting of the DC load during the overlap period u. This is known as commutation overlap.

Procedure

1. Make the connections as per the circuit diagram..
2. Keep the multiplication factor of the CRO’s probe at the maximum position.
3. Switch on the thyristor kit and firing circuit kit.
4. Keep the firing circuit knob at the 180 ° position.
5. Vary the firing angle in steps.
6. Note down the voltmeter reading and waveform from the CRO.
7. Switch off the power supply and disconnect.

Circuit Diagram

 

                                                                            

                                                                                                          

 Tabular Column:

S.No.	Firing angle a (degree)	Output voltage Vo  (volts)	Non conducting Period ‘ t ’ sec	Observed angle ao (degree)

FORMULA :

                                                       t

Firing angle     α₀ = --------- x 180

                                                       10ms

t= non-conducting period of thyristor.

Nature of Graph

 

in

                                            2p

                  

              p                                                              3p                          t

                                                                             

V_T

                                                                                                                      

     

                                                                                                                          

            p                              2p                    3p                                         t

                                                                                                                         

Result

Viva Questions

1 .What are converters?

2. Explain the working principle of single phase fully controlled Converter?

3. What ate the applications of single phase fully controlled Converter?

Single Phase inverter

Theory

            It consists of four IGBTs S1, S2, S3, S4 and four inverse parallel diodes D1, D2, D3, D4. The diodes are essential to conduct the reactive current, and thereby to feed back the stored energy in the inductor to the DC source. These diodes are known as feed back diodes. For many industrial applications the output AC voltage of the inverter must be sinusoidal in shape and the amplitudes and frequency must be controllable. This is achieved by PWM of the inverter switches.

            The switching sequence of the inverter switches in this case, is obtained by comparing a sinusoidal control signal, of adjustable amplitude and frequency with a fixed frequency triangular carrier. The frequency of the triangular carrier wave determines the switching frequency of the inverter switches. The frequency of the sinusoidal control signal decides the fundamental frequency of the inverter output voltage, and is also called the modulating frequency. The sinusoidal pulse width modulation can be programmed to have either bipolar voltage switching or unipolar voltage switching. The unipolar voltage switching has the advantage of effectively doubling the switching frequency as compared to the bipolar voltage switching.

 Procedure

1. Ensure that the circuit breaker and pulse release ON/OFF toggle switch are in OFF position.

2. Connect the R-L load across the output terminals Lo and No provided in the front panel. Include an ammeter to measure the current and voltmeter to measure the voltage.

3. Connect an AC input at the input terminals L and N provided in the front panel.

4. With the pulse ON/OFF switch and circuit breaker in OFF condition give the power to the inverter module. This will ensure the control power supply to all the control circuitry.

5. Set the amplitude of the reference sine wave to minimum value.

6. Keeping the pulse release ON/OFF switch in OFF position, switch ON the power supply to the bridge rectifier.

7. Release the gating signals to the inverter switches by turning ON the pulse release ON/OFF switch.

8. Observe the triangular carrier and the reference sine waveforms on the CRO. Measure the amplitude and the frequency of the triangular carrier through CRO and note it down. Adjust the sine wave frequency to about 50Hz.

9. Connect the CRO probes to observe the load voltage and current waveforms.

10. Observe the load voltage and load current waveforms. Sketch the waveforms on a graph sheet to scale for one cycle period of the inverter output frequency. Measure the amplitude of the voltage pulses.

11. Measure the output voltage either by using a digital multimeter.

12. Calculate the modulation index ma and the rms output voltage Vo.

13. Increase the amplitude of the reference sine wave and note down its value.

14. Repeat steps 8 to 13 for various amplitude of reference sine wave and tabulate the readings. Plot the characteristics of modulation index versus output voltage.

Circuit Diagram

 

Tabulation

S.no	Output voltage (v)	Time (ms)

FORMULAE USED:

            ma = Vsine / Vtri

            Vo = ma x Vs

            Where,

            ma = Modulation index ,Vsine = Amplitude of the sine wave ,Vtri = Amplitude of the triangular wave , Vo = Output voltage ,          Vs = DC supply voltage.

           

Nature of the Grap

 

 Result

           

 Viva Questions

             

 1. What is the function of an inverter?

 2. What are the different types of inverters?

 3. Why thyristors are not preferred for inverters?

 4. What is meant by PWM control?

 5. What are the different types of PWM control?

 6. What are the advantages of PWM control?

Three Phase inverter           

Theory

            The most frequently used three phase inverter circuit of three legs, one for each phase.

 For this configuration, output transformer is not required. Also, this circuit uses six IGBTs. The   inverter configuration is also termed as six step bridge inverter. In inverter terminology, a step is defined as a change in the firing from one IGBT to the next IGBT in proper sequence. For one cycle of 360°, each step would be of 60° for a six step inverter. This means that IGBT’s would be gated at regular intervals of 60°.

            Basically, there are two possible schemes of gating the IGBT. In one scheme, each IGBT conducts for 180° and in the other scheme, each IGBT conducts for 120°. In 180° mode operation, pair in each leg, i.e. T1, T4; T3, T6; and T5, T2 are turned ON with the time interval of 180°. It means that IGBT T1 conducts for 180° and IGBT T4 for the next 180° of a cycle. IGBTs in the upper group, i.e. T1, T3, T5 conduct at an interval of 120°. It means that if IGBT T1 is fired at 0°, then T3 must be triggered at 120° and T5 at 240°. Same is true of lower group of IGBT.

Procedure

             

1. Make the connection as per the circuit diagram.
2. Connect the gating signal from the inverter module.
3. Switch ON D.C 24 V.
4. Keep the frequency knob to particulars frequency.
5. Observe the input and output waveforms for 180° conduction mode and 120° conduction mode in the CRO.
6. Obtain the output waveform across the load Rheostat. 

7.    Circuit Diagram:

	LAMP        LOAD

Tabular Column:                                                                                                                            

S.No.	Output Voltage (V)	Time (ms)

Waveforms

Result

Viva Questions

 1. What is the use of three phase inverter?

 2. Define step.

 3. What are the different conduction methods of three phase inverter?

 4. What is the function of capacitor connected at the input terminal of an inverter?

             

5. What is the function of feedback diodes in an inverter?

6. What is the switching sequence for three phase inverters in 180° conduction?

Experiment No. 09

Objective

Simulation of Converter / Chopper using SPICE/MATLAB.

Objectives

1.To study about the Three phase Thyristor

2. To simulate the output waveforms of Three Phase Thyristor Converter.

3.To study the opertion of a chopper

4. To simulete Chopper fed DC motor Drive for continuous and Discrete power supply.

5.To plot the responses

Apparatus Required:

MATLAB tool

Theory:

Converter:

Three phase full converter is a fully controlled bridge controlled rectifier using six thyristors connected in the form of a full wave bridge configuration. All the six thyristors are controlled switches which are turned on at a appropriate times by applying suitable gate trigger signals.

The three phase full converter is extensively used in industrial power applications upto about 120kW output power level, where two quadrant operations is required. The figure shows a three phase full converter with highly inductive load. This circuit is also known as three phase full wave bridge or as a six pulse converter.

            The thyristors are triggered at an interval of (∏/3) radians (i.e. at an interval of 30°). The frequency of output ripple voltage is 6f_s and the filtering requirement is less than that of three phase semi and half wave converters.

            

Procedure:

1.    Open MatLab  click on Simulink icon

2.    Go to Help è Simulink Help è Demos è simulink èSim Power Systems

3.    Click on Power Electronic Models

4.    Double click on Three Phase Thyristor Converter

5.    Run the simulation and observe the waveform

Circuit Diagram:

 

Expected Waveform:

Results:

Viva Questions:

1. Define convrter?

2. Explian 3 phase thyristor converter

3. What are the application of converter

Theory:

Chopper Introduction:

A chopper is a static device that converts fixed dc input voltage to a variable dc output voltage directly. A chopper is considered as dc equivalent of an ac transformer since it behaves in an identical manner. The choppers are more efficient as they involve in one stage conversion. The choppers are used in trolley cars, marine hoists, forklift trucks and mine hauler. The future electric automobiles are likely to use choppers for their speed control and braking. The chopper systems offer smooth control, high-efficiency, fast response and regeneration. The chopper is the dc equivalent to an ac transformer having continuously variable turns ratio. Like a transformer, a chopper can be used to step down or step up the fixed dc input voltage.

Principle of Operation:
            A chopper is a high-speed on/off semiconductor switch. It connects source to load and disconnects the load from source at high-speed. In other words, the principle of chopper is application of fixed dc voltage intermittently to the load. This is achieved by continuously triggering ON and triggering OFF the power switch (SCR) at rapid speed. The duration for which the SCR remains in ON and OFF states are called ON time and OFF time respectively. By varying the ON time and OFF time of the SCR, the average voltage across the load can be varied. The terms DC–DC converters and choppers are one and same. In the texts usually these terms are interchanged. The Choppers can be operated in either a continuous or discontinuous current conduction mode. They can be built with and without electrical isolation.

Applications of Choppers:

·         They are used for DC motor control (battery-supplied vehicles), solar energy conversion and wind energy conversion. 

·         Choppers are used in electric cars, airplanes and spaceships, where onboard-regulated DC power supplies are required.

·         In general, Chopper circuits are used as power supplies in computers, commercial electronics, electronic instruments.

Classification of Choppers:

(a) Depending upon the direction of the output current and voltage, the converters can be classified into five classes namelyClass A [One-quadrant Operation]
Class B [One-quadrant Operation]
Class C [Two-quadrant Operation]
Class D

(b) Based on the output voltage of the output, the choppers are classified as

(i) Step-Down Chopper
In this case the average output voltage is less than the input voltage. It is also known as step down converter

(ii) Step-Up Chopper
Here the average output voltage is more than the input voltage. It is also known as step up converter

(iii) Step-Up/Down Chopper
This type of converter produces an output voltage that is either lower or higher than the input voltage

(c) Depending upon the power loss occurred during turn ON/OFF of the switching device, the choppers are classified into two categories namely

(i) Hard switched Converter
Here the power loss is high during the switching (ON to OFF and OFF to ON) as a result of the non zero voltage and current on the power switches.

(ii) Soft switched or resonant converters
In this type of choppers, the power loss is low at the time of switching as a result of zero voltage and/or zero current on the switches.

Procedure:

1.    Open MatLab  click on Simulink icon

2.    Go to Help è Simulink Help è Demos è simulink èSim Power Systems

3.    Click on Power Electronic Models

4.    Double click on Chopper Fed DC motor Drive

5.    Run the simulation and observe the waveform.

Circuit Diagram:

Circuit Description

The DC motor is fed by the DC source through a chopper which consists of GTO thyristor and free-wheeling diode D1. The motor drives a mechanical load characterized by inertia J, friction coefficient B, and load torque TL. The hysteresis current controller compares the sensed current with the reference and generates the trigger signal for the GTO thyristor to force the motor current to follow the reference. The speed control loop uses a proportional-integral controller which produces the reference for the current loop. Current and Voltage Measurement blocks provide signals for visualization purpose.

Expected Waveform

Expected Waveform:

Result:

Viva Questions:

1.    What is the Difference between Chopper and transformer?

2.     What are the applications of Chopper?

3.    Classify chopper

4.    Explain the principle of operation of Chopper.

Experiment No. 10

Objective

Simulation of PWM Inverter using SPICE/MATLAB.

Objectives

1.To study about single phase half bridge and fulll bridge PWM inveters

2. To study about  three phase three level PWM inverter

3. To simlate the output Waveforms for single phase and three phase PWM inverters

Apparatus Required:

MATLAB Package

Theory:

Single-phase voltage source half-bridge inverters: The single-phase voltage source half-bridge inverters are meant for lower voltage applications and are commonly used in power supplies. Low-order current harmonics get injected back to the source voltage by the operation of the inverter. This means that two large capacitors are needed for filtering purposes in this design. Only one switch can be on at time in each leg of the inverter. If both switches in a leg were on at the same time, the DC source will be shorted out.

The full-bridge inverter: It is similar to the half bridge-inverter, but it has an additional leg to connect the neutral point to the load.^[8] Figure 3 shows the circuit schematic of the single-phase voltage source full-bridge inverter.

To avoid shorting out the voltage source, S1+ and S1- cannot be on at the same time, and S2+ and S2- also cannot be on at the same time. Any modulating technique used for the full-bridge configuration should have either the top or the bottom switch of each leg on at any given time. Due to the extra leg, the maximum amplitude of the output waveform is Vi, and is twice as large as the maximum achievable output amplitude for the half-bridge configuration

Three phase three level PWM inverter: Single-phase VSIs are used primarily for low power range applications, while three-phase VSIs cover both medium and high power range applications. Switches in any of the three legs of the inverter cannot be switched off simultaneously due to this resulting in the voltages being dependent on the respective line current's polarity. States 7 and 8 produce zero AC line voltages, which result in AC line currents freewheeling through either the upper or the lower components

Procedure:

1.    Open MatLab  click on Simulink icon

2.    Go to Help è Simulink Help è Demos è simulink èSim Power Systems

3.    Click on Power Electronic Models

4.    Double click on Single Phase PWM inverter

5.    Run the simulation and observe the waveform

6.    Repeat same procedure for Three phase three level PWM inverter

Circuit Diagram for Half bridge Invereter:

Expected Waveform:

Circuit Diagram for full Bridge inverter:

Expected Waveform:

Circuit Diagram for three phase PWM Converter:

Expected Waveform:

Result:

Viva Questions:

1.    Explain voltage source half bridge inverter and full bridge inverter

2.    What are the applications of inverters?

3.    Explain three phase three level inverters?

 Experiment No. 11

Objective

To simulate Multilevel inveter using Simulink

Objectives

1.To study about  Mutilevel inverter

2. To plot the output responses of multilevel inverter

Apparatus Required

MATLAB Package

Theory

A relatively new class called multilevel inverters has gained widespread interest. Normal operation of CSIs and VSIs can be classified as two-level inverters because the power switches connect to either the positive or the negative DC bus. If more than two voltage levels were available to the inverter output terminals, the AC output could better approximate a sine wave. For this reason multilevel inverters, although more complex and costly, offer higher performance. A three-level neutral-clamped inverter is shown in bellow Figure

Control methods for a three-level inverter only allow two switches of the four switches in each leg to simultaneously change conduction states. This allows smooth commutation and avoids shoot through by only selecting valid states.^[9] It may also be noted that since the DC bus voltage is shared by at least two power valves, their voltage ratings can be less than a two-level counterpart.

Carrier-based and space-vector modulation techniques are used for multilevel topologies. The methods for these techniques follow those of classic inverters, but with added complexity. Space-vector modulation offers a greater number of fixed voltage vectors to be used in approximating the modulation signal, and therefore allows more effective space vector PWM strategies to be accomplished at the cost of more elaborate algorithms. Due to added complexity and number of semiconductor devices, multilevel inverters are currently more suitable for high-power high-voltage applications. This technology reduces the harmonics hence improves overall efficiency of the scheme.

Procedure

1.    Open MatLab  click on Simulink icon

2.    Go to Help è Simulink Help è Demos è simulink èSim Power Systems

3.    Click on Power Electronic Models

4.    Double click on multi-level multi Phase PWM inverter

5.    Run the simulation and observe the waveform

Circuit Diagram:

Expected Waveform:

Results:

Viva Questions

1. Viva questions: what is the difference between two level and multilevel inverters?

2. What are the application of multilevel inverters?

3. which technique is used for the analysis of multilevel inverters?