pe lab manual

Power Electronics Lab Manual

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2Power Electronics Lab Manual

Circuit Diagram:

Fig. 1: Circuit diagram for SCR Characterstics


3

Expt No. Date: …………….

Study of Characteristics of SCR , MOSFET & IGBTa)Characteristics of SCR

Aim: To obtain the V-I Characteristics of SCR and to determine the latching current, holding

current.

Apparatus Required:

Table 1

S.No. Name of the equipment Range Qty

Theory:

Silicon Controlled Rectifier: The Silicon Control Rectifier (SCR) consists of four layers of

semiconductors, which form NPNP or PNPN structures. It has three junctions,

labeled J1, J2, and J3 and three terminals. The anode terminal of an SCR is connected to the

P-Type material of a PNPN structure, and the cathode terminal is connected to the N-Type

layer, while the gate of the Silicon Control Rectifier SCR is connected to the P-Type material

nearest to the cathode.

Forward blocking mode: In this mode of operation the anode is given a positive potential

while the cathode is given a negative voltage keeping the gate at zero potential i.e.

disconnected. In this case junction J1 and J3 are forward biased while J2 is reversed biased

due to which only a small leakage current flows from the anode to the cathode until the

applied voltage reaches its breakover value at which J2 undergoes avalanche breakdown and

at this breakover voltage it starts conducting but below breakover voltage it offers very high

resistance to the flow of current and is said to be in off state.

Forward conduction mode: SCR can be brought from blocking mode to conduction mode

in two ways - either by increasing the voltage across anode to cathode beyond

breakover voltage or by applying of positive pulse at gate. Once it starts conducting

no more gate voltage is required to maintain it in on state. There is one way to turn it


off i.e. Reduce the current flowing through it below a minimum value called holding

current.

Tabular Column:

V-I Characteristics

Table No: 2 Table No: 3

Model graph:

IG1= VAK (Volts) IA (mA)

IG2= VAK (Volts) IA (mA)


5

Fig. 2:V-I Characteristics of SCR

Reverse blocking mode: SCRs are available with reverse blocking capability. Reverse

blocking capability adds to the forward voltage drop because of the need to have a

long, low doped P1 region. (If one cannot determine which region is P1, a labeled

diagram of layers and junctions can help). Usually, the reverse blocking voltage rating

and forward blocking voltage rating are the same. The typical application for reverse

blocking SCR is in current source inverters.

Latching Current: Latching current (IL) is the minimum principal current required to

maintain the Thyristor in the on state immediately after the switching from off state to

on state has occurred and the triggering signal has been removed.

Holding Current: Holding current (IH) is the minimum principal current required to

maintain the Thyristor in the on state.

Procedure:V-I Characteristics:-

1. Make all connections as per the circuit diagram.

2. Initially keep V1 & V2 at minimum position and R1 & R2 maximum position.

3. Adjust Gate current Ig to some value(2.5/5.0mA) by varying the V1 or R1.


4. Now slowly vary V2 and observe anode to cathode voltage VAK and anode current

IA.

5. Tabulate the readings of anode to cathode voltage VAK and anode current IA.

6. Repeat the above procedure for different Gate current Ig.

Gate triggring and finding Vg and Ig:-

1. Keep all positions at minimum.

2. Set anode to cathode voltage VAK to some value say 15V.

3. Now slowly vary the V1 voltage till the SCR triggers and note down the reading of

gate current(IG) and Gate Cathode voltage(VGK) and rise of anode current IA

4. Repeat the same for different Anode to Cathode voltage and find VAK and IG

values.

To find latching current:-

1. Keep R2 at middle position.

2. Apply 20V to the anode to cathode by varying V2

3. Rise the Vg voltage by varying V1 till the device turns ON indicated by sudden

rise in IA . The current at which SCR triggers is the minimum gate current required

to turn ON the SCR.


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4. Now set R2 at maximum position, then SCR turns OFF, if it is not turned off reduce

V2 up to turn off the device and put the gate voltage.

5. Now decrease the R2 slowly, to increase the anode current gradually in steps.

6. At each and every step, put OFF and ON the gate voltage switches V1. If the

Anode current is greater than the latching current of the device, the device stays

ON even after switch S1 is OFF, otherwise device goes to blocking mode as soon

as the gate switch is put OFF.

7. If IA>IL then, the device remains in ON state and note that anode current as

latching current.

8. Take small steps to get accurate latching current value.

To find holding current:-

1. Now increase load current from latching current level by varying R2 & V2

2. Switch OFF the gate voltage switch S1 permanently (now the device is in ON

state)

3. Now increase load resistance(R2), so that anode current starts reducing and at

some anode current the device goes to turn off .Note that anode current as holding

current.


4. Take small steps to get accurate holding current value.

5. Observe that IH<IL

Precautions:

1.All the connection should be tight.

2. Ammeter is always connected in series in the circuit while voltmeter is parallel to the

conductor.

3. The electrical current should not flow the circuit for long time, Otherwise its

temperature will increase and the result will be affected.

4. It should be care that the values of the components of the circuit is does not exceed

to their ratings (maximum value).

5. Before the circuit connection it should be check out working condition of all the

Component.

Result:


9

Viva-voce:

1. Define holding current,latching current, ON state resistance,break down voltage.

2. Write an expression for anode current?

3. Mention the applications of S.C.R?


Remarks

Signature of the faculty

Circuit Diagram:


11

Fig. 3: Circuit diagram for MOSFET Characterstics


b) Study of MOSFET Characteristics

Aim: To obtain the various characteristics of MOSFET.

Apparatus Required:

Table 4


Theory: A metal oxide semiconductor field effect transistor is a recent device developed by

combining the areas of field effect concept and technology. It has three terminals called drain,

source and gate. MOSFET is a voltage controlled device. As its operation depends upon the

flow of majority carriers only, MOSFET is uni polar device. The control signal or gate

current less than a BJT. This is because of fact that gate circuit impedance in MOSFET is

very high of the order of 109 Ω. This larger impedance permits the MOSFET gate be driven

directly from microelectronic circuits. Power MOSFETs are now finding increasing

applications in low-power high frequency converters.

The transfer characteristics of MOSFET shows the variation of drain current ID as a

fuction of gate to source voltage VGS. The device is in OFF state upto some voltage called

threshold device voltage. The output characteristics of Power MOSFET indicate the variation

of Drain current ID as a function of Drain source voltage VDS as a parameter. This device

combines into advantages of IGBT and BJT.

Procedure:

Transfer Characteristics:


2. Switch on the regulated power supply. Keep VDS constant say 10V. Vary VGS

in steps and note down the corresponding drain current ID

3. Tabulate the readings in the table.

4. Plot a graph of ID against VGS.


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Observations:

Table 5: Output Charcteristics Table 6: Transfer Charcteristics

S. No.

VGS1 VGS2

VDS (V) ID(mA) VDS (V) ID(mA)

Model Graphs:

VDS

VGS (V) ID(mA)


Fig. 4: Output Characterisitcs Fig. 5: Transfer Characterisitcs

Output Characteristics:

1. Make the connections as shown in the circuit diagram.

2. Initially set VGS to some value say 10V.

3. Slowly vary VDS and note down the values of ID and VDS.

4. At particular value of VGS there a pinch off voltage between drain and source. If

VDS< VP device works in the constant resistance region and ID is directly

proportional to VDS. If VDS>VP device works in the constant current region.

5. Repeat above procedure for different values of VGS and draw graph between ID

and VDS.

Precautions:

1.All the connection should be tight.


conductor.

3. The electrical current should not flow the circuit for long time, Otherwise its temperature

will increase and the result will be affected.

4. Care should be taken such that the values of the components of the circuit does not exceed


5. Before the circuit connection , check out the working condition of all the components.

Result:

Viva-voce:

1.What is the difference between MOSFET and BJT?

2.What are the two types of MOSFET?


15


3. How are MOSFETs suitable for low power high frequency applications?

4. What are the merits of MOSFET?

5. What are demerits of MOSFET?

6.What are the applications of MOSFET?


17

Remarks


Circuit Diagram:

Fig. 6: Circuit diagram for IGBT characterstics


c) Study of IGBT Characteristics

Aim: To obtain the Output and Transfer Characteristics of IGBT.

Apparatus Required:

Table 7


Theory :

It is a new development in the area of power MOSFET technology. This device

combines in to advantages of both MOSFET and BJT. So an IGBT has high input impedance

like as MOSFET and low ON state power like BJT. Further IGBT is free from second

breakdown problem present in BJT. IGBT is also known as metal oxide insulated gate

transistor.

It was also called as insulated gate transistor. The static characteristics or output

characteristics of IGBT shows plot of collector current IC vs collector –emitter voltage VCE

for various values of gate emitter voltage. In the forward direction the shape of output

characteristics is similar to that of BJT and have the controlling parameter is gate-emitter

voltage VGE because IGBT is a voltage controlled device. The device developed by

combining the areas of field effect concept and technology.

Procedure:

Transfer Characteristics:



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2. Initially keep V1 & V2 at minimum position and R1 & R2 middle position.

3. Set VCE to some say 10V.

4. Slowly vary gate emitter voltage VGE by varying V1.

5. Note down IC and VGE readings for each step.

6. Repeat above procedure for 20V & 25V of VCE and plot the graph between IC

& VGE.

Tabular Column:

Table No. 8 Ouput Characteristics

S.No. VGE1 VGE2

VCE (Volts) IC (mAmps) VCE (Volts)IC

(mAmps)

Table No. 9 Transfer Characteristics

VCE

VGE (Volts) IC (mAmps)

Model Graphs:


Fig. 7 Output Characteristics Fig. 8 Transfer Characteristics

Output Characteristics:

1. Initially set VGE to some value say 5V by varying V2.

2. Slowly vary V2 and note down IC and VCE readings.

3. At particular value of VCE there will be a pinch off voltage VP between collector and

emitter.

4. Repeat above procedure for different values of VGE and draw graph between IC and VGE.

Precautions:

1. All the connection should be tight.


conductor.



4. It should be care that the values of the components of the circuit is does not exceed to

their ratings (maximum value).


Component.

Result:

Viva Voce:

1. In what way IGBT is more advantageous than BJT and MOSFET?


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2. What are merits of IGBT?

3. What are demerits of IGBT?


4. What are the applications of IGBT’s?

5. How is IGBT turned off?

6. What is threshold voltage?


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Remarks


Circuit Diagram:

Fig. 1 R-C Triggering circuit


Expt No: Date: …………

Gate Firing Circuits of SCR

a)R-C Triggering

Aim: To observe the output waveforms of Resistance- Capacitance firing circuit of SCR..

Apparatus Required:

Table 1


Theory:It includes variable resistor, two diodes, SCR (Silicon Controlled Rectifier),

Capacitor, Load resistor.The circuit diagram of an RC Triggering R-C-Diode circuit giving

full half-cycle control (180 electrical degrees).

On the positive half-cycle of SCR anode voltage the capacitor charges to the trigger

point of the SCR in a time determined by the RC time constant and the rising anode voltage.

The top plate of the capacitor charges to the peak of the negative voltage cycle through diode

D2 on the negative half-cycle, resetting it for the next charging cycle.

Procedure:


2. Give the AC power supply from the source.


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3. Connect resistive load of 200Ω between two points.

4. Switch ON Power supply and observe the wave forms of input & output at a time

in the CRO.

5. Slowly vary the control Resistor RC, so that firing angle can vary from 0-180°.

6. Observe various voltage waveforms across load, SCR and other points, by varying the

load resistance.

7. Compare practical obtained voltage waveform swith theoretical waveforms and observe the

firing angle in R-C Triggering.

Waveforms:

(a) (b)

Fig. 2: Output voltage waveforms for RC half wave triggering circuit of (a) high value

j(b) low value of R


Precautions:



conductor.






Component.

Result:

Viva Voce:

1.What is the maximum firing angle of RC-triggering and why?

2.What are the limitations of RC triggering circuit?


27

Remarks


Circuit Diagram:

Fig. 3: Resistance triggering circuit


b)Resistance Triggering

Aim: To observe the output waveforms of resistance firing circuit of SCR.

Apparatus Required:

Table 2


Theory:It includes one fixed resistor, variable resistor, diode, SCR(Silicon Controlled

Rectifier), Load resistor. The circuit diagram of an R Triggering consistsof Simple resistor;

diode combinations trigger and control SCRs over the full 180 electrical degree ranges,

performing well at commercial temperatures. These types of circuits operate most

satisfactorily when SCRs have fairly strong gate sensitivities. Since in a scheme of this type a

resistor must supply all of the gate drive required to turn on the SCR, the less sensitive the

gate, the lower the resistance must be, and the greater the power rating.

It provides phase retard from essential zero (SCR full “on”) to 90 electrical degrees of

the anode voltage wave (SCR half “on”).Diode D1 blocks reverse gate voltage on the

negative half-cycle of anode supply voltage. It is necessary to rate blocking to at least the

peak value of the AC supply voltage and the trigger voltage producing the gate current to fire

IGF are in phase. When EAC = Em, at the peak of the AC supply voltage, the SCR can still

trigger with the maximum value of resistance between anode and gate.

Procedure:



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2. Connect the AC power supply from the source indicated in the front panel.

3. Connect Load i.e., Rheostat of 200Ω between two points.

4. Switch ON power supply and observe the wave forms of input & output at a time

in the CRO.

5. Slowly vary the control Resistor R, so that firing angle can vary from 0-90°.

6. Observe various voltage waveforms across load, SCR and other points.

Waveforms:

(a) (b) (c)

Fig. 4: Waveforms across gate, load and SCR for Resistance firing circuit of an SCR in

a half wave circuit at (a) No triggering of SCR (b) α=900 (c) α<900


7. Compare practical obtained voltage waveforms with theoretical waveforms and

observe the firing angle in Resistance Triggering.

Precautions:



conductor.






Component.

Result:.

Viva Voce:

1. What is the maximum firing angle of R-triggering circuit and why?

2. What are the disadvantages of R triggering?

3. Mention different methods of trigerring SCR?


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4. Why gate triggering is preferred?

Remarks Signature of the facult

Circuit Diagram:

Fig. 5 UJT triggering circuit


c) UJT Triggering

Aim: To obtain firing of SCR using UJT Relaxation Oscillator and observe the output

wavwforms.

Apparatus Required:

Table 3


Theory:Uni-Junction Transistor: UJT exhibits negative resistance characteristics; it can be used as

relaxation oscillator. The external characteristics RB1 and RB2 are resistances which are small

in comparison with internal resistances R1 and R2 of the UJT base. The emitter potential V is

varied depending on the charging rate of capacitance C. The charging resistance Rc should be

such that the load line intersects the device only in the negative resistance region. η is called

as the intrinsic standoff ratio. It is defined as

UJT is a highly efficient switch .It’s switching time is in a range of nano seconds. The rise

time output pulse will depend on the switching speed of the UJT and duration will be

proportional to the time constant RB1C of the discharge circuit.

The output pulses of UJT are identical in magnitude and time period

The value of η is specified for each device . For UJT η=0.63


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Procedure:

1. First observe the waveforms at different points in circuit and also trigger output

T1 and T1` observe the pulses are synchronized.

2. Now make the connections as per circuit using AC source, UJT Relaxation

Oscillator, SCR’s and Loads.

3. Observe the waveforms across the load and SCR and other points, by varying the

variable resistor Rc and resistance load, observe firing angle of SCR.

4. Use differential module for observing two waveforms (input and output)

simultaneously in channel 1 and channel 2.

5. Check the waveforms for large value of RC and small value of RC and also

triggering points of SCR.

For Relaxation Oscillator:

1. Short the CF capacitor to the diode bridge rectifier to get filtered AC Output.

2. We get equidistance pulses at the output of pulse transformer.

3. The frequency of pulse can be varied by varying the potentiometer.

4. Observe that capacitor charging and discharging time periods and calculate

frequency and RC time constant of UJT Relaxation Oscillator by using given

formulas

Precautions:



conductor.







35

Component.


Result:

Viva Voce:

1. Why is an UJT used in SCR firing circuit?

2. Why is the isolation needed between Thyristor and firing circuit?

3. What are the applications of UJT trigger circuits?

4. What are the merits of UJT firing circuit over RC triggering circuit?

\


37

Remarks


Circuit Diagrams:

(a) Class-A Commutation (b) Class-B Commutation

(c) Class-C

Commutation

(d) Class-D

Commutation


(e) Class-E Commutation

Fig. 1: Commutation Circuits

Expt No. Date: …………

Study of Forced Commutation Circuits

Aim: To verify the different types of forced commutation circuits of SCR by connecting

a resistive load.

Apparatus Required:

Table 1


Theory: Commutation is the process of turning off the SCR and it normally causes the

transfer of current flow to other parts of circuit. Commutation can be divided into

a) Natural commutation

b) Forced commutation

a) Natural commutation: If the source voltage is AC, the SCR current goes through a

natural zero and reverse voltage appears across the SCR. The device is automatically turns

off due to the natural behavior of the source voltage. This is known as natural commutation

or line commutation.

b) Forced commutation: In some SCR circuits the input voltage is DC and the

forward current of the SCR is DC and the forward current of the SCR is forced to zero by


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external or additional circuitry called as commutation circuitry to turn off SCR. This

technique is called forced commutation and normally applied in DC to DC converters .

Forced Commutation circuits can be classified as

i. Class-A Commutation (Series resonant commutation circuit)

ii. Class-B Commutation (Parallel resonant commutation circuit)

iii. Class-C Commutation ( Complementary commutation circuit)

iv. Class-D Commutation (Auxiliary Commutation)

v. Class-E Commutation (External Pulse Commutation.

Waveforms:

Class-A Commutation

Class-B Commutation


Procedure:

Class-A Commutation:

1. Connect the circuit as shown in the circuit.

2. Connect Trigger output T1 to gate and cathode of SCR T1

3. Switch on the DC supply to the power circuit and observe the voltage waveform

across load.

4. Repeat the same for different values of L,C and R.

Class-B Commutation:


2. Connect Trigger output T1 to gate and cathode of SCR T1

3. Switch on the DC supply to the power circuit and observe the voltage waveform

across load.

4. Repeat the same for different values of L,C and R.

Class-C Commutation:


2. Connect T1 and T2 from firing circuit to gate and cathode of Thyristors T1 and T2.

3. Observe the waveforms across R1,R2 and C by varying frequency and also duty cycle

potentiometer.

4. Repeat the same for different values of C and R.

Class-D Commutation:


2. Connect T1 and T2 gate pulses from the firing circuit to the corresponding SCRs’in

Power circuit.


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3. Initially keep the trigger ON/OFF at OFF position initially charge the capacitor, this can be

observed by connecting CRO across the capacitor.

4. Now switch ON the trigger output and observe the voltage waveform across the

load, T1, T2 and capacitor.Note down the voltage waveforms at different frequency of

chopping and also at different duty cycles.

5. Repeat the experiment for different values of load Resistance, commutation inductance

and capacitance.

Class-C Commutation:

Class-D Commutation:


Class-E Commutation:


2. Connect the trigger output T1 from the firing circuit to the SCR.

3. Connect T2 to the Transistor base and emitter points.

4. Switch on the Power Supply and External DC supply.

5. Switch on the trigger output and observe and note down waveforms. Repeat the

Same by varying frequency and duty cycle.

Precautions:



conductor.







43

Component.

Class-E Commutation


Result:

Viva Voce:

1.What is meant by commutation?

2.What are the different types of commutation techniques?

3.What is meant by impulse commutation?


45

4.What is meant by external pulse commutation?

5.When the circuit is said to be under damped circuit?

6.In which type of converter forced commutation is employed?

Remarks


Circuit Diagram:

Fig. 1: Half Controlled Bridge Converter with R load


Fig. 2: Half Controlled Bridge Converter with R-L load


Single Phase Half Controlled Bridge Converter

Aim: To obtain the output waveform of single phase half controlled bridge converter with R

and RL Loads.

Apparatus Required:

Table 1



47

Theory:

The circuit arrangement of a 1-ph converter is shown in figure 1. In the positive half

cycle thyristor T1 is forward biased. When SCR T1 is fired at ωt = α, the load is connected to

the input supply through T1 and D2 during the period from α ≤ ωt ≤ π+α the input voltage is

negative and freewheeling diode DM is forward biased. DM conducts to provide continuously

current in case of inductive loads. In the negative half-cycle of input voltage T2 is forward

biased and triggering of T2 at ωt = π +α will reverse bias DM and is turned OFF. Load is

connected to supply through T2 and D1.

The converter has a better power factor due to the freewheeling diode and is

commonly used in applications up to 15KW where one quadrant operation is acceptable.

The half controlled bridge has the inherent freewheeling action and analysis is more

or less the same with or without a freewheeling diode is connected across the load. In

practical it is always adjustable to provide a freewheeling diode in a half-controlled converter

so that the commutation of SCR’s is assumed inductive loads.

Tabular Column:

Table 2

Model Calculations:

Load type

Input Voltage (Vin)(volts)

Firing angle in Degrees

Output voltage (V0) Output Current (I0)

Theoretical(V) Practical(V) Theoretical(A) Practical(A)


Procedure:

1.Make all connections as per the circuit diagram

2.Connect first 30V AC supply from Isolation Transformer to circuit

3.Connect firing pulses from firing circuit to Thyristors as indication in circuit

4.Connect resistive load 200Ω / 5A to load terminals and switch ON the MCB and

IRS switch and trigger output ON switch

5.Connect CRO probes and observe waveforms in CRO, Ch-1 or Ch-2, across load.

6..By varying firing angle gradually up to 1800 and observe related waveforms

7.Measure output voltage and current by connecting AC voltmeter & Ammeter

Tabulate all readings for various firing angles.

8.For RL Load connect a large inductance load in series with Resistance and observe

all waveforms and readings as same as above.

9.Observe the various waveforms at different points in circuit by varying the Resistive

Load and Inductive Load.

10.Calculate the output voltage and current by theoretically and compare with it

practically obtained values.

Precautions:


49



conductor.






Component.

6. Use only isolated power sources (either isolated power supplies or AC power through

isolation power transformers). This helps using a grounded oscilloscope and reduces the

possibility of risk of completing a circuit through your body or destroying the test equipment.

Waveforms:


Fig. 2 : Single Phase Semi Converter output voltage waveforms

Result:


51

Viva Voce:

1. What is meant by half controlled rectifier?

2. What is the effect of adding free wheeling diode?

3.Give at least five application of phase controlled rectifier?

4.What is meant by firing angle?

5.What is other name for single half controlled rectifier?

6.What is meant by pulse number?

Remarks


Circuit Diagram:


Fig. 1:Fully Controlled Bridge Converter with R load

Fig. 2:Fully Controlled Bridge Converter with R-L load


53

Expt No: Date: …………

Single Phase Fully Controlled Bridge Converter

Aim: To observe the output waveforms of a single phase fully controlled bridge converter

with R and RL Loads.

Apparatus Required:

Table 1

S.No. Name of the equipment Range Type Qty

Theory:

A single phase full bridge converter using four SCR’s is shown in figure1. The load is

assumed to be R and RL. Thyristor pair T1 and T2 is simultaneously triggered and π radians

after pair T3 and T4 is gated together.

During the positive half cycle SCR’s T1 and T1I are forward biased and when there

two thyristors are fired simultaneously at wt = α, the load is connected to the input through T1

and T1I. In this case of inductive loads during the period π <wt < π+α the input voltage is

negative and freewheeling diode Dm is forward biased. Dm conducts to provide the

conductivity of current in the inductive load. The load current is transferred from T1 and T1I

to DM and thyristors T1 and T1I are turned off due to line or natural commutation.

During the negative half cycle of the input voltage thyristors T2 and TI2 are forward

biased. The firing of thyristors T2 and T2I simultaneously at wt = π+α will reverse bias DM. the

diode DM is turned off and the load is connected to the supply through T2 and T2I .


Tabular Column:

Table 2

Free wheelLoad

type

Input

Voltage

(V in)

Firing

angle in

Degrees


Theoretical Practical Theoretical Practical

Model Calculations:

Procedure:

1. Make all connections as per the circuit diagram


55

2. Connect firstly 30V AC supply from Isolation Transformer to circuit

3. Connect firing pulses from firing circuit to Thyristors as indication in circuit

4. Connect resistive load 200Ω / 5A to load terminals and switch ON the MCB and

IRS switch and trigger output ON switch.

5. Connect CRO probes and observe waveforms in CRO, Ch-1 or Ch-2, across load

and device in single phase half controlled bridge converter.

6. By varying firing angle gradually up to 1800 and observe related waveforms

7. Measure output voltage and current by connecting AC voltmeter & Ammeter

8. Tabulate all readings for various firing angles.

9. For RL Load connect a large inductance load in series with Resistance and

observe all waveforms and readings as same as above.

10. Observe the various waveforms at different points in circuit by varying the

Resistive Load and Inductive Load.

11. Calculate the output voltage and current by theoretically and compare with it


Precautions:



conductor.






Component.

6.Use only isolated power sources (either isolated power supplies or AC power through




Result:


57

Viva Voce:

1.What is a full controlled rectifier?

2. How can we control the output voltage of a single-phase full converter?

3. What is the type of commutation used in a single phase full controlled converter?

4. What is the effect of adding free wheeling diode?

5. What is rectification mode and inversion mode?

6.What are the applications of Single phase fully controlled rectifiers?

Remarks



Circuit Diagram:

Fig. 1: 1-Ф A.C. Voltage Controller with R load

Fig. 2: 1-Ф A.C. Voltage Controller with R-L load



59

Single Phase A.C. Voltage Controller

Aim: To observe the output wave forms of 1-phase A.C.voltage controller with R and RL

loads using anti parallel connection of SCR’s.

Apparatus Required:

Table 1

S.No. Name of the equipment Range Type Qty

Theory: AC voltage controller’s are thyristor based devices ,which converts the fixed Ac

voltage into variable AC voltage with same frequency .The circuit diagram of Single phase

AC voltage controller is shown in figure .It consists of two SCR’s connected in anti parallel.

The input and output voltage waveforms are also shown. The SCR’s are gate controlled and

gate pulses are obtained from firing unit.

For R-Load: For the first half cycle of input voltage waveform SCR T1 conducts and gives

controlled output to load. During the other half cycle of input voltage waveform SCR T2

conducts .During the Positive half cycle T1 is triggered at a firing angle of wt= α .T1 starts

conducting and source voltage is applied to the load from α to π. At wt= π both Vo and Io

falls to zero. Just after wt= π, T1 is reverse biased and therefore it is turned off by self

commutation. During the negative half cycle of T2 is triggered at wt= π+α, then T2 conducts

from wt = π+α.


Tabular Column:

Table 2: For R-Load

Table 3: For R-L Load

Model Calculations:

Procedure:

AC voltage controller with two thyristors:

S.No.Input Voltage

(V in)

Firing

angle in

Degrees

Output voltage (V0r) Output Current (I0r)


S.No.Input Voltage

(V in)

Firing

angle in

Degrees

Output voltage (V0r) Output Current (I0r)



61


2.Connect firstly 30V AC supply from Isolation Transformer to circuit

3.Connect firing pulses from firing circuit to Thyristors as indication in circuit


. IRS switch and trigger output ON switch

5.Observe waveforms in CRO, across load by varying firing angle gradually up to

1800.


7.Tabulate all readings for various firing angles.







A.C. voltage controller with TRIAC:



3.Connect firing pulse from firing circuit to TRIAC as indication in circuit



5.Observe waveforms in CRO, across load by varying firing angle gradually up to

1800.


7.Tabulate all readings for various firing angles.






practically obtained values

Waveforms: (i) For R-Load


(ii) For R-L Load

Precautions:


63



conductor.






Component.




Result:

Viva Voce:

1.What is ac voltage controller?

2.What are the applications of ac voltage controllers?

3. What are the two types of control?

4. What are the merits and demerits of voltage controllers?


5. Why is the trigger source for the two Thyristor isolated from each other in a single-phase


65

voltage controller?

6. What is the difference between cycloconverters and ac voltage controllers?

Remarks


Circuit Diagrams:


Fig.1: Single Phase Cycloconverter with R-load


67

Fig. 2: Single phase Cycloconverter with R-L LoadExpt No. Date: …………

Single Phase Cycloconverter

Aim: To verify the operation of single phase Cyclo Converter with R and RL Loads and to

observe the output and input waveforms.

Apparatus Required:

Table 1



Theory:

The circuit diagram of 1-φ cyclo converter with R and RL load are shown in fig.

Construction ally there are four SCR’s T1, T2, T3 &T4.Out of them T1, T2 are

responsible for generating positive halves forming the positive group. The other two T3, T4

are responsible for negative haves forming negative group. This configuration and waveforms

are shown for ½ and 1/3 of the supply frequency. Natural commutation process is used to turn

off the SCR’s.

A) For R-Load:

During the half cycle when point A is positive with respect to O, SCR T1 is in

conducting mode and is triggered at wt =α then current flows through positive point A-

T1-load-negative O. In the negative half cycle when B point is positive with respect to the

point O,SCR T1 is automatically turned off due to natural commutation and SCR T2 is

triggered at wt = π+α. In this condition the current flows through B-T2-load-O. The flow

of the current direction is same as in the first case. After two positive half cycles of load

Tabular Column:

Table 2: For R-load

Sl.

No

Input

Voltage

(V in)

Firing

angle in

Degrees

Frequency

Division

V o

(V)

I o

(A)

Input

frequency

f s

Output

frequency

f o=fs / 2

f o / f s

Table 3: For R-L Load

Sl.

No

Input

Voltage

(V in)

Firing

angle in

Degrees

Frequency

Division

V o

(V)

I o

(A)

Input

frequency

f s

Output

Frequency

f o=fs / 3

f o / f s


69

voltage and load current SCR T4 is gated at wt=2π+α when O is positive with respect to

B. In this condition the load current flows through O-load-T4-B.Thus the direction of

load current is reversed. In the next half cycle when O is positive with respect to A when

wt=3π, T4 turnoff due to natural commutation and at wt=3π+α T3 is triggered. In this

condition the load current flows through O-load-T3-A. The direction of load current is

same as previous case. In this manner two negative half cycles of load voltage and load

current, equal to the number of two positive half cycles are generated. Now T1 is again

triggered to fabricate further two positive half cycles of load voltage and so on. Like this

the input frequency 50Hz is reduced to ½ at the output across the load. The input and

output waveforms are shown in figure.

The frequency of the output voltage can be calculated by:

Frequency ( fo )=(Time period)-1

B) For RL-Load:-

When A is positive with respect to O forward biased SCR T1 is triggered at wt=α and

the current start to flow through A-T1-R-L-O. Load voltage becomes zero at wt=π but load


current will not become zero at this angle due to inductance. It becomes zero at wt =β which

is called extinction angle. So it is naturally commutated at wt=β. After half cycle point B

positive with respect to point O. Now at angle wt=π+α. T2 is triggered and the load current

takes path from B-T2-R-L_o and its direction is positive as in the previous case. The load

current decays zero at wt =π+β and SCR T2 is naturally commutated. In the half cycle when

O is positive with respect to B point, T4 is triggered instead of T1 at an angle of wt= (2π+α).

Now the load current flows through O-L-R-T4-B but the direction of load current reversed.

When the load current becomes zero at an angle wt= (2π+α) , T4 naturally commutated

because the voltage is already reversed at wt = 3π.When wt = (3π+α) and point O, is positive

with respect to point A,T3 is triggered then the current flows through O-L-R-T3-A , and the

direction of load current is same in previous case. In the next half cycle again T1 will

triggered like this we get one cycle of output frequency for two cycles of input frequency,

when the frequency division switch is at 2. The waveforms of load voltage and load current

are shown in fig.

The frequency of load voltage can be calculated by fo=(Time period)-1

Waveforms:


71

Fig. 3: Output Voltage waveforms for step down cycloconverter with R load at α=00 and

α=600

Fig. 4: Output Voltage waveforms for step down cycloconverter with R-L load

Procedure:


2. Connect firstly (30V-0-30V) AC supply from Isolation Transformer to circuit


4. Connect resistive load 200Ω / 5A to load terminals.

5. Set the frequency division switch to (2,3,4,…9) your required output

frequency.

6. Switch ON the MCB and IRS switch and trigger output ON switch.

7. Observe waveforms in CRO, across load by varying firing angle gradually up

to 1800 and also for various frequency divisions(2,3,4,…9).

8. Measure output voltage and current by connecting AC voltmeter & Ammeter

9. Tabulate all readings for various firing angles.

10. For RL Load connect a large inductance load in series with Resistance and

observe all waveforms and readings as same as above.


11. Observe the various waveforms at different points in circuit by varying the

Resistive Load and Inductive Load.



Precautions:


2. Ammeter is always connected in series in the circuit while voltmeter is parallel to

the conductor.






Component.


isolation power transformers). This helps using a grounded oscilloscope and reduces

the possibility of risk of completing a circuit through your body or destroying the test

equipment.


73

Result:

Viva Voce:

1. What is meant by cycloconverter?

2. What are the applications of Cycloconverter?


3. What are the merits of Cycloconverter?

4. What are demerits of Cycloconverter?

Remarks


Circuit Diagram:


75

Fig. 1: Circuit Diagram for Single Phase Series Inverter


Single Phase Series Inverter with R and RL Loads

Aim: To study the behavior of modified series inverter by varying load resistance at

different frequencies.

Apparatus Required:

Table 1


Theory:

This circuit which converts DC power into AC power is called inverter. If the

thyristor commutation circuit of the inverter is in series with the Load, then the inverter is

called “Series are tightly coupled. In this circuit, it is possible to turn-on-thyristor Tp before

the current through thyristor Tn has become zero and vice-versa. Therefore, the Modifed

Series Inverter can be operated behond the resonance frequency (fr) of the circuit. Inverter is


operated at the resonance frequency (fr) if the load current waveform has low frequency and

should not have zero current interval. The inverter’s resonance frequency depends on the

values of L, R and C in the circuit.

Procedure:


2. Give the DC power supply 30V to the terminal pins located in the power circuit




5. By varying the frequency pot, observe related waveforms

6. If the inverter frequency is increases above the resonant frequency of the power

circuit commutation fails. Then switch OFF the DC supply , reduce the inverter

frequency and try again.

Tabular Column:

Table 2 For R= , L=

S.NoInput

Voltage(v)Firing Angle(α)

Output

Voltage(VO)

Output

Current (A)


77

7. Repeat the above same procedure for different value of L,C load and also above

the wave forms with and without fly wheel diodes.

8. Total output waveforms entirely depends on the load, and after getting the perfect

wave forms increase the input supply voltage up to 30V and follow the above

procedure.

9. Switch OFF the DC supply first and then Switch OFF the inverter.( Switch OFF

the trigger pulses will lead to short circuit)

Precautions:



the conductor.






Component.

Result:


Viva Voce:

1.What is meant by inverter?

2.What are the different types of inverters?


79

3.What are the application of series inverters?

4.In which aspect inverters are classified?

5.What is the other name for series inverter?

6. What are the merits and demerits of series inverter?


Remarks


Circuit Diagram:

Fig. 1: Circuit Diagram for Parallel Inverter


81


Single Phase Parallel Inverter with R and RL Loads

Aim: To study the performance of center tapped transformer type parallel inverter at different

frequencies.

Apparatus Required:

Table 1


Theory:

The circuit diagram of 1-ph Parallel Inverter is shown in fig., SCR1 and SCR2 are

main thyristors. Supply voltage Vdc appears across the left half of the transformer primary

winding OA. Terminal O is positive w.r.t.A. By transformer action terminal B will be at

potential of 2Vdc w.r.t A. Thus capacitor C will get charged twice the supply voltage. The

load voltage will be positive and will have a magnitude VL . At the end of half period SCR2

is firing , capacitor C will be immediately apply a reverse voltage of 2Vdc across SCR1 and

turns off it.


Similarly the Vdc applies to right half of the primary winding and capacitor gets

charged with 2Vdc in reverse direction. Now the load voltage is negative and hence the

current. Since the commutating capacitor is in parallel with SCRs, so it is called parallel

inverter.

Tabular Column:

Table No: 2

VDC(V) TON(Sec) TOFF(Sec) Frequency(HZ) Vload(v)

Waveforms:


83

Fig. 2: Waveforms across load and SCR of Parallel Inverter

Procedure:

1. Make all connections as per the circuit, and give regulated power supply 30V/5A.

2. Give trigger pulses from firing circuit to gate and cathode of SCR’s T1 & T2.

3. Set input voltage 15V, connect load across load terminals.

4. Now switch ON the DC supply, switch ON the trigger output pulses.

5. Observe the output voltage waveforms across load by varying the frequency pot.

6. Repeat the above same procedure for different value of L,C load values.

7. Switch off the DC supply first and then switch off the inverter.

(switch off the trigger pulses will lead to short circuit)

Precautions:



the conductor.







component.

Result:


85

Viva voce:

1. To what voltage will the capacitor gets charged?

2. What is the need of the transformer is the circuit?

3. What type of commutation is employed in this circuit?


4. What are the applications of parallel inverter?

5. What are the merits and demerits of parallel inverter?

Remarks


Fig

. 1:

Th

ree

Ph

ase

Hal

f C

ontr

olle

d B

rid

ge C

onve

rter

wit

h R

-L lo

ad


87

Expt No. Date:

…………

Three Phase Half Controlled Bridge Converter

Aim: To observe the output waveforms of three phase half controlled bridge converter with

R & RL Load.

Apparatus Required:

Table 1


Cir

cuit

Dia

gram

:


Theory :

For large power dc loads, 3-ph ac to dc converter are commonly used. The various

types of three phase controlled converter are 3-ph half wave converter, 3-ph half wave

converter is rarely used in industry because it introduces de component in the supply circuit.

If diodes are replaced by 3-thyristors, a semi converter bride is obtained.

Free wheeling mode of operation of bridge connected rectifiers can be realized half of

its thyristor with diodes. Therefore, circuit of three phase half-controlled bridge converter

contains three thyristor in three arms and diodes in the other three arms.For α<600 the

continuous conduction mode is possible. For firing angles α>600 the discontinuous

conduction mode occurs. It can be observed from the waveforms that the output voltage

becomes zero during a part of the output voltage period, because of the free wheeling action.

It is easily noted from the waveforms that the freewheeling period is . Therefore the supply

current flows for the period (Π-α) in each half cycle. As α increase the duration of the supply

current pulse decreases. Therefore, the harmonic content in the source current increases

as the firing angle increases.

Tabular Column:

Table 2

S.No.

Input

Voltage (V

in)

Firing

angle in

Degrees


Theoretical(v) Practical(v)Theoretical

(A)

Practical

(A)

Model Calculations:


89

For large firing angle delays, commutation failure may take place due to the limited

time available in symmetrical half controlled converter circuit configuration, if the current is

assumed to be continuous. This may result in half weaving effect

Procedure:



3.Connect resistive load 200Ω / 5A to load terminals and switch ON the MCB

4.Observe waveforms in CRO, across load and device in three phase half controlled bridge

converter.

5.By varying firing angle gradually up to 1800 and observe related waveforms

6.Measure output voltage and current by connecting DC voltmeter & Ammeter

7.Now increase the input supply voltage by changing tapping at Isolation Transformer.

Observe waveforms and readings, changing the supply voltage up to 230V. Tabulate all

readings at various angles and various voltages.

8.For RL Load connect a large inductance load in series with Resistance and observe all

waveforms and readings as same as above.


9.Observe the various waveforms at deferent points in circuit by varying the Resistive Load

and Inductive Load.

10.Calculate the output voltage and current by theoretically and compare with it practically

obtained values.

Waveforms:


91

Fig. 2: Output Voltage Waveforms for 3-Ф Semi Converter at α=00, 600, 900


Precautions:

1.Do not conduct the experiment without three phase isolation transformer. If you try to

conduct experiment without isolation transformer the instrument may be damaged due to

short circuit exists between single phase & three phase supply while making measurement

using CRO.

2.Do not attempt to observe load voltage and input voltage simultaneously, if does so input

voltage terminal directly connected to load terminals due to the non isolation of both channels

of the CRO.

Result:

Viva Voce:

1.What are the advantages of three phase circuit over single phase circuit?

2.What is the total harmonic distortion in a three phase semiconverter?

3. Why output voltage is more at lesser value of firing angle?

4.Whis the difference between three phase semiconverter and three phase fully controlled

converter?

5.What are the application of Three phase semiconverter?

Remarks



93

Circuit Diagram:

Fig1: Four Quadrant Chopper Circuit With Field Supply



Chopper Controlled DC Motor

Aim: To analyze the operation of four quadrant chopper drive by controling the speed

of the dc motor.

Apparatus:

Table 1


Theory:

Chopper converts fixed DC voltage to variable DC voltage through the use of

semiconductor devices. The DC to DC converters have gained popularity in modern

industry. Some practical applications of DC to DC converter include armature voltage

control of DC motors converting one DC voltage level to pulse width modulated voltage,

and controlling DC power for wide variety of industrial processes. The time ratio

controller (TRC) is a form of control for DC to DC conversion.

In four quadrant dc chopper drives, a motor can be made to work in forward-motoring

mode (first quadrant), forward regenerative breaking mode (second quadrant), reverse

motoring mode (third quadrant) and reverse regenerative breaking mode (fourth quadrant).

The circuit shown offers four quadrant operation of a separately-excited dc motor. This

circuit consists of a DC Power Supply, four choppers, four diodes and a dc motor. Its

operation in the four quadrants can be explained as under.

Four Quadrant diagram:


95

Fig 2: Four Quadrant Diagram


Fig 3: Four Quadrant Chopper circuit

Forward motoring mode (I quadrant):

During this mode or first-quadrant operation, chopper CH2, CH3 are kept off,

CH4 is kept on whereas CH1 is operated. When CH1, CH4 are on, motor voltage is

positive and positive armature current rises. When CH1 is turned off, positive armature

current free-wheels and decreases as it flows through CH4, D2. In this manner controlled

operation in first quadrant is obtained.

Forward regenerative breaking mode (II quadrant):

A dc motor can work in the regenerative-breaking mode only if motor generated emf

is made to exceed the dc source voltage. For obtaining this mode CH1, CH3 and CH4 are

kept off whereas CH2 is operated. When CH2 is turned on, negative armature current

rises through CH2, D4, Ea, La, ra. When CH2 is turned off, diodes D1, D4 are turned on

and the motor acting as a generator returning energy to dc source. This results in forward

regenerative-breaking mode in the second-quadrant.


97


Reverse motoring mode (III quadrant):

This operating mode is opposite to forward motoring mode. Chopper CH1, CH4

are kept off, CH2 is kept on whereas CH3 is operated. When CH3 and CH2 are on,

armature gets connected to source voltage Vs so that both armature voltage and armature

current iaare negative. As armature current is reversed, motor torque reversed and

consequently motoring mode in third quadrant is obtained. When CH3 is turned off,

negative armature current freewheels through CH2, D4, Ea, La, ra; armature current

decreases and thus speed control is obtained in third quadrant. Note that during this mode

polarity of Ea is opposite to that shown in circuit diagram.

Reverse Regenerative-braking mode (IV quadrant):

As in forward braking mode, reverse regenerative-braking mode is feasible only if

motor generated emf is made to exceed the source voltage. For this operating mode, CH1,

CH2 and CH3 are kept off whereas CH4 is operated. When CH4 is turned on, positive

armature current ia rises through CH4, D2, ra, La, Ea. When CH4 is turned off, diodes D2,

D3 begin to conduct and motor acting as generator returns energy to dc source. This leads

to reverse regenerative-braking operation of the dc separately excited motor in fourth

quadrant.

The chopper circuit provided is made to work in the following manner:

Forward Rotation:

During this mode chopper is operating in I quadrant (Current & Voltage are

positive) however chopper jumped to IV quadrant momentarily because current doesn't

become zero instantaneously. Therefore in forward motoring current is always positive

but voltage may be positive or negative. In this way chopper operated in I and IV

quadrants.

Reverse Rotation:

During this mode chopper is operating in III quadrant (Current & Voltage are

negative) however chopper is jumped to II quadrant momentarily because current doesn't

become zero instantaneously. Therefore in reverse motoring current is always negative but

voltage may be positive or negative. In this way chopper operated in III and II quadrants.


99


Procedure:

Keyboard settings

Stop/Set key:

This key is used to stop the process. And also this key is used to move the curser to set

the parameters (frequency, duty cycle and Fw/Rw).

INR key: This key is used to increase the parameters (f, Dcy or Fw/Rw) by one.

DCR key: This key is used to decrease the parameters (f, Dcy or Fw/Rw) by one.

RUN key: This key is used to run at set parameters.

Note: The parameters of the curser positions are varied by pressing INR, DCR keys. The

curser can be brought to different parameters (frequency, duty cycle, Q1&Q4/Q3&Q2) using

SET key. When the process is under RUN the parameters can't be changed (INR, DCR keys

are inactive). The parameters can be changed only after STOP key is pressed and the process

return to SET after wait.When RUN key is pressed the motor goes to RUN mode after wait

mode. The parameters can be changed only after STOP key is pressed. The process return to

SET after wait.

1. Keep the toggle switch to SET QUAD position.

2. Power circuit connections are made as shown in the circuit diagram.

3. Connect motor terminals to respective points in the power circuit as shown in the circuit

diagram. Field of the motor to field terminals of the unit.. Armature to the respective

terminals in the circuit.

4. Voltmeter and ammeter are connected internally as shown in the circuit..

5. Triggering pulses are connected internally to respective IGBTs..

6.Connect the power scope to monitor current and voltage waveforms (if provided) otherwise

use CRO.

7. Check the connections and conform the connections made are correct before switching on

mains supply.

8. Connect three pin power cord from the four quadrant chopper power unit to the single

phase three pin power mains

9. Switch on the field supply to the motor.

10. Switch on the single phase power supply to the four quadrant chopper triggering circuit.

11. Keeping power supply voltage knob to minimum position sett frequency, duty cycle,

directions of the motor..


101

Tabular Column:

Forward Motoring Mode:

Table No:2

S.No. Duty Cycle Speed In rpm

Reverse Motoring Mode:

Table No: 3

S.No. Duty Cycle Speed In rpm


12. Enter RUN key.

13. DC power supply voltage must be increased now from 0 up to suitable value (say 100-

150V) by switching on MCB.

14. When RUN key is pressed the chopper is gone for wait mode,, during this mode the

chopper duty cycle is adjusted to less than 10% for a time interval.. After that the

chopper goes to RUN mode, during RUN the chopper duty cycle is adjusted to the sett

value.

15. Observe the speed of the motor in rpm..

16. Now reduce the supply voltage to minimum value..

17. Enter STOP key..

18. When STOP key is pressed the chopper is gone for wait mode,, during this mode the

chopper duty cycle is adjusted to less than 10% for a time interval.. After that the

chopper goes to SET mode, during SET the chopper frequency, duty cycle, chopper

directions (Fw & Rw) can be set.

19. Do the experiment for different duty cycles.

20. Observe the load voltage & load current waveforms using power scope. Load the motor

21. Load the motor slowly (maximum 1A) & study the performance of the motor.

22. Every time reduce the load when you are setting new duty cycle.

23. Release the load. Reduce power supply voltage.

24. Switch OFF power supply.

25. Switch OFF firing circuit & field supply to the motor at the end.

26. Remove the connections

Precautions:



conductor.






Component.


103


Result:

Viva Voce:

1.What is meant by step up chopper?

2.Explain how forward motoring operation is attained with four quadrant chopper in a D.C

motor?

3.Explain how forward breaking operation is attained with a four quadrant chopper in a D.C

motor?

4. Explain how reverse motoring operation is attained with a four quadrant chopper in a D.C

motor?

5. Explain how reverse breaking operation is attained with a four quadrant chopper in a D.C

motor?

6.What is meant by step down chopper?

Remarks



105

Circuit Diagram:

Fig. 1 : Non-Circulating current type Single Phase Dual Converter

Fig. 2 : Circulating current type Single Phase Dual Converter



Single-Phase Dual Converter

Aim:

To construct a single phase dual converter and to apply reversible voltage to load.

Apparatus Required:

Table 1


Theory:

Dual converter consists of two converters both are connected to the same load. The

purpose of a dual converter is to provide a reversible DC voltage to the load. It is needed for

DC motor drives where reversal is required. Dual converter provides four quadrant operations

hence the name dual. The two modes of operations are the non-circulating current mode and

circulating current mode. In the former only one bridge is triggered. When reversal of output

voltage is required, the firing pulses for concreting bridge are stopped and second bridge is

gated. Since the conducting SCR’s in the first bridge will turn off only when the current goes

to zero, a small dead time must be allowed before the second bridge is gated otherwise: the

AC input will be shorted through the two bridges.

In the circulating current mode, both bridge are gated simultaneously, one operating

in the rectifying mode and the other in the inverting mode to avoid short circuits. This

scheme requires fully controlled bridges. The internal voltage of rectifier is higher and that of

inverter is lower than the output voltage. This can be done by two ways 1) by keeping supply

voltage V constant and firing bridge 1 (P- converter) at α and bridge 2 (N-converter) at (π-α).

By keeping firing angle constant and maintaining supply voltage at rectifier bridge greater

than supply voltage at inverter bridge.

Model Waveforms:


107

Fig. 3: Voltage waveforms for Non-Circulating current type Dual Converter

Fig. 4: Voltage waveforms for Circulating current type Dual Converter

The dual converters can be operated with or without a circulating current. In this case of

operation without circulating current, only one converter operates at a time and carries the


load current and the other converter is completely blocked by inhibiting gate pulses.

However, the operation with circulating current has the following advantages.

The circulating current maintains continuous conduction of both converters over the whole

control range, independent of the load.Since one converter always operates as a rectifier and

the other converter operates as an inverter, the power flow in either direction at any time is

possible.Since both converters are in continuous conduction the time response for changing

from quadrant to another is faster.

Procedure:

Dual Converter in Non-Circulatory Current Mode:

I) P-Converter is ON & N-Converter is OFF:

1) Connections are made as per the circuit diagram.

2) Connect rheostat at 50Ω/8A.

3) Connect CRO across load.

4) Apply AC input voltage using isolation transformer.

5) Made P-converter ON & OFF the N-converter.

6) Vary firing angle observe load voltage waveforms on CRO.

7) Note down load voltage in steps by varying firing angle α using multimeter.

II) N-Converter is ON & P-Converter is OFF:




4) Apply AC input voltage using isolation transformer.

5) Made N-converter ON & P-converter OFF.

6) Vary firing angle observe load voltage waveforms on CRO.


Firing angle of N-converter = П – firing angle of P-converter = П – α

Tabular column:

Dual converter in non circulating current mode:


109

P-converter is ON & N Converter is OFF:

Table 2

S.No Firing angle in degrees

(N-converter) π-α

Load voltage VL(DC)

in volts

N-Converter is ON & P-Converter is OFF:

Table 3

S.No Firing angle in degrees

(N-converter) π-α

Load voltage VL(DC)

In volts

Dual Converter in Circulatory Current Mode:

Table 4

S.No.

Ffiring angle α in

degrees

( P-Converter)

Firing angle П-α

in degrees

(N-converter)

Load voltage VL

(DC) in volts

III) Dual Converter in Circulatory Current Mode:




4) Apply AC input voltage using isolation transformer. Say 30V range.

5) Made N-converter ON & P-converter ON.

6) Vary firing angle, observe load voltage waveforms on CRO.



Precautions:



conductor.






Component.

Result:

Viva voce:

1.What is meant by dual converter?

2.What are types of dual converters?


111

3.What is the phase shift we have to provide between each converter in circulating current

mode of operation?

4.What are the application of dual converter?


5.what are the merits and demerits of dual converter?

Remarks


Circuit Diagram:


113

Fig:1 Circuit Diagram for single phase IGBT Based PWM Inverter

Exp no: Date:………….

Single Phase IGBT Based PWM Inverter


Aim: To study the operation of IGBT based PWM inverter.

Apparatus Required:

Table 1

S.No. Name of the equipment Range Quantity

Theory:

A device that converts DC power in to AC power at output voltage and frequency is

called an inverter. Some industrial applications of inverters are for adjustable speed AC

drives, inductive heating, stand by aircraft supplies, UPS, HVDC, transmission lines etc.

Schematic diagram of a single phase inverter is given in the fig. The current can be

supplied to the load by proper gating the IGBTs. Only two IGBTs will be on at any one time.

Load voltage is PWM signal.


115

Model Waveform:

Fig 2Model waveform for Single phase bridge inverter


The power circuit is IGBT based full bridge inverter shown in figure. When T1, T2 conduct,

load voltage is +Vs and when T3, T4 conduct load voltage is –Vs. The frequency of the

output voltage can be controlled by varying the time period. For inductive loads the diodes

connected in anti-parallel with thyristors will allow the current to flow when the main

thyristors are turned off. These diodes are called feedback diodes. The modulation technique

used is sinusoidal pulse width modulation technique. The modulation index can be varied by

the parameter setting through keyboard.

The AC load voltage is controlled by controlling modulation index. Modulation index

is the ratio of maximum amplitude of sine wave to maximum amplitude of triangular wave.

When modulation index is set keeping amplitude of triangular wave constant the amplitude of

sine wave is varied. This will happen in the internal circuit. The speed of the motor can also

be varied by varying the frequency of the inverter circuit. A keyboard is provided to set the

frequency and the modulation index. The various parameters can be displayed by the liquid

crystal display.

Procedure:

Keyboard settings

Stop/Set key: This key is used to stop the process. And also this key is used to move the

cursor to set the parameters (frequency and modulation index).

INR key: This key is used to increase the parameters (f or M) by one.

DCR key: This key is used to decrease the parameters (f or M) by one.

RUN key: This key is used to run at set parameters.

1. Circuit connections are made as shown in the circuit diagram.

2. Connect the required load.

3. Check all the connections and confirm connections made are correct before switching

on the equipment.

4. Keep the DC Voltage knob at minimum position.

5. Switch on firing circuit switch.

6. Switch on the MCB.

7. Set frequency and modulation index at suitable value. Press RUN key.

8. Adjusting input DC voltage to 100V to 200V slowly.

9. Observe the load voltage waveforms using CRO.

10. Record the frequency of the inverter circuit & the variation in AC voltage with reference

to the modulation index.


117

Tabular Column:

Table 2

S.No Modulation Index R-Load RL-Load


11. Reduce the DC voltage to minimum value.

12. Press STOP key.

13. Set new modulation index. Press RUN key.

14. Tabulate the readings in the table.

15. Slowly reduce the DC voltage to zero. Switch off all the switches when the voltage is

completely reduced.

16. Remove the connections.

17. Do the experiments for R-L load.

Precautions:



conductor.






Component.

Result:

Viva Voce:

1.What is the principle of operation of single phase bridge inverter?

2.Explain about PWM?

3.Explain about different types of PWM techniques?


119


4.What are merits and demerits of Single phase bridge inverters?

5.What are the applications of single phase bridge inverters?

RemarksSignature of the faculty


121

Circuit Diagram:

Fig. 1: DC Jones chopper circuit



DC Jones Chopper

Aim: To analyze the “ DC Jones Chopper” with R & RL loads.

Apparatus Required:

Table 1


Theory:

In many industrial applications, it is required to connect a fixed voltage DC source

into a variable voltage DC source. A DC chopper converts directly from fixed DC to variable

DC and is also known as DC to DC converter. A chopper can be considered as DC equivalent

to an AC transformer with a continuously variable turns ratio. Like a transformer, it could be

used to step down or step up a DC voltage source. Choppers are widely used for traction

motor control in electric automobiles, trolley cars, marine hoists, forklift trucks and mine

haulers. They provide smooth acceleration control, high efficiency and fast dynamic

response. Chopper can be used in regenerative braking of DC motors to return energy back to

the supply and this feature results in energy savings for transportation systems with frequent

stops. Chopper can also be in DC voltage regulators.

The Jones chopper is another example of class-D commutation in which a charged

capacitor is switched by an auxiliary SCR to commutate the main SCR.

Tabular Column:

Table 12.2


123

Model Calculations:

S.NO VIN(v) TON(Sec) TOFF(Sec)DUTY CYCLE

δ = TON / TVO(v) IO(A)


Procedure:


2. Give the DC power supply 10V to the terminal pins located in the power circuit




5. By varying the frequency and duty cycle, observe related waveforms

6. Measure output voltage and current by connecting DC voltmeter & Ammeter

7. Observe waveforms and readings, changing the frequency and duty cycle, and

Tabulate all readings



Precautions:



conductor.






Component.


125

Waveforms:

Fig. 2: Voltage Waveforms across Capacitor, SCR, auxiliary SCR and load


Result:

Viva Voce:1. What are choppers?

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

3. What are different control strategies found in choppers?

4. What are the advantages of DC choppers?

5. Explain the principle of operation of a chopper?

6. What are the disadvantages of choppers?

7. What are the applications of dc choppers?


127

Remarks

Signature of the

faculty

Fig

:1

Th

ree

Ph

ase

Fu

lly C

ontr

olle

d B

rid

ge C

onve

rter

Wit

h R

-L L

oad



Three Phase Fully Controlled Bridge Converter with R,RL Loads

Aim: To analyze the three phase fully controlled full wave bridge rectifier for

1.R load

2.R-L load.

Apparatus Required:Table 1

S.No Name of the equipment Specifications Quantity

Theory:

For any current to flow in the load at least one device from the top group

(T1, T3, T5) and one from the bottom group (T2, T4, T6) must conduct. It can be argued

Cir

cuit

Dia

gram

:


129

as in the case of an uncontrolled converter only one device from these two groups will

conduct.Then from symmetry consideration it can be argued that each thyristor conducts

for 120° of the input cycle. Now the thyristors are fired in the sequence T1 → T2 → T3 →

T4 → T5 → T6 → T1 with 60° interval between each firing. Therefore thyristors on the

same phase leg are fired at an interval of 180° and hence can not conduct simultaneously.

This leaves only six possible conduction mode for the converter in the continuous

conduction mode of operation. These are T1T2, T2T3, T3T4, T4T5, T5T6, T6T1. Each

conduction mode is of 60° duration and appears in the sequence mentioned. The

conduction table shows voltage across different devices and the dc output voltage for each

conduction interval. Each of these line voltages can be associated with the firing of a

thyristor with the help of the conduction table-1. For example the thyristor T1 is fired at

Experimental Observations:

FOR R LOAD: Table No: 2

S. No.Firing anglein degrees

Loadvoltage VDC

in volts

Current inamp

FOR R-L LOAD:Table No: 3

S. No.Firing anglein degrees

Loadvoltage VDC

in volts

Current inamp


the end of T5T6 conduction interval. During this period the voltage across T1 was vac.

Therefore T1 is fired α angle after the positive going zero crossing of vac. Similar

observation can be made about other thyristors. The phasor diagram also confirms that all

the thyristors are fired in the correct sequence with 60° interval between each firing. shows

the waveforms of different variables

If the converter firing angle is α each thyristor is fired “α” angle after the positive going

zero crossing of the line voltage with which it’s firing is associated. Once the conduction

diagram is drawn all other voltage waveforms can be drawn from the line voltage

waveforms Similarly line currents can be drawn from the output current and the

conduction diagram. It is clear from the waveforms that output voltage and current

waveforms are periodic over one sixth of the input cycle. Therefore this converter is also

called the “six pulse” converter. The input current on the other hand contains only odds

harmonics of the input frequency other than the triplex (3rd, 9th etc.) harmonics.

The advantage of fully controlled bridge rectifier is the capability of wide

voltage variation between Vdcmin minimum to Vdcmax maximum volts. Such rectifiers

find application in DC motor loads for both motoring and electrical braking of the motor.

Procedure:

1. The connections are made as shown in the circuit of fully controlled rectifier with R load using

Isolation transformer and rheostat.

2. Connect input terminals N, R, Y & B of isolation transformer to respective terminals N, R, Y &

B of firing circuit.


131

3. Connect output terminals R, Y & B of isolation transformer to respective terminals R, Y & B

of power module.

4.Connect input terminals N, R, Y & B of isolation transformer to respective terminals N, R, Y &

B of firing circuit.

5. Connect CRO across the load. Use 10:1 CRO Probe or Power Scope.

6. The gate cathode terminals of the three SCR’s are connected to the respective points on the

firing unit.

7. Check all the connections and conform connections made are correct before switching on the

instrument.

8. Keep the firing angle knob to minimum position. Switch on three phase supply, power unit as

well as firing unit.

Model Waveforms:

Fig2: Votage and Current Waveforms for a 3-phase Full Converter at different firing

angles


9.Vary firing angle gradually. The output wave forms are seen on a CRO.

10. Trace the load voltage waveforms for any one firing angle.

11. The firing angle is varied and DC output voltage is noted.

12. Bring the firing angle knob to minimum (anticlockwise) position.

13. Switch off MCB, firing unit & three phase AC mains.

14. Experiment may be repeated by connecting R-L loads.

Precautions:

1.Do not conduct the experiment without three phase isolation transformer. If you try to conduct

experiment without isolation transformer the instrument may be damaged due to short circuit

exists between single phase & three phase supply while making measurement using CRO.

2.Do not attempt to observe load voltage and input voltage simultaneously, if does so input voltage

terminal directly connected to load terminals due to the non isolation of both channels of the

CRO.

Result:

Viva Voce:

1.What is need of three phase isolation transformer in a three phase fully controlled rectifier?


133

2.What is the effect of source inductance in a three fully controlled converter?

3.Compare the three phase full controlled and three phase semi converter?

4.What are the applications of three phase fully controlled converter?


5.Explain how the triggering pulses are given in a three phase fully converter?


135

Remarks


pe lab manual

Engineering

anode current

latching current

current il

forward voltage

current ih

flow of current

current flowing

gate voltage