Power Electronics Lab Demo

Power Electronics Lab DemoBy : Alok Kumar MishraAsst. ProfessorDepartment of EEEExperiment No.123SCR V-I Characteristics To Draw the V-I Characteristics of SCR.

4 Aim of the Experiment5Apparatus RequiredSl. No.InstrumentRangeTypeQuantity1.SCR Characteristics Study Unit--012.Patch cords for connecting--As reqd.6Circuit Diagram

SCR (Silicon Controlled Rectifier) is a 3-terminal, 4-layer, 3-junction p-n-p-n semiconductor switching device.

7SCR

Like a diode an SCR is an unidirectional device that blocks current from cathode to anode. Unlike a diode SCR also blocks current from anode to cathode until it is trigger into conduction by a proper gate signal between gate and cathode.8VI Characteristics of SCR

9Reverse Blocking Mode

In this mode when a ve voltage is applied across anode to cathode a reverse leakage current starts flowing from cathode to anode due to minority charge carrier.

When Vak increases to VBR (Reverse Breakdown Voltage) Junction J1 & J3 breakdown due to avalanche.

At this voltage VBR SCR is associated with high voltage & high current, so the device will get damage.

10Forward Blocking Mode

In this mode when a +ve voltage is applied across anode to cathode a forward leakage current starts flowing from anode to cathode due to minority charge carrier.

If Vak increases to VB0 (Forward Breakover Voltage) Junction J2 breakdown due to avalanche & the device gets turned on.

Generally VB0 is very high, hence this method is not employed for triggering the SCR.11Forward Conduction Mode

In this mode a +ve voltage is applied across anode to cathode.

When a small voltage is applied across gate to cathode Ig starts flowing from G to K hence electron will flow in opposite direction & Junction J2 breakdown due to avalanche, at Vak which is much less than VB0 & the device get turned on ( when Ia > IL ).

V I Characteristics of an SCR is drawn between anode to cathode voltage (Vak) and anode current Ia.

In this exp. we only draw the forward blocking & forward conduction part.

Make the connection as per circuit diagram.

Switch on the power supply.

Give a suitable gate current ( 5 6 ) mA from 15V source.

Keep the rheostat in middle position.

12ProcedureGradually increase Vak from 40V source (start from 0 volt).

Note down Vak & Ia

At one point we notice that Vak reduces to very low value & Ia increases to a high value.

Note down Vbo (forward breakover voltage ) & IL (Latching Current).

Keep Vak at Vbo from 40V source & increase the load resistance gradually from rheostat.

Ia starts decreasing keep on increasing the load resistance gradually.

13Procedure Contd Note down Vak & Ia. At one point we notice that SCR stops conducting. Note down Recovery voltage & IH (Holding Current). Repeat above step for a different gate current, we notice that Vbo will change. Draw the graph between Vak & Ia. Mark the point IH , IL ,Vbo.

14Procedure ContdSl.No.Ig1 = (mA)

Ig2= (mA)

Vak (V)Ia (mA)Vak (V)

Ia (mA)

1.2.3. . ...........

.....

.....

15Tabulation End ..Experiment No.21617Triggering method of SCRStudy of Different method of triggering of SCR.

RC Triggering MethodUJT Triggering MethodCosine - Triggering Method

18 Aim of the Experiment19Apparatus RequiredSl. No.InstrumentRangeTypeQuantity1.RC triggering Unit--012.UJT triggering Unit--013.Cosine triggering Unit

-

-

01

4.Patch cords for connecting--As reqd.20TriggeringTriggering means taking an SCR from forward blocking mode to forward conduction mode.

There are different method triggering i.e. a) Forward voltage triggering b) Gate triggering c) dv/dt triggering d) Light triggering e) Temperature triggering

Generally we use gate triggering. (practically)

In gate triggering a gate pulse is applied across gate & cathode when SCR is forward biased(i.e. SCR is in Forward blocking mode).

In some cases (HVDC) Light triggering is also used. 21Firing Angle

In the +ve half SCR is FB

In the -ve half SCR is RB

If we give a gate pulse in the +ve half SCR will turn on

If we replace SCR with a diode it will remain on in in entire +ve half & off in entire -ve half.

Firing angle is the angle measured from the instant that gives largest average output voltage to the instant it is triggered.22R & RC Triggering In Resistance(R) triggering the range of firing angle is 0 < < 90 .

The limited range of firing angle or triggering angle in Resistance(R) triggering can be overcome by using RC (Resistance - Capacitance) triggering.

23Circuit Diagram

24RC Half wave TriggeringThe above circuit is called a half wave trigger circuit because this is a half wave controlled rectifier circuit.

The transformer step down the ac supply 230v to12v.

In the +ve half of the transformer output the SCR is FB, if SCR is triggered then it get short circuited and we get the output voltage across the load.

To trigger the SCR capacitor voltage Vc is used, which will appear across the gate and cathode when it is charged. 25RC Half wave Triggering Contd.. At wt = 0 let the voltage reduce to some lower value oa . In the +ve half cycle the capacitor charges with upper plate +ve , through load, R1, R, C & T/F from the initial ve voltage -oa.

In the ve half cycle the capacitor C charges to peak supply voltage Vm at wt= -/2, through D2, R1, load, T/F .

26RC Half wave Triggering Contd.. When capacitor voltage Vc = Vgt SCR turns on.

Vgt = minimum gate trigger voltage, i.e. the minimum G to K voltage to turn on the SCR.

To change the firing angle change the value of R .

For higher value of R the charging time of the capacitor is more ( from oa to Vgt ) and is more & output voltage is less, and vice versa. 27Procedure for RC Triggering Make the connection as per circuit diagram, connect multimeter or dc voltmeter across the load.

Keep the potentiometer (R) in maximum position & switch on the supply.

For high value of R charging time is more hence we will not get the pulse & output voltage.

Decrease the value of R, observe the gate pulse across gate cathode terminal & trace the gate pulse from the CRO.

28Procedure for RC Triggering Contd..Observe the load voltage (Vo), SCR voltage waveform on CRO & trace it. Gradually vary the potentiometer (R), note down firing angle & corresponding load voltage & gate pulse. Calculate the firing angle from CRO for different R. Calculate the theoretical value of load voltage (Vo) by using the formula Vo = Vm /2 ( 1 + cos ) { for HWR } Note down the practical value Vo from voltmeter reading. Calculate the error.

29Circuit Diagram

30RC Full wave TriggeringFull wave triggering means we trigger the SCR in both the half cycle & we get the output voltage in both the half cycle . In this full wave triggering circuit the diode rectifier output voltage Vd is

In both the half cycle the SCR is FB, if SCR is triggered (in both the half cycle) then it get short circuited and Vd will appear across the load.

In the same way Vc is used to trigger the SCR.

31RC Full wave Triggering Contd.. When capacitor voltage Vc = Vgt SCR turns on in both the half cycle. The firing angle can be changed by changing R. Here initial voltage across the capacitor is zero. In the beginning of the of the cycle capacitor charges from zero through load, R1, R, C ,Vd.

32Procedure for RC Full wave Triggering Same as half wave triggering. To calculate the theoritical value of load voltage (Vo) change the formula to Vo = Vm / ( 1 + cos ) { for FWR }33UJT (Uni Junction Transistor) An UJT is made up of an n-type silicon base to which p-type emitter is embedded. It has 3 terminal emitter E, base-one B1, base-two B2 Between bases B1 & B2 the unijunction behaves like an ordinary resistance. RB1 & RB2 are the internal resistances respectively from bases B1 & B2 to eta point A.

34UJT Contd When VBB is applied between B1 & B2 then VAB1 = VBB (RB1 / (RB1 + RB2) ) = VBB = intrinsic stand-off ratio Let a voltage Ve is applied between E & B1 As long as Ve < VBB E - B1 unijunction is RB & Ie is ve When Ve = VBB + VD at point B Ie is +ve & E - B1 unijunction begins to conduct. At point B emitter starts injecting holes to B1 It has 3 terminal emitter E, base-one B1, base-two B2 Because of increase number of charge carrier the resistance RB1 of E - B1 unijunction decreases. As a result potential of eta point A falls & therefore Ie increases. This exhibit a ve resistance region ( an increase in Ie with decrease in Ve ) At point C entire base region is saturated & RB1 does not decrease any more. A further increase in Ie will increase Ve. This ve resistance region is used to trigger the SCR. 35UJT Oscillator Triggering R1 & R2 is small compared to RB1 & RB2 .

When VBB is applied capacitor C begins to charge through R towards VBB. When the capacitor voltage Vc = Ve reaches VBB + VD, E - B1 junction breaks downs. As a result UJT turns ON and C rapidly discharges through low resistance R1.

The voltage drop across R1 used to trigger the SCR.

36Circuit Diagram

37Procedure Same as RC triggering for UJT Triggering In this circuit In place of R1 a pulse transformer is connected for amplification and isolation of gate pulse. Make the connection as per circuit diagram, connect multimeter or dc voltmeter across the load. Keep the potentiometer (R =220K) in maximum position & switch on the supply. For high value of R charging time is more hence we will not get the pulse & output voltage. Decrease the value of R, observe the gate pulse across gate cathode terminal & trace the gate pulse from the CRO.

38Procedure Same as RC triggeringfor UJT Triggering Contd..Observe the load voltage (Vo) waveform on CRO & trace it. Gradually vary the potentiometer (R), note down firing angle & corresponding load voltage & gate pulse. Calculate the firing angle from CRO for different R. Calculate the theoretical value of load voltage (Vo) by using the formula Vo = Vm /2 ( 1 + cos ) { for HWR } Note down the practical value Vo from voltmeter reading. Calculate the error.

39Circuit Diagram

40Procedure for UJT Full wave TriggeringSame as half wave triggering. For full wave remove the connection P to Q & R to S and join P to M & N to S. To calculate the theoretical value of load voltage (Vo) change the formula to Vo = Vm / ( 1 + cos ) { for FWR }41COSINE FIRING PULSE GENERATOR

42Procedure for COSINE Triggering t1 is a sine wave when it is phase shifted by 90 it becomes cosine. Similarly 270 phase shift means it is -cos. Comparator 1 gives o/p when VR is more +ve than t2 (same for comp.2). Clock pulse generator gives pulse when there is a change from 0 to +ve. When CK1and CK2 is given to SR FF they generates the respective o/p according to the T T. Carrier oscillator gives train of pulses.43Waveforms for cosine firing scheme

44How to calculate the firing angle

45TabulationSl.No.Type of ConnectionxyVmaxVo(Theoretical)(From Formula)Vo(Practical)(From Voltmeter Reading)%Error1.UJT HW2.UJT - FW3.RC HW4.RC FW5.COSINEEnd ..Experiment No.34647AC RegulatorTo trace the output waveform of a single Phase AC Voltage Controller Using TRIAC.

48 Aim of the Experiment49Apparatus RequiredSl. No.InstrumentRatingQuantity1.Triac Power Circuit-012.MultimeterDigitalType01

3.CRO(Dual Trace)30MHz01

4.Patch cords for connecting-As reqd.50Triac SCR is an unidirectional device conduct only from A to K by giving a proper gate signal in Forward blocking mode. A TRIAC is a bidirectional device which can conduct from A to K and K to A with a proper gate signal. As it can conduct in both direction the terms A & K are not applicable to triac. Its three terminals are MT1 (Main terminal 1), MT2 (Main terminal 2) & Gate.When a +ve gate voltage w.r.t MT1 is applied in ve half it conducts from MT1 to MT2. When a +ve gate voltage w.r.t MT2 (or -ve voltage w.r.t MT1) is applied in -ve half it conducts from MT1 to MT2. 51Triac Contd

52AC Regulator S1 & S2 get turn OFF naturally at & 2 respectively due to resistive load as current falls below IH . In this experiment we use one Triac in place of two switch S1 & S2.

53Circuit Diagram

54Circuit Diagram

55Procedure Make the connection as per circuit diagram. Generate the firing pulse from the triggering unit and trace the o/p voltage for different firing angle from the CRO. Calculate the firing angle using the formula = (X/Y) 180 Calculate Vrms using the formula Vrms = Vm/2[ {( )+ (sin2)/2 }] = Vs/[ {( )+ (sin2)/2 }]

56TabulationSl. No.xy

Vrms (Theoretical)Vrms (Practical)

1.2.3....

...

...

...

...

End ..56Experiment No.457581Controlled RectifierTo trace the output waveform of a single Phase half/full controlled bridge with R, R-L, R-L-E Load with & without FD

59 Aim of the ExperimentSl.No.InstrumentRatingQuantity1.Isolation transformer230/115V,11KV012.1 Full Controlled Converter-01

3.1 Converter Firing Circuit-

01

4.Load Resistance100/2A01

5.Load Inductance

80mH01

6.DC Voltmeter0-75V01

7.DC Ammeter

0-2A01

8.Patch cords for connecting-As reqd.60Apparatus Required61Circuit Diagram

Phase control Rectifier converts fixed ac to variable dc.

CLASSIFICATIONDepending upon the device used : Uncontrolled all device are diode Controlled all device are SCR Half Controlled The device are half SCR and half diode.Depending upon the type of supply : Single phase Half wave Full wave Centre tapped Bridge type Three phase Half wave Full wave

In this experiment we will do the single phase full wave bridge type controlled and half controlled rectifier with different load.

62Phase Controlled Rectifier63Phase Controlled Rectifier Contd

In +ve half T1T2 are FB when a gate pulse is applied to T1T2 at a firing angle it conduct and Vo = Vin. For R load o/p current waveform is same as Vo, hence at t = , Vo = 0 and io = 0 hence T1T2 turns off naturally as io < iH . In -ve half T3T4 are FB and process repeats

For R-L load due to the inductor which opposes sudden change in current, current rises gradually and let it reduce to zero at > .Up to , T1T2 remain ON as io > iH i.e. +ve and hence -ve voltage appears across the o/p. Same process repeats in -ve half.For RLE (dc motor) load T1T2 is FB only if Vin >E hence must be greater than 1 and less than 2. Same procedure repeats similar to RL load only the change is when all device are not conducting we get Vo = E.

64Phase Controlled Rectifier ContdWith Free Wheeling Diode : When we connect the FD across the load a b and c d are connected (in circuit diagram).No change in o/p voltage waveform for R load.For RL and RLE load -ve part of o/p voltage will be removed because when the Vo tries to just become -ve FD get FB and thyristor current shifts to FD and SCR turns OFF and current freewheels through FD. Vo =0 when FD is conducting.FD is used to improve the performance of the o/p voltage.

65Phase Controlled Rectifier ContdSingle phase Semi controlled Bridge rectifier :

Same circuit diagram only T2 T4 is replaced by D2 D1.In +ve half T1 D2 are FB when gate pulse given to T1 it conducts.In -ve half T3 D1 are FB when gate pulse given to T3 it conducts.For R load V0 waveform remain unchanged.For RL load and RLE load at t = D2 is RB and turns OFF and T1 also turns OFF there is no -ve o/p voltage waveform beyond .If we connect the FD across the load i.e. connect a b and c d .No change for R load.For RL and RLE load if load current is assumed continuous at t = FD gets FB and load current freewheels through FD.

66Phase Controlled Rectifier Contd Make the connection as per circuit diagram For half controlled, connect the gate cathode terminal of 2 SCRs to the respective point on the firing module & for full controlled connect 4 SCRs to the respective point on the firing module.Keep the firing angle knob at 180(min. position) & switch on the power supply. Switch on the power circuit & vary the firing angle.For full controlled bridge trace the o/p voltage waveform from CRO at different firing angle and different load.Simultaneously note DC o/p voltage & current through the load.

67ProcedureCalculate from CRO and Vm and calculate the avg. o/p voltage from the given formula.

Vo(avg) = Vm/ [ 1+cos ] R load Vo(avg) = Vm/ [ 1+cos ] R-L load with FD Vo(avg) = 2Vm/ [ cos ] R-L load without FD

Semicontrolled rectifier : Vo(avg) = Vm/ [ 1+cos ] R-L load with FD Compare the theoretical value of Vo(avg) calculated using the formula with practical value(voltmeter readings).Repeat above procedure with FD.Repeat above procedure for half controlled bridge.

68Procedure Contd69TabulationSl.No.Firing angle ()Idc (Amp)Theoretical VdcPractical Vdc

Full Controlled rectifier with R load (without FD) 1.....2.....Full Controlled rectifier with R load (with FD) 1.....2.....Full Controlled rectifier with R-L load (without FD) 1.....2.....Full Controlled rectifier with R-L load (with FD) 1.....2.....70TabulationSl.No.Firing angle ()Idc (Amp)Theoretical VdcPractical Vdc

Semi Controlled rectifier with R load (without FD) 1.....2.....Semi Controlled rectifier with R load (with FD) 1.....2.....Semi Controlled rectifier with R-L load (without FD) 1.....2.....Semi Controlled rectifier with R-L load (with FD) 1.....2.....Experiment No.57172Series Inverter To observe and trace the output waveforms of series inverter for different frequencies.

73 Aim of the Experiment74Apparatus RequiredSl. No.InstrumentRatingQuantity1.Series inverter module-012.MultimeterDigitalType01

3.CRO(Dual Trace)30MHz01

4.

Load Resistance (Rheostat)50, 5A01

5.Patch cords for connecting-As reqd.75Commutation Once the SCR is in conduction gate looses its control now the device can be turned off if IA is reduced below IH, the device goes to forward blocking mode. How ? Consider a circuit Case - I

There is a natural zero at A, io = 0 due to resistive load as io < IH SCR OFF naturally i.e. natural commutation.76 Commutation Contd Commutation means the SCR has regain its forward blocking capability after forward conduction. (i.e. thyristor turn off process) Case II

When SCR in ON Vo=Vdc , io = Vdc/R ( there is no natural zero )

In the case we have to use forced commutation.

In this experiment the series inverter uses class A type commutation. In series inverter the commutating component are connected in series i.e. L,C.

77Class A Commutation

In class A commutation R,L,C is so designed that the overall circuit must be under damped.

For an under damped circuit i.e < 1, n = 1/ (LC), = R/2(C/L)

The nature of current waveform is shown above.

As there is a natural zero at A SCR turns OFF. 78Circuit Diagram

79Procedure T1 & T2 are FB always. Gate pulse given to T1 it conducts and it forms an under damped circuit consisting of L1, R ,C2. +ve o/p voltage appears across the load. And T1 turns off automatically due to the natural zero.(self commutation) When gate pulse given to T2 process repeats similar to T1 and ve voltage appears across o/p. (as current direction is reverses) In this exp. make circuit connections as shown in the circuit diagram. Connect rheostat as load with input DC voltage at 24V.7980Procedure Contd Connect the CRO probe across the rheostat.

Switch on the D.C. power supply and give triggering pulses to the thyristors.

Vary the frequency of the inverter circuit in steps. For each step note down output voltages.

Observe and trace load voltage waveforms from CRO.

Tabulate the readings in the table. 81TabulationSL.NO.No. of X DivisionTime periodT in ms Frequency in HzLoad Voltage in volts1.2.3.4...........

.....

.....

End ..Experiment No.682833Controlled RectifierTo trace the output voltage waveform of Three Phase full wave fully controlled and semi controlled converter with R, R-L, R-L-E Load with & without FD.

84 Aim of the ExperimentSl.No.InstrumentRatingQuantity1.3 Isolation transformer-

012.3 Converter Module-01

3. Converter Firing Circuit-

01

4.Load Resistance100/2A01

5.Load Inductance

-01

6.Autotransformer0-230V/5A01

7.Patch cords for connecting-As reqd.85Apparatus Required86Circuit Diagram

For high power application 3 rectifier are used. (1 up to 15KW are used)

87Fundamentals

Diode whose anode potential is highest will conduct i.e. D2.

Diode whose cathode potential is min will conduct i.e. D2.883 Half Wave Rectifier

The 3 phase voltages (VA, VB, VC) are displaced by 120.

The diode whose anode potential is highest must conduct first. Each diode conduct for a duration of 120 in 360 cycle.

VA is more +ve from /6 to (5)/6 D1 conduct when D1 conduct V0 = VA

VB is more +ve from (5)/6 to (9)/6 D2 conduct, V0 = VB

VC is more +ve from (9)/6 to 2 D3 conduct, V0 = VC

89Output voltage waveform of 3 HWR

It gives a 6-pulse o/p voltage per cycle.

Each diode conduct for 120 and diode conduct in pair i.e. one from +ve group and one from -ve group, i.e. at the o/p we will always get a line voltage between any two line.

903 Full Wave Diode Bridge Rectifier

The diode in +ve and ve group are so named that they conduct in a serial sequence in no. i.e. D1-D2, D2-D3, D3-D4, D4-D5, D5-D6, D6-D1 & again D1-D2.Here we draw the line voltage.

we know that the line voltage leads the respective phase voltage by 30.

913 Full Wave Diode Bridge Rectifier Contd..

From /6 to /2 the line voltage VAB is most +ve hence A connected to D1 and B connected to D6 conduct & we get V0 = VAB Similar to diode rectifier it gives a 6-pulse o/p voltage.Each SCR conduct for 120 and SCR conduct in pair for 60 one from +ve group & one from -ve group.Conducting seq. is same T1-T2, T2-T3, T3-T4, T4-T5, T5-T6, T6-T1

923 Full Wave Controlled Rectifier Firing angle will measure from t = /6 (bcoz T1 get FB from /6).

Let = 30, when we give a gate pulse at /3 to T1 it conduct together with T6, as T6 is already conducting. When we give a gate pulse to T2 at (2)/3 it conduct with T1 up to & so on.For R-load if < 60 the current is continuous.933 Full Wave Controlled Rectifier Contd

For R-load if > 60 the current becomes discontinuous.

943 Full Wave Controlled Rectifier Contd

For R-L load if the load inductor is assumed to be high then even if >60 load current is continuous and we get the ve voltage also all device will conduct for 120.

953 Full Wave Controlled Rectifier Contd

963 Half Controlled Converter SCR from ve group T4 T6 T2 is replaced with D4 D6 D2 Only the SCR to be triggered at an interval of 120.

For < 60 o/p voltage waveform is continuous if > 60 it is discontinuous. As the o/p voltage tries to become ve FD conducts & V0 = 0.

973 Half Controlled Converter Contd

Make the connection as per the circuit diagram.For half controlled, connect 3 SCRs and three diodes. In case of a full converter six SCRs are connected as controlled switches. Switch on the power circuit and vary the firing angle.Trace the waveforms for various firing angle from CRO.Simultaneously note DC o/p voltage and current through load. 98ProcedureCalculate from CRO and Vm and calculate the avg. o/p voltage from the given formula.

Vo(avg) = 3Vm/ [cos ] R load, < 60 Vo(avg) = 3Vm/ [ 1+cos ( /3 + ) ] R load > 60 Vo(avg) = 3Vm/ [ cos ] R-L load

Semicontrolled rectifier : Vo(avg) =33 Vm/2 [ 1+cos ] R-L load with FD Compare the theoretical value of Vo(avg) calculated using the formula with practical value(voltmeter readings).

Repeat above procedure with FD.

Repeat above procedure for half controlled bridge.99Procedure ContdSl.No.Firing Angle ()Type of LoadPractical value in volt (Vo)Theoretical value in volt (Vo)

% Error1.2.3.4.30...R Load with FDR-L Load with FDR Load without FDR-L Load with FD5.6.7.8.75...R Load without FDR-L Load with FDR Load without FDR-L Load without FD100Tabulation End ..Experiment No.7101102Jones chopperDC Jones chopper to control o/p average dc voltage at constant frequency with different duty cycle.

103 Aim of the Experiment104Apparatus RequiredSl. No.InstrumentRatingQuantity1.DC Chopper power circuit-012.DC Chopper firing circuit-013.DC regulated power supply0-30V/2A01

4.

Rheostat50 / 2A 01

5.CRO30MHz01

6.Patch cords for connecting-As reqd.DC Chopper converts fixed dc to variable dc voltage105DC Chopper

106Circuit Diagram

T1 is the main SCR i.e. when T1 is on V0 = Vin and when T1 is OFF V0=0To get a chopped o/p voltage we have to ON and OFF T1 in a cycle.T1 is always FB when a gate pulse is applied it turns ON.How to turn OFF no natural zero current.To turn OFF T1 we have to use forced commutation, here C, T2, D1, L1 is used to turn OFF the main SCR T1.T2 is called the auxiliary SCR, when T2 is turn ON T1 OFF.Here we will use CLASS-D commutation (voltage commutation.)

107ExplanationIn voltage commutation a reverse voltage is applied to a conducting thyristor for a min time tq.Let us assume that initially C is charged to a voltage Vdc with upper plate +ve.When SCR T1 is triggered, load current flow through T1, L2, Load & Vin.The capacitor discharge current flow through C, T1, L1,D it forms a LC resonant circuit and charges to Vdc with lower plate +ve.When T2 is ON the reverse capacitor voltage appears across T1 and turns OFF.

108Explanation Contd Voltage commutationIn this experiment we have to trace the o/p voltage waveform for different i/p voltage at different duty cycle.Make the interconnections in the power circuit as given in the circuit diagram, connect DC supply from a fixed DC source.Initially set the input DC supply to 10V. Connect a resistive load.Connect the respective trigger outputs from the firing circuit to the respective trigger outputs from the firing circuit to the respective SCRs in the power circuit.

109ProcedureObserve the voltage waveform across load. A chopper DC waveforms is clearly observed.

Observe the voltage across load, capacitor, main SCR and auxiliary SCR by varying duty cycle.

Now vary the DC supply up to a rated voltage (30V DC). Trace the waveforms at different duty cycle. Connect voltmeter and ammeter and note down values in the table.110Procedure ContdSl. No.VinToN(In ms)ToFF(In ms)

Duty CycleVo1.2.3.4.5.111Tabulation End ..Experiment No.8112113Different Commutation Technique of SCR To rig up different Commutation Circuit of SCR and trace the o/p waveform from CRO.

114 Aim of the Experiment115Apparatus RequiredSl. No.InstrumentRatingQuantity1.Forced Commutation study unit-012.

Rheostat50 / 2A 01

3.CRO30MHz01

4.Patch cords for connecting-As reqd.5.CRO Probe

-

As reqd.

116Commutation Once the SCR is in conduction (gate looses its control) gate pulse can be removed to reduce the gate circuit power loss, now the device can be turned off if IA is reduced below IH, the device goes to forward blocking mode.

How ?

It depends on the nature of the circuit.

Thyristor turns off means the thyristor has regain forward blocking capability after forward conduction this process is known as commutation. 117Commutation Contd

Consider a circuit Case - I There is a natural zero at A, io = 0 due to resistive load as io < IH SCR turns off naturally i.e. natural commutation.118 Commutation Contd Case II

When SCR in ON Vo=Vdc , io = Vdc/R ( there is no natural zero )

In the case we have to use forced commutation.

Forced commutation means some external mechanism has to be used to turn off the SCR

Forced commutation are of two type 1. Voltage commutation 2. Current commutation119Voltage Commutation

When SCR is conducting with help of an external capacitor a reverse voltage is applied to the device for a minimum time tq and SCR become RB & the current through the SCR is zero which is less than IH & device gets commutated.120Current commutation

When device is conducting it is carrying a current iA in the direction i.e. A to K.

A current Ix is allowed to flow in the opposite direction i.e. K to A in the conducting SCR

Net current in the direction A to K reduces & when the current reduces below IH device turns off.121ClassificationCLASS A & B Current Commutation

CLASS C, D & E Voltage Commutation

CLASS A Self Commutation or natural Commutation

(because they do not require an external SCR to OFF main SCR)

CLASS B, C, D, E Auxiliary Commutation

(because they need an external SCR to OFF the main SCR)122Class A Commutation In class A commutation R,L,C is so designed that the overall circuit must be under damped.

For an under damped circuit i.e. < 1, n = 1/ (LC), = (R/2)(C/L)

The nature of current waveform is shown above.

As there is a natural zero at A SCR turns OFF naturally.

123Class B Commutation

Initially the capacitor charges to a voltage Vs with polarity shown.

When T1 ON load current starts flowing through Vs, T1, Load.(Capacitor voltage remains at Vs) When TA ON Capacitor forms a resonant circuit C, L,TA and reverse its polarity and charges to Vs with - + polarity, and the resonant current becomes zero TA turns OFF. Vc with polarity - + Vs FB the diode D, as T1 is conducting a resonant current starts flowing in opposite direction i.e. from K to A of SCR T1, C, L, D. Hence the net current through T1 reduces to zero & turns OFF.124Class C Commutation

Initially T1 is on by giving a gate pulse, a load current starts flowing through Edc, R1, T1. Another current ic flow through T1 through Edc, R2, C, T1 which charges Vc to Edc with polarity shown.When T2 is ON a reverse voltage Vc falls across T1 through T2 which turns OFF T1. Now two current flow, Edc, R1, C, T2 and cap. charges to Edc with reverse polarity, another current Edc, R2, T2 flow. When T1 ON T2 turns OFF as a reverse voltage appears across T2 through T1. This type of commutation is called complementary commutation because turning ON T1 turns OFF T2 and turning ON T2 turns OFF T1.125Class D Commutation

Initially it is assumed that capacitor is charged to a voltage of Edc with polarity shown & T1 & T2 are initially OFF.

When T1 is ON a load current starts flowing through T1 & V0 = Edc, and capacitor gets a discharging path through T1 and a resonant current also starts flowing through C, T1, L,D which changes the polarity of capacitor voltage to Edc. When T2 turns ON a reverse voltage appears across T1 i.e. the Edc and turns OFF T1.126Class E Commutation

This commutation is also called external pulse commutation becuase an external source is needed to turn OFF the main SCR.

T1 is FB and when it is triggered it starts conducting and V0 = V1. To turn OFF T1 a base current is given to ON the transistor. As a result a reverse voltage V2 appear across T1 and turns off thyristor T1.127Procedure Make the connections as per circuit diagram.

Connect trigger outputs to gate and cathode of respective SCRs.

Switch on the DC supply to the power circuit and also switch on the firing circuit.

Observe the voltage waveforms across the load resistance and trace those waveforms.

Observe the voltage waveforms at different frequencies of chopping and also at different duty cycles. Repeat the experiment for different commutation circuits. End ..Experiment No.91281293- InverterTo trace the phase and line voltage waveform of a IGBT based 3- PWM inverter.

130 Aim of the Experiment131Apparatus RequiredSl. No.InstrumentRatingQuantity1.Inverter module-012.Rheostat100 / 2A 013.CRO30MHz014.DC Power Supply0 60V015.Patch cords for connecting-As reqd.6.MultimeterDigital type01132Inverter Inverter is a device converts dc to ac.

VSI (Voltage source inverter) A Voltage source inverter is that inverter which takes a constant dc supply as its input.

A 3 VSI, takes constant dc as input and generate 3 voltage having a phase displacement of 120.

A basic 3 inverter is a six-step bridge inverter. It uses a minimum of 6 thyristor or 6 IGBT. A step is defined as a change in the firing from one thyristor to the next thyristor in proper sequence.133Inverter Contd For 360 each step must be of 60 for a 6 step 3 inverter.

It means that the thyristor must be gated at interval of 60 in proper sequence, so that a 3 ac voltage is produced at the output of a six-step inverter.

A 3, 6-step inverter has either 6 thyristor or 6 IGBT & 6 diodes.

Basically there are two modes of gating the thyristor, in one mode thyristor conduct for 180 each & in other mode thyristor conduct for 120 each.

But in both the mode gating signals are applied & removed at 60 intervals of the output voltage waveform.

134Circuit Diagram

Each SCR conduct for 180 of a cycle.

SCR in each arm/phase e.g. T1 & T4 are switched ON after a time interval of 180 between the two SCR.

It means T1 conducts for first 180 of the cycle then T4 conducts for next 180 of the cycle.

SCR in the upper group or in the +ve group conduct at an interval of 120.

i.e. T1 is ON at t = 0 & T3 is ON at t = 120, then T5 is ON at t = 240. Same things for lower group.

The sequence of firing SCR is

T1 T2 T3 T4 T5 T6

135180 Conduction Mode of 3 VSI136180 Conduction Mode of 3 VSI Contd

In step I 0 < t < /3

Equivalent ckt. is Req = (3R)/2,

Vs = I (3R)/2 => I = (2Vs)/(3R)

Apply current division

Vao = R I R/(2R) = Vs/3

Vco = Vs/3 Vbo = -2V/3

Line voltage

Vab = Vao Vbo = Vs Vbc = -Vs, Vca = 0

Analysis is same for all other step.137180 Mode

Circuit diagram is same.

Here SCR conduct for 120 only, here also each step is of 60 duration.

Here if the upper thyristor of a phase conduct for 120 then there is a 60 commutation gap during which none of the thyristor of that phase conduct.

If T1 is triggered at t = 0, T1 conducts upto t = 120, then from t = 120 to 180 none of T1 & T4 conduct. At t = 180 T4 is triggered and so on..

Here in any one step only two thyristor is conducting.

138120 Conduction Mode of 3 VSI139120 Conduction Mode of 3 VSI Contd

The triggering sequence is same T1 T2 T3 T4 T5 T6

Step - I 0 < t < /3 ,

Eq. ckt.

Only T1 & T6 are conducting.

Vao = Vs/2 Vbo = -Vs/2 , Vco = 0 Vab = Vs

Vbc = -Vs/2 , Vca = -Vs/2 Analysis is same for all other step.

140120 Mode

Make circuit connections as shown in the circuit diagram.

Apply DC source of 60V to the inverter module.

Switching ON the inverter firing unit.

Switch ON the DC power supply.

Vary the frequency of the inverter circuit in steps. For each step observe and trace load voltage waveforms from CRO.

141ProcedureSl.No.Mode of OperationLine Voltage From CRO(in volt)Line Voltage From Multimeter(in volt)

Phase Voltage From CRO(in volt)

Phase Voltage From Multimeter(in volt)

1.180 (f1)2.180 (f2)

3.120 (f1)

4.120 (f2)

142Tabulation End ..Experiment No.10143144CycloconverterTo trace the output voltage waveform of a step down Cycloconverter with R load.

145 Aim of the Experiment146Apparatus RequiredSl. No.InstrumentRatingQuantity1.1 Cycloconverter power module-012.Rheostat100 / 2A 013.CRO30MHz014.Thyristor Firing Circuit-015.Patch cords for connecting-As reqd.6.MultimeterDC, Digital type01147Cycloconverter Converts AC AC

Cycloconverter is a device which converts fixed frequency ac voltage to variable frequency variable magnitude ac voltage.

TYPE 1. Step up Cycloconverter (f0 > fs) 2. Step down Cycloconverter (f0 < fs) f0 = output frequency fs = supply frequency148Circuit Diagram

149Cycloconverter Contd In +ve half of supply voltage i.e. a is +ve w.r.t. o and b is ve w.r.t. o.

In this half TH1 and TH4 are FB, if TH1 is triggered +ve o/p voltage appears across the output, if TH4 is triggered -ve o/p voltage appears across the output.

In -ve half of supply a is -ve w.r.t. o and b is +ve w.r.t. o, TH2 is FB and TH3 is FB.

If TH3 is triggered +ve voltage appear across o/p and TH2 is ON ve voltage appear across the o/p.

For step down Cycloconverter fo < fs & To > Ts

150Cycloconverter Contd Let to draw the waveform of f0 = (1/4)fs

TH1 & TH3 ON alternately for four half cycle and for next four half cycle TH2 & TH4 ON alternately.

In the first cycle TH1 is ON at a firing angle Vo = Vao appear across the output, at t = , TH1 OFF naturally due to R load as io = 0 at t = .

In next half cycle TH3 is FB triggered at an angle Vo = Vbo, & the process repeats. Here To = 4Ts 1/fo = 4/fs fo = fs/4 stepdown151Output Waveform

152Procedure Make the connections as shown in the circuit diagram with R load for f/2 frequency.

Connect the gate cathode terminals of the thyristors to the respective points on the firing module.

Switch ON the unit.

Trace the output waveforms from CRO.

Vary the firing angle and note the AC output voltage across the load.

Repeat the above procedure for frequency f/4 & f/8153TabulationSl.No.Firing Angle ()Frequency f/2 modeFrequency f/4 mode

Frequency f/8 mode

Load Voltage (Vo)Theoretical

Load Voltage % ErrorLoad Voltage (Vo)Theoretical

Load Voltage

%

ErrorLoad Voltage (Vo)

Theoretical

Load Voltage

%

Error

1.2.3.4.153154Calculation To find out the theoretical value of load voltage use the formula : Calculate the firing angle using the formula : = (X/Y) 180

Calculate Vrms (load voltage) using the formula : Vrms = Vm/2[ {( )+ (sin2)/2 }] = Vs/ [ {( )+ (sin2)/2 }] End ..155Thank You