Power Electronics Manual

POWER ELECTRONICS LAB MANUAL

BMS INSTITUTE OF TECHNOLOGYAvalahalli, Doddaballapur Main Road, Bangalore-64

POWER ELECTRONICS LABORATORY MANUAL

DEPARTMENT OF ELECTRONICS & COMMUNICATION

NAME:………………………………………………… USN:…………………………………………………… BRANCH AND SEM:…………………………………..

BATCH:…………………………………………………

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INTRODUCTION

Power electronics deals with the application of solid state electronics for the control and

conversion of electric power.Convertion techniques require the switching ON and OFF of

power semiconductor devices. Low level electronics circuits which normally consist of

integrated circuits and discrete components generate the required gating signals for power

devices.Integrated circuits and discrete componenets are now being replaced by

microprocessors.

An ideal power device should have no switching on and off limitations in terms

of turn on time, turn off time, current and voltage handling capabilities. Power

semiconductor technology is rapidly developing fast switching power devices with

increasing voltage and current limits. Power switching devices such as power BJTS,

power MOSFETS, IGBTS, SCRs, TRIACs, and other semiconductor devices are finding

increasing applications in wide range of products. With availability of faster switching

devices the applications of modern microprocessors in synthesizing the control strategy

for gating power devices to meet the conversion specifications are widening the scope of

power electronics .

Power electronics combine power, electronics and control. Control deals with

the steady state and dynamic characteristics of closed loop system. Power deals with the

static and rotating power equipment for the generation, transmission and distribution of

electric power. Electronics deals with the solid state devices and circuits for signal

processing to meet the desired control objectives. Power electronics may be defined as

the applications of slid state electronics for the control and conversion of electric power .

It is primarily based on the switching of power semi conductor devices.

Power electronics have already found an important place in modern technology

and are now used in great variety of high power products including heat control, light

controls, motor control, power supplies, vehicle propulsion systems.Power semiconductor

devices can be operated as switches by applying control signals to the gate terminal of

thyristors and the required output is obtained by varying the conduction time of these

switching devices .When a power semiconductor device is in normal conduction mode

there is a small voltage drop across the device.

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CONTENTS

Sl. No.

Name of the Experiment Page No.

1. Syllabus

2. Static Characteristics of SCR and DIAC.

3. Static Characteristics of MOSFET and IGBT.

4. Controlled HWR and FWR using RC triggering circuit.

5. SCR turnig off using i) LC circuit ii) Auxiliary Commutation.

6. UJT firing circuit for HWR and FWR circuits.

7. Generation of firing signals for thyristors/ trials using digital circuits / microprocessor.

8. AC voltage controller using TRIAC-DIAC cmbination.

9 Single phase Fully Controlled Bridge Converter with R and R-L loads.

10.Voltage (Impulse) commuted chopper both constant frequency and variable frequency operations.

11. Speed control of a separetely exited DC motor.

12. Speed control of Universal motor.

13. Speed control of Stepper motor.

14. Parallel/series inverter.

15. VIVA Questions

POWER ELECTRONICS LABORATORY

Sub code: 06ECL48 IA Marks: 25

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No.of Practical Hrs/Week:03 Exam Hrs:03Total No.of Practical Hrs: 42 Exam marks: 50

1. Static Characteristics of SCR and DIAC.

2. Static Characteristics of MOSFET and IGBT.

3. Controlled HWR and FWR using RC triggering circuit.

4. SCR turnig off using i) LC circuit ii) Auxiliary Commutation.

5. UJT firing circuit for HWR and FWR circuits.

6. Generation of firing signals for thyristors/ trials using digital circuits / microprocessor

7. AC voltage controller using TRIAC-DIAC cmbination.

8. Single phase Fully Controlled Bridge Converter with R and R-L loads.

9. Voltage (Impulse) commuted chopper both constant frequency and variable frequency operations.

10. Speed control of a separetely exited DC motor.

11. Speed control of Universal motor.

12. Speed control of Stepper motor.

13. Parallel/series inverter.

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EXPERIMENT:01

STATIC CHARACTERISTICS OF SCR AND DIAC

Aim: a)To plot the static characteristics of the given SCR and find the latching and holding current.

Equipments/Apparatus Used:

1) SCR/DIAC Module 1No

2) Ammeter 0-500mA(DC) 1No0-50mA(DC) 1No

3) Multimeter 1No

Theory:

SCR: The Silicon Controlled Rectifier(SCR) also called the thyristor is one of the most widely used semi controlled switching device. It has a p-n-p-n structure and 3 terminals- anode (A), cathode (K), and gate(G).A graph of the anode current (iA) Vs anode to cathode voltage (VAK) for different values of gate current (iG ) is called as the static characteristics.When the SCR is reverse biased, a very small amount of reverse current in the order of a few micro amperes flows through it.Latching current is the minimum anode current required to turn on SCR without gate current. Holding current is the maximum anode current with gate being open, at which SCR is turned off from on condition.

Circuit Diagram:

Experimental procedure:

1. Connections are made as shown in the circuit diagram.

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2. Switch on the power supply. Keep the gate current (Ig) at convenient value (say 5 mA) by varying V2.

3. Gradually increase anode to cathode voltage (VAK) by varying V1.

Note down the value of anode current (Ia) and anode to cathode voltage (VAK) at each step till the SCR fires.

4. With gradual increase in VAK, SCR fires and the ammeter will show a sudden increase in anode current Ia called Latching Current (IL). This current and (VAK) are noted down.

5. Increase the supply voltage V1 in steps, note the voltage (VAK) and anode current (IA) and tabulate it.

6. Whole procedure is repeated for different values of gate current (Ig).

To find the Holding Current:

7. The gate current (Ig) made to zero (i.e.switch off V2), reduce the anode to cathode voltage (VAK) gradually and note down the anode current, at which SCR turns off (i.e. at which (VAK) will show sudden increase). This anode current is known as holding current (IH).

8. Draw the graph of VAK v/s IA for different gate currents.

Tabular Column:

IG= IG=

VAK (Volts) IA (Amps)

Specimen Calculations:

Forward Resistance Rf= =

Graphs to be drawn:

6

VAK (Volts) IA (Amps)

∆A

ΔIdId

in amps

VAK volts

IA

in Amps

IG1IG2

where IG2>IG1

IL

IH


Conclusion:

AIM B) To plot the static characteristics of DIAC


1) SCR/DIAC Module 1No

2) Ammeter 0-500mA(DC) 1No0-50mA(DC) 1No

3) Multimeter 1No

THEORYDIAC: A DIAC is a dual trigerred diode that will produce an output for every half cycle. It is effectively two seperate diodes connected internally.

Circuit Diagram:

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PROCEDURE

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

2.keep R2 at maximum position.

3.Keep V1 potentiometer at minimum position.

4.Now switch on the circuit and vary V1 in steps of 5V upto 25V and then vary in steps

of 1V.

5.At a particular value ,the device conducts. This can be seen by the sudden increase in

the ammeter reading.

6.Vary V1 further and tabulate the readings.

TABULAR COLUMN

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Voltage (V) Current(I)

CONCLUSION:

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EXPERIMENT:02

STATIC CHARACTERISTICS OF MOSFET AND IGBT

AIM: a)To plot the transfer characteristics of MOSFET .


1) MOSFET and Firing Module 1No

2) Ammeter 0-200mA 1No

3) Multimeter 1No

4) Voltmeter 0 – 10 V 1No

Theory : MOSFET (Metal oxide semiconductor FET) is a voltage controlled device where as BJT is a current controlled device. It has very high input impedance and works at high switching frequency. In general MOSFET are of two types, Enhancement type and Depletion type. N –channel enhancement type has n substrate with p impurities on either sides. A thin layer of metal oxide is deposited over the left side of the channel. A metallic gate is deposited over the oxide layer. As silicon dioxide is an insulator, gate is insulated from the channel. For this reason MOSFET is sometimes called as Insulated gate FET.The steady state transfer characteristics of power MOSFET, involves a quantity called Transconductance (gm). This is a graph of ID versus the control input VGS.

Gm= Transconductance= Change in I D Change in VGS.

VDS = constant

A graph of ID versus VDS is known as the output characteristics. There are three regions of operation:1. Cut-off-region, where VGS < VT

2. Saturation region , where VDS > VGS - VT ; VGS › VT

3. Linear region, where VDS < VGS - VT ; VGS › VT

In the linear region , VDS is small and the current increases linearly with VDS.

Circuit Diagram:

Experimental Procedure:

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A) Transconductance Characteristics

1) Make circuit connection as shown in the circuit diagram.

2) Initially keep V1 and V2 zero.

3) Set Vds = 10V (say) by varying V1.

4) Vary V2 such that VGS varies in steps of 0.5 volts. Note down VGS and ID

readings and tabulate.

5) The minimum gate voltage VGS which is required to turn on the MOSFET (i.e. ID

increases) is called threshold voltage Vgs( Vth).

6) Vgs is varied up to 6 volts and the readings are noted down.

7) Reduce both voltages V1 and V2 to zero.

8) Repeat the step 3 to 6 for different values of VDS up to 20v(say)

9) Plot the graph of Id vs VGS.

B) Drain Characteristics: -

1) With V1 and V2 kept at zero value.

2) Set Vgs= 3.5V(more than threshold voltage).

3) Slowly vary V2 and note down the readings of Id and Vds.

[If VDS is lower than Vp (pinch off voltage), the device works in the constant resistance region (ie., linear region).

If VDS is more than Vp, a constant Id flows from the device and this

Operating region is called the constant current region.]

4) In both region, Id and Vds readings are note down and tabulated.

5) Repeat the step 2 to 6 for VGS=3.8V and note down ID & VDS.

6) Plot the graph of ID vs VDS for different values of Vgs.

Tabular Column:

Transconductance Characteristics:

VDS1= VDS2 =

VGS Volts ID mA VGS Volts ID mA

Drain Characteristics:

VGS1= VGS2 =

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VDS Volts ID mA VDS Volts ID mA


Transconductance gm = =_____________ at constant VDS.

Output resistance Ro = =_______________ at constant VGS.

Graphs to be drawn:

TransConductance Characteristics: -

Drain Characteristics: -

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Δvgs

Vgs in volts

Δvgs

Vgs in volts

ΔIdId

in amps

ΔIdId

in amps

ΔId

Vds in volts

Id

in Amps

VGS = 3V

VGS= 3.5V

PINCH OF VOLTAGE VP

CONSTANT CURRENT REGION

CONSTANT RESISTANCE REGION

ΔVds


Conclusion:

AIM B) To plot the transfer characteristics of IGBT.

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1) IGBT module 1No

2) Ammeter 0-200mA 1No

3) Multimeter 1No

4) Voltmeter 0 – 10 V 1No

Theory: IGBT is a three terminal device having a collector(C) and emitter(E) and the

gate(G). The collector and emitter are the power terminals. Gate and Emitter are the

control terminals. An IGBT has the the advatage of both BJT and MOSFET. Like

MOSFETS , it has very high input impedance and draws very small gate current and

like a BJT , has ON resistance which is lower even at higher blocking voltages. IGBT

does not have a second break down problem like the BJT. IGBT can also be designed

to block negative voltages.

Circuit Diagram:

Experimental Procedure

a) Trans conductance characteristics

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1) Circuit connection is made as shown in circuit diagram.

2) Initially keep V1 and V2 at zero position.

3) Set VCE= 10V (say) by varying V2 and note down IC and VGE readings for every 0.5 volts and tabulated.

[The minimum gate voltage VGE which is required for conduction to start in the IGBT (ie.Ic is increased) is called threshold voltage VGE (th).If VGE is less than VGE

(th) only very small leakages current flow from collector to emitter.]

4) If VGE is greater than (VGE (th)), collector current (Ic) increases, note down Ic and Vge values by varying Vge up to 15vlots.

5) Repeat the step 2 to 4 for different values of VCE and note down Ic and VGE.

6) Plot the graph of Vge vs Ic.

b) Output Characteristics

1) Initially set VGE (say 5V) by varying V2.

2) Slowly vary V1 and note down Ic and VcE.

[For a particular value of Vce there is a pinch off voltage up between collector and emitter.If VCE is lower than Vp, the device works in the constant resistance region.]

3) If VCE is more than up constant Ic flows from the device and this operating region is called constant current region. Note down Ic and VCE by varying VCE

up to 35V.

4) Repeat the step 2 to 4 for different value of VGE and note down Ic and VCE.

5) Plot the graph of IC and VCE.

Tabular Column:

Transconductance Characteristics:

VCE = VCE =

VGE volts IC mA VGE volts IC mA

Output Characteristics:

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VGE =

VCE volts IC mA

VGE

VCE volts IC mA



Trance conductance gm = =____________ at constant Vds.

Output resistance Ro = =______________ at constant Vgs

Graphs to be drawn:

a) Trans Conductance Characteristics: -

b) Drain Characteristics: -

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Vge in volts

Δvge

Icin amps

ΔIc

ΔIc

VCE in volts

Ic

in Amps

VGE= 3v

VGE= 3.5V

PINCH OF VOLTAGE VP

CONSTANT CURRENT REGION

CONSTANT RESISTANCE REGION

ΔVCE


Conclusion:

EXPERIMENT:03

Controlled HWR and FWR using RC triggering circuit.

AIM: To observe the waveforms of load voltage VL (half wave rectification & full wave rectification) and VSCR on the CRO and to plot load voltage VL VS delay angle α.

EQUIPMENT/APPARATUS REQUIRED:1.Module 1 No2.CRO 1 No

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THEORY:To obtain controlled output voltages, phase controlled thyristors are used.The output voltage of thyristor rectifiers is varied by controlling the delay or firing angle of thyristors. Phase controlled rectifiers are less expensive and simple.In RC-triggering circuit, the firing angle can be varied from nearly 0° to almost 180° by varying the resistance.

CIRCUIT DIAGRAM:

RC HALF WAVE RECTIFIER CIRCUIT:

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RC FULL WAVE RECTIFIER TRIGGERING CIRCUIT:

PROCEDURE:

1.Connections are made as shown in the circuit diagram2. R is varied gradually to get different gate currents.3.Load voltage VL is noted down for different values of R and the corresponding delay angle is measured on CRO.4. Variation of VL and VSCR is observed for different values of R on CRO.5. A curve of VL vs α is plotted.

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TABULAR COLUMN:

For RC trigerred half wave rectifier

t(ms)

α β VL VTH

GRAPHS:

VARIOUS WAVEFORMS FOR HALF CONTROL

20

VS

VL

VT


TABULAR COLUMN:

For RC trigerred full wave Rectifier

t(ms)

α β VL VTH

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VARIOUS WAVEFORMS FOR FULLY CONTROL

SPECIMEN GRAPH:

VL

22

VS

VL

VT


α (degrees)

CONCLUSION

EXPERIMENT :04

SCR turnig off using i) LC circuit ii) Auxiliary Commutation.

AIM: To SCR turn off SCR using 1. LC circuit2. Auxillary commutation

APPARATUS REQUIRD:1. Module 1 No2. CRO 1 No

THEORY:CLASS B Commutation (LC circuit): In this type of commutation, reverse voltage is applied to thyristor by the over swinging of an underdamped that is a circuit connected across thyristor. Capacitor charges up to the supply voltage before the trigger pulse is applied to gate. When Thyristor is trigerred two currents flow, a load

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current through external circuit and pulse current through LC circuit and thyristor in opposite dirction. This resonant current tends to turn off thyristor.AUXILLARY COMMUTATION: This type of commutation is popular due to design flexibility. There are many choppers and inverters under this class. T2 must be trigerrred first in order to charge up capacitor C. T2 is commutated off owing to lack of current. When T1 is trigerred, current flows in two paths, load current through R1 and commutating current through C,T1,L and D. The charge on capacitor is reversed and held with hold off diode D1.

CIRCUIT DIAGRAM:

Class B Commutation ( LC circuit )

Procedure1. Make the interconnections as shown in the circuit diagram, connect trigger output

T1 to Gate and cathode of SCR T1. 2. Switch on the DC supply to the power circuit and observe the voltage wave forms

across load by varying the frequency and potentiometer.

CLASS - B

VSCR

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T1


t

VC1

t

VL

t

TABULAR COLUMN

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Ton Toff Height Frequency Duty Cycle

Auxillary Commutation

PROCEDURE1. Make the connections as given in the circuit diagram.

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2. Connect T1 and T2 gate pulse from the firing circuit to the corresponding SCRs in the power circuit. Initially keep the Trigger ON/OFF at OFF position to initially charge the capacitor, this can be observed by connecting CRO across the Capacitor.

3. Now Switch ON the trigger O/P switch and observe the Voltage wave forms across the load,T1, T2 and Capacitor. Note down the voltage waveforms Across the load, T1, T2 and capacitor.

4. Note down the voltage waveforms at different frequency of chopping and also at different duty cycle.

5. Repeat the experiment for different values of load resistance, commutation inductance and capacitance.

Ton Toff Height Frequency Duty Cycle

CLASS - D

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VL

t

VSCR1

t

VSCR2

t

VC

t

CONCLUSION

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EXPERIMENT:05

UJT firing circuit for HWR and FWR circuits

AIM:1. To obtain synchronised trigerring pulses and to trigger SCR at any given

angle.2. To study and test the performance of synchronized UJT firing circuit for HWR

and FWR circuits.

APPARATUS REQUIRED:1.UJT relaxation oscillator 1No2.Firing circuit module 1No3.SCR 1No4.CRO 1No5.Rheostat as load 1No6.Connecting wires.

THEORY: A Synchronized UJT trigger circuit is required for triggering of SCR’s in converter circuits, where the supply across SCR and that given to UJT trigger circuit must be the same. UJT is commonly used for triggering signals for SCR. It has three terminals called emitter (E), base1(B1) and base2(B2). The electrical resistance between B1 and B2 is called the internal base resistance. The p-n junction may be treated as a diode.Phase Controlled Thyristors are used to obtain controlled output voltages. A phase controlled thyristor is turned on by applying a short pulse to its gate and turned off due to natural or line commutation.Single phase converters are a classification of phase control converter. The UJT is commonly used for generating triggering signals for SCRs. The triggering voltage should be designed to sufficiently turn on the thyristor.

Circuit DiagramI. To obtain synchronised trigerring pulses and to trigger SCR at any given angle

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PROCEDURE:

1.Connections are made as shown in the diagram.2.Observe the waveforms on the CRO.3.To trigger SCR at different firing angles, vary DRB settings and note the DC voltage

across load RL for each setting.

II. To study and test the preference of synchronized UJL firing circuit for HWR and FWR circuits.

PROCEDURE:

1.Make connections as shown in the circuit.2. Gate pulses are given through the UJT firing circuit.3. Vary the firing angle and note down the output voltage waveforms.4.Repeat the above steps for full wave rectification with the corresponding circuit.

GRAPHS

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CONCLUSION:

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t

VG

t

Vin

VC

Vfwr

T1

VLfwr

VLhwr

VZ


EXPERIMENT 06

Generation of firing signals for thyristors/ trials using digital circuits / microprocessor

AIM: To generate firing signals for Thyristors using digital firing circuits.

APPARATUS REQUIRED:1.DFC 1No2.CRO 1No

THEORY: The circuit shows the simple block diagram of a digital firing circuit, which can be used for triggering thyristors that are used in single phase AC regulators and controlled rectifiers. It has n-bit down counter. The counter is present to the decimal equivalent of n-bit present binary input and enabled at each zero crossing of the synchronizing signal. The counter starts counting at the rate of clock signal produced by the oscillator, for example if the counter is mod 16, then in one half cycle the oscillator should provide 16 pulses.

CIRCUIT DIAGRAM:

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Fixed frequency oscillator

N-bit Counter

Flip-FlopF/F

Logic circuit + modulator + driver stage

ZCD Carrier frequency oscillator (~5 KHz)

A A

LOAD

RESET

A A

Sync Signal

DC 5V supply5V supply

TM TA

TP

TN

B

Fc

Preset

EN Reset

C


PROCEDURE

1.The circuit connections are made as shown in the circuit diagram.2.The values of duty cycle and α is selected.3.The waveforms across AC reference, ZCD,clock,Tm,TA and oscillator are measured

and observed.4.The obseved waveforms are plotted on the graph.

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CONCLUSION

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EXPERIMENT:07

AC voltage controller using TRIAC-DIAC cmbination.

AIM: To conduct a suitable experiment to control AC voltage using TRIAC-DIAC combination.

APPARATUS REQURIED:1.AC Voltage controller 1No2.Lamp with holder 1No3. C.R.O. 1No4.Isolation transformer5. Connecting wires etc.,

THEORY:AC voltage controller has single or three phase ac input having fixed voltage and frequency. The output has the same number of phases and frequency as input , but the rms value of the output is variable. TRIAC is a bidirectional semiconductor switching device which can be turned on by giving a gate pulse. The circuit works on the phase control method. It consists of phase shifting network comprising of R and C. The firing of TRIAC is determined by the relative phase difference between line and gate control voltage. Adjusting the value of R changes the phase difference between line and control voltage and thus changes the voltage available to the load. DIAC is used to trigger the TRIAC.

CIRCUIT DIAGRAM

FRONT PANEL DIAGRAM

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PROCEDURE

1.Make the connections as given in the circuit diagram. 2. Switch ON the mains supply.3.Vary the firing angle and potentiometer to observe the variation in lamp brightness and also note down the voltage variation across the lamp.4.Plot output voltage vs the firing angle.

Tabular Column:

Vo (volts) Firing angle α (degrees)

Expected Graph

CONCLUSION:

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EXPERIMENT:08

Single phase Fully Controlled Bridge Converter with R and R-L loads.

Aim: To study the output waveform of single phase FWR with R and RL loads.

Equipments required:

1) FWR bridge Module 1No

2) Firing Module 1No

3) Load Resistance 200 Ohm 1No

4) Inductive Load 1No

5) CRO 1No

6) Multimeter 1No

Theory:

A controlled rectifier converts an alternating voltage to a variable DC voltage. The DC voltage so obtained is not pure as from the battery, but contains ripple superimposed on the DC components . Diodes form the bridge of the FWR. Transformer steps down the voltage to a lower value suitable for rectification. First, the resistance is connected as the load for observing the output waveform. In practice most loads are inductive to certain extent. Therefore RL load is connected to observe the load current. With RL load, load current shape is different from the load voltage whereas it is the same as that of voltage waveform in resistive load.

CIRCUIT DIAGRAM:

FWR with R load:

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FWR with RL load:

Experimental procedure:

1. Make the connections as per the diagram. 2. Switch the AC mains supply.3. Vary the firing angle knob to get suitable triggering in both half cycles.4. Output current and output voltage is measured for different settings of

firing angle.5. Step 3 is repeated for the following load conditions.

R=50Ω and for R-L load R=50Ω and L=25mH to 150mH.

Tabular Column:

SL NO Input Voltage-Vin Firing angle Output voltage Output Current

GRAPHS

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Conclusion:

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EXPERIMENT:09Voltage (Impulse) commuted chopper both constant frequency and variable frequency operations.

AIM:

THEORY: In many industrial applications, it is required to convert a fixed-voltage DC source into a variable-voltage DC source. A DC chopper converts directly from DC to DC and is also known as a DC-to-DC converter. A chopper can be considered as DC equivalent to 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, fork-lift trucks and mine haulers. They provide smooth acceleration control, high efficiency and fast dynamic response. Choppers 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. Choppers are also used in DC voltage regulators.

CIRCUIT DIAGRAM

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PROCEDURE1) To begin with switch ON the power supply to the unit. 2) Observe the trigger output signals by varying Duty cycle and Frequency

Potentiometer. 3) Now make the interconnections in the power circuit as given in the circuit diagram. 4) Connect a Resistive load. Connect respective trigger outputs from the firing circuit

to the respective SCRs in the Power Circuit.5) Initially keep the ON/OFF switch in the firing circuit in OFF position. 6) Switch ON the DC supply. Apply Main SCR trigger pulses by pressing the ON/OFF

switch to ON position. 7) Observe the voltage waveforms across load. We can observe the chopped DC

waveform. If the commutation fails we can see only the DC voltage. In that case switch OFF the DC supply, Switch OFF pulses and check the connections and try again.

8) Observe the voltage across load, across Capacitor, across Main SCR and auxiliary SCR by varying Duty cycle and frequency Potentiometer. Draw the wave forms at different duty cycle and at different Frequency. Connect Voltmeter and Ammeter and note down values in the table.

Frequency Constant [PWM& Duty cycle varied]

Duty cycle (%) Ton (ms) Toff (ms) O/p Voltage (v)

Duty Cycle Constant [frequency varied]

Frequency (Hz) Ton (ms) Toff (ms) O/p Voltage (v)

GRAPH

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CONCLUSION

EXPERIMENT:10

Speed control of a separetely exited DC motor

42

Vdc

VL


AIM: To study the speed control of a DC motor.

APPARATUS REQUIRED:1.DC motor 1No2.Module 1No

THEORY: DC motors are used in adjustable speed drives and position control application. DC motors are preferred when wide speed control range is required. Phase controlled converters provide an adjustable dc output voltage from a fixed AC input voltage.

CIRCUIT DIAGRAM

PROCEDURE1.First switch on the firing circuit and verify output and their phase sequence. Also vary firing angle potentiometer and observe trigger outputs.2.Now make power circuit connections. Replace the motor by a rehostat of 50ohms/5A.3.Connect the trigger output from the firing circuit to the corresponding SCR’s gate and cathode. Initially keep input signal to a low voltage of say 30V.4.Switch on mains and observe voltage waveforms across load by varying the firing angle potentiometer.5.If the unit is working correctly,switch OFF mains. Connect field terminals of a DC motor to feild supply points in the power circuit.6.Initially keep the firing angle potentiometer at 180° and rheostat resistance at

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maximum.7.Switch on supply and trigger outputs.8.Vary firing angle potentiometer and input voltage up to 230V in steps and tabulate the readings.

TABULAR COLUMN

1.Firing angle α varying

Speed (rpm) α (degrees) Voltage (v)

I (A)

11.Varying field; α= constant (90deg), R L= constant(max)

Voltage (v) Speed (rpm)

111.Varying load R L ; α= constant (90deg)

Voltage (v) I (A) Speed (rpm)

R=V/I (Ω)

GRAPHS:

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speed

firing angle

EXPERIMENT 11

SPEED CONTROL OF UNIVERSAL MOTOR

AIM: To study the working of universal motor.

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APPARATUS REQURIED:1.TRIAC firing module2.Universal motor3.CRO4.Tachometer5.AC voltmeter6.AC ammeter7.Connecting wires and patch chords.

THEORY: The universal motor is a rotating electric machine similar to a DC motor but designed to operate either from direct current or single-phase alternating current. The stator and rotor windings of the motor are connected in series through the rotor commutator. Therefore the universal motor is also known as AC series motor or an AC commutator motor. The universal motor can be controlled either as a phase-angle drive or as a chopper drive. In the phase-angle application, the phase-angle control technique is used to adjust the voltage applied to the motor. A phase shift of the gate's pulses allows the effective voltage, seen by the motor, to be varied. The phase-angle drive requires just a TRIAC. In the chopper application, the Pulse Width Modulation (PWM) technique is used to adjust the voltage applied to the motor. Modulation of the PWM duty cycle allows the effective voltage, seen by the motor, to be varied. Compared to a phase-angle drive, a chopper drive requires a more complicated power stage with an input rectifier, a power switch and a fast power diode. The advantage is higher efficiency, less acoustic noise and better EMC behaviour.

CIRCUIT DIAGRAM:

46


PROCEDURE:1. Make the inter connections in the power circuit as given is the circuit

diagram.

2. Switch ON the firing circuit and observe the trigger outputs . Make sure that the firing pulses are proper before connecting to the power circuit.

3. Then connect the trigger output from firing circuit to corresponding SCR’s / Triac. In the power circuit initially set the AC input to 30 volts.

4. Switch ON the MCB. Switch ON the Trigger outputs switch. Select the SCR/ triac selection switch and observe the output waveforms across ‘R’ load by varying the firing angle potentiometer.

5. If the output wave form is proper then you can connect the motor & increase the input voltage to rated value 0 – 230V gradually.

6. Vary the firing angle and note down O/P voltage and speed of the motor.

TABULAR COLUMN

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FIRING ANGLE

SPEED IN RPM

OUTPUT VOLTAGE(V)

OUTPUT CURRENT(A)

SPECIMEN GRAPH

Conclusion

EXPERIMENT 12

SPEED CONTROL OF STEPPER MOTOR

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AIM:To study the working of stepper motor.

APPARATUS REQUIRED: 1. Firing Modular 1No2. Stepper Motor 1No3. Isolation Transformer1No4. CRO 1No5. Tachometer 1No

THEORY: Stepper motors are electro mechancial motion devices , which are used primarily to convert information in digital form to mechanical motion. These motors rotate at a predetermined angular displacement in response to a logic input. Whenever stepping from one position to another is required, the stepper motors are generally used. They are used as drivers for paper in line printers and in other computer peripheral equipment such as in positioning of the magnetic disk ahead.

CIRCUIT DIAGRAM:

RED

V = 4 X Motor VoltageRs = 3 X Rm (Motor resistance/ Phase)Suitable for slow RPM

SWITCHING LOGIC SEQUENCE

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LOGIC FREEWHEEING DIODES

green

BLUE

black

SWITCHING TRANSISTORS

WHITE

BLACK

Rs

Rs

-- ve

+ ve

A1

A2

B1

B2


To change the direction read sequence from bottom to top.

PROCEDURE:1.Connect A1, A2, B1 and B2 leads of stepper motor to the corresponding output terminal points. And two common terminals to +V supply. Switch ON the mains supply to the unit. Check the power supplies. The unit displays WELCOME STEPPER MOTOR After few seconds it displays STOP S/R R/S H/F

RPM 1 FOR FULL2. Press 1NC / DEC key to select STEP or RPM ( Continuous rotation) mode.3. After selecting RPM/STEP mode press SET key to select the mode.4. Now 1 blinks. This corresponds to number of rotation or number of steps selected. Press 1 NC/DEC key to select the speed or steps.5. Press SET key to set the rpm / number of steps. Now FOR blinks. This corresponds to direction of rotation (Forward). Press 1 NC/DEC key to select the direction of rotation

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and press SET key to select.6. Now FULL blinks. This is corresponds to Full step. Press 1 NC/DEC key to select Half step / Full step mode and press SET key to select Half / Full step mode.7. Press RON / STOP key, the stepper motor rotates at the set speed if RPM is selected or it moves the number of steps set and stops.8.Repeat the same for Full step mode. Repeat the same for Reverse direction.

EXPERIMENT 13

PARALLEL/ SERIES INVERTER

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AIM: To realize parallel and series inverter

APPARATUS REQUIRED: 1.Power supply. 2.Inverter module 3.firing circuit4.CRO 5. connecting wires.

THEORY: A dc-ac converter is also known as an inverter. The function of an inverter is to change a dc input voltage to a symmetric ac output voltage of desired magnitude and frequency. The output voltage could be fixed or variable at a fixed or variable frequency.Inverters are widely used in industrial applicatoins,eg.. Variable speed ac motor drivers, induction heating, stand by power supplies,UPS, etc.Series Inverters are based on resonant current oscillation. The commutating components and switching devices are placed in series with the load to form an underdamped circuit.Parallel Inverters are the dual of series inverters. It is supplied from the current source so that the circuit offers a high impedance to the switching current. Since the current is continuously controlled, this inverter gives a better short circuit protection under fault condition.

CIRCUIT DIAGRAM:

PROCEDURE:1.Switch on the power supply to the firing circuit module and check trigger pulses by varying the frequency.2. Make the interconnections of the power circuit as shown in the circuit diagram. Now

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connect trigger outputs from the firing circuits to Gate and Cathode of SCR`s T1 and T2.3.Switch on dc supply to inverter and observe voltage waveforms across the load.4.Vary the frequency to the firing circuit and observe the waveforms.5.Repeat the same with diferent values of L and C given on the module.

Resonance frequency:- fr = 1 1 _ R 2 2II LC 4L2

WAVEFROMS

t f1 > fr

CIRCUIT DIAGRAM

PARALLEL INVERTER

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t

t

T1

T2

eo

fi = fr

fi<fr

eo

eo


D1

SCR1

KVL

RL

L C

+ SCR2

D2

PROCEDURE:

1. Switch on the firing circuit. Observe the trigger outputs TP and TN by varying frequency potentiometer and by operating ON/OFF switch.

2. Then connect input DC supply to the power circuit. Connect trigger outputs to Gate and Cathode of SCR TP & TN.

3. Make the interconnections as shown in circuit diagram. Connect load between load terminals.

4. Connect free wheeling diodes in the circuit. To begin with set input voltage to 15V. Apply trigger pulses to SCR and observe voltage waveforms across load. Output voltage is square wave only. Then remove freewheeling diode connections and observe the waveforms.

5. Keeping frequency minimum, the duty cycle and output voltage is calculated.6. Keeping frequency maximum,the duty cycle and output voltage is calculated.

WAVEFORMS:

TRIGGER OUTPUTS

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-

Vdc

T1


OUTPUT

Conclusion:

55

T2

+VDC

--VDC


VIVA QUESTIONS

1. What is POWER ELECTRONICS?

2. What are the peripheral effects of power electronics equipment?

3. What is a thyristor?

4. Name the most popular thyristor.

5. What is a latching current?

6. What is a holding current?

7. What is the ratio of latching current to holding current in a thyristor?

8. What are the different methods of turning-on of a thyristor with its gate

disconnected?

9. Can a forward voltage be applied to an SCR soon after its anode current has

fallen to zero? Explain.

10. What are the necessary conditions for turning-on of an SCR?

11. When cathode is positive w.r.t anode in an SCR then what is the number of

blocked P-N junctions?

12. A thyristor is a AC or DC switch?

13. How can a conducting thyristor be turned-off?

14. What is turn-on & turn-off time of SCRs?

15. Is turn-off time of a SCR constant? What factors influence its value?

16. What is MOSFET?

17. Name the three terminals of MOSFET.

18. What are the two types of power MOSFET’s? Which one is commonly used?

19. What is threshold voltage?

20. Mention some features of MOSFET.

21. Explain the switching characteristics of MOSFET.

22. Give comparison of MOSFET with BJT.

23. Give some applications of MOSFETs.

24. What is IGBT? What are its other names?

25. Explain input & transfer characteristics if IGBT.

26. Why are IGBTs becoming popular in their applications to controlled

converters?

27. Enumerate some applications of IGBT.

28. What is ‘pinch of voltage’?

29. What is UJT? Mention its applications.

30. Whan an UJT exhibits negative resistance region?

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31. When an UJT is used for triggering an SCR, what is the waveshape of the

voltage obtained from UJT circuit?

32. What is the function of connecting a zener diode in an UJT circuit?

33. What is the ‘intrinsic stand-off ratio’ of a UJT?

34. What is the peak voltage of a UJT?

35. What is the valley point voltage of a UJT?

36. Explain the working of an UJT oscillator.

37. Explain how the circuit operation is influenced if the charging resistor is small

so that the capacitor voltage reaches Ujt threshold voltage twice in each half

cycle.

38. Bring out the differences between UJT firing circuit with R and RC firing

circuits.

39. What do you mean by synchronous and asynchronous mode?

40. What is commutation?

41. What are the two general types of commutation?

42. Give the various commutation techniques used for thyristors.

43. Explain the working of class B-commutation.

44. Bring out the differences between voltage commutation and current

commutation.

45. What is the difference between self and natural commutation.

46. Explain the output waveform of class B-commutation.

47. What is the principle of complementary commutation?

48. Explain the working principle of auxiliary commutation.

49. What is the purpose of connecting and antiparallel diode across the main

thyristor with or without a series inductor?

50. Why does the commutation capacitor in a resonant pulse commutation get

overcharged?

51. How is the voltage of the commutation capacitor reversed in a commutation

circuit?

52. What is controlled rectifier?

53. What is a converter?

54. What is a delay-angle control of converters?

55. What are the effects of removing the freewheeling diode in single-phase semi

converter?

56. How are gate-turn-off thyristors turned on and off?

57. What is isolation transformer?

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58. How the speed of DC Motor is controlled using single-phase half controlled

bridge rectifier?

59. What is the base speed of DC Motors?

60. What is the purpose of a converter in DC drives?

61. Which are the parameters to be varied for speed control of DC series motors?

62. Why are the DC series motors mostly used in traction application?

63. What is the speed regulation of DC drives?

64. What is the magnetization characteristics of DC motors?

65. Do the output ripple voltages of converters depend on the delay angle?

66. What is a DC chopper?

67. What are the performance parameters of a chopper?

68. What is the purpose of the commutation circuit of a chopper?

69. What are the advantages of chopper-fed DC drives?

70. What is the principle of regenerative braking of DC chopper-fed DC motor

drives?

71. What is stepper motor?

72. Explain the principle operation of stepper motor?

73. What is half step and full step motor?

74. What are the different types of stepper motor? Mention the difference

between each.

75. What is meant by “STEP ANGLE”?

76. Give the applications of stepper motor?

77. Mention the advantages and disadvantages of stepper motor?

78. What is TRIAC?

79. The firing circuit of TRIAC is based on which method?

80. Explain how a TRIAC may sometimes operate in the rectifier mode.

81. What are the types of Induction Motor?

82. What is the slip at starting of Induction Motor?

83. What are the various means for speed control of Induction Motors?

84. What is a snubber network?

85. What is the difference between an SCR & a TRIAC?

86. What are the advantages & disadvantages of TRIAC A.C switch?

87. What are the effects of load inductance on the performance of a.c voltage

controllers?

88. What are the advantages & disadvantages of a.c voltage controllers?

89. What are the advantages & disadvantages of inverse-parallel Thyristor a.c

switches?

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90. What is an autotransformer?

91. What is the base frequency an Induction Motor?

92. What is synchronous speed?

93. What is a field-weakning mode of Induction Motor?

94. What is a static switch?

95. What are the differences between a.c & d.c switches?

96. What are the advantages of static switches over mechanical or

electromechanical switches?

97. What type of commutation is required for a.c & d.c switches?

98. Explain the features that the firing circuit for Thyristors should possess?

99. Describe the resistance firing circuit used for triggering SCRs?

100. Is it possible to get a firing angle greater than 900 with resistance firing?

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Documents

Power Electronics Manual