DEPARTMENT OF ELECTRONICS AND COMMUNICATION & … · 3 Controlled HWR and FWR using RC triggering...

APPROVED BY AICTE NEW DELHI, AFFILIATED TO VTU BELGAUM

DEPARTMENT OF ELECTRONICS AND COMMUNICATION &

ENGINEERING

POWER ELECTRONICS LABORATORY

LAB MANUAL – 10ECL78

2016-2017 VII SEMESTER

Prepared by: Reviewed by: Approved by:

Sathisha B M Kavitha M V Dr. A.A. Powly Thomas

Assistant professor Head of the Department Principal

Dept. of ECE Dept. of ECE GCEM

GCEM GCEM

81/1, 182/1, Hoodi Village, Sonnenahalli, K.R. Puram, Bengaluru, Karnataka-

560048.

INDEX

S.No Title Page No

1.

Syllabus

i

2.

Course objective

ii

3.

Course outcome

ii

4.

Do’s & Don’t’s

iii

5.

List of experiments

iv

6.

Viva questions

77

i

SYLLABUS

POWER ELECTRONICS LAB

Subject Code: 10ECL78 IA Marks: 25

No. of Practical Hrs/Week: 03 Exam Hours: 03

Total 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 turn 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 combination.

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

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

10. Speed control of a separately exited DC motor.

11. Speed control of universal motor.

12. Speed control of stepper motor.

13. Parallel / series inverter.

Note: Experiments to be conducted with isolation transformer and low voltage.

ii

Course Objectives:

To study and analyze V-I characteristics of SCR and DIAC.

To study and analyze V-I characteristics of MOSFET and IGBT.

To study and analyze various waveforms across different circuit elements in Full and Half wave

rectifier using RC Firing circuit.


rectifier using UJT Firing circuit.


rectifier using Digital Firing circuit.

To study and analyze various waveforms across different circuit elements various waveforms of AC

voltage regulator using TRIAC and DIAC combination.

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

To study and analyze Voltage (Impulse) commutated chopper

To study and analyze Speed control of a separately exited DC motor, Universal motor and stepper

motor.

To study and analyze various waveforms across different circuit elements in Parallel / series

inverter

Course Outcomes:

The student will have the Ability:

To conduct and analyze V-I characteristics of SCR and DIAC.

To conduct and analyze V-I characteristics of MOSFET and IGBT

To conduct and analyze Full and Half wave rectifier using RC Firing circuit

To conduct and analyze Full and Half wave rectifier using UJT Firing circuit

To conduct and analyze Full and Half wave rectifier using Digital Firing circuit.

To conduct and analyze AC voltage regulator using TRIAC and DIAC combination

To conduct and analyze Single phase Fully Controlled Bridge Converter with R and R-L loads

To conduct and analyze Voltage (Impulse) commutated chopper

To conduct and analyze Speed control of a separately exited DC motor, Universal motor and

stepper motor.

To conduct and analyze Parallel / series inverter.

iii

DOS

It is COMPULSORY to wear covered shoes when entering the lab.

You must keep your bags at the designated area. Bags should NOT be placed on or under the

workbench.

Chairs and stools should be kept under the workbenches when not in use. Sit upright on chairs or

stools, keeping feet on the floor.

Follow the instructions of your lab demonstrator while conducting the experiments.

In an emergency, all power in this lab can be switched off by pressing the button on the main

breaker panel. It is to be used for emergencies ONLY

DON’TS

Do NOT transfer equipment to other workbench or other labs without permission

Do NOT touch any equipment until you are told to do so.

Wearing a ring or watch can be hazardous in an electrical lab since such items make good

electrodes for the human body

NO food and drinks are allowed in this lab

NEVER touch any equipments or components with wet or damp hands

iv

List of Experiments

Exp

No

Experiment Nmae Page No

1 Static characteristics of SCR and DIAC. 01-04

2 Static characteristics of MOSFET and IGBT. 05-09

3 Controlled HWR and FWR using RC triggering circuit

10-14

4 UJT firing circuit for HWR and FWR circuits 15-20

5 Generation of firing signals for thyristors/ trials using digital

circuits / microprocessor. 21-27

6 AC voltage controller using triac – diac combination.

28-32

7 SCR turn off using i) LC circuit ii) Auxiliary Commutation 33-34

8 Single phase Fully Controlled Bridge Converter with R and

R-L loads. 35-47

9 Speed control of a separately exited DC motor.

48-52

10 Speed control of universal motor.

53-58

11 Speed control of stepper motor.

59-63

12 Voltage (Impulse) commutated chopper both constant

frequency and variable frequency operations 64-71

13 Parallel / series inverter.

72-80

1

Experiment No: 01 Date:

SCR AND DIAC CHARACTERISTICS

CHARACTERISTICS OF SCR:

AIM: To plot the characteristics of an SCR and to find the forward resistance, holding current and latching

current.

APPARATUS:

SCR & DIAC Characteristics Study unit.

THEORY: A Silicon Controlled Rectifier (SCR) is a four layer, three terminals and three junction device,

which is basically a rectifier with a control terminal called Gate. Like diode, it is also a uni-directional

device and forward conduction is from anode to cathode. Since SCR use silicon for its construction so it

is called silicon controlled rectifier. Where it operates as a rectifier, it is mainly used as a switch.

There are three terminals namely Anode (A), Cathode (K), and Gate (G). Four layers, with alternatively

P-type and N-type silicon semiconductors forming three junctions. If SCR is forward biased with G=0,the

Junctions J1 and J3 are forward biased and J2 is reverse biased .If +VAK is small, leakage current flows

(IFX) until +VAK is large enough to cause reverse biased junction J2 to break down. The forward voltage

at this point is called forward break over voltage VF (BR).

If a negative voltage is applied to anode and a positive voltage is applied to cathode of the SCR,

the junction J2 is forward biased and J1 and J2 are reverse biased with very small leakage current flow

called reverse blocking current. If the reverse voltage is now increased, J1 and J3 break down in the zener

or avalanche mode and IR is not limited, hence SCR could be damaged or destroyed. The region of the

reverse characteristics before reverse breakdown is called reverse blocking region. CIRCUIT DIAGRAM:

2

PROCEDURE:-

I. V – I Characteristics:

1) Make the connections as given in the circuit diagram including meters.

2) Now switch ON the mains supply to the unit and initially keep V1 &V2 at minimum.

3) Set load potentiometer RL in the minimum position.

4) Adjust IG –IG1 say 10 mA by varying VG or gate current potentiometer Rg.

5) Set load potentiometer Rg in the minimum position. Adjust IG –IG1 say 10 mA by varying VG or gate

current potentiometer RG. Slowly vary VL and note down V AK and IA readings for every 5 Volts and

entered the readings in the tabular column.

6) Further vary VL till SCR conducts, this can be noticed by sudden drop of VAK and rise of IA readings.

7) Note down this readings and tabulated. Vary VL Further and note down IA and VAK readings. Draw

the graph of VAK V/s IA.

8) The forward resistance or on state resistance can be calculated from the graph by using formula

Ron-State = ΔVAK/ΔIA Ω.

Tabular Column:

IG = IG1 =___ mA IG = IG2 =____ mA

V AK IA VAK IA

II. To find latching current:

1) Apply about 20 V between Anode and Cathode by varying VA. Keep the load potentiometer RL

at minimum position. The device must be in the OFF state with gate open.

2) Gradually increase Gate voltage - Vg till the device turns ON. This is the minimum gate current

( Ig min) required to turn ON the device.

3) Adjust the gate voltage to a slightly higher. Set the load potentiometer at the maximum resistance

position. The device should comes to OFF state, otherwise decrease VA till the device comes to

OFF state.

4) The gate voltage should be kept constant in this experiment. By varying RL, gradually increase load

current IA in steps. Open and close the Gate voltage VG switch after each step.

5) If the anode current is greater the latching current of the device, the device stays on even

after the gate switch is opened. Otherwise the device goes into blocking mode as soon as the

gate switch is opened.

6) Note the latching current. Obtain the more accurate value of the latching current by taking small

steps of IA near the latching current value.

3

III. To find Holding current: 1) Increase the load current from the latching current level by load pot RL or VL.

2) Open the gate switch permanently. The Thyristor must be fully ON.

3) Now start reducing the load current gradually by adjusting RL.

4) If the SCR does not turns OFF even after the RL at maximum position, then reduce VL.

Observe when the device goes to Blocking mode.

5) The load current through the device at this instant, is the holding current of the device. Repeat

the steps again to accurately get the Ih. Normally Ih < Il.

Ideal graph of V-I characteristics of SCR

+ I A

IL Ig2 Ig1 Ig0 VBO

Ih

-VAK +VAK

-I A

RESULT: The values of VAK and IAK are noted down, graph is plotted and SCR forward

resistance, Latching and Holding Currents are found.

4

CHARACTERISTICS OF DIAC:

AIM: To study and plot the characteristics of an DIAC.

APPARATUS:

SCR & DIAC Characteristics Study unit.

CIRCUIT DIAGRAM:

PROCEDURE:

1) Make the connections as given in the circuit diagram.

2) Keep potentiometer also at minimum position.

3) Next switch ON the unit & VL power supply.

4) Vary VL insteps of 5V and note down the corresponding Ammeter reading.

5) Vary insteps of 5V up to 25Volts. After that vary insteps of 1V.

6) At a particular value of voltage the device conducts. This can be noticed by the sudden increase of

Ammeter reading.

7) This is the device breakdown voltage, vary VL further and note down the corresponding V/I

readings in the tabular column.

8) Reverse the supply to the DIAC and repeat the above procedure to find the reverse break down

Voltage.

Note :- If Voltage from V1 is not sufficient to turn on Diac, connect V1 and V2 supplies in series

(connect V1- to V2 +)and use V1 + as + and V2 – as – .

TABULAR COLUMAN

V I

Volts mA

RESULT: The values of VAK and IAK are noted down, graph is plotted and SCR forward

resistance, Latching and Holding Currents are found.

I

- 20mA +

- Rg

V = 0-200V

V1 +

V

-

5


IGBT AND MOSFET CHARACTERISTICS

AIM: To plot the static characteristics of MOSFET & IGBT.

APPARATUS:

IGBT & MOSFET Characteristics Study unit.

Characteristics of MOSFET:

THEORY:

In n-channel depletion type MOSFET with gate positive with respect to source ID, VDS and VGS

are drain current, drain source voltage and gate-source voltage. A plot of variation of ID with VDS for a

given value of VGS gives the Drain characteristics or Output characteristics. If VGS & VDS are positive,

ID is positive for n channel MOSFET and VGS is negative for depletion mode. VGS is positive for

enhancement mode.

MOSFET can be operated in three regions

Cut-off region,

Saturation region (pinch-off region) and

Linear region.

In the linear region ID varies linearly with VDS. i.e., increases with increase in VDS. Power

MOSFETs are operated in the linear region for switching actions. In saturation region ID almost remains

constant for any increase in VDS.Transfer characteristic gives the variation of ID with VGS for a given

value of VDS. ID is the drain current with shorted gate. As curve extends on both sides VGS can be

negative as well as positive.

CIRCUIT DIAGRAM:

6

Trans Conductance Characteristics:-

PROCEDURE: 1) Make the connections as shown in the circuit diagram with meters.

2) Initially keep V1 and V2 Minimum. Set V1= VDS1= say 10V.Slowly vary V2(VGS) and note

down ID and VGS readings for every 0.5 Volts. and enter in the tabular column.

3) The minimum gate voltage VGS which is required for conduction to start in the MOSFET

is called Threshold Voltage VGS(Th).

4) If VGS is less than VGS (Th) only very small leakage current flows from Drain to

Source. If VGS is greater than VGS(Th), the Drain current depends on magnitude of the

Gate Voltage. VGS varies from 2 to 5Volts.

Repeat the same for different values of VDS and draw the graph of I D V/S VGS.

TABULAR COLUMN:-

V1= VDS1 = 10 Volts. V1 = VDS2 = 30 Volts

V GS Volts ID ma VGS Volts ID ma

IDEAL GRAPH :

ID

V DS = 15V

ID

(on)

V GS (Th) V GS (on) V GS

3.5V

Drain Characteristics:-

PROCEDURE: 1) Initially set V2 to VGS1= 3.5 Volts.

2) Slowly vary V1 and note down ID and VDS.For a Particular value of VGS1 there is a pinch

off voltage ( Vp) between drain and source as Shown in figure.

3) If VDS is lower than Vp, the device works in the constant resistance

region and ID is directly proportional to VDS. If VDS is more than Vp, constant ID

flows from the device and this operating region is called constant current region.

4) Repeat the above for different values of VGS and note down ID V/S VDS

5) Draw the graph of I D V/S VDS for different values of VGS.

VDS = 25V

V DS = 15V

7

TABULAR COLUMN:-

V2 = VGS = 3.5 Volts V2 = VGS=3.8 Volts

VDS Volts ID ma VDS Volts ID ma

IDEAL GRAPH :

ID in

mA

V GS = 3.6V

V GS = 3.55V

V GS = 3.5V

V DS

Characteristics of IGBT

Theory:

An output characteristic is a plot of collector current IC versus collector to emitter voltage VCE for

given values of gate to emitter voltage VGE. A plot of collector current IC versus gate-emitter voltage VGE

for a given value of VCE gives the transfer characteristic. Figure below shows the transfer characteristic.

The Controlling parameter is the gate-emitter voltage VGE in IGBT. If VGE is less than the threshold

voltage VT then IGBT is in OFF state. If VGE is greater than the threshold voltage VT then the IGBT is in

ON state. IGBTs are used in medium power applications such as ac and dc motor drives, power supplies

and solid state relays.

CIRCUIT

DIAGRAM:

8

Transfer Characteristics:-

PROCEDURE:

1) Make the connections as shown in the circuit diagram with meters.

2) Initially keep V1 and V2 minimum. Set V1= VCE1= say 10V.Slowly vary V2 (VGE) and note

down IC and VGE readings for every 0.5 Volts. and enter in the tabular column.

3)The minimum gate voltage VGE which is required for conduction to start in the IGBT

is called Threshold Voltage VGE(Th).If VGE is less than VGE (Th) only very small

leakage current flows from Collector to Emitter. If VGE is greater than VGE(Th), the Collector

current depends on magnitude of the Gate Voltage. VGE varies from 5 to 6Volts.

4) Repeat the same for different values of Vc and draw the graph of Ic V/S VGE.

TABULAR COLUMN:-

V1= VCE1 = ------Volts. V1 = VCE2 = ----Volts

V GE Volts ID ma VGE Volts ID ma

9

IDEAL GRAPH :

IC

V CE = 15V

IC

(on)

V GE (Th) V GE (on) V GE

Collector Characteristics:-

PROCEDURE:

1) Initially set V2 to VGE1= 5 Volts. Slowly vary V1 and not down IC and VGE. For a Particular value of

VGE1 there is a pinch off voltage ( Vp) between Collector and Emitter as Shown in figure.

2) If VGE is lower than Vp, the device works in the constant resistance region and IC is directly

proportional to VGE. If VGE is more than Vp, constant IC flows from the device and this

operating region is called constant current region.

3) Repeat the above for different values of VGE and note down IC values.

4) Draw the graph of IC V/S VGE for different values of VGE.

TABULAR COLUMN:-

V2 = VGE = 5.0 Volts V2 = VGE=5.2Volts

VCE Volts IC mA VCE Volts IC mA

VCE = 25V

V DS = 15V

10

IDEAL GRAPH :

IC in V GE = 5.25V

mA

V GE = 5.2V

V GE = 5.15V

V GE = 5.1V

V CE

RESULT: The transfer characteristics & collector characteristics are obtained and their

respective graphs are plotted and output resistance and Trans conductance are found.

11


RC HALF AND FULL WAVE FIRING CIRCUIT

AIM: To study the static characteristics of RC Half and Full wave firing circuit .

APPARATUS:

RC Firing circuit Study unit

THEORY:

In the negative half cycle of the AC supply, diode D2 is forward biased. It will short circuit the

potentiometer “R’’ and the capacitor “C’’ is charged to negative peak voltage through D2 as shown in fig

(a). with its upper plate negative with respect to its lower plate . In the positive half cycle, D2 is reverse

biased. The capacitor “C’’ will charged through “R’’ to the trigger point of the thyristor in a time

determined by the RC time constant and the rising anode voltage(see fig(b)). The diode D1 will isolate and

protect the gate cathode junction against reverse (negative) voltage.

As soon as the capacitor voltage become sufficiently positive to forward bias. Diode D1 and the

gate cathode junction of thyristor will be turned on. As soon as the thyristor is turned on, the voltage across

it reduced to a very low value and the gate current goes to zero.

R-C HALF WAVE FIRING CIRCUIT:

Circuit diagram :

12

Connection Diagram for RC Half wave Firing Circuit :

PROCEDURE:


2) Connect a Resistance of 50 ohms between the load points.

3) Vary the control pot(R) and observe the voltage waveforms across load, SCR and at different

points of the circuit.

4) Note down the output voltage across the Load for different values of firing angle in degree.

5) Calculate the theoretical and practical output voltage.

6) Draw the graph for input wave form, output waveforms across SCR and Load.

TABULAR COLUMN:

Sl no. Firing Angle Load Voltage

Theoretical

[Vm/2( 1 +cos)

In Volts

Practical in Volts

13

VARIOUS WAVEFORMS FOR RC HALF WAVE CONTROL

Wave forms across Input and load:

Wave forms across Load and SCR:

Firing Angle = X/Y180

Vin

Vload

Vload

Vscr

14

RC FULL – WAVE FIRING CIRCUIT

Circuit diagram:

Connection Diagram for RC Full wave Firing

15

PROCEDURE : 1) Make the connections as given in the circuit diagram.

2) Connect a Rheostat of 100 ohms between the load points.

3) Vary the pot and observe the voltage waveforms across load, SCR and at different points of the

circuit.

4) Note down the output voltage across the Load for different values of firing angle in degree.

5) Calculate the theoretical and practical output voltage.

6) Draw the graph for input wave form, output waveforms across SCR and Load.

VARIOUS WAVEFORMS FOR RC FULL WAVE CONTROL:

Wave forms across Input and load:

Wave forms across Load and SCR:

RESULT:- Half and full wave R & RC triggering circuit have been rigged up and output waveforms have

been plotted.

Vin

Vload

Vscr

Vload

16


HALF AND FULL WAVE RECTIFIER USING UJT FIRING CIRCUIT


APPARATUS:

UJT Firing circuit Study unit

Theory:

UJT is highly efficient switch. The switching time is in the range of nanoseconds. Since UJT

exhibits negative resistance characteristics it can be used as relaxation oscillator. The circuit diagram is as

shown with R1 and R2 being small compared to RB1 and RB 2 of UJT.

When VBB is applied, capacitor ‘C’ begins to charge through resistor ‘R’ exponentially towards

VBB. During this charging emitter circuit of UJT is an open circuit. The rate of charging is1 RC . When

this capacitor voltage, which is nothing but emitter voltage, VE reaches the peak pointVP VBB VD , the

emitter base junction is forward biased and UJT turns on. Capacitor ‘C’ rapidly discharges through load

resistance R1 with time constant2 R1C . When emitter voltage decreases to valley point Vv , UJT turns

off. Once again the capacitor will charge towards VBB and the cycle continues. The resistor R in the circuit

will determine the rate of charging of the capacitor. If R is small the capacitor charges faster towards VBB

and thus reaches VP faster and the SCR is triggered at a smaller firing angle. If R is large the capacitor

takes a longer time to charge towards VP the firing angle is delayed.

Circuit Diagrams:-

Single phase Half wave Rectifier:

T1

20

230V

AC Supply 0

20

Single phase Full wave Rectifier:- T1

20

230V

AC Supply 0

T2

20

R – Load

50Ω/25Watt

R- Load

50Ω/25Watt

17

PROCEDURE:

I. Firing of SCR using UJT for half wave rectifier:-

1) Switch ON the mains supply observe and note down the wave forms at the different points in the circuit

and also the trigger O/ps – T1, & T1’.

2) Make sure that the pulse transformer O/ps T1 & T1’ are proper and synchronized.

3) Now make the connections as given the circuit diagram for Half wave rectifier using AC source,UJT

relaxation Oscillator, SCR(T1)and a suitable load (50Ohms/25wattResistor)

4) Now switch ON the mains supply, observe and note down the output waveforms across load And SCR.

5) Draw the wave forms at different firing angle – 120, 90 & 60 for both Half & full wave rectifier

6) In the UJT firing Circuit the firing angle can be varied from 150degree to 30degree approximately.

7) We cannot vary from exact 0degree to 180degree as we vary in single phase converter firing circuit.

8) This is one of the simplest method of SCR triggering.

9) We can also fire SCR’s in the different Power circuits like Single phase half controlled converter, Single

phase AC voltage controller using both SCR and Triac.

II. Firing of SCR using UJT for Full wave rectifier:-

1) Switch ON the mains supply observe and note down the wave forms at the different points in the circuit

and also the trigger O/ps – T1, & T1’.

2) Make sure that the pulse transformer O/ps T1 & T1’ are proper and synchronized.

Now make the connections as given the circuit diagram for Fullwave rectifier

using ACsource, UJT relaxationOscillator, SCR’s(T1andT2)and a suitable

load(50Ohms/25wattResistor)

3) Now switch ON the mains supply, observe and note down the output waveforms across load And SCR.

4) Draw the wave forms at different firing angle – 120, 90 & 60 for both Half & full wave rectifier

5) In the UJT firing Circuit the firing angle can be varied from 150degree to 30degree approximately.

6) We cannot vary from exact 0degree to 180degree as we vary in single phase converter firing circuit.

7) This is one of the simplest method of SCR triggering.

8) We can also fire SCR’s in the different Power circuits like Single phase half controlled converter ,Single

phase AC voltage controller using both SCR and Triac.

a. UJT Relaxation Oscillator:

1) To Study oscillator using UJT, short CF to the diode bridge rectifier to get filtered DC output.

2) Now we will get the equidistant pulses at the o/p of pulse transformer.

3) The frequency of the pulses can be varied by varying the potentiometer RC.

4) Draw the wave forms at different points by varying RC.

Firing Angle = X/Y180

18

Connection Diagram for Half wave rectifier using UJT Firing Circuit

Table1:- Synchronized UJT triggering circuit for HWR:-


Theoretical

[Vm/2( 1 +cos)

In Volts

Practical in Volts

AC

RC

DZ

CF

C

T1

20VAC 0V 20VAC

MAINS

B1 EMITTER

T2

B2

T1

T2

R Load

19

VARIOUS WAVEFORMS FOR UJT HALF WAVE RECTIFIER

Wave forms across Input and load

Wave forms across Load and SCR (maximum α)

Wave forms across Load and SCR (minimum α)

Vscr

Vload

Vin

Vload

Vload

Vscr

20

Connection Diagram for Full wave rectifier using UJT

VARIOUS WAVEFORMS FOR UJT FULL WAVE RECTIFIER

Wave forms across Load and SCR

Vscr

Vload

21

Table 2; Synchronized UJT triggering circuit for FWR:-


Theoretical

[Vm/( 1 +cos)

In Volts

Practical in

Volts

NOTE:- a) At lower firing angles SCR may not turn on due to the fact that UJT takes some time (charging time)

to generate first firing pulse.

b)At higher firing angles SCR may not turn on due to the fact that VAK is insufficient to turn-on SCR.

RESULT:- UJT Relaxation oscillator and UJT HALF & FULL WAVE CONTROLLED CIRCUITS is

constructed and its performance is studied.

22


HALF AND FULL WAVE DIGITAL FIRING CIRCUIT

AIM: To study the static characteristics of Half and Full wave Digital firing circuit .

APPARATUS:

Digital Firing circuit Study unit

BLOCK DIAGRAM OF THE DIGITAL FIRING CIRCUIT.

_

A A

Preset

R (‘N’no. of counting bits)

_

Fixed freq. Clock N – bit max Flip – Flop B Logic ckt.+ hjTPTPTP

min S

En + driver stage

R Reset

Reset Load

Fc TM TA

Sync.

Signal (~ 10V) _

C

DC 5V

Supply

_

A A

Fixed Freq.

Oscillator (ff.)

N –bit

Counter-3

stage

Flip – Flop

(F/F)

Logic ckt. +

Modulator + 74121 monostable

oscillator

ZCD

Carrier Frequency

Oscillator.

(~ 5 kHz)

23

Different waveforms for Digital firing unit

AC REF

ZCD

RESET CLK

COUNTER O/P

CLOCK GEN.

O/P

fC oscillator

output

24

TRIGGER OUTPUTS

O/P TP

O/P TN

O/P TM

O/P TA

T1&T1’

T2&T2’

25

PROCEDURE FOR TRIGGERING OF SCR :-

1. Firing of SCR(Half wave converter) using digital firing circuit :- .

10) Switch ON the mains supply observe and note down the wave forms at the different points In the circuit . 11) Connect Tp and Tn o/ps to 1 and 2 input of pulse transformer isolation circuit.

12) Make sure that the pulse transformer outputs are proper and synchronized. 13) We will get the pulse transformer isolated trigger o/ps at T1, T1

1 , T2 and T2

1, connect the trigger o/p T1 or T2 to Gate

and Cathode of SCR.

14) Now make the connections as given the circuit diagram for Half wave rectifier using AC source, Digital

firing circuit , SCR and a load (100 Ohms/25watt) supplied along with this unit. 15) Select 180° / 100% selector switch to 180* mode(Converter expt. Mode).

16) Now switch ON the mains supply, observe and note down the output waveforms across load And SCR by

varying firing angle in steps of 10degrees.

17) Draw the wave forms at different firing angle – 120, 90, 60,30,0 degrees for half wave rectifier

18) In the firing Circuit the firing angle can be varied from 180degree to 0 degrees.

19) This is one of the triggering method of SCR .


phase AC voltage controller using both SCR, and Triac..

CIRCUIT DIAGRAM:-

Single phase Half wave Rectifier:

T1

20

0

NOTE: - If there is no output even after all the proper connections, switch OFF the

Supply and interchange the connections at step down transformer

terminals. This is to make the power circuit and firing to synchronize.

R – Load

100Ω/25Watt

26

VARIOUS WAVEFORMS FOR SINGLE PHASE HALF WAVE RECTIFIER

Wave forms across Input and load (α=90°)

Wave forms across Load and SCR (α=90°)

PROCEDURE FOR TRIGGERING OF SCR :-

2. Firing of SCR(Full wave converter) using digital firing circuit :- .

1) Switch ON the mains supply observe and note down the wave forms at the different points In the circuit .

2) Connect Tp and Tn o/ps to 1 and 2 input of pulse transformer isolation circuit.

3) Make sure that the pulse transformer outputs are proper and synchronized.

4) We will get the pulse transformer isolated trigger o/ps at T1, T11

, T2 and T21, connect the trigger o/p T1

or T2 to Gate and Cathode of SCR.

5) Now make the connections as given the circuit diagram for full wave rectifier using AC source, Digital

firing circuit , SCRs and a load (100 Ohms/25watt) supplied along with this unit.

6) Select 180° / 100% selector switch to 180* mode(Converter expt. Mode).

Vin

Vload

Vload

Vscr

27

7) Now switch ON the mains supply, observe and note down the output waveforms across load And SCR by

varying firing angle in steps of 10degrees.

8) Draw the wave forms at different firing angle – 120, 90, 60,30,0 degrees for full wave rectifier

9) In the firing Circuit the firing angle can be varied from 180degree to 0 degrees.

10) This is one of the triggering method of SCR .


phase AC voltage controller using both SCR, and Triac..

CIRCUIT DIAGRAM:-

Single phase Full wave Rectifier:-

T1

20

0

T2

20

NOTE: - If there is no output even after all the proper connections, switch OFF the

Supply and interchange the connections at step down transformer

terminals. This is to make the power circuit and firing to synchronize.

VARIOUS WAVEFORMS FOR SINGLE PHASE FULL WAVE RECTIFIER

Wave forms across Input and load (α=90°)

R- Load

100Ω/25Watt

Vload

Vin

28

Wave forms across SCR1(T1) and SCR2(T2) (α=90°)

RESULT:- Half and full wave Digital triggering circuit have been rigged up and output waveforms have

been plotted.

VT1

VT2

29


AC VOLTAGE CONTROLLER USING TRIAC-DIAC


APPARATUS:

Light Dimmer circuit Study unit

THEORY:

AC VOLTAGE CONTROLLERS

If a Thyristor switch is connected between AC supply and load, the power flow can be controlled

by varying the rms value of AC voltage applied to the load, and this type of power circuit is known as an

AC voltage controller. The most common application of AC voltage controller are; Industrial heating, on-

load transformer tap changing, light controls, speed control of polyphase induction motors and AC magnet

controls. For power transfer, two types of control are normally used :

1. On – Off control

2. Phase – angle control

In On – Off control, thyristor switches connect the load to the AC source for a few cycles of input voltage

and then disconnect it for another few cycles. This is trivial in nature and will not be discussed here.

In Phase control, thyristor switches connect the load to the AC source for a portion of each type of input

voltage. This mode of control will be the subject of discussion in this section.

Since the input voltage is AC, thyristors are line commutated; and phase – control thyristors,

(converter grade thyristors), which are relatively inexpensive and slower than fast – switching thyristors,

are normally used. For applications upto 400Hz, if TRIAC’s are available to meet the voltage and current

ratings of a particular application, TRIAC’s are more commonly used.

Circuit diagram:

R1 100

Gate TRIAC

47K VR1

Diac (DB3)

R2 250

0.1F C1 C2

0.1F

MT1

LAMP

50V AC

MT2

30

Connection diagram:

PROCEDURE:


2) Connect the lamp at load terminals.

3) Switch ON the mains supply, vary the firing angle potentiometer and observe the variation in lamp

brightness and also note down the voltage variation across the lamp.

4) Observe the waveform across the lamp and Triac.

Different waveforms for AC voltage controller using triac-Diac

Voltage waveforms across input and load(Lamp)

Vin

Vload

31

Voltage waveforms across Load(lamp) and Device(Triac)

Firing Angle α = X/Y180

TABULAR COLUMN

For Lamp Load:-

Sl. No. Input Voltage

AC in volts

Firing angle

In Degrees

Output voltage

AC in Volts

1

2

3

.

.

.

.

50V

50V

50V

.

.

.

.

180

150

120

.

.

.

.

RESULT:-AC voltage controller using TRIAC and DIAC have been rigged up and output waveforms

have been plotted.

Vload

Vtriac

32

Experiment No: 07 Date: SCR TURN OFF METHODS

AIM: To study the commutation circuit Using LC circuit and Auxillary Commutation.

APPARATUS:

Forced commutation circuit Study unit

THEORY:

CLASS – B COMMUTATION: (Self Commutation by an LC circuit):-

In this type of commutation reverse voltage is applied to the thyristor by the over swinging of an

Under damped LC circuit connected across the Thyristor. Capacitor charges up to the supply voltage

before the trigger pulse is applied to the gate. When the thyristor is triggered, two currents flow, a load

current through the external circuit and a pulse of current through LC circuit and thyristor in opposite

direction. This resonant current tends to turn – off the thyristor.

CIRCUIT DIAGRAM:

L

T1 C

24v

R

PROCEDURE: -

1)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, potentiometer. Duty cycle potentiometer is of no use in this experiment.

3)Repeat the same for different values of L, C and R

33

DIFFERENT WAVEFORMS FOR CLASS – B COMMUTATION

Vscr

V load

V load

VC

34

CLASS – D COMMUTATION: (Auxilliary Commutation by an LC circuit):-

This type of commutation is popular due to the design flexibility. There are many choppers and Inverters

under this class.T2 must be triggered first in order to charge up capacitor ‘C’ is charged, T2 is commutated off

owing to lack of current. When T1 is triggered, current flows in two paths, load current through RL and

Commutating current through ‘C’ T1, L and D1. The charge on the capacitor is reversed and held with The hold –

off diode D1. At any desired instant T2 may be triggered when then Places ‘C’ across T1 via T2 and T1 is turned

off.

CIRCUIT DIAGRAM:

PROCEDURE:


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.

4) Note down the voltage waveforms Across the load, T1, T2 and capacitor.

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

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

35

DIFFERENT WAVEFORMS FOR CLASS – D COMMUTATION

RESULT:-SCR Turn off methods using LC and Auxilliary tyristor have been rigged up and output

waveforms across different circuit elements have been plotted.

Vscr1

V

Load

Vc

Vscr2

36


SINGLE PHASE CONTROLLED CONVERTER

AIM: To study the different wave forms across the circuit elements in Single phase controlled converter .

APPARATUS:

Single phase controlled converter circuit Study unit.

SINGLE PHASE FULLY CONTROLLED CONVERTER FOR R-L LOAD WITH FREE

WHEELING DIODE :

THEORY:

The circuit arrangement of a single phase fully controlled converter is shown in fig(a). During the

positive half cycle, thyristor T1 & T1¹ are forward biased and When these two thyristor are fired

simultaneously at wt = , the load is connected to the input supply through T1 and T1¹. during the period

≤ wt ≤ ( + ). the input voltage is negative and the free wheeling diode Dm is forward biased. Dm

conducts to provide the continuity of current in case of inductive loads. The load current is transferred from

T1 & T1¹ to Dm; and thyristors T1 & T1¹ are turned off due to line or natural commutation.

During the negative half cycle of input voltage, thyristor T1 & T1¹ are forward biased. The firing of

thyristor T2 & T2¹ simultaneously at wt = (+) will reverse bias Dm. The diode Dm is turned off and

load is connected to the supply through T2 & T2¹. This converter has better power factor due to free

wheeling diode. The average output voltage can be found from Vdc(av) = Vm/ (1+ Cos ).

CIRCUIT DIAGRAM:

PROCEDURE:

1) Switch ON the Mains Supply to the Firing circuit.

2) Observe all the test points by varying the firing angle using potentiometer and trigger o/Ps ON/OFF

switch.

3) Then observe the trigger o/p s and phase sequence.

4) Make sure that all the trigger o/p s are proper before connecting to the power circuit.

5) The trigger o/p pulse width varies as we vary the firing angle.

6) Next make the connections in the power circuit.

7) Connect 30V taping of the isolation transformer secondary to the power circuit.

37

8) Connect the R-load (150 Ohms Rheostat) between load points.

9) Connect trigger pulses from single phase converter firing unit T1,T1’ and T2,T2’ to Corresponding

SCR’s(T1,T1’and T2,T2’) in the power circuit.

10) Switch ON the supply to the isolation transformer.

11) Switch ON the MCB in the power circuit , switch ON the trigger o/ps in the firing unit.

12) Note down the output voltage & output current from Voltmeter and Ammeter provided in the power

circuit for different firing angle (0-180 degree).

13) Draw the waveforms across load and devices for different firing angle.

14) Repeat the same for different input voltage up to maximum voltage(230Volts) as provided in the

isolation transformer.

15) Repeat the same for R-L load (connect Inductive load 0-150mH in series with Resistive load) with and

without freewheeling diode and note down the waveform.

TABULAR COLUMN

SL NO Input

Voltage-

Vin

Firing

angle

Output

voltage

Output Current Calculated o/p

Voltage (Volts)

Vo=

Vm/ (1+ Cos )

WAVEFORM:

38

SINGLE PHASE FULLY CONTROLLED CONVERTER FOR R-L LOAD WITH OUT FREE

WHEELING DIODE :

THEORY:

When the single phase fully controlled converter is connected with R-L load , During the positive

half cycle, thyristor T1 & T1¹ are forward biased and When these two thyristor are fired simultaneously

at wt = , the load is connected to the input supply through T1 and T1¹. Due to inductive load T1 and T1’

will continue to conduct till wt = ( + ) even though the input voltage is already negative .During

negative half cycle of the input voltage thyristors T2 & T2’ are forward biased and firing of thyristors T2

& T2’ at wt = ( + ) will apply the supply voltage across thyristors T1 & T1¹ as reverse blocking voltage.

T1 & T1¹ will be turned off due to line or natural commutation.

During the period ( +) the input voltage Vs and input current is positive and the power flows from the

supply to load .The converter is said to be operated in rectification mode.During period from to +

,the input voltage Vs is negative and the input current is positive ,and there will be reverse power from the

load to supply. The converter is said to be operated in inversion mode.

The average output voltage can be found from Vdc = 2Vm/Cos.

CIRCUIT DIAGRAM:

PROCEDURE :


2) Observe all the test points by varying the firing angle using potentiometer and trigger o/Ps ON/OFF

switch.

3) Then observe the trigger o/p s and phase sequence.

4) Make sure that all the trigger o/p s are proper before connecting to the power circuit.


6) Next make the connections in the power circuit.

7) Connect 30V taping of the isolation transformer secondary to the power circuit.

8) Connect the R-load (150 Ohms Rheostat) between load points.

9) Connect trigger pulses from single phase converter firing unit T1,T1’ and T2,T2’ to Corresponding

SCR’s(T1,T1’and T2,T2’) in the power circuit.

10) Switch ON the supply to the isolation transformer.

11) Switch ON the MCB in the power circuit , switch ON the trigger o/ps in the firing unit.



39

13) Draw the waveforms across load and devices for different firing angle.





TABULAR COLUMN

SL NO Input

Voltage-

Vin

Firing

angle

Output

voltage

Output Current

WAVEFORM:

Note : In case of fully controlled bridge the triggering angle should not increase beyond max (approx.160

degree) to allow conducting SCR sufficient time to turnoff. The maximum value of firing is obtained from

the relation

E = Vm Sin( + ) ,there =-sin (E/Vm)

Where E is counter e.m.f. generated in the inductor.

Io=Vo/RL for continuous & constant current.

40

SINGLE PHASE SEMI CONTROLLED CONVERTER FOR R-L LOAD WITH FREE

WHEELING DIODE :

THEORY:

The circuit arrangement of a single phase semiconverter. During the positive half cycle, thyristor T1

is forward biased. When thyristor T1 is fired at wt = ά, the load is connected to the input supply through T1

and D2 during the period ά ≤ wt ≤ п. During the period from a ≤ wt ≤ (п + ά), the input voltage is negative

and the free wheeling diode Dm is forward biased. Dm conducts to provide the continuity of current in case

of inductive loads. The thyristor T1 and the Diode D2 are turned off. During the negative half cycle of

input voltage, thyristor T2 is forward biased and firing of thyristor T2 at wt = ( п + ά) will reverse boas

Dm, the diode Dm is turned off and load is connected to the supply through T2 and D1.

This converter has a better power factor due to the freewheeling diode and is commonly used in

application up to freewheeling diode and is commonly used in applications up to 15KW, where one-

quadrant operation is acceptable.

The Half controlled Bridge has the inherent fly wheel action, and analysis is more or less the same

with or without a fly wheel diode connected across the load. In practice it is always advisable to provide a

fly wheel diode in a half controlled bridge so that the commutation of SCR’s is assured with inductive load.

CIRCUIT DIAGRAM:

PROCEDURE :


2) Observe all the test points by varying the firing angle and trigger o/p s ON/OFF switch.

3) Then observe the trigger o/p s and phase sequence. Make sure that all the trigger o/p s are proper

before connecting to the power circuit.


5) Next make the connections in the power circuit for single phase half controlled converter.

6) Connect 30V tapping of the transformer secondary to the power circuit.

7) Connect the R-load (150 ohms/5Amps) between load points.

8) Connect firing pulses from the firing circuit to the respective SCR’s (T1 & T2) in the power circuit.

9) Switch ON the MCB, switch ON the trigger o/ps and note down the voltage wave forms across load

and devices.



41

11) Draw the waveforms across load and device for different firing angle.





TABULAR COLUMN:

SL NO Input

Voltage-Vin

Firing angle Output

voltage

Output

Current

Calculated o/p

Voltage

Vm//(1+COS)

WAVE FORMS:

42

SINGLE PHASE SEMI CONTROLLED CONVERTER FOR R-L LOAD WITHOUT FREE

WHEELING DIODE :

THEORY:

The circuit arrangement of a single phase semiconverter. During the positive half cycle, thyristor T1

is forward biased. When thyristor T1 is fired at wt = ά, the load is connected to the input supply through T1

and D2 during the period ά ≤ wt ≤ п. During the period from a ≤ wt ≤ (п + ά), the input voltage is negative

and the free wheeling diode Dm is forward biased. Dm conducts to provide the continuity of current in case

of inductive loads. The thyristor T1 and the Diode D2 are turned off. During the negative half cycle of

input voltage, thyristor T2 is forward biased and firing of thyristor T2 at wt = ( п + ά) will reverse boas

Dm, the diode Dm is turned off and load is connected to the supply through T2 and D1.

This converter has a better power factor due to the freewheeling diode and is commonly used in

application up to freewheeling diode and is commonly used in applications up to 15KW, where one-

quadrant operation is acceptable.

The Half controlled Bridge has the inherent fly wheel action, and analysis is more or less the same

with or without a fly wheel diode connected across the load. In practice it is always advisable to provide a

fly wheel diode in a half controlled bridge so that the commutation of SCR’s is assured with inductive load.

CIRCUIT DIAGRAM:

PROCEDURE :


2) Observe all the test points by varying the firing angle and trigger o/p s ON/OFF switch.

3) Then observe the trigger o/p s and phase sequence. Make sure that all the trigger o/p s are proper

before connecting to the power circuit.


5) Next make the connections in the power circuit for single phase half controlled converter.

6) Connect 30V tapping of the transformer secondary to the power circuit.

7) Connect the R-load (150 ohms/5Amps) between load points.

8) Connect firing pulses from the firing circuit to the respective SCR’s (T1 & T2) in the power circuit.

9) Switch ON the MCB, switch ON the trigger o/ps and note down the voltage wave forms across load

and devices.



43

11) Draw the waveforms across load and device for different firing angle.



Repeat the same for R-L load (connect Inductive load 0-150mH in series with Resistive load) with and


TABULAR COLUMN

SL NO Input

Voltage-Vin

Firing angle Output

voltage

Output

Current

WAVE FORMS:

RESULT:-Single phase controlled converters have been rigged up with R and RLload and output

waveforms across different circuit elements have been plotted.

44


SPEED CONTROL OF DC MOTOR


APPARATUS:

DC motor driving Study unit

Aim of the experiment :- To study the speed control of DC shunt motor using 1-Ph

Half controlled rectifier.

Equipment’s Required :- 1) Single phase converter firing circuit.

2) D C Motor speed control unit ( Power circuit).

3) Power scope or CRO.

4) DC Shunt motor. 0.5 H.P. / 220 V.

5) Rheostat - 50 Ohms/5 Amps.

6) Tachometer.

D2 D4

Connect trigger pulses T1& T2 from Single phase converter firing unit.

DC MOTOR SPEED CONTROL UNIT (POWER CIRCUIT)

M

C

B

ON

T1 T2

Dm

A

50E / 5A

+

V M

A

AA

F

I

E

L

D

0.5HP

DC Shunt

Motor

-

+ -

FIELD SUPPLY

AC –IN from

Isolation Transformer

AC -OUT

45

PROCEDURE FOR ARMATURE CONTROL:-

1) Switch ON the mains supply to the single phase converter firing circuit.

2) Observe the test points and trigger outputs. Verify the trigger outputs and their phase sequence.

3) Vary the firing angle potentiometer and observe the trigger outputs. The pulse train width

will increase as we decrease the firing angle from 180* to 0*. It is 0 at 180* and maximum

at 0* and 50% at 90* Soft start and stop feature is provided for trigger outputs. When we

press the ON/OFF switch the trigger outputs will start at 180* and slowly increased to the

firing angle set by firing angle potentiometer. The acceleration time is set in the factory.( 10

Seconds ).

4) When we release the ON/OFF switch, the trigger outputs will slowly decreased to

180*from the set firing angle. The deceleration time is set in the factory. ( _ 2 seconds ).The

deceleration time is very short compared to acceleration time.

5) Make sure that all the trigger outputs are proper before connecting to the power circuits.

6) Make the connections in the power circuit as given in the circuit diagram.

7) Connect the AC input to the power circuit through isolation transformer.

8) Initially keep the input supply at low voltage say 60 Volts.

9) Connect the trigger outputs from firing circuit to the corresponding SCR’s Gate and

Cathode.

10) Initially connect a Rheostat of 50 Ohms/5 Amps.

11) Switch ON the trigger O/Ps observe the voltage waveforms across load by varying the

Firing angle Potentiometer.

12) Compare with the expected waveforms. If the unit is working properly switch OFF the

trigger O/Ps and switch OFF the MCB.

13) Connect Field terminals of DC motor to the field supply points in the power circuit.

14) Then connect Armature terminal of the DC motor through the rheostat and the Ammeter

provided in the unit to the output of rectifier.

15) Switch ON the field supply first. Set the field voltage to some value – 200Volts.This voltage

can be measured using the Voltmeter provided in the rectifier.

16) Set the input voltage to 100Volts. Initially keep the firing angle pot at 180°.

17) Initially keep the resistance at maximum position and cut off once the DC motor starts.

This is to limit the starting current.

18) Switch ON the MCB and trigger O/Ps .

19) Vary the firing angle potentiometer and note down the O/P Voltage, Output Current and

measure the speed of the DC motor using hand held Digital tachometer for different values of

Firing angle.

20) Note down these values in the tabular column. And also observe the Voltage waveforms.

We can observe that back emf will increase as the speed increases. Next vary the input

voltage upto 230Volts in steps and note down the readings in the tabular column.

21) Apply the load to the DC Shunt motor till the rated current of the motor(3Amps)

22) Adjust the load in steps to a maximum current of 2 Amps (do not exceed 2 amps) 23) At each load note down the speed O/P Voltage, Output Current and measure the speed of the

DC motor using hand held Digital tachometer for different values of Firing angle.

46

PROCEDURE FOR FIELD CONTROL: - 1) Keep the armature Voltage constant (150V).

2) Vary the field voltage from Min100V appx. to Max 200V appx. Using 400 ohms / 1A Rheostat and

note down the speed for different values of field voltage.

3) We can observe that speed increases as we decrease the field voltage.

4) Decrease the firing angle to 180º and switch off the MCB in the power circuit and switch off the

firing unit and power circuit.

TABULAR COLUMN FOR ARMATURE CONTROL:-

Sl No Field

Voltage

Vin volts Firing

angle

Va Volts Ia amps Speed

1 200V 150V 180

2 200 V 150V 150

TABULAR COLUMN FOR FIELD CONTROL:-

Sl No Vin volts Armature

Voltage

Field

Voltage

Firing

angle

Ia amps Speed

1 150V 150V 100V 180 2 150 V 150V 120V 150

Different waveforms for speed control of DC motor using single phase half controlled converter.

Voltage waveform across armature without load(α=90°)

Back EMF

47


Voltage waveform across armature with load(α=90°)


48

RESULT:- Speed control of DC motor using single phase SCR half controlled converter is

constructed and its performance is studied.

Experiment No: 10 Date: SPEED CONTROL OF UNIVERSAL MOTOR


APPARATUS:

Universal motor driving Study unit

1) Using- SCR’s : -

T1

230V MCB T2

AC AC input AC output

Isolation transformer

2 ) Using TRIAC : -

AC input AC output

V M

V M


230V

AC

MCB

A

A

Universal

Motor/Induction

motor

Universal

Motor/Induct

ion Motor

TR

49

TABULAR COLUMN:-

Sl.No Input Voltage –Vin AC

Volts

Firing Angle- α Output Voltage-

Vo AC Volts

Speed -RPM

PROCEDURE FOR SPEED CONTROL OF UNIVERSAL MOTOR USING DC VOLTAGE

CONTROL :-

1) Make the inter connections in the power circuit as given is the circuit

diagram for different experiments using SCR for DC control of universal motor.

2) Connect Universal motor at load points. 3) Switch ON the firing circuit and observe the trigger outputs by varying the firing angle

potentiometer and by operating ON/OFF switch and SCR/TRIAC selector switch .

4) Make sure that the firing pulses are proper before connecting to the power

circuit.

5) Then connect the trigger outputs T1 & T2 from firing circuit to corresponding

SCR’s Select the SCR / Triac selection switch to SCR for DC voltage control

using SCR.

6) In the power circuit Initially set the AC input to 60 volts from single phase

isolation

transformer/single phase auto transformer.

7) Connect AC/DC voltmeter and Ammeter provided in the unit and select switch for

DC Voltage/Current measurement.

8) Switch ON the MCB and Switch ON the Trigger outputs switch.

9) Vary the firing angle from 180 degree to 0 degree and note down O/P

voltage,current and speed of the motor using Tachometer.

10) If the output is proper then you can increase the input voltage to rated value

0 – 230V gradually ,and note down O/P voltage , current and speed of the

motor using Tachometer.

50

B) Speed control of Universal motor using DC Voltage control

1. Single phase Half wave converter : -

AC input AC output


2. Single phase Half controlled bridge rectifier : -

Dm

TABULAR COLUMN:-

Sl.No Input Voltage –Vin AC

Volts

Firing Angle- α Output Voltage-

Vo DCVolts

Speed -RPM

Result:- Speed control of Universal motor(AC/DC) And Induction motor is constructed and

its performance is studied.

V M

230V

AC MCB

V

M


230V

AC MCB

T1 T2

D3 D4

T1

Universal

motor

A

+ -

+

-

A

+ -

+

-

51


SPEED CONTROL OF STEPPER MOTOR


APPARATUS:

Stepper motor driving circuit Study unit

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.

2) Switch ON the mains supply to the unit.

3) Check the Power supplies.

The unit displays WELCOME STEPPER MOTOR

After few seconds it displays STOP S/R R/F H/F

RPM 1 FOR FULL

Stop - Corresponds to RUN/STOP selection.

S/R - Corresponds to Step/RPM ( Continuous rotation) selection.

R/F - Corresponds to Reverse/Forward - direction selection.

H/F - Corresponds to Half step/Full step selection.

4) Now RPM blinks. Press INC / DEC key to select STEP or RPM ( Continuous

rotation) mode.

5) After selecting RPM/STEP mode press SET key to select the mode. Now 1 blinks.

This is corresponds to number of rotation or number of steps selected.

6) Press INC/DEC key to select the speed or steps.

7) Press SET key to set the rpm / number of steps. Now FOR blinks. This is corresponds

to direction of rotation Forward. Press INC/DEC key to select the direction of rotation

and press SET key to select.

8) Now FULL blinks. This is corresponds to Full step.

9) Press INC/DEC key to select Half step / Full step mode and press SET key to select

Half / Full step mode.Now the setting is over.

10) Press RUN / STOP key, the stepper motor rotates at the set speed if RPM is selected

or it moves the number of steps set and stops.

11) Again pressing RUN/ STOP key the motor stops if it is in RPM mode or it again

moves the number of set steps and stops.

12) Set the step mode, 1 step, FORWARD and Half step mode.

13) Check the output status by LED indication for each step and verify with the

switching logic sequence as given in the below truth table.

14) Repeat the same for Full step mode. Repeat the same for Reverse direction.

53

CIRCUIT DIAGRAM

Red

V = 4 X Motor Voltage

Rs = 3 X Rm (Motor resistance/ Phase)

Suitable for slow RPM

SWITCHING LOGIC SEQUENCE

Full step

A1 A2 B1 B2

Red Black Blue Green

1 0 0 1

0 1 0 1

0 1 1 0

1 0 1 0

Q1 Q2 Q3 Q4

Half step

A1 A2 B1 B2

Red Black Blue Green

1 0 1 0

1 0 0 0

1 0 0 1

0 0 0 1

0 1 0 1

0 1 0 0

0 1 1 0

0 0 1 0

To change the direction read sequence from bottom to top.

LOGIC CONTROLLER

USING MICROCONTROLLER FREEWHEEING DIODES

Black

Blue

Green

SWITCHING TRANSISTORS

WHITEE

BLACK

Rs

Rs

- ve + ve

A1

A2

B1

B2

54

STEPPER MOTOR SPECIFICATION

The stepper motor fitted on the panel and all the connections are brought to a connector. The dial is fitted to the shaft of the

stepper motor. The inner dial marking in steps and outer marking in degrees.

Step angle : 1.80 + or – 0.1

0

3 Kg Cm = 0.1 NM

Steps per revolution : 200

Half step = 0.90

VOLTAGE -5V

THEORY:-

The stepping motor is an electromagnetic device which converts digital pulses into discrete mechanical

rotational movements. In rotary step motor, the output shaft or motor rotates in equal increments, in

response to a train of input pulses.

CHARACTERISTICS:

Construction:

Stepping Motor is basically a Motor with two phases, eight salient poles, toothed iron rotor and a

permanent magnet. This rotor is known as hybrid rotor. The rotor is suspended in the stator by means of

sealed ball bearings. All parts of the motors are precision machined for better performance and accuracy of

steps.

Step Angle: 1.8 0 + or – 0.1

0 non –cumulative.

Holding Torque : 2.8 Kg. Cm.

Dynamic Torque: Dynamic torque is mainly controlled by the electronics control

Circuits. Torque will drop down as the speed increases.

Residual Torque or Detent Torque: Because of the presence of permanent magnet in the

Rotor.

Working Temperature and insulation Class: Temperature of stepping motors may rise 500

C

above ambient. It is observed that body temp. Generally stabilizes at about 850 C to 90

0 C for continuous duty cycle.

The insulation used is of class B type which can withstand hot spot temp. of 1300

C. For better

heat dissipation motors duly fitted with heat sinks are recommended. This reduces the temp

by about 100 C to 15

0 C.

Working of Stepping Motor:

The stepping action is caused by sequential switching of supply to the two phases of motor as shown in the

switching logic sequence table. The specified torque of any Stepping motors is the torque at stands still

(holding torque). This torque is directly proportional to the current to rated level within the time given for

one step.

This is mainly due to L/R time constant of winding. The drop in current level causes drop in torque as the

speed increases. In order to improve torque at high speed it is necessary to maintain current at the rated

level.

Never exceed rated current of the motor.

Stepper Motors differ form conventional DC Servo motors in the following respects:

55

1. There is no control winding in Stepping Motors Both windings are identical.

2. The stepping rate (Speed of rotation) is governed by frequency of switching and not by

supply voltage.

3. A Single pulse input will move the shaft of Motor by one step. Thus number of steps

can be precisely controlled by controlling number of pulses.

4. When there is no pulse input, the rotor will remain locked in the position in which the

last step was taken since at any time two winding are always energized which lock the

rotor electromagnetically.

5. Stepping Motors can be programmed in there parameters namely: a) direction,

b) Speed, c) Number of Steps.

6. Stepping Motor is brushless so no wear & tear.

7. Load & no load condition makes no difference in running currents of the motor.

PROCEDURE:

1) To begin with switch ON the DC Chopper firing unit.

2) Observe the test point signals and Trigger output signals by varying Duty cycle and

Frequency Potentiometer. Be sure the trigger outputs are proper before connecting to

the power circuit.

3) Now make the interconnections in the power circuit as given in the circuit diagram

for voltage commutated chopper(Impulse commutated chopper)

4) Connect DC supply from a Regulated DC power supply unit(30V/2A).

5) Connect a Resistive load (100 Ohms/2Amps Rheostat)

6) Connect respective trigger outputs from the firing circuit to the respective SCRs

in the Power Circuit.

7) Initially keep the ON/OFF switch in the firing circuit in OFF position.

8) Switch ON the DC supply. Apply Main SCR trigger pulses by pressing the ON/OFF

switch to ON position.

9) Observe the voltage waveforms across load.

10) We can observe the chopped DC waveform.

11) If the commutation fails we can see only the DC voltage.

12) In this case switch OFF the DC supply, Switch OFF pulses and check the

connections and try again.

56

13) Observe the voltage across load, across Capacitor, across Main SCR and

auxiliary SCR by varying Duty cycle and frequency Potentiometer.

14) Draw the wave forms at different duty cycle and at different Frequency.

15) Connect Voltmeter and Ammeter and note down values in the table.

16) Connect R-L Load and repeat the above procedure for with and without

freewheeling diode.

Result:- Driving circuit for Speed control of Stepper motor is constructed and its

performance is studied.

57

Experiment No: 12 Date: IMPULSE COMMUTATED CHOPPER


APPARATUS:

DC chopper circuit Study unit

.

TABULAR COLUMN VOLTAGE COMMUTATED CHOPPER:-

Sl.

No.

V in

volts

Ton

msec

T off

msce

Duty cycle

=Ton/T

in %

Vo

volts

Io

amps

Vdc

T1

T2

LOAD

C

D1 L

Dm

58

Actual Waveforms for Voltage commutated chopper

Vscr1

V Load

Vscr2

b) Waveforms

V Load

59

THEORY:-

DC CHOPPER 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. 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. Choppers are also used in DC voltage regulators.

Circuit Diagram

a) Circuit Diagram

VDC

60

Accessories Required :-

1) Rheostat-100 Ohms/2A.

2) DC power supply -30V/2A (Single)

3) C.R.O.-20MHZ.

Procedure:-

1) To begin with switch on the power supply to the firing circuit. Check that

trigger pulses by varying the frequency.

2) Make the interconnections of the power circuit as shown in the circuit diagram.

3) Now connect trigger outputs from the firing circuits to Gate and Cathode of

SCR`s T1 and T2.

4) Connect input from a 30V/2A Regulated power supply. Switch on the input

DC supply. Now apply trigger pulses to the SCR`s and observe voltage wave

from across load.

5)Vary the frequency and observe the wave forms.

6) If the inverter frequency increases above the resonant frequency of the power

circuit commutation will fail.

7)Then switch OFF the DC supply, reduce the inverter frequency and try again if

you will not get the result. Check the input fuse and try again.

8) Repeat the same with different values of L , C and load.

9) And also observe the waveforms with and without fly wheel diodes. The output

waveform is entirely depending on load.

10) Plot a graph of frequency verses output voltage. (Output voltage varies with frequency)

11. Tabulate the readings in the table (refer given table).

12 . Draw the load voltage waveform.

To switch OFF the inverter. Switch OFF the input supply first and then trigger pulses.

Resonance frequency:- fr.= 1 1 _ R2

2 π LC 4L2

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TABULAR COLUMN:-

Sl No. Frequency

inHZ

Load

Voltage(AC)

volts using

multimeter

RESULT:-DC chopper circuit have been rigged up with L and C and output waveforms across different

circuit elements have been plotted.

62


SERIES AND PARALLEL INVERTER


APPARATUS:

RC Firing circuit Study unit

MODIFIED SERIES INVERTER

D1 T1

C1 (C2)

L1

Lm

50ohms/2A

L2 C11 (C2

1)

VDC

0-30V RPS

D2 T2

CIRCUIT DIAGRAM

Low frequency

eo

LOAD

F

U

S

E

S

W

I

T

C

H

63

DIFFERENT WAVEFORMS FOR SERIES INVERTER AS OBSERVED

ON DIGITAL STORAGE OSCILLOSCOPE

Capacitor-10Microfarad L= 10mh R=15 Ohms app.

WAVEFORM ACROSS LOAD (AT LOW FREQUENCY

WITH OUT FLY WHEEL DIODES)

High frequency

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WAVEFORM ACROSS LOAD (AT HIGH FREQUENCY

WITH OUT FLY WHEEL DIODES

WAVEFORM ACROSS LOAD (AT LOW FREQUENCY

WITH FLY WHEEL DIODES)

WAVEFORM ACROSS LOAD (AT HIGH FREQUENCY

WITH FLY WHEEL DIODES)

65

Accessories Required :-

1) Rheostat-100 Ohms/2A.

2) DC power supply -30V/2A (Single)

3) C.R.O.-20MHZ.

THEORY :

The circuit is a typical class C Parallel inverter.

Assume TN to be ON and Tpto be OFF. The bottom of the commutating capacitor is charged to twice the

supply voltage and remains at this value until TP is turned on.

When TP is turned on, the current flows through lower half of the primary, TP and commutating inductance

L. Since voltage across C cannot instantaneously, the common SCR cathode point raises approximately to

2Vdc and reverse biases TN. Thus TN turns off and C discharges through L, the supply circuit and then

recharges in the reverse direction. The autotransformer action makes C to charge making now its upper

point to reach +2Vdc Volts ready to commutate Tp, When TN is again turned on and the cycle repeats.

The major purpose of commutating inductor – L is to limit commutating capacitor charging current during

switching.

Free wheeling diodes DP and DN assist the inverter in handling a wide range of loads and the value of C

may be reduced since the capacitor now does not have to carry the reactive current. To dampen the

feedback diode currents within the half period, feedback diodes are connected to tapings of the transformer

at 25V tapings.

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 from DC Regulated

power supply(30V/2A) .

3) Connect trigger outputs to Gate and Cathode of SCR TP & TN.

4) Make the interconnections as shown in circuit diagram.

5) Connect load between load terminals(50ohms/2A).

6) Connect free wheeling diodes in the circuit.

7) To begin with set input voltage to 15V. Apply trigger pulses to SCR and

observe voltage waveforms across load.

8) Output voltage is square wave only. Then remove freewheeling diode connections

and observe the waveforms.

9) Then vary the load, vary the frequency and observe waveforms. To switch OFF

the inverter switch OFF DC input supply only. Switch OFF the trigger pulses will

lead to short circuit. 10) Since the parallel inverter works on forced commutation, there is a chance of

commutation failure.

11) If the commutation fails, there is a dead short circuit in the input DC supply,

which will leads to the blown off the input fuse. Please check the fuse if the

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commutation fails. Preferably connect the input DC supply from the 30V/2A regulated

DC power supply unit which has over current tripping facility thereby protect the DC

supply unit. 12) If the commutation fails, switch off the DC supply first and then trigger Outputs.

Check the connections again.

CIRCUIT DIAGRAM:

D1

30V

SCR1

0

25V

0V RL

L C

25V

30V

SCR2

30V

D2

TRIGGER OUTPUTS

T1

T2

+VDC

--VDC

_

VDC

+

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DIFFERENT WAVEFORMS FOR PARALLEL INVERTER

AS OBSERVED ON DIGITAL STORAGE OSCILLOSCOPE

LOAD VOLTAGE WAVEFORM FOR WITHOUT

FREEWHEELING DIODE

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VOLTAGE WAVEFORM AT PRIMARY OF TRANSFORMER

WITHOUT FREEWHEELING DIODE

LOAD VOLTAGE WAVEFORM FOR WITH

FREEWHEELING DIODE

VOLTAGE WAVEFORM AT PRIMARY OF TRANSFORMER

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WITH FREEWHEELING DIODE

VOLTAGE WAVEFORM ACROSS SCR’s TP&TN

WITH FREEWHEELING DIODE

RESULT:-Series and Parallel Inverter circuits have been rigged up with L and C and output waveforms

across different circuit elements have been plotted.

DEPARTMENT OF ELECTRONICS AND COMMUNICATION & … · 3 Controlled HWR and FWR using RC triggering...

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Transcript of DEPARTMENT OF ELECTRONICS AND COMMUNICATION & … · 3 Controlled HWR and FWR using RC triggering...