06ecl78 Pe Lab

A LAB MANUAL ON

POWER ELECTRONICS

Subject Code: 06ECL78

(As per VTU Syllabus)

PREPARED BY

DEPARTMENTOF

ELECTRONICS & COMMUNICATION ENGINEERING

CONTENTS

EXPT. NO.

NAME OF THE EXPERIMENTPAGE NO.

01 STATIC CHARACTERISTICS OF SCR AND DIAC

02 STATIC CHARACTERISTICS OF MOSFET AND IGBT

03 CONTROLLED HWR AND FWR USING RC TRIGGERING CIRCUIT

04 SCR TURN OFF USING LC CIRCUIT AND AUXILIARY COMMUTATION

05 UJT FIRING CIRCUIT FOR HWR AND FWR CIRCUITS

06GENERATION OF FIRING SIGNALS FOR THYRISTORS/ TRIALS USING DIGITAL CIRCUITS/MICROPROCESSOR

07 AC VOLTAGE CONTROLLER USING TRIAC / DIAC COMBINATION

08SINGLE PHASE FULLY CONTROLLED BRIDGE CONVERTER WITH R AND R-L LOADS

09VOLTAGE(IMPULSE) COMMUTATED CHOPPER BOTH CONSTANT FREQUENCY AND VARIABLE FREQUENCY OPERATIONS

10 SPEED CONTROL OF A SEPERATELY EXITED DC MOTOR

11 SPEED CONTROL OF UNIVERSAL MOTOR

12 SPEED CONTROL OF STEPPER MOTOR

13 PARALLEL / SEREIS INVERTER

Power Electronics Laboratory Manual - 06ECL78

Ex.No:01 STATIC CHARACTERISTICS OF SCR

AIM:

To find the characteristics of SCR and to find the forward resistance, holding. Current and latching current

APPARATUS

Sl. No. Components Details Specification Qty

1. SCR 2P4M 1 No.

2. Ammeter 250A, 10 mA. 2 Nos.

3. Dual Regulated power supply 30V/2A 1No.

4. Resistance 1k /10w 1No.

5. Resistance 5k/10w 1No.

6. Diode BY127 1No.

7. Multimeter, Connecting Board

THEORY

Silicon controlled rectifier (SCR) –also called, as thyristor is one of the most widely used semi-controlled switching devices in power electronics. It has a p-n-p-n structure and three terminals – anode (A), cathode (K) and gate (G). SCR can be turned on by injecting a current into the gate only when its anode is positive with respect to cathode i.e. when it is forward biased. Then current flows through it in the direction anode to cathode, It does not conduct when reverse biased even if gate current is injected. Therefore, SCR is a current controlled Uni.-directional switch.

With iG = 0, when the device is forward biased, a very small forward leakage current of few A flows as long as VAK < VBO. When VAK becomes equal to VBO, the device turns on causing a sudden drop in VAK. This turns on damages the device. Then iA abruptly increases to a value limited by any impedance connected in series with the device. The device remains in on state if IL IA where IL is called the latching current. A conducting SCR can be brought to off state by first reducing the anode current below IH where IH is called holding current and then by applying reverse voltage across the device for duration not less than the turn off time specified by the manufacturer. With forward bias, when a gate current is injected, the device turns on at a lower forward voltage than VBO. This is the normal way of turning on the device. The forward voltage at which the device turns on reduces with increase in gate current. After turn on, the gate has no control over the device. Therefore SCR is called semi-controlled switch. This is the Main disadvantage of the device when compared to fully –controlled switches like IGBT or MOSFET.

CIRCUIT DIAGRAM

PROCEDURE

Department of Electronics & Communication Engg. – CMRIT 1


1. Make the connections as shown in the circuit diagram. Keep the v1 and v2 minimum.

2. Adjust the gate current to say 40 µA by varying v1.

3. Slowly vary v2 and note down VAK and IA and enter the readings in the tabular Column given.

4. Vary v2 till the SCR conducts.

5. Further vary and note all values. Draw the graph of VAK vs IA.

6. Repeat the same for different values of IG.

PROCEDURE TO FIND HOLDING CURRENT

1. Turn on the SCR, open the gate

2. Keep V2 to about 20v

3. Slowly reduce V2 and note the reduction in IA. At certain value of voltage, IA Suddenly goes to 0, this current is called holding current.

PROCEDURE TO FIND LATCHING CURRENT

1. Keep the gate current to a value such that SCR is fired.

2. Keep V2 such that current through SCR is just one division higher than the holding Current

3. Now remove the gate supply and check if current falls back to zero, if it falls to Zero then connect back the gate and increase V2 to get still higher current and again remove the gate and check if current falls to zero, if it falls back to zero then repeat the procedure until the current is stable at the adjusted value.

4. This minimum value of current where gate does not have any effect is the latching Current.

TABULAR COLUMN

IH =___________

IL = ___________

Forward resistance = (VAK)/IA

Rf = ……….

EXPECTED GRAPH


IG1 = …………A IG2=………A

VAK (V) IA (mA) VAK (V) IA (mA)SATURATION REGION

NEGATIVE RESISTANCE REGION

CUTTOFF REGION

VRA

AVAKANCHE BREAKDOWN


RESULT:



Ex.No:02 STATIC CHARACTERISTICS OF MOSFET AND IGBT

AIM:

To plot the static characteristics of IGBT and MOSFET.

APPARATUS

Sl. No. Components Details Specification Qty

1. IGBT. and MOSFET

2. Ammeter 100mA & 25mA Each 1 No.

3. Regulated power supply 0 to 30 v

4. Decade resistance box 2 Nos.

5. Digital Multimeter

THEORY

Metal oxide semiconductor field effect transistor (MOSFET) is a voltage - controlled device and requires only a small input current. The gate is isolated from the channel by a thin oxide layer. The three terminals are called gate, drain and source. The switching speeds are very high and the switching times are of the order of nanoseconds. MOSFETs find wide applications in low – power high-frequency converters. MOSFETS should be protected from static charges. The two types of MOSFETs are 1) Depletion type and 2) Enhancement type .As a depletion MOSFET remains on at zero gate voltage whereas an enhancement type MOSFET remains off at zero gate voltage, the enhancement type MOSFETs are generally used as switching devices in power electronics.

Insulated Gate Bipolar Transistor (IGBT) is one of the widely used fully controlled switching devices in power electronics. The device has three terminals- gate (G), emitter (E) and collector (C). These three terminals are respectively similar to the base, emitter and collector of a BJT. IGBT is also a voltage-controlled device like MOSFET. The Current rating of a single IGBT can be up to 400A, 1200V, and the switching frequency can be up to 20kHz.IGBTs are finding increasing applications in the medium power applications such as DC and AC motor drives, power, solid-state relays, and contactors.

An IGBT is turned on by just applying a positive gate voltage to open the channel for n- carriers and is turned off by just removing the gate voltage to close the channel.

IGBT combines the advantages of BJT and MOSFETs as shown below

Fully controlled device.

Peak current capability and ruggedness

Small gate input current of the order of few nano-amperes. In other words very high input impedance.

Low on-state conduction losses like BJTs

Lower switching losses.

Absence of second breakdown phenomenon, which is serious problem in BJTs.

Absence of thermal runaway.

Higher switching speeds compared to BJTs.

Relatively simpler drive circuits as in case of MOSFETs.


R1

1k

- A +

C

E

R2

1k

V1 0 - 30 V

0 - 100mA

G IGBT

IRG2P4C20U

R3

10k

V2 0 - 30 V


CIRCUIT DIAGRAM

CIRCUIT DIAGRAM TO FIND CHARACTERISTICS OF IGBT

CIRCUIT DIAGRAM TO FIND CHARACTERISTICS OF MOSFET


G

0 - 25mA

D

- A +

V2 0 - 30V

R2

1k

R3 10k

V1 0 - 30 V

3N200 S

R1

1k

R1

1k

- A +

C

E

R2

1k

V1 0 - 30 V

0 - 100mA

G IGBT

IRG2P4C20U

R3

10k

V2 0 - 30 V


PROCEDURE

TRANSFER CHARACTERISTICS OF IGBT

1. Make the connections as shown in the circuit diagram. Keep the v1 and v2 minimum.

2. Slowly vary v1 and set VCE equal to say 10v and slowly vary v2 (VGE.) and note the Ic in the tabular column given.

3. Repeat the same for different values of Vc and draw the graph of Ic Vs VGE.

COLLECTOR CHRACTERISTICS IGBT

1. Keep v2 to VGE = 5v. Slowly vary V1 and note down Ic and VCE.for particular. Values of VGE1 pinch off voltage (Vp) between collector and emitter.

2. Repeat the same for different values of VGE and note down Ic Vs VCE. Plot the Graph Ic Vs VCE for different values of VGE.

TABULAR COLUMNS

TRANSFER CHARACTERISTICS

VCE1(V) VCE2(V)

VCE (V) IC(mA) ) VCE (V) IC(mA) )

PROCEDURE

TRANSCONDUCTANCE CHARACTERISTICS

1. Make the connections as shown in the diagram.

2. Set V1 equal to say 10v. Slowly vary V2 (VGS) and note down ID and VGS readings for every 0.5 v.

3. Repeat the same for different values of VDS and draw the graph of ID Vs VGS.

DRAIN CHARACTERISTICS

1. SET V2 to VGS equal to 3.5v, slowly vary V1 and note down ID and VDS.

2. Repeat for different values of VGS and plot the graph ID Vs VDS.



TABULAR COLUMN

TRANSCONDUCTANCE CHARACTERISTICS

VDS1(V) VDS2(V)

VGS (V) ID(mA) ) VGS (V) ID(mA) )

DRAIN CHARACTERISTICS

VGS1(V) VGS2(V)

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

EXPECTED GRAPH

Transfer Characteristics of depletion type MOSFET.

ID

VP 0

VGS



Output Characteristics of depletion type MOSFET.

ID

VGS2

VGS1

0 VDS

Transfer Characteristics of IGBT

IC VCE1

0

VGE

Output Characteristics of IGBT

Ic

VGE2

VGE1

0

VCE

RESULT:



Ex.No:03 CONTROLLED HWR AND FWR USING RC TRIGGERING CIRCUIT

AIM:

To study RC Triggering for FWR and HWR.

APPARATUS

1. RC TRIGGERING MODULE

2. Multimeter -1NO.

3. LAMP 100W – 1NO.

4. FUSE UNIT – 1NO.

5. CRO with PROBE – 1NO.

THEORY

R and RC firing circuits are simple and economical. They can be used to trigger SCRs in rectifiers and AC voltage controllers. But they are not used commercially because the turn on angle of the SCR realized using these circuits is not thermally stable.

In the RC firing circuit the firing angle can be varied from nearly 0o to almost 180o by varying the pot. During each negative half cycle of the input voltage, the capacitor charges to the peak supply voltage through D1 is provided in order to by pass R1 and R2 during each negative half cycle of the supply voltage so that the capacitor charges fast to the negative peak value of the supply . When the SCR anode voltage becomes positive, the capacitor starts charging through the pot so as to make the top plate positive with respect to the bottom plate. When the positive capacitor voltage becomes equal to Vgt the SCR turns ON. The time taken for the capacitor to charge up to Vgt can be increased by increasing the pot resistance. Then, the firing angle increases. If the pot resistance is decreased, then, the capacitor voltage reaches the value Vgt earlier during the half cycle, and the firing angle will be lower. Diode D2 is provided to prevent breakdown of cathode gate junction during negative half cycles.

When the SCR turns ON, the voltage drop across C during conduction of SCR keeps it almost discharged till the beginning of the next half cycle of the supply voltage.



CIRCUIT DIAGRAM


R1

470K

C10.1µf

LAMP

R2

1kS

CR

TY

N6

16

1N5408

1N5408

A

K

G

I/P 230V AC

(50Hz)

HALF WAVERECTIFIER


PROCEDURE

1. Observe and study the module.

2. Connect the circuit as shown in the circuit diagram

3. Vary the pot meter R1 to get minimum alpha and note down the corresponding average value of the voltage and wave form across the load.

4. Repeat the steps 1 to 3 for different values of alpha by varying the pot meter Resistance.

5. Switch off the power.

TABULAR COLUMN

EXPECTED GRAPH

RESULT:


Time (ms)Delay angle

α

Conduction angle

β

voltage

(v)


Ex.No:04 SCR TURN OFF USING LC CIRCUIT AND AUXILIARY COMMUTATION

AIM:

To study the operation of LC SCR turn off circuit and auxiliary SCR turn off Circuit by observing waveforms of voltages and current at different points in the Circuit

APPARATUS:

1. MODULE ECO4B

2. MODULE ECO4A

3. RPS 0 TO 30V, 2A

4. RPS + - 15V, 500Ma

5. CRO WITH PORBE

THEORY

A Thyristor is normally switched on by applying a pulse of gate signal, when a thyristor is in conduction mode, its voltage drop is small (0.25 to 2v), Once the thyristor is turned on and the output requirements are satisfied it is usually necessary to turn off. The turning off means that the forward conduction of the thyristor has ceased and reapplication of positive voltage to the anode will not cause current flow without applying gate signal. “Commutation “ is the process of turning off a thyristor and it normally causes transfer of current flow to other parts of the circuit. A commutation circuit normally has additional components to accomplish the “TURN OFF”. There are, in general two modes of commutation: (i) Natural commutation, (ii) Forced commutation.

The commutation techniques use LC resonance or and under damped RLC circuit to force the current or voltage of a thyristor zero there by turning off the power device. Power electronics uses semiconductor devices as switches for turning on and off to the load. The study of commutation techniques reveals the transient voltage and current waveforms of LC circuits under various conditions. It helps in understanding dc transient resonance or and under damped RLC circuit to force the current or voltage of a thyristor zero there by turning off the power device. Power electronics uses semiconductor devices as switches for turning on and off to the load. The study of commutation techniques reveals the transient voltage and current waveforms of LC circuits under various conditions. It helps in understanding dc transient under switching conditions.

L-C Commutation Circuit

This circuit is also called Class B commutation circuit. The commutating components L and C1 are connected across the SCR. Thus, they need not form an oscillatory circuit with the load. Besides, the commutating components do not carry the load current. Initially, the capacitor C1 remains charged to the supply voltage V with the polarity shown in the circuit diagram.

When the SCR T1 is triggered, load current i V/RLoad flows. Since SCR is on, the stored energy in the capacitor drives an oscillatory current through the loop formed by C1, L and Ti. The SCR current iT=io +ic. When ic -V/RLoad, the net current through SCR becomes zero, causing it to turn off . Figure 2 shows waveforms of voltages and currents in the circuit of Figure 1. It is clear from the figure that, communication is possible only if the peak value of the capacitor current I peak is greater than V/ RLoad. The waveform of load voltage is similar to that offload current since the load is resistive. When load is inductive, a freewheeling diode is connected across it. Drawing Waveform of SCR voltage has been left as an exercise for the student.

LC COMMUTATION



Auxiliary Commutation Circuit

This is also called class D commutation circuit. In this, SCR T1 is called the main SCR, and SCR T2 is called auxiliary SCR. First T2 is turned on so that the capacitor charges to the supply voltage through the load with the polarity shown in figure. After this, T1 is triggered. Then, it carries the current iT = io - ic. Here, ic is the oscillatory discharging current of the capacitor which flows through the path T1-L-D1. Because of this current, the capacitor voltage becomes negative and remains with this polarity due to the diode D1. At any time when T1 has to be turned off, the auxiliary SCR T2 is triggered. Due to this, the capacitor voltage appears as reverse voltage across T1 causing it to turn off. Therefore, this method of turning off is also called voltage commutation.

AUXILIARY COMMUTATION


R1

1 E

C1Q2

L1INDUCTOR

R

12

C1 (1UF)

L1

10uH

I10-30v

1 2

1 2

RLoad (250 ohms)

IN5408

(TYN616)

AK1

(TYN616)AK2

AK2


RESULT



Ex.No:05 UJT FIRING CIRCUIT FOR HWR AND FWR CIRCUITS

AIM:

To study the Triggering of SCR using UJT relaxation oscillator circuit for HWR.

APPARATUS

1. UJT TRIGGERING MODULE

2. Multimeter -1NO.

3. LAMP 100W – 1NO.



THEORY:

UJT works very satisfactorily as relaxation oscillator output pulses of variable time periods is possible by suitably increasing the emitter to base-1 voltage from valley voltage level to peak voltage level, either by employing a constant current source or by charging a capacitor by a resistor. If constant current source is employed then the emitter voltage rises linearly as Vc = 1/c idt. If RC network is employed between b1 and emitter then Vc = v (1-etRC) though the capacitor voltage Vc is exponential, the time period is fixed by RC circuit. Hence simple RC network can be easily employed to obtain output pulses.

In phase controlled rectifier circuits trigger pulses to SCR must be synchronized or phase locked with a.c supply frequency. This helps constant D.C output voltage across the load for a specified triggering angle. On the other if synchronization or phase locking is not achieved then even for a minute variation in a.c supply frequency, triggering angle of SCR varies from instant to instant, resulting in variation in a.c output voltage across the load



CUIRCUIT DIAGRAM

UJT TRIGGERING CIRCUIT


D1

D4 D2

D5

DIODE ZENER 20V

R115K/1W

R2470K POT

R3680E

R3

33K/2W

Q1

UJT N (2N2646)

D4

DIODE

C10.022uF

D3

T1

TRANSFORMER CT

1 5

6

4 8

230V AC I/P

A

B

U V

W

X

B2

B1


POWER CIRCUIT (HWR)

TYN616

G

KA

LAMP

A

B

230 V AC I/P

POWER CIRCUIT (FWR)

D3

D2D4

D1

LAMP

SCR

230V AC I/P

A

B



PROCEDURE

Testing the Triggering circuit

1. Make the connections as in fig 1 and fig 2.

2. Do not connect the pulse transformer (PT) o/p to the gate of SCR now. Keep the

Potentiometer in the middle of the range

3. Switch on power to the triggering circuit and using the CRO verify the waveforms at

Points V, W & X

4. To observe waveform at point U, Use the potential divider n/w provided on the

Module, switch off the power supply

Testing the power circuit

1. Connect the pulse transformer o/p G1, K1 to G1, K1 of SCR

2. Connect the two channels of CRO through attenuator, switch on the power

3. Operate the potentiometer to get max. Firing angle (α), observe the waveforms of

SCR voltage and voltage across lamp. It is seen that when α is max. the brightness of

Lamp is minimum. Enter the values in the observation table. Now adjust the

Potentiometer to get different firing angles (α), observe the corresponding waveforms

And tabulate it.

TABULAR COLUMN


SL

NO

Time(t)

ms

Conduction

Angle (Degrees)

Firing angle

(Degrees)

DC load

Voltage (V)


Ex.No:06 GENERATION OF FIRING SIGNALS FOR THYRISTORS/ TRIALS USING DIGITAL CIRCUITS/MICROPROCESSOR

AIM:

To study the operation of digital SCR triggering circuit which can be used in a single-phase rectifier or AC voltage controller.

APPARATUS

1. STUDY MODULE - EC050 – 1

2. STEP DOWN TRANSFORMER - 0 TO 30V -1

3. LAMP

4. CRO WITH PROBE

THEORY

In this experiment we study the operation of a simple digital triggering circuit that can be used for triggering SCRs in single –phase controlled rectifiers and AC voltage controllers. The panel diagram shows various blocks in the digital triggering scheme.

The circuit has a 4-bit MOD- 16 pre-settable count down counter. The counter can be preset to any one of sixteen states using four switches marked as D3, D2, D1, and D0. The clock signal required for the counter is generated using an oscillator. The frequency of this oscillator is adjusted such that there are 16 pulses of clock in half-cycle of A.C. supply voltage at 50 HZ. In other words, the oscillator outputs sixteen pulses in 10ms. The circuit has a zero –crossing detector (ZCD) that loads a starting count into the counter and also starts the counter to count down at every zero crossing of the supply voltage. When the counter counts down to the state 0000, then, BORROW signal is output by the counter. This BORROW signal, along with signals coming from sine-to-square wave converter and high-frequency carrier signal generator are processed by using flip-flop and a control logic circuit to generate the triggering pulses at the secondaries of pulse transformers. The firing angle of the SCR is given by

= N/24 180o Where N = decimal equivalent of the starting count of the counter.

BLOCK DIAGRAM DIGITAL TRIGGERING CIRCUIT

POWER CIRCUIT



TYN616

G

KA

LAMP

A

B

230 V AC I/P

PROCEDURE:

1. Carefully observe the study module

2. Connect the 30v AC supply and DC dual power supply to the triggering circuit as shown (DO NOT CONNECT THE TRIGGERING PULSES TO THE POWER CIRCUIT NOW. THIS SHOULD BE DONE AFTER CHEKING THE TRIGGERING CIRCUIT)

3. Switch on both AC and DC power supply to the module. Preset the starting count to 1000.This is indicated by the LEDS PROVIDED on the panel. This is equivalent to decimal 8.

4. Now check the waveforms at different stages of the triggering circuit using CRO and compare them with the waveforms given in the figure 2, 3, 4.

5. Change the starting count to some other value and observe how the firing angle varies.

6. Switch off the power supply.

7. Connect the power circuit shown in figure 5

8. Switch on power supply and vary the firing angle by changing the starting count value as per the table 1. At each firing angle, measure the average load voltage by operating the AC/ DC coupling switch in the CRO.Record the readings. Observe and record the waveforms of SCR voltage and load voltage for any one firing angle.

TABULAR COLUMN

Starting count in binary Firing angle Average load voltage.

0010

0100

0110

1000

1100

EXPECTED WAVEFORMS:



WAVEFORMS ACROSS LAMP LOAD

RESULT:



Ex.No:07 AC VOLTAGE CONTROLLER USING TRIAC / DIAC COMBINATION

AIM: –

To vary the RMS voltage across an incandescent lamp using TRIAC and DIAC combination

APPARATUS

1. Single phase AC Voltage Controller MODULE

2. Multimeter 1NO.

3. LAMP 100W – 1NO.



THEORY

The versatility of the Triac and the simplicity of its use make it ideal for a wide variety of applications involving a.c. phase control .A phase controlled circuit using Triac is shown above where use is made of a Diac. The Diac ensures that the Triac receives a clean, fast trigger pulse. During the positive half cycle (when p is positive), the Triac requires a positive gate signal for turning it on. This is provided by the capacitor C when its voltage is above the breakdown voltage of the Diac. The capacitor discharges through the Triac gate. When the Triac triggers, the voltage drop across PQ will be zero and the capacitor voltage will be reset to zero. A similar operation takes place in the negative half cycle and a negative gate pulse will be applied when the Diac breaks down in the reverse direction. The charging rate of capacitor C can be changed by varying the resistance R and hence the firing angle can be controlled.

CIRCUIT DIAGRAM


DIAC DB3

MT1

MT2MT2G

TRIA

C B

TA12

C20.1uF

R1470K

R26.8K

LAMP

AC I/P (50 Hz)

CRO


PROCEDURE

1. Carefully observe the module and familiarize with terminals given on the panel

2. Make connections as shown in the circuit diagram

3. Set the pot (470k) for maximum resistance. Switch on power. Observe the waveform Of load voltage and TRIAC voltage on CRO. Now the conduction angle is Closer to 180 degrees. The voltage across the lamp load is maximum. Measure the Firing angle in degrees from the waveforms and voltage across the lamp load using DMM and enter the values in table.

4. Vary the firing angle by operating the pot (470k) goes to the lowest possible value .At each value of firing angle, repeat the measurements as done in the previous step and Record the values in table 1.

5. Record the waveforms of load voltage and TRIAC voltage for some intermediate Value of firing angle and for the lowest value of firing angle.

TABULAR COLUMN

R = Resistance of the lamp load.

Delay angle ()

Vrms (V) Irms (A) Conduction angle ()

Time (t) ms

EXPECTED WAVEFORMS

RESULT:



Ex.No:08 SINGLE PHASE FULLY CONTROLLED BRIDGE CONVERTER WITH R AND R-L LOADS

AIM:

To study the operation of single – phase fully controlled bridge rectifier connected to R and RL Load.

APPARATUS:

1. MODULE ECL1-9/10/12, 12a2. RPS + - 15V3. STEP DOWN TRANSFORMER 0 TO 15V4. CRO & PROBE

THEORY

The circuit arrangement of single – phase fully controlled bridge is shown in fig1. The bridge is said to be fully controlled since all four devices in the bridge are controlled. If two of these devices are replaced by diodes, then the bridge is said to be semi-controlled or half controlled. During positive half cycles of the input voltage, thyristor T1 and T3 are forward biased; and when these two thyristor are fired simultaneously at wt = , then, the load is connected to the input supply through T1 and T3. If the load is inductive, then, due to the energy trapped in this, thyristor T1 and T3 will continue to Conduct beyond wt = p, even though the input voltage has become negative. During the negative half cycles of the input voltage, thyristor T2 and T4 are forward biased; and firing of thyristor T2 and T4 will apply the supply voltage across thyristor T1 and T3 as reverse blocking voltage, causing them to turn off. This is called line commutation or natural commutation. When T1 and T3 turn off, the load current is transferred from T1 and T3 to T2 and T4.

During the period from to p, the input voltage vs and input current is are positive; and the power flows from the supply to the load. The circuit is said to be operated in the rectification mode. During the period p to p+, the input voltage vs is negative and input current is positive; and there will be reverse power flow from the load to the supply. The circuit is said to be operated in the inversion mode.

CONTROL CIRCUIT



POWER CIRCUIT

PROCEDURE

TESTING THE FIRING CIRCUIT

1. Connect dual power supply to study the module. Connect an AC voltage in the range 15 v to 30 v.

2. Keep the pot meter provided in the module in the middle position and check the waveforms at different test points marked in the panel diagram. DO NOT CONNECT YET THE PULSE TRANSFORMER OUTPUT TO THE POWER CIRCUIT NOW.

3. Turn the pot meter over its range and observe how the firing angle varies.

4. Turn the pot to zero and switch off AC and DC power to the firing circuit.

TESTING THE POWER CIRCUIT

1. Using the patch cords make the connections as shown in the circuit diagram.

2. Switch ON power to both modules.

3. Operate the pot meter over its range to vary the voltage applied to the lamp. Record the DC values of load voltage and current for different firing angles in the tabular column given.

TABULAR COLUMN

SL NO FIRING ANGLE LAMP VOLTAGE(V) LAMP CURRENT(A)

EXPECTED WAVEFORMS



RESULT:



Ex.No:09 VOLTAGE (IMPULSE) COMMUTATED CHOPPER BOTH CONSTANT FREQUENCY AND VARIABLE FREQUENCY OPERATIONS

AIM: To study the operation of Chopper circuit.

APPARATUS:

1. MODULE ECO4B

2. MODULE ECO4A

3. RPS 0 TO 30V, 2A

4. RPS + - 15V, 500mA

5. CRO WITH PORBE

CIRCUIT DIAGRAM

VOLTAGE COMMUTATED

CHOPPER

THEORY:


12

C1 (1UF)

L1

10uH

0A

1 2

1 2

RLoad (250 ohms)

IN5408

( TYN616)AK1

(TYN616)AK2

AK2


Chopper is DC-DC converter. It is also called DC transformer. If the average output voltage of chopper is less than its input DC voltage, then it is said to be step-down chopper. If the output voltage is higher than the input voltage then, it is said to be step-up chopper. In this experiment , study the working of a step-down chopper.

To control the operation of high frequency switch we use industry standard SG3524 Regulating PWM IC to control the SCR switch.

A DC chopper is a static device (switch) used to obtain variable DC from a source of constant DC voltage. Therefore, chopper may be thought of as a dc equivalent of an ac transformer since they behave in an identical manner. Besides the saving in power, the d.c chopper offers greater efficiency, faster response, lower maintenance, small size, smooth control, and for many applications, lower cost than motor generator sets or gas tubes approaches.

Applications: solid state choppers are widely used in trolley cars, battery – operated vehicles, traction-motor control, control of a large number of dc motors from a common d.c bus with a considerable improvement of power factor, control of induction motors, marine hoists, forklift trucks and mine haulers.

Principle of chopper operation

A chopper is a thyristor on/off switch that connects load to and disconnects it from the supply and produces a chopped load voltage from a constant input supply voltage. Fig 1 illustrates the principle of a chopper. The chopper is represented by an SCR inside a dotted square. It is triggered periodically and is kept conducting for a period Ton, and is blocked for a period Toff. The chopped load voltage waveform is shown Fig 2

During the period Ton, when the chopper is on, the supply terminals are connected to the load, terminals. During the interval Toff, when the chopper is off, load current flows through the freewheeling diode DF. As a result, load terminals are short circuited by Df and load voltage is therefore, zero during Toff. In this manner, a chopped D.C. voltage is produced at the load terminals.

Equations:

The average load –voltage Eo is given by

E0 = Edc {Ton/(Ton + Toff)}

Where

Ton = on –time of the chopper

Toff = off-time the chopper

T = Ton + Toff = chopping period

If = Ton/t be the duty cycle, then above equation becomes,

E0 = Edc Ton/T

E0 = Edc.



Control strategies:

The equation E0 = Edc. , Shows that average value of output voltage, E0 can be controlled by periodic opening and closing of the switches. The two types of control strategies for operating the switches are employed in D.C choppers. They are:

1) Time – ratio control (TRC)

2) Current limit control

In time ratio control, the value of Ton/T is varied. This is effected in two ways.

They are variable frequency operation and constant frequency operation.

1. Constant frequency system:

In this type of control strategy, the on time Ton is varied but the chopping frequency f (f = 1/T, and hence the chopping period T) is kept constant. This control strategy is also called as the pulse-width modulation control.

Fig 3 illustrates the principle of pulse width modulation .The chopping period T is kept constant. In Fig4a, Ton = 1/4T, so that duty cycle =25%. In Fig4b, Ton = 3/4T, so that duty cycle = 75%. Hence, the output voltage Eo can be varied by varying the on time Ton.

3) Variable frequency system

In this type of control strategy, the chopping frequency f is varied and either- a) on time, Ton is kept constant or b) off time, Toff is kept constant. This type of control strategy is also called as frequency modulation control.

Fig5 illustrates the principle of frequency modulation. As shown in Fig5a, chopping period T is varied but on-time Ton is kept constant. The output voltage waveforms are shown for two different duty cycles. In Fig5b chopping period T is varied but Toff is kept constant.

Frequency modulation – control strategy has the following major disadvantages compared to pulse width modulation control.

1) The chopping frequency has to be varied over a wide range for the control of output voltage in frequency modulation. Filter design for such wide frequency variation is therefore quite difficult.

2) For the control of duty cycle, frequency variation would be wide. As such, there is a possibility of interference with signaling and telephone lines in frequency modulation technique.

3) The large off time in frequency modulation technique may make the load current discontinuous, which is undesirable.

Thus, the constant frequency system (PWM) is the preferred scheme for chopper drives.



TABULAR COLUMN

SLNO

T OFF

T ON DUTY CYCLE =

TON/(TON+TOFF)

Vth = V * DUTY CYCLE

AVERAGE LOAD VOLTAGE

EXPECTED WAVEFORMS



Ex.No:10 SPEED CONTROL OF A SEPERATELY EXITED DC MOTOR

AIM:

o control the speed of a separately excited DC Motor by varying the armature voltage using a controlled rectifier.

APPARATUS:

1. E5L1 – 72. EE5L1- 7a 3. RPS 0 to 30V4. CRO and PROBE

THEORY

In this experiment we study the operation of simple circuits, which can be used for open loop speed control of a separately exited DC motor.

The power circuit has a half controlled bridge rectifier to supply variable DC voltage to the motor armature, a diode bridge to supply fixed DC voltage to the field coil of the motor, a relay for the field failure protection and a free wheeling diode. A current transformer is provided to sense the field current. In case of field failure, the relay opens the armature circuit preventing damage of semiconductor devices that will be caused by heavy current flow.

The firing circuit produces triggering pulses required to turn on SCRs connected in the power circuit. Operating the potentiometer P can vary the angle. When P is turned to maximum, then, the firing angle will be minimum and vice versa. When power supply is switched on, the soft start capacitor, which is of high value, charges slowly. Due to this, slowly changes from a value closer to 180o to a lower value determined by the position of wiper of the potentiometer. This causes soft start of the motor. When power is switched off, the soft start capacitor discharges through the impedance of the supply via the diode.

CIRCUIT DIAGRAM

CONTROL CIRCUIT



POWER CIRCUIT

PROCEDURE

Make the connections of firing circuit and power circuit as shown in the figure


1. Connect dual power supply and 15v AC. DO NOT CONNECT FIRING CIRCUIT TO POWER CIRCUIT.

2. Keep the pot meter in middle position and check waveforms at A, B, C, D, E, F & G.

3. Change the pot meter position and observe the change in firing angle.

4. Turn off DC & AC supply.




1. Connect firing circuit to power circuit.

2. Switch ON power to both. Now the field ON indicator should glow.

3. Vary the pot and see the variation in speed. Keep the pot in middle of its range and record the waveform of armature voltage.

4. By varying P record armature voltage and speed for different values of firing angles.

5. Switch off power.

TABULAR COLUMN

SLN FIRING ANGLE () ARMATURE VOL(V) SPEED (RPM)

EXPECTED WAVEFORMS



Ex.No:11 SPEED CONTROL OF UNIVERSAL MOTOR

AIM: Speed control of a Universal Motor and induction Motor

APPARATUS:

1. EC08A-12. EC08B –1 (With 15V Step down transformer)3. RPS 0 to 30V-1, 0 to - +15v 4. CRO and PROBE-15. DMM-1

THEORY

A universal motor is basically an electric motor capable of working on either dc or single-phase ac at approximately the same speed and output. The stator and rotor windings of the motor are connected in series through the rotor commutator. Therefore the universal motor is also known as an ac series motor or an ac commutator motor. Speed of this motor can be varied by varying the applied voltage by using either a controlled rectifier or an ac voltage controller. Universal motors are generally designed to operate at high speeds in the range 10000 to 20000 r.p.m. The universal motors normally work on AC supply but their performance is much better on DC supply. The inherent disadvantage of universal motors is that normally they operate at high speed and at low speed universal motors are very bulky.

Single –phase induction motor speed can be controlled over a limited range by varying the applied voltage using an ac voltage controller. The ac motors have a number of advantages, they are almost 20 to 40% lighter than equivalent dc motors, they are inexpensive and have low maintenance compared to dc motors but they require complex control circuits.

CONTROL CIRCUIT



POWER CIRCUIT

PROCEDURE

Make the connections of firing circuit and power circuit as shown in the figure


1. Connect dual power supply and 15v AC. DO NOT CONNECT FIRING CIRCUIT TO POWER CIRCUIT.

2. Keep the pot meter in middle position and check waveforms at A, B, C, D, E, F & G.

3. Change the pot meter position and observe the change in firing angle.

4. Turn off DC & AC supply.


1. Connect firing circuit to power circuit.

2. Switch ON power to both. Now the field ON indicator should glow.

3. Vary the pot and see the variation in speed. Keep the pot in middle of its range and record the waveform of armature voltage.

4. By varying P record armature voltage and speed for different values of firing angles.

5. Switch off power.



POWER CIRCUIT WAVE FORMS



Ex.No:12 SPEED CONTROL OF STEPPER MOTOR

AIM: Speed control of stepper motor.

APPARATUS

1. STEPPER MOTOR –12. MODULE EE5L1-9 –13. RPS – 0 TO +12V,1A –14. CRO With probe –1

THEORY

Stepping motor or stepper motor is a brush less electromagnetic device, which converts digital pulses into discrete mechanical rotational movements. The output shaft of the motor rotates through a specific angle (called a step) for each electrical pulse received from its control unit. Stepper motors are used in digitally controlled position controlled systems in open-loop mode. The input command is in the form of train of pulses to turn the motor shaft through a specific angle. Stepper motors have a wide range of applications like printers, floppy drives, X – Y plotters, in positioning of work table and tools in numerically controlled machining equipment, in robots etc.

There are basically three types of stepper motors: (a) Variable Reluctance (VR) type; (b) Permanent Magnet type; (c) Hybrid type. The motor used in the experiment has the following specifications.

Two phases, bifilar wound, with eight salient poles, toothed iron rotor (hybrid type).

No. Of leads: 6

Step angle: 1.8 + 0.1

Step per revolution: 200.

Supply voltage: 12 V DC.

Winding resistance: 75

Torque: 1 Kg-cm or 0.1 N-m or 13.9 Oz-in.

Stepper motors differ from conventional D.C. servomotors in the following respect.

There is no control winding in stepper motors.

The stepping rate (speed of rotation) is governed by frequency of switching and not by supply voltage.

A single pulse input will move the shaft of motor by one step.

When there is no pulse input, the rotor will remain locked in the position corresponding to the last pulse input, since, at any time, two windings of the motor are always energized which lock the rotor electro-magnetically.

Stepper motors are brush less, so there is no wear and tear.

Load and no load conditions make no difference in current drawn by the motor.



The switching logic sequence to operate the motor is given in the table. Here, Red, Orange, Green, Blue are the leads of the motor windings. A, B, C, D are the outputs of the controller. The sequence given in table should be repeated continuously to rotate the shaft continuously. To change the direction of rotation, the logic sequence should be applied from bottom to top. The study module has digital sequence generator built using the IC CD4027 (Dual J-K Master/ Slave Flip Flop) that outputs four digital signals namely A,B,C,D according to table. The state of these outputs can be observed by the respective LEDs. When a digital output is 1, the corresponding LED glows and the respective MOSET driven by it turns on. This causes current to flow through the respective winding of the motor. The sequence generator is driven by a clock signal generated by the oscillator built using the timer IC 555.

A (RED) B (ORANGE) C (BLUE) D (GREEN)

0 1 0 1

0 1 1 0

1 0 1 0

1 0 0 1

Switching Logic sequence.

CIRCUIT DIAGRAM:

PROCEDURE



STEP CONTROL

1. Connect the circuit as given in the circuit diagram

2. Connect the 12v dc supply.

3. Select the STEP mode and FORWARD direction.

4. Switch on power. Press STEP control 25 times and record the approximate value of angle of rotation of the shaft. At every press of STEP Control button observe the states of the digital signals A, B, C, D.

5. Now select the reverse direction. Repeat the previous step.

CONTINUOUS CONTROL

1. Select CONTINUOUS and FORWARD. Operate the SPEED pot over its range and observe how the speed of motor changes.

2. Select CONTINUOUS and REVERSE. Operate the SPEED pot over its range and observe how the speed of motor changes in opposite direction.

3. Now connect one of the channels of CRO to the clock output of the controller. For different positions of Speed pot, record the clock frequency and measure the RPM of motor.

4. Enter the readings in the tabular column. Calculate the RPM using the formula

Speed in RPM = (f x 60) /200)

Where 200 is the number of steps per revolution of motor specified by the manufacturer.

TABULAR COLUMN

Step control of stepper motor

FORWARD REVERSE

Approx. angle of rotation after 25 pulses Approx. angle of rotation after 25 pulses

Continuous running of stepper motor.

Slno Speed pot position Clock Frequency, f Calculated RPM Measured RPM

1 0

2 2

3 4

4 6

5 8

6 10

RESULT:

Ex.No:13 PARALLEL / SEREIS INVERTER

AIM:



To study the operation of series and parallel inverters

APPARATUS:

1. MODULE2. LAMP3. RPS – 0 – 30V4. STEP UP TRANSFORMER 0- 230V5. CRO & PROBE.

THEORY

DC to AC converters are known as inverters. 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 of variable at a fixed or variable frequency. The inverter gain may be defined as the ratio of the ac output voltage to the dc input voltage.

The output voltage waveforms of ideal inverters should be sinusoidal. However, the waveforms of practical inverters are nonsinusoidal and contain certain harmonics. For low and medium power applications, square wave or quasi-square wave voltages may be acceptable and for high power applications low distorted sinusoidal waveforms are required. Inverters are widely used in industrial applications for e.g., variable speed ac motor drives, induction heating, standby power supplies and uninterruptible power supplies. The input may be a battery, fuel cell, solar cell or other dc source.

Parallel Inverter

Referring to the circuit diagram, when T1 is turned ON, the left half side of the primary winding of transformer is connected to supply and at the same time the capacitor C is charged via the right half side of the winding to 2V. When T2 is triggered, the charged capacitor is connected across T1, turning it OFF. Now, the right half of the primary winding is connected to the supply through T2, and the capacitor charges from 2V to -2V via the left half of the winding. When T1 is triggered again, T2 tur Ns OFF because of reverse voltage applied by the charged capacitor. This cycle continues. We get more or less rectangular ac voltage waveform across the load.

Series Inverter

Referring to the series inverter circuit, the load is resistive. L and C are chosen such that the resulting R-L-C circuit is under-damped. For a given R, and L, the condition for the circuit to be under-damped i = R/2L < wo = 1/LC

The value of external capacitance is chosen to satisfy the above-mentioned condition. With C = 1F,

The circuit is under damped. When T1 is turned ON, an exponentially decaying sinusoidal pulse current flows through the load charging the capacitor with the polarity . When this current falls to zero, T1 turns OFF on its own. Now, T2 can be turned ON . Then, the capacitor discharges through the load causing an exponentially

Decaying negative half-cycle of sinusoidal current through the load. T2 turns OFF on its own when this current falls to zero.

CIRCUIT DIAGRAM


TY

N61

6

1

23

C11 µf

R1100 Ohm

L1

10mH

VDC

TYN616

1

23

0 - 30V

T1

T2

SERIES INVERTER


PROCEDURE

1. Set up the circuit as shown in the circuit diagram

2. Switch on the DC supply.

3. Switch ON the firing pulses.

4. Observe the firing pulse on the CRO


TY

N61

6

1

23

INVERTER TRANSFORMER

L = 10mH

VDC

TY

N61

6

1

23

C = 3.3uF

PARALLELINVERTER

0 - 30V

0V30V 30V

LAMP

0V 240V

2AT1 T2


5. Observe the output voltage across the load resistor R.

6. Vary the frequency by using the pot meter in the inverter control circuit and note down the output waveform and the voltage.

7. Tabulate the readings in a tabular column

8. Repeat the above steps for both parallel and series inverter.

TABULAR COLUMN

SERIES INVERTER & PARALLEL INVERTER

V in = ………volts

SLNO

FREQUENCY (HZ)

OUTPUT

VOLTAGE ACROSS LOAD R(V)

F in = ………Hz

SLNO

INPUT VOLTAGE

OUTPUT

VOLTAGE ACROSS LOAD R(V)



WAVEFORMS FOR SERIES INVERTER

RESULT


06ecl78 Pe Lab

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Transcript of 06ecl78 Pe Lab