Government Polytechnic Muzaffarpur Name of the Lab: Power Electronics & Drives Lab

Subject Code: 1620607

Experiment-1

Aim: To obtain V-I characteristics of thyristor.

Apparatus required:

i. Trainer kit ii. Patch chord

iii. Millimetre

Circuit diagram:

VBO= forward break over voltage

VBR= reverse break over voltage

Ig= gate current

Characteristic curve:

Observation table:

Gate current Ig= Ig1= ….mA

VAK(V) IA(mA)

Procedure:

i. Connections are made as shown in the circuit diagram. Set R1 and R2 to mid position

and V1 and V2 to minimum.

ii. Set the gate current Ig = Ig1 (such that forward break over voltage is between 15V to

20V), by varying R2 and V2.

iii. Slowly vary V1 in steps of 2V and note down VAK and IAK at each step till SCR

conducts. (Note down maximum VAK, which is forward break over voltage just before

SCR conducts).

Finding latching current

i. Ensure that the SCR is in the state of conduction.

ii. Start reducing anode voltage (VAK) in steps of 2V; simultaneously check the state of

SCR by switching off gate supply V2. If SCR switches off just by removing gate

terminal, and switches on by connecting gate supply, then the corresponding anode

current IA is the latching current (IL) for the SCR.

Finding holding current:

i. Ensure that the SCR is in the state of conduction.

ii. Switch off the gate supply permanently.

iii. Start reducing anode voltage (VAK) in steps of 2V; simultaneously check the state of

SCR. If SCR switches off. Note down the anode current (IA) just before it drops to

zero, which will be IH.

iv. Reverse the anode voltage polarity.

v. Vary VAK in steps of 5V till 25V and note down VAK and IA values at each step.

vi. Plot forward and reverse characteristics using the above-tabulated values. Find the

SCR forward resistance using the graph.

vii. Repeat the above procedure for the forward and reverse characteristics of SCR for a

gate current Ig = Ig2.

Result:

The values of VAK and IAK are noted down, plotted and SCR forward resistance is found. The

values obtained are verified.

Government Polytechnic Muzaffarpur Name of the Lab: Power Electronics & Drives Lab

Subject Code: 1620607

Experiment-2

Aim: To study the operation of single phase fully controlled converter using R and RL load

and to observe the output waveforms.

Apparatus required:

i. Power Thyristors

ii. Rheostat

iii. CRO

iv. Transformer

v. Connecting wires

Circuit diagram:

Model graph:

Observation table:

S

No.

Triggering

angle α degree

Output voltage Time period

1

2

3

Procedure:

i. Single Phase Fully Controlled Bridge Rectifier.

ii. Make the connections as per the circuit diagram.

iii. Connect CRO and millimetre (in dc) across the load.

iv. Keep the potentiometer (Ramp control) at the minimum position (maximum

resistance).

v. Switch on the step down ac source.

vi. Check the gate pulses at G1-K1, G2-K2, G3-K3, & G4-K4 respectively.

vii. Observe the waveform on CRO and note the triggering angle ‘α’ and note the

corresponding reading of the millimetre. Also note the value of maximum amplitude

Vm from the waveform.

viii. Set the potentiometer at different positions and follow the step given in (6) for every

position.

ix. Tabulate the readings in observation column.

x. Draw the waveforms observed on CRO.

Theory:

A fully controlled converter or full converter uses thyristors only and there is a wider control

over the level of dc output voltage. With pure resistive load, it is single quadrant converter.

Here, both the output voltage and output current are positive. With RL- load it becomes a

two-quadrant converter. Here, output voltage is either positive or negative but output current

is always positive. Figure shows the quadrant operation of fully controlled bridge rectifier

with R-load. Fig shows single phase fully controlled rectifier with resistive load. This type of

full wave rectifier circuit consists of four SCRs. During the positive half cycle, SCRs T1 and

T2 are forward biased. At ωt = α, SCRs T1 and T3 are triggered, then the current flows

through the L – T1- R load – T3 – N. At ωt = π, supply voltage falls to zero and the current

also goes to zero. Hence SCRs T1 and T3 turned off. During negative half cycle (π to 2π).

SCRs T3 and T4 forward biased. At ωt = π + α, SCRs T2 and T4 are triggered, then current

flows through the path N – T2 – R load- T4 – L. At ωt = 2π, supply voltage and current goes

to zero, SCRs T2 and T4 are turned off. The Fig-3, shows the current and voltage waveforms

for this circuit. For large power dc loads, 3-phase ac to dc converters are commonly used.

The various types of three-phase phase-controlled converters are 3 phase half-wave

converter, 3-phase semi converter, 3-phase full controlled and 3-phase dual converter. Three-

phase half-wave converter is rarely used in industry because it introduces dc component in

the supply current. Semi converters and full converters are quite common in industrial

applications. A dual is used only when reversible dc drives with power ratings of several MW

are required. The advantages of three phase converters over single-phase converters are as

under: In 3-phaseconverters, the ripple frequency of the converter output voltage is higher

than in single-phase converter. Consequently, the filtering requirements for smoothing out the

load current are less. The load current is mostly continuous in 3-phase converters. The load

performance, when 3- phase converters are used, is therefore superior as compared to when

single-phase converters are used.

( )( )

Result:

Thus the operation of single phase fully controlled converter using R and RL load has been

studied and the output waveforms has been observed.

Government Polytechnic Muzaffarpur Name of the Lab: Power Electronics & Drives Lab

Subject Code: 1620607

Experiment-3

Aim: To study the operation of parallel inverter.

Apparatus required:

i. Parallel inverter kit

ii. Inductor transformer

iii. CRO

This module consists of two units

i. Firing circuit

ii. Power circuit

Circuit diagram:

Model graph:

Observation table:

S

No.

Frequency Voltage amplitude(V) Time period(ms)

1 Minimum

2 maximum

Procedure:

i. Switch on the firing circuit. Observe the trigger outputs TP and TN by varying

frequency potentiometer and by operating ON/OFF switch.

ii. Then connect input DC supply to the power circuit. Connect trigger outputs to Gate

and Cathode of SCR TP & TN.

iii. Apply trigger pulses to SCR

iv. Observe voltage waveforms across load. Output voltage is square wave only.

v. Vary the load, vary the frequency and observe waveforms.

Theory:

The circuit is a typical class C Parallel inverter. Assume TN to be ON and TP to 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 rises approximately to 2V dc and reverses

bias 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 +2V dc volts ready to commutate TP, When TN is again turned on and the cycle

repeats. Freewheeling 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 tapping of the transformer at 25V tapping.

Firing circuit:

This unit generates two pairs of pulse transformer isolated trigger pulses to trigger two SCR’s

connected in centre tapped transformer type parallel inverter. Frequency of the inverter can

be varied from 75Hz to 200 Hz approximately.

Power circuit:

This unit consists of two SCR’s, two freewheeling diodes, commutation inductor,

commutation capacitor and a centre tapped transformer to be inter connected to make parallel

inverter. All the points are brought out to the front panel. A switch and fuse is provided for

input DC supply. All the devices are mounted on proper heat sink. Each device is protected

by snubber circuit.

Result:

Thus the operation of a parallel inverter is studied and the output waveforms are measured

and drawn.

Government Polytechnic Muzaffarpur Name of the Lab: Power Electronics & Drives Lab

Subject Code: 1620607

Experiment-5

Aim: To study the operation of series inverter.

Apparatus required:

i. Series inverter module

ii. Loading rheostat-50Ω

iii. CRO

iv. Connecting wires

This unit consists of power circuit and firing circuit sufficient to build and study the modified

series inverter.

Circuit diagram:

Model graph:

Observation table:

S No. Amplitude(Volts) TON(ms) TOFF(ms)

1

2

Procedure:

i. To begin with switch on the power supply to the firing circuit check that Trigger

pulses by varying the frequency.

ii. Connections are made as shown in the circuit diagram. Now connect trigger outputs

from the firing circuits to gate and cathode of SCRs T1 & T2.

iii. Connect DC input from a 30V/2A regulated power supply and switch on the input DC

supply.

iv. Now apply trigger pulses to SCRs and observe voltage waveform across the load.

v. Measure VRMS & frequency of output voltage waveform.

Firing Circuit:

This part generates two pairs of pulse transformer isolated trigger two SCR’s connected as

series inverter. ON/OFF switch is provided for the trigger pulses which can be used to switch

ON the inverter. Frequency of the inverter can be varied from 100 Hz to 1 KHz

approximately.

Power Circuit:

This part consists of two SCRs & two diodes. A centre tapped inductor with tapings and 4

capacitors. Input supply terminals with ON/OFF switch and a fuse is provided. All the

devices in this unit mounted on a proper heat sink, snubber circuit for dv/dt protection and a

fuse in series with each device for short circuit protection. All the points are brought out to

front panel for inter connections. They have to be interconnected as shown in the circuit

diagram. Fly wheeling diodes can be connected across SCR’s and its effect can be observed.

Theory:

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

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

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

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

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

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

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

values of L, R and C in the circuit.

Result:

Thus the operation of a series inverter is studied.

Government Polytechnic Muzaffarpur Name of the Lab: Power Electronics & Drives Lab

Subject Code: 1620607

Experiment-6

Aim: To observe the operation of class D commutated technique.

Apparatus required:

i. Force commutation trainer kit

ii. Patch chord

iii. CRO

Circuit diagram:

Model graph:

Observation table:

S No. Duty cycle (%) Output voltage(V) Time period(ms)

1

2

3

Procedure:

i. Patch the voltage commutated chopper as per the circuit diagram.

ii. Connect the CRO probe across the commutated chopper.

iii. Give the input dc voltage (0-30)v, 2amps from the external power supply.

iv. Switch ON the trainer then switch ON the input dc supply circuit breaker.

v. After then switch ON the trigger OFF-ON position.

vi. From the capacitor output waveform we can measure the turn on time and turn off

time of main SCR as well as auxiliary SCR.

vii. Verify the unity and frequency of the triggering circuit using parts provided on the

triggering circuit.

viii. Also observe the voltage across main SCR and auxiliary SCR and load.

ix. Take the turn on and turn off time at main so auxiliary SCR from the capacitor

waveform at various values of unity cycle and frequency and tabulate them.

x. Also find out the peak value of current through the capacitor.

Theory:

Mode I:

Main SCR is triggered to make source current to flow in two path one is load current and

other path with triggering of SCR load get connected to supply and load voltage.

Mode II:

At a desired instant the auxiliary SCR is to be triggered for turning OFF the main SCR T1

with the switch ON, T2 reverse capacitance voltage appears across T1 which reverse biases it

and turn it OFF.

Mode III:

SCR T2 turn OFF since the capacitance is slightly changed after the freewheeling diode set

frequently forward biased.

Result:

Thus the operation of class D commutated technique has been obtained.

Government Polytechnic Muzaffarpur Name of the Lab: Power Electronics & Drives Lab

Subject Code: 1620607

Experiment-9

Aim: To study the speed control of a dc motor by varying armature applied voltage through

phase controlled converter.

Apparatus required:

i. DC motor control unit

ii. DC armature

iii. DC voltmeter

iv. CRO

v. DC motor

Circuit diagram:

DC Motor Speed Control Unit (Power Circuit) 230V/5A

This power circuit consists of two SCR’s and three diodes. These devices can use to build

single phase half wave converter, single phase full wave converter and single phase half

controlled bridge converter, and also single phase AC voltage controller power circuits. Each

device in the unit is mounted on an appropriate heat sink and is protected by snubber circuit.

Short circuit protection is achieved using glass fuses. A circuit breaker is provided in series

with the input supply for over load protection and to switch ON/OFF the supply to the power

circuit. The Gate and Cathode of each SCR’s brought out on the front panel for firing pulse

connection. A digital voltmeter and an ammeter is mounted on the front panel to measure the

armature voltage and current.

Procedure:

Switch ON the mains supply to the single phase converter firing circuit. Observe the test

points and trigger outputs. Verify the trigger outputs and their phase sequence. Vary the firing

angle potentiometer and observe the trigger outputs. The pulse train width will increase as we

decrease the firing angle from 1800 to 0

0. It is 0

0 to 180

0 and 50% at 90

0 soft start and stop

feature is provided for trigger outputs. When we press of ON/OFF switch the trigger outputs

will start at 1800 and slowly increased to the firing angle set by firing angle potentiometer.

The acceleration time is set in the factor (10 seconds). When we release the ON/OFF switch

the trigger outputs will slowly decreased to 1800 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. Make sure that all the trigger outputs are proper before connecting to the

power circuits. Make the connections in the power circuit as given in the circuit through

isolation transformer. Initially keep the input supply at low voltage say 30 volts. Connect the

trigger outputs from firing circuit to the corresponding SCR’s Gate and Cathode. Initially

connect a Rheostat of 50 Ohms / 5amps. Switch ON the trigger outputs observe the voltage

waveforms across load by varying the firing angle potentiometer. Compare with the expected

waveforms, if the unit is working properly switch OFF the trigger outputs and switch OFF the

MCB. Connect field terminals of DC motor to the field supply points in the power circuit.

The connect armature terminal of the DC motor through the rheostat and the rheostat and the

ammeter provided in the unit to the output of rectifier. Switch ON the field supply. Set the

field voltage to some value – 150Volts. This voltage can be measured using the voltmeter

provided in the rectifier. Set the input voltage to 100Volts. Initially keep the firing angle pot

at 1800. Initially keep the resistance at maximum position and cut off once the DC motor

starts. This is to limit the starting current. Switch on the MCB and trigger outputs. Vary the

firing angle potentiometer and note down the output voltage, output current and measure the

speed of the 52 DC motor for different values of firing angle. 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 up to 230 volts in steps and note

down the readings in the tabular column.

Armature control:

S No. Output voltage(V) Duty cycle (%) Frequency Speed(RPM) Current

1

2

3

Result:

Thus the speed control of DC motor is performed by varying armature voltage through phase

controlled converter.