basic electrical lab manual

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UNIVERSITY OF PETROLEUM AND

ENERGY STUDIES

BASIC ELECTRICAL LAB

MANUAL

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List of Experiments

1. To verify Thevenin’s theorem.

2. To Verify Super Position theorem.

3. To verify Maximum Power transfer theorem.

4. Study phenomenon of resonance in RLC series Circuit.

5. To study LCR parallel circuits in resonance condition.

6. Measurement of Efficiency of a single phase transformer

by load test.

7. To connect, start; reverse the direction of rotation and

measure speed of a 3-phase Induction motor.

8. To determine load characteristics of a shunt wound DC

Generator.

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Experiment-1

Object: To verify the Thevenin’s theorem.

Apparatus required: Voltmeter, d.c. supply, d.c. ammeter, rheostats, resistors,

keys and connecting wires etc.

Theory/Principal: Sometimes we want to determine the response (Current,

Voltage or power delivered) in a single load resistance in network by a simple

equivalent circuit. Determining the response in the load resistance then becomes

much easier. The use of this theorem is very helpful & time saving when we have

to find the response in any branch of a given network shown in Fig below for

different values of load resistance.

According to the Thevenin’s theorem “Any linear network containing energy

sources (generator) & resistances(impedances) can be replaced by an equivalent

circuit consisting of a voltage source VTH in series with RTH. The value of ‘VTH’ is the

open circuit voltage between the terminals of the network and RTH is the

impedance measured between the terminals with all energy sources treated as

open circuit. Current across the load resistance (RL) is given by the equation

IL=VTH/(RTH+RL)

Sl. No

Name of Apparatus

Type Range/Rating Quantity Make Remarks

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Procedure: Step1: Remove the load resistance or the branch from original given

circuit, where response or loop current have to be measured.

Step 2: Calculation of VTH or VOC : VOC = V.R2/(R1+R2) =VTH

Step 3: Calaculation of RTH: (1) Theoretically: By series-parallel or star-delta trans-

formation we can calculate the thevenin resistance.

Here RTH=R1IIR2+R3=(R1 R2/R1+R2)+R3

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(ii) Practically: (i) We can measure the Thevenin resistance by measuring short

circuit current ISC through AB terminal by connecting an ammeter. RTh=VTH/ISC

(ii) We can measure the Thevenin resistance by using digital multimeter.

Step 4: Now make the equivalent circuit and connect the supply = to VTH or VOC,

Thevenin resistance RTH & load resistance RL in series.

Step 5: Now measure/ Calculate the load current IL through load resistance.

IL= VTH/(RTH+RL)

Theoretically Practically Supply voltage

Thevenin voltage VTH or VOC

Short circuit current ISC Thevenin Resistance from RTH

Circuit Load Current IL

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8. Result:

1.The value of open circuit voltage (VOCVTH) is …… volts.

2.The value of Thevenin’s resistance is ……......ohms.

3. The value of current across load is ………….amps.

From the reading, it is found that measure value of current flowing through the

load IL are the same as determined by Thevenin’s theorem.

9. Precautions:

1. All connections should be tight.

2. All steps should be followed carefully.

3. Readings and calculation should be taken carefully.

4.Don’t touch the live terminals.

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Experiment-2

Objective: Find the current i1, i2 & i3 as shown in fig1 using Superposition

theorem.

Fig. 1

Accessories required:

1. AC power supply (12V,5V), (0-15V)voltmeter, (0-250mA)ammeter.

2. Single point patch chords for inter connections.

3. Digital multimeter

Theory.

Super position theorem states that in a network of linear resistances containing

more than one source of e.m.f the current which flows at any points is the sum of

all the currents which would flow at that point if each e.m.f source replaced for

the time being by resistances equal to their internal resistances.

Procedure:

1. Connect the circuit as shown in fig 2 through patch cords. Consider only

one voltage source at a time, first 12 volt.

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2. Switch ‘ON’ the instrument using ON/OFF switch provided on the front

panel.

3. Note down the respective currents i1, i2, & i3 by connecting ammeter of

250mA range in series of resistances R1, R2 & R3 in table 1.

Fig 2.

4. Similarly note down voltages V1, V2, &V3, one by one by Connecting

voltmeter of range 15V across the each resistance in table1

5. Now connect the circuit again by removing the B1 voltage source, through

patch cords. Consider only one voltage source at a time, second 5 Volt.

6. Repeat the steps 2 to 5

7. Connect the circuit as shown in Fig 1, to measure net algebraic sum of

current when both the voltage sources (12 volt and 5 Volt) connected

simultaneously.

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8. Again repeat the steps 2 to 5.

9. Compare the values of columns 4 &5, 8 &9 and 12 &13.

OBSERVATION TABLE:

Table 1

Measureand R1= R2= R3=

In fig2 In

fig3

Sum of

fig2

&fig3

In

fig1

In

fig2

In

fig3

Sum

of

fig2

&fig3

In

fig1

In

fig2

In

fig

3

Sum

of fig2

&fig3

In

fig1

VoltageVi(=1

2,3) volts

Current

I1(=1,2,3)mA

Sources and Error:

1. In AC power supplies there are always variations present at output due

to fluctuations in mains input. These fluctuations may disturb the output

result, so be careful when you are performing verification of super

position theorem in AC circuits.

2. Resistance tolerance: 10%

3. Check AC output voltage every time when you are performing the

experiments.

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Experiment-3

Object :- To verify the maximum power transfer theorem.

Apparatus required:- Accumulator, D.C. Voltmeter (0-15V), D.C. ammeter (o-

25Amp.), Resistance box, Rheostat of suitable rating and connecting wires.

Apparatus details:-

S.No. Name of

Apparatus

Type Range/Rating Quantity Make Remarks

Theory/Principle:- According to the maximum power transfer theorem as applied

to D.C. network, a resistive load will abstract maximum power from a network

when the load resistance is equal to the resistance of the network as viewed from

the output terminals, with all energy sources replaced by their internal resistance.

In the case of A.C. network load impedance should be complex conjugate of

source impedance.

Circuit diagram:- the circuit diagram and graph power and load resistance is

shown below in the figure.

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Procedure:-

1. Connection diagram is shown above in figure, where R is fixed at some suitable value, load resistance RL varies and ammeter and voltmeter readings are noted.

2. This process is repeated for different values of R. 3. For every value of RL, curve is plotted between power consumed in the load

resistance and load resistance RL and from the curve so drawn the value of RL for maximum power drawn in determined.

Observation table:-

Value of R =

Calculations:-

1. The value of current in load resistance in -----ohms.

2. The value of voltage in load resistance is---volt.

3. The value of power consumed is -----watts.

4. The value of load resistance is--------ohms.

Result:- It will be found that power consumed (VL, IL) will be maximum when RL

becomes equal to R. This verifies the maximum power transfer theorem.

Precautions:-

1. All the connections should be tight. 2. Readings should be taken carefully for accurate result. 3. Do not touch the live terminals because touching the live

terminals is injurious to health. 4. Switch off after taking the readings

S.No. Resistance RL

in ohms

Current in

amperes (IL)

Voltage VL in

volts

Power consumed P= VL IL

watts

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Experiment:4

Object:- Study of phenomenon of resonance in R-L-C series circuit.

Apparatus required:- A.C. voltmeter, A.C. Ammeter, Kit for RLC Circuits.

Apparatus detail:-

Sl.No. Apparatus Type Range/Rating Quantity Make Remarks

Theory/Principle:- Consider an A.C. circuit containing a resistance R, and

inductance L and a capacitance C connected in series as shown in Fig. below

At resonance voltage, XL=XC (in magnitude only) So , at resonance (i) Not reactance is zero i.e., X= 0 (ii) Impedance of the circuit, Z= R (iii) The current flowing through the circuit is maximum and in phase with the applied voltage. The magnitude of current is equal to V/ R. (iv) The Voltage drop across the inductance is equal to the voltage drop across the capacitance. (v) The power factor is unity. When this condition exists, the circuit is said to be in resonance the frequency at which it occurs is known as resonance frequency. It is denoted by fr then XL = Xc

; L = 1/ C ; 2 frL = 1/2 frC ; fr = 1/2 LC. From the above expression it is obvious that the value of resonance frequency depends on the parameter of the two energy-storing elements.

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Circuit diagram:- Circuit diagram is shown below in Fig:-

Procedure:- An non-inductive resistor R, an inductor L and a Capaciter ‘C ‘are

connected in series, connect a voltmeter and an ammeter, as shown in fig above

, such that the current I= V/R does not exceed the safe value and with a small

capacitor the supply is switched on and all readings are recorded. The frequency

is gradually increases reading being noted at several intervals. The curves are

plotted for current and frequency capacitance which is shown in fig below

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Observation Table:- R= ------ohms ; Supply voltage V1= ----Volts (A.C.)

Inductance, L = ______ Capacitance, C = _________

Calculation:- The resonance frequency fr = → fr = …..Hz

Result:-

The power factor rises in a similar way as the circuit current i.e., it rises first to a maximum value of unity at resonance point and then falls. At resonance the circuit behaves like a pure resister.

Precautions:-

1. Connect the voltmeter and ammeter as per the circuit diagram, 2. Increase the value of capacitor slowly after fixed interval of time. 3. Take reading accurately. 4. Measure power factor with every reading. 5. Do not touch the live wire during practical.

S.No. Frequency (in Hz) I (in mA)

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EXPERIMENT-5

Objective: To study LCR parallel circuits in resonance condition.

Apparatus required:

1. LCR Kit

2. Function Generator or Audio frequency oscillator model-712.

3. AC voltmeter(0-5V) & mili ammeter(0-25mA).

Theory:

Resonance:

An AC circuit is said to be in resonance when the applied voltage and the resulting

current are in the phase . A parallel resonant circuit consists of an inductor L in

parallel with a capacitor C as shown in Figure. R is a small resistance associated

with the coil. The capacitor C is assumed to be lossless. The tuned circuit is driven

by a voltage source V. Such a parallel-tuned circuit is commonly used in tuned

amplifiers, oscillators, etc.

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PROCEDURE:

1. Connect Voltmeter to ‘V’ sockets and Milliammeter to mA sockets.

2. Connect output terminals on function generator to sockets marked signal

output.

3. Keep function generator selector switch at sine wave.

4. Keep dial at minimum i.e. 100c/s and with amplitude control set voltmeter

reading 3 to 4 volt.

5. Note frequency from the dial and current from the miliammeter.

6. Change the frequency from the dial in steps of 100 or so and note down the

corresponding current till the characteristics of parallel resonance is obtained.

7. Plot a graph between frequency and current in mA.

8. Repeat the steps 4 to 7 for different R & L.

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OBSERVATIONS:

R= ------ohms ; Supply voltage V1= ----Volts (A.C.)

Inductance, L = ______ Capacitance, C = _________

Sl.No. Frequency (Hz ) Current(mA)

Precautions:-

1. Connect the voltmeter and ammeter as per the circuit diagram,

2. Increase the value of capacitor slowly after fixed interval of time.

3. Take reading accurately.

4. Measure power factor with every reading.

5. Do not touch the live wire during practical.

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Experiment :6

Object:-Measurement of efficiency of a single phase transformer by load test.

Apparatus required:- A.C. ammeters, A.C. voltmeters, Wattmeters, Single phase

transformer, Lamp Load in series with an inductor DPIC switch, connecting wires.

Apparatus details:

S.No. Apparatus Type Range/Rating Quantity Make Remarks

Theory/ Principle: The efficiency of a transformer is given by the expression

= (Output power/ Input power) x 100

or = P2 / P1 x 100 The transformer efficiency can be obtained by direct measurement of output and

input does not give accurate result, as the power losses are quite law ( of the

order of 1-4%) . The difference between the reading of output and output

instrument is then so small that an instrument error as low as 0.5% would cause

an error of the order of 15-20% in the power losses. There is a wastage of large

amount of power and no information is available from such a test about the

proportion of copper and iron losses.

Circuit diagram:-

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Procedure:-

1. Make the connection according to the circuit diagram as shown above in fig.

2. Connect the wattmeters, ammeters and voltmeters both in primary as well as in secondary winding and connect a load across the output of the secondary winding under test according to the circuit diagram.

3. The supply is switched on to the circuit through DPIC switch. 4. The readings of voltmeters, ammeters and wattmeters , on input (primary)

and output (secondary) are noted for different loads and repeat these readings at least three time and then take their mean.

Observation table:-

S.No. V1 in volts

I1 in amps.

P1 in watts

V2 in volts

I2 in amps.

Cos P2 in watts =

V2I2Cos 2

= P2/P1 x 100

Calculations:- Power input (Primary) side, P1 = ………watts.

Power in output (Secondary) side P2= V2 I2Cos = ……..watts.

Transformer efficiency = P2/P1 x 100 or =…….

Result:- The efficiency of the given single phase transformer is ……….

Precautions:-

1. All connections should be tight. 2. All apparatus should be of suitable range. 3. Never touch live conductors or terminals. 4. Readings should be taken accurately.

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Experiment:7

Object:- To connect , start and reverse the direction of rotation of a 3-phase

induction motor. Measure the speed of motor.

Apparatus required:- Three phase induction motor, Star delta starter, TPIC switch

, Screw driver, Plier, Connecting wirers, Tachometer etc.

Theory /Principle:-

Motor is connected to a three phase ac supply mains through star-delta starter

and TPIC switch, as shown in figure. The direction of rotation of 3-phase induction

motor can be reversed by interchanging any two terminals at the TPIC switch.

Circuit diagram:-

Procedure:-

The connections of a 3-phase induction motor are made to the star-delta starter

and to the TPIC switch. The TPIC switch is closed and the motor is started by

taking the lever of the starter to the start (star) position and then with a jerk to

the run position (or delta connections). The direction of the rotation of the motor

is observed. Say, it is in clock-wise direction. Measure the speed.

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Now the motor is stopped by pushing the stop button and supply to the motor is

removed by opening TPIC switch. The two leads of motor is interchanging to the

TPIC switch. TPIC switch is closed and motor is started again. The direction of

rotation of the motor is observed.

Observation and Result: The direction of motor is found reversed when two leads of motor are interchanged. Precautions:-

1. All connections should be tight. 2. Never touch the live terminals. 3. Before change the connection OFF the supply properly. 4. Increase the load gradually. 5. Don’t wear the loose dress during the experiment, it may be dangerous. 6. Always use the starter of proper rating.

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Experiment-8

Objective- Determination of a load characteristic of a shunt wound DC

generator.

Apparatus required: D.C shunt generator(coupled with D.C shunt motor),

Ammeters, Voltmeters, Loading rheostat, Connecting wire etc.

Apparatus details :

S.No. Apparatus Type Range/Rating Quantity Make Remarks

Theory/principle: A d.c shunt generator is a self excited generator. A d.c shunt

motor with constant speed characteristics can be used as a prime mover. The

motor and generator are firmly coupled. To enable the generator to develop the

voltage, it is driven by motor using starter. During this process, there is no

connection of load terminals of the generator. The field terminals of the

generator and the field resistance of the shunt generator is varied using rheostat

in the field circuit, as shown in fig. As soon as the terminal voltage on no load;

read by the voltmeter V2 reached the rated no load voltage, the shunt field

resistance is kept fixed at this value. Load resistance(rheostat) is now connected

across the load terminals. Any variation in the field and consequently the terminal

voltage. Due to voltage drop in the armature resistance as well as due to

armature reaction, there will be a drop in terminal voltage from its no-load value,

with different values of load, i.e, load current, Load characteristics of a d.c

generator depicts the variation of terminal voltage(V) with load current (I), when

both the speed and the exciting current are kept constant. For the D.C shunt

generator we plot a curve between load current and terminal voltage. In this

generator terminal voltage reduces due to:

1.Armature reaction.

2. Resistive drop in windings and brushes.

3.Weal excitaition.

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By increasing load current as shown in graph.

From the load characteristics of d.c. shunt generator it is clear:

(i) The terminal voltage of a d.c shunt generator decreases with increase in

load current.

(ii) In the beginning the load current reaches a certain value (much higher

than full load current0 further decrease in load resistance causes a

decrease in load current rather than increase and the external

characteristics turn back. This is due to predominant demagnetizing

armature reaction effect and armature voltage drop over the effect of

decrease in load resistance.

Procedure: 1.Connect all the equipments as circuit diagram. 2. Start the motor. 3. Now, read the ammeter & voltmeter readings varying the load resistance. 4.Note the readings in observation table and plot a graph. 5.Switch off the main supply.

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Observation table:

S.No. Load current IL amp

Terminal voltage VT volt

1 2

3

Result: The characteristics of D.C shunt generator is found as observed in

observation table and corresponding graph.

Precautions:

1. All connection should be tight.

2. Reading should be taken carefully.

3. Readings should be taken, increasing the load current.

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VIVA-VOCE QUESTIONS

BASICS OF EXPERIMENTS 1. How an ammeter is connected in an electrical ckt? 2. How a voltmeter is connected in an electric ckt? 3. How do you measure speed of a motor. 4. How a wattmeter is connected in an electric ckt? 5. How do you provide loading to an electric motor in electrical eng. Lab?

NETWORK THEOREMS

1. Define the linear ckt. 2. Define the non linear ckt. 3. Define bilateral ckt. 4. Define unilateral ckt 5. What do you mean by Active Network? 6. What do you mean by passive network? 7. What is the of the circuit in maximum power transfer theorem? 8. Where maximum power transfer theorem is useful.

RESONANCE

1. What is resonance? 2. What do you mean by resonance curve? 3. What is Q-factor? 4. What are important characteristics of parallel resonance?

TRANSFORMER

1. How can we reduce the eddy current losses? 2. Give the name of material by which the core of transformer is made and

why. 3. Give the applications of Dynamically induced emf 4. Give the application of statically induced emf. 5. How can we determine the HV and LV winding of transformer? 6. What is the unit of rating of transformer and why? 7. What will happen if transformer is connected to rated D.C supply and why?

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3 – PHASE INDUCTION MOTOR

1. What is the slip? 2. What is a synchronous speed? 3. Why the starter is necessary tor starling a 3 induction motor? 4. How can we reverse the rotation of 3 induction motor?

DC SHUNT MACHINE

1. What should be happened, if there is no back emf? 2. Where armature control method is used in dc machine? 3. Where field control method is used in dc machine? 4. What is the constructional difference between D.C motor and D.C

generator? 5. What is the function of commutator in a D.C. generator? 6. What is the function of commutator is a D.C. motor. 7. Where we use lap winding? 8. Where we use ware winding?

basic electrical lab manual

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