Government Girls` Polytechnic, Bilaspur

Government Girls` Polytechnic, Bilaspur

Government Girls’ Polytechnic, Bilaspur

Name of the Lab:Electrical&Electronic

Measurement Lab

Practical: Electrical & Electronics Measurement

Lab

Class :3

rd

Semester ( ET&T )

Teachers Assessment:10 End Semester Examination: 50

Experiment no 1

OBJECTIVE :. Self-inductance measurements: Ammeter and Voltmeter method, Self-Inductance

Measurement by General fourarms‟s bridge network method.

MATERIAL REQUIRED :- Inductance, Ammeter and Voltmeter, General fourarms‟s bridge network method

THEORY:- Inductance Meters instruments for measuring the inductance of circuits with lumped parameters, the windings of transformers and chokes, and inductance coils. Their principle of operation depends on the method of measurement.

The “voltmeter-ammeter” method (Figure 1) is used for measuring relatively large inductances

(from 0.1 to 1,000 henrys [H]) when the resistance of the windings is significantly lower than the inductance. In this case L x

= U

/(π f · I), where U is the voltage, I is the current in the circuit whose inductance is being measured, and f is the frequency of the alternating current, usually 50 hertz

(Hz). Such inductance meters have an error of 2

–3 percent.An AC measuring bridge (for frequencies of 100, 400, and 1,000 Hz) with standard capacitance or, much less frequently,standard inductance (Figure 2) is the main component of inductance meters. When the bridge is balanced,

r

x

= (R

1

· R

2

)/R

3

L

x

= C

s

· R

1

· R

2 where r

x

is the ohmic resistance of the wire of the inductance coil winding. Bridge measurement methods have an error of 1

–3 percent; the range of measurement is 0.1 to 1,000 H.

Resonance methods are based on the resonant properties of an oscillatory circuit composed of the inductance being measured (L

x

) and a standard capacitance C

s

(Figure 3). By varying C„ the circuit can be tuned to resonance with the generator (at frequencies of 10 kHz to 1.5 MHz); the inductance is then calculated using the formula L

x

= (2.53 × l0

)(f

2

0

· c), where L

x

is the inductance in mH, f

0

is the resonance frequency in kHz, and C = C

s

+ C

c

is the total capacitance of the circuit in picofarads. Resonant inductance meters have an error of 2

–5 percent; the range of measurement is 0.05 μ.H to 100 mH.

Electrical & Electronics Measurement Lab Manual:3 rd

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For each value of Rx, find the expected value of current that will flow (assuming a voltage source of 10 V) by dividing 10 V by the value of Rx.

3- Based on the value of current expected to flow, calculate the required value of the DRB to convert the ammeter to the suitable full scale deflection (FSD).

4- Set the DRBX to the value of Rx.

5- Calculate the required value of DRBV to a value suitable for a FSD of voltage of 10 V.

6- Connect the circuit as shown in Figure 2 below. Remember that the voltmeter shown might include DRBV in series. Also remember that the ammeter shown might include DRBA in parallel.

CIRCUIT DIAGRAM :

Figure 1

. Diagram of the measurement of inductance by the “voltmeter-ammeter” method:(A) ammeter, (V) voltmeter, (L x

) inductance, (l) current, (U) voltage

Figure 2 Diagram of resonance method of inductance measurement: (L

c

) coupling loop, (L

x

) inductance being measured,

(C

c

) self-capacitance of coil, (C

s

) standard capacitance

Procedure:-

1) Study the circuit provided on the front panel of the kit.

2) Connect unknown inductance LX1 in the circuit. Make all connections to complete the bridge.

3) Put the supply ON

4) Set the null point of galvanometer by adjusting variable resistance R3.

5) Note value of R2, R3, and C4 by removing connection by patch cords.

6) Calculate theoretical value of LX1 using L=R2R3C4

7) Measure value of LX2 by LCR meter and compare it.

8) Repeat process for LX2.

Result:-

The unknown inductance is measured using Hay‟s bridge and is found to be___

Precautions :

1.connections should be tight.

2.Instrument should be handled carefully.

Electrical & Electronics Measurement Lab Manual:3 rd

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Experiment No :-2

Object:- Measurement of the unknown inductance by using Hay

‟s bridge method.

Apparatus:- Multimeter

LCR meter

Hay‟s bridge kit,

Patch cords.

Theory:-

The hays bridge is the modification of the Maxwell Bridge. This bridge uses a resistance in series with the standard capacitor. The bridge has four resistive arms in which the arms one is consists of the resister R1, Lx .The arm 2 is consists of the variable resistance

R3.The low value of the resistance is obtain by the low resistive arms of the bridge. The value of R4 and C4 is the standard value of the capacitor and resistance.

By using the unknown inductance having a resistanceR1. R2, R3,R4-is the known non-inductive resistance and C4 is standard value of the capacitor. The unknown value of inductance and Quality factor of the Bridge is obtained by formula.

Lx = (R2R3C4) /(1+

2R42C4

2) Quality factor (Q)=(1/

2R42C42)

Basic AC bridges consist of four arms, source excitation and a balanced detector.

Commonly used detectors for AC bridges are:

(1) Head phones

(2) Vibration galvanometers

(3) Tunable amplifier detectors

Vibration galvanometer is extremely useful at power and low audio frequency ranges. Vibration galvanometers are manufactured to work at various frequency ranging from 5 KHZ to 1 KHZ. But one most commonly used between

200HZ.

Advantage-1) This Bridge gives very simple expression for unknown for High Q coil.

2) This bridge also gives a simple expression for Q factor.

Disadvantage-1)The hays bridge is suited for the measurement of the High Q inductor.

2)It is used to find the inductor having the q value of the smaller then 10.

.

Procedure:-

1) Study the circuit provided on the front panel of the kit.

2) Connect unknown inductance LX1 in the circuit. Make all connections to complete the bridge.

3) Put the supply ON

4) Set the null point of galvanometer by adjusting variable resistance R3.

5) Note value of R2, R3, and C4 by removing connection by patch cords.

6) Calculate theoretical value of LX1 using L=R2R3C4

7) Measure value of LX2 by LCR meter and compare it.

8) Repeat process for LX2.

Circuit Diagram:-

Electrical & Electronics Measurement Lab Manual:3 rd

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Figure 1. Diagram of a bridge for measuring inductance: (U) current source; (G) galvanometer; (R

(R

2

), and (R

3

) ohmic resistances; (C x

,) standard capacitance; (L x

) inductance being measured

3

),

Result:-

The unknown inductance is measured using Hay‟s bridge and is found to be___

Precautions :

1.connections should be tight.

2.Instrument should be handled carefully.

Electrical & Electronics Measurement Lab Manual:3 rd

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Experiment no 3

Object:

To measure self inductance of two coils, mutual inductance between these and the coefficient of coupling

Apparatus required

:

Ammeter 0-2am,0-10A MI Voltmeter 0-500V MI100V

Theory:

When the two coils are connected for additive polarity the fluxes produced by the current in the two coils will aid each other and hence the impedance is high. This require the equivalent inductance to be very high. In this case, the mutual inductance terms will have the same sign as that of the self inductance terms. Thus, if the two coils having inductance L

1

and L

2

respectively and a mutual inductance of M between them are connected for additive polarity, the equivalent inductance

L

1

= L

1

+ L

2

+ 2M

– (1)

When the two coils are connected for subtractive polarity, the two fluxes will oppose each other and the inductance and hence the impedances are low. In this case the mutual inductance terms will have the opposite sign as that of the self inductance terms. Hence the equivalent inductance

L

11

= L

1

+ L

2

- 2M

– (2)

From (1) and (2), we can find out M, L

1

and L

2

. Thus the coefficient of coupling is calculated by the formula

K

M

L

1

L

2

Procedure:

First the polarity is to be checked. For this connect the 0

– 10A ammeter and 0 –

500V voltmeter in the circuit. Then switch on the supply with autotransformer in its minimum position. Then vary the autotransformer and note the variation of readings in the ammeter and voltmeter. If the current variation is high, for a small voltage the polarity is subtractive. Otherwise, the polarity is additive. After checking the polarity put the required range of meters in the circuit.

For additive polarity, give the rated voltage (ie 220 + 200 = 420V) and note the ammeter and voltmeter readings.

For subtractive polarity, pass the rated current (7.5A) by adjusting the autotransformer and note the ammeter and voltmeter readings.

For measuring the resistance, keeping the current limiting rheostat at its maximum position supply is switched on. Then the ammeter and voltmeter readings are noted.

Circuit diagram:

Electrical & Electronics Measurement Lab Manual:3 rd

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For additive polarity

For subtractive polarity

Tabulation

V

1

Volts

Additive Polarity

I

1

Amp

Z

1

Ω

V

2

Subtractive Polarity

Volts

I

2

Amp

Z

2

Ω

Resistance Measurement

V

3

Volts

I

3

Amp

R

Ω

Electrical & Electronics Measurement Lab Manual:3 rd

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Calculation

L

1

= L

1

+ L

2

+ 2M ………….. (1)

L

11

= L

1

+ L

2

-

2M ………….. (2)

L

1

+ L

11

= 2 (L

1

+ L

2

)

L

1

+ L

2 = (

L

1

+ L

11

) / 2 ………….. (3)

(L

1

L

11

)/4 = M ………….. (4)

Z

1

= V

1

/ I

1

Z

2

= V

2

/ I

2

R = V

3

/ I

3

X

1

Z

1

2

L

1

= X

1

/ 2 f

R

2

Z

2

2

X

L

2

2

= X

2

/ 2 f

R

2

L

1

L

2

N

1

N

2

2

E

1

E

2

Solve for L

1

L

2 and M.

Coefficient of coupling

K

M

L

1

L

2

Result Self inductance of primary coil L

1

= __________________

Self inductance of primary coil L

2

= __________________

Mutual inductance M = __________________

Coefficient of coupling K = __________________

Precautions :

1.connections should be tight.

2.Instrument should be handled carefully.

Electrical & Electronics Measurement Lab Manual:3 rd

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Experiment no 4

Aim

:- Measurement of the unknown inductance by using Maxwell bridge method.

Apparatus:-

Digital multimeter,

Patch cords.

R2=100

=1M,

R3=9.97K

,

C4=1

f

LX1=318mH

LX2=73 mH

Theory:-

The Maxwell‟s bridge is used an inductance is measured by comparison with a standard variable capacitance. One of the ratio arms has a résistance and the capacitance in the parallel.

In this bridge at the balance in condition there is no current is flow in the galvanometer.henced the balance equation for the bridge using the admittance of the arm 1 instead of the impedance.

ZX=(Z2* Z 3*Y1)

Where the Y1 is the admittance of the arm-1.

Z2=R2

Z3=R3

Y1=(1/R1+j)

By separating the real and imaginary term the unknown value of the resister (Rx) and the unknown value of the capacitor (Cx) has given below.

Rx=(R2*R3/R1).

LX= (R2*R3*C1)

Advantage-

1) This bridge is very useful for measurement of a wide range of a inductance at the power and audio frequencies.

2) The frequency does not appear in any of the two equations.

Disadvantage-

1) This bridge requires a variable standard capacitor, which may be

Vary expensive if the calibration to a high degree of the accuracy.

2) The bridge is limited the measure the low Q value.

Procedure:-

1) Study circuit on kit from panel.

2) Connect unknown inductance LX1 in circuit. Make all possible connections to complete the network. Switch the supply on.

3) Set null point of galvanometer by adjusting variable resistance R3

4) Note values of R2, R3, C4 by removing their connections. Calculate theoretical values of LX using L1=R2R3C4.

5) Measure actual value of LX1 using LCR meter. Compare this value with calculated. also calculate Q factor by using above equation.

Circuit Diagram :

Electrical & Electronics Measurement Lab Manual:3 rd

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Result:-

Unknown inductance measured using Maxwell‟s bridge is found to be

LX1=____

Precautions :

1.connections should be tight.

2.Instrument should be handled carefully

Electrical & Electronics Measurement Lab Manual:3 rd

semester(ET&T) 9

Experiment No-05

Aim

:- Measurement of medium resister by the voltmeter and ammeter method.

Apparatus:- DC ammeter(0-500mA)

DC Voltmeter (0-5V)

Dc power supply (0-30V)

Variable Resistance -100 ohm.

Connecting wires.

Theory: -

Two types of the connections are done one employed for the ammeter voltmeter method as shown in the figure voltmeter and ammeter are connected in series, where ammeter measures the total current flowing through the circuit and voltmeter measures the voltage across the unknown resistance .The voltmeter should have ideally infinite resistance and ammeter should have ideally zero resistance so that it will measure total current flowing through the unknown resistance. But practically it is not possible and measured value Rm of the resistance is the sum of resistance of ammeter and actual resistance.

Rm=R1+Ra

Where

R1=Actual resistance.

Ra=resistance of the ammeter.

It is clear from the expression that the value of measured resistance is equal to actual resistance when ammeter has zero resistance.

Procedure:-

1) Make the connections as per circuit diagram.

2) Switch on the supply and note down the readings of ammeter and voltmeter.

3) Calculate the value of the unknown resistance by ohms low.

4) Perform the procedure for the other case similarly.

Circuit Diagram:-

Electrical & Electronics Measurement Lab Manual:3 rd

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Observation Table:-

Result: - Hence we study the measured and the actual vale of the unknown resistance is found .

Precautions :

1.connections should be tight.

2.Instrument should be handled carefully

Electrical & Electronics Measurement Lab Manual:3 rd

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Experiment No.-06

Aim:

- Measurement of the medium resistance by using whetstone bridge method.

Apparatus:

- Power supply,

Resistor: - 10K_-1no,5K_-1no,11K_-1no

Unknown resistor=100_,

Pot =1K-1no.

Wheat stone bridge kit.

Digital multimeter-1no,

Patch codes.

Theory-

A very important device used in the measurement of medium resistances is the

Wheat stones bridge .it is an accurate and reliable instrument .The wheat stone bridge is an instrument based on the principle of null indication and comparison measurements .

The basic circuit of a wheat stone bridge is shown in fig . it has four resistive arms, consisting of resistances P,Q,R and S together with a source of emf and a null detector , usually a galvanometer G or other sensitive current meter is used. The current through the galvanometer depends on the potential difference between point‟s b and d

.The bridge is said to be balanced when there is on current through the galvanometer or when the potential difference across the galvanometer is zero. this occurs when the voltage from point „b‟ to point „a‟ equals the voltage from point „d‟ to point „a‟ or by referring to other battery terminal , when the voltage from point „d‟ to point „c‟ equals the voltage from point „b‟ to point „c‟.

For bridge balance;

I1P=I2 R ……………. (.1)

I1=I3=E/P+Q (2)

I2=I4=E/R+S (3)

E=emf of battery.

Combining equ (1) and (2) we get

P/P+Q=R/R+S (4)

QR=PS (5)

Equ . (5) Shows the balance condition of wheat stone bridge. If three of the resistances are known then fourth may be determined by formula…

R=S*P/Q

Where R is the unknown resistance, S is called the standard arm resistor and P and Q are called the ratio arms.

Procedure: -

1) Connect the patch cord as per the circuit diagram.

2) Note the resistance of P and Q using multimeter.

3) Adjust the resistance of P, Q, R, S

4) Switch on the power supply and adjust the resistance S such that galvanometer shows the zero deflection.

5) Now calculate R, R=P*S/Q

Electrical & Electronics Measurement Lab Manual:3 rd

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Circuit Diagram:-

Observation Table:

-

Result:- Hence we have studied the low resistance by using whetstone bridge.

Precautions :

1.connections should be tight.

2.Instrument should be handled carefully

Electrical & Electronics Measurement Lab Manual:3 rd

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Experiment No:-07

Aim:

- Measurement of the low resistance by using Kelvin Double bridge method.

Apparatus:

- Regulated dc supply-1no

Standard resistance coil-1no

Kelvin‟s double bridge kit.

Digital multimeter-1no,

Patch codes.

Theory: -

Kelvin‟s bridge is a modification of whetstone‟s bridge and always used in measurement of low resistance. It uses two sets of ratio arms and the four terminal resistances for the low resistance consider the ckt. As shown in fig. The first set of ratio P and Q. The second set of ratio arms are p and q is used to connected to galvanometer to a pt d at an Approx. potential between points m and n to eliminate the effects of connecting lead of resistance r between the known std. resistance „s‟ and unknown resistance R

.The ratio P/Q is made equal to p/q. under balanced condition there is no current flowing through galvanometer which means voltage drop between a and b, Eab equal to the voltage drop between a and c, Eamd.

Now Ead=P/P+Q ; Eab=I[R+S+[(p+q)r/p+q+r]] -------------- (1)

Eamd= I[R+ p/p+q[ (p+q)r/p+q+r]] --------------------------- (2)

For zero deflection->

Eac=Ead

[ P/P+Q]I[R+S+{(p+q)r/p+q+r}]=I[R+pr/p+q+r] ---- (3)

Now, if P/Q=p/q

Then equa… (3) becomes

R=P/Q=S -

(4)

Equation (4) is the usual working equation. For the Kelvin‟s Double Bridge .It indicates the resistance of connecting lead r. It has no effect on measurement provided that the two sets of ratio arms have equal ratios. Equation (3) is useful however as it shows the error that is introduced in case the ratios are not exactly equal. It indicates that it is desirable to keep r as small as possible in order to minimize the error in case there is a diff. between the ratio P/Q and p/q.

R=P/QS

Circuit Diagram: -

Observation Table: -

Electrical & Electronics Measurement Lab Manual:3 rd

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Procedure: -

1) The circuit configuration on the panel is studied.

2) Supply is switched on and increased upto 5v.

3) The unknown resistance is connected as shown .

4) The value of P,Q was selected such that a. P/Q=p/q

5) S was adjusted for proper balance and balance value of s was balanced.

6) The value of known resistance was calculated.

Result- The observed value of unknown resistance is

Precautions-

1) Check all the connections before turning ON the power supply.

2) Do not exceed the value of 5v.

3) Note the readings accurately.

Electrical & Electronics Measurement Lab Manual:3 rd

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Experiment No-08

Aim:- Measurement of the high resistance by using loss of charge method.

Apparatus:- Multimeter

–1no

Ammeter-(0-200ma)-1no

Voltmeter

–(0-30v)-1no

Capacitor-10uf-1no

Resister-100K-1no

Power supply-(0-30v)-1no

.

Theory:-

In this method the resistance which is measured is connected in parallel with the capacitor C and the electronic voltmeter V. The capacitor is the charged up to some suitable voltage by means of the battery having the voltage V and is then allowed to discharge through the resistance.

The terminal voltage is observed over the considerable period of the time during discharge.

Let ,

V=initial voltage on the charged capacitor .

.v=instantaneous discharging voltage.

I=the discharging capacitor current through the unknown resistor at time “t”.

Q=the charge still remaining in the capacitor.

I=dq/dt=-cdv/dt since[I=V/R]

V/R+C dv/dt=0 (1)

1/RC dt+1/V.dV=0 integrating both sides t/RC+logev+K=0 (2)

K is const. of integration

At initial condition

When T=0 , v=V from equ. (2)

K=-logeV now equ. (2) becomes t/RC+logev-logeV=0 therefore. Loge (v/V)=-t/RC v/V=e-t/RC v=V*e-t/RC

Taking log on both sides logev=logeV+logee-t/RC

R=t/C*loge(V/v)

R=0.4343*t/C*log10(V/v)

Procedure:-

1) Connections is make as per the circuit diagram.

2) Close the switch s and keep s2 open the capacitor charge by own leakage method.

3) New open reading and voltmeter .as its own resistance.

4) Note down the reading of the voltmeter Vs equal interval of the time

Circuit Diagram:-

Electrical & Electronics Measurement Lab Manual:3 rd

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Observation Table:-

Result:- High resistance of the resistance is calculated by using loss of charge method.

Precautions-

1) Check all the connections before turning ON the power supply.

2) Do not exceed the value of 5v.

3) Note the readings accurately.

Electrical & Electronics Measurement Lab Manual:3 rd

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Experiment no-9

Aim

:- Measurement of the unknown capacitance sharing bridge method.

Apparatus:-

Sharing bridge kit digital multimeter, patch cords,

Theory:-

The schering bridge one of the most important ac bridge is used the extensively for the measurement of capacitors.

In the schering bridge the arm 1 now contains a parallel combination of the resistor and the capacitor and standard arm contain only one capacitor. The standard capacitor is usually a standard high quality mica capacitor. in the balance condition of the bridge the sum of the phase angles of the arms 1 and 4 is equal the sum of the phase angle of arms 2 and 3.at the balance in condition there is no current flow in the galvanometer.

The balance equation is derived in the usual manner, and by substituting the corresponding impendence and the admittance the value of the unknown capacitor and the resister is find as given below.

Cx=C3(R1/R2).

Rx=R2(C1/C2)

Procedure: -

1) Study the circuit provided on the front panel on the kit.

2) Connect the unknown capacitance of the position given.

3) Set the null point of galvanometer by adjusting the variable

4) Calculate the value of unknown capacitance by formula given

Circuit Diagram

Electrical & Electronics Measurement Lab Manual:3 rd

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Result: -The values of unknown capacitance is measured by shearing bridge

Precautions-

1) Check all the connections before turning ON the power supply.

2) Do not exceed the value of 5v.

3) Note the readings accurately.

Electrical & Electronics Measurement Lab Manual:3 rd

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Experiment no-10

Aim:-

To study of DC potentiometer.

Apparatus:-

Power supply,(0-50v)

Battery,(0-30V)

Resister-100

Theory:-

The potentiometer although not consider a bridge has circuit that using a simple circuit thermo becomes identical to the whetstone bridge. The potentiometer is a device for measuring the voltage while presenting a very high impedance to the voltage source under the Test.

The variable resister R1 is the precision device that can be set to an accurate value. the resister is adjusted so that no current is flow through the galvanometer. which is the similar to balancing the bridge.

At this point the zero current flow several important characteristics of the potentiometer cab be determine.

Circuit Diagram

Result:- To study the working of the DC potentiometer.

.

Precautions-

1) Check all the connections before turning ON the power supply.

2) Do not exceed the value of 5v.

3) Note the readings accurately.

Electrical & Electronics Measurement Lab Manual:3 rd

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Experiment no 11

OBJECTIVE: . Study of C.R.O. Voltage measurement on C.R.O.

Current measurement on C.R.O. Frequency measurement on C.R.O. Phase difference measurement on C.R.O.

APPARATUS REQUIRED : Cathode-ray oscilloscope, millimeter, and oscillator.

Theory

INTRODUCTION: The cathode-ray oscilloscope (CRO) is a common laboratory instrument that provides accurate time and amplitude measurements of voltage signals over a wide range of frequencies. Its reliability, stability, and ease of operation make it suitable as a general purpose laboratory instrument. The heart of the CRO is a cathode-ray tube shown schematically in Fig. 1.

The cathode ray is a beam of electrons which are emitted by the heated cathode (negative electrode) and accelerated toward the fluorescent screen. The assembly of the cathode, intensity grid, focus grid, and accelerating anode (positive electrode) is called an electron gun. Its purpose is to generate the electron beam and control its intensity and focus. Between the electron gun and the fluorescent screen are two pair of metal plates - one oriented to provide horizontal deflection of the beam and one pair oriented ot give vertical deflection to the beam. These plates are thus referred to as the horizontal and vertical deflection plates. The combination of these two deflections allows the beam to reach any portion of the fluorescent screen. Wherever the electron beam hits the screen, the phosphor is excited and light is emitted from that point. This coversion of electron energy into light allows us to write with points or lines of light on an otherwise darkened screen.

Electrical & Electronics Measurement Lab Manual:3 rd

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In the most common use of the oscilloscope the signal to be studied is first amplified and then applied to the vertical (deflection) plates to deflect the beam vertically and at the same time a voltage that increases linearly with time is applied to the horizontal (deflection) plates thus causing the beam to be deflected horizontally at a uniform (constant> rate. The signal applied to the verical plates is thus displayed on the screen as a function of time. The horizontal axis serves as a uniform time scale.

The linear deflection or sweep of the beam horizontally is accomplished by use of a sweep

generator that is incorporated in the oscilloscope circuitry. The voltage output of such a generator is that of a sawtooth wave as shown in Fig. 2. Application of one cycle of this voltage difference, which increases linearly with time, to the horizontal plates causes the beam to be deflected linearly with time across the tube face. When the voltage suddenly falls to zero, as at points (a)

(b) (c), etc...., the end of each sweep - the beam flies back to its initial position. The horizontal deflection of the beam is repeated periodically, the frequency of this periodicity is adjustable by external controls.

To obtain steady traces on the tube face, an internal number of cycles of the unknown signal that is applied to the vertical plates must be associated with each cycle of the sweep generator. Thus, with such a matching of synchronization of the two deflections, the pattern on the tube face repeats itself and hence appears to remain stationary. The persistance of vision in the human eye and of the glow of the fluorescent screen aids in producing a stationary pattern. In addition, the electron beam is cut off (blanked) during flyback so that the retrace sweep is not observed.

CRO Operation: A simplified block diagram of a typical oscilloscope is shown in Fig. 3. In general, the instrument is operated in the following manner. The signal to be displayed is amplified by the vertical amplifier and applied to the verical deflection plates of the CRT. A portion of the signal in the vertical amplifier is applied to the sweep trigger as a triggering signal. The sweep trigger then generates a pulse coincident with a selected point in the cycle of the triggering signal. This pulse turns on the sweep generator, initiating the sawtooth wave form. The sawtooth wave is amplified by the horizontal amplifier and applied to the horizontal deflection plates.

Usually, additional provisions signal are made for appliying an external triggering signal or utilizing the 60 Hz line for triggering. Also the sweep generator may be bypassed and an external signal applied directly to the horizontal amplifier.

CRO Controls

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The controls available on most oscilloscopes provide a wide range of operating conditions and thus make the instrument especially versatile. Since many of these controls are common to most oscilloscopes a brief description of them follows.

CATHODE-RAY TUBE

Power and Scale Illumination: Turns instrument on and controls illumination of the graticule.

Focus: Focus the spot or trace on the screen.

Intensity: Regulates the brightness of the spot or trace.

VERTICAL AMPLIFIER SECTION

Position: Controls vertical positioning of oscilloscope display.

Sensitivity: Selects the sensitivity of the vertical amplifier in calibrated steps.

Variable Sensitivity: Provides a continuous range of sensitivities between the calibrated steps.

Normally the sensitivity is calibrated only when the variable knob is in the fully clockwise position.

AC-DC-GND: Selects desired coupling (ac or dc) for incoming signal applied to vertical amplifier, or grounds the amplifier input. Selecting dc couples the input directly to the amplifier; selecting ac

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send the signal through a capacitor before going to the amplifier thus blocking any constant component.

HORIZONTAL-SWEEP SECTION

Sweep time/cm: Selects desired sweep rate from calibrated steps or admits external signal to horizontal amplifier.

Sweep time/cm Variable: Provides continuously variable sweep rates. Calibrated position is fully clockwise.

Position: Controls horizontal position of trace on screen.

Horizontal Variable: Controls the attenuation (reduction) of signal applied to horizontal aplifier through Ext. Horiz. connector.

TRIGGER

The trigger selects the timing of the beginning of the horizontal sweep.

Slope: Selects whether triggering occurs on an increasing (+) or decreasing (-) portion of trigger signal.

Coupling: Selects whether triggering occurs at a specific dc or ac level.

Source: Selects the source of the triggering signal.

INT - (internal) - from signal on vertical amplifier

EXT - (external) - from an external signal inserted at the EXT. TRIG. INPUT.

LINE - 60 cycle triger

Level: Selects the voltage point on the triggering signal at which sweep is triggered. It also allows automatic (auto) triggering of allows sweep to run free (free run).

CONNECTIONS FOR THE OSCILLOSCOPE

Vertical Input: A pair of jacks for connecting the signal under study to the Y (or vertical) amplifier.

The lower jack is grounded to the case.

Horizontal Input: A pair of jacks for connecting an external signal to the horizontal amplifier. The lower terminal is graounted to the case of the oscilloscope.

External Tigger Input: Input connector for external trigger signal.

Cal. Out: Provides amplitude calibrated square waves of 25 and 500 millivolts for use in calibrating the gain of the amplifiers.

Accuracy of the vertical deflection is + 3%. Sensitivity is variable.

Horizontal sweep should be accurate to within 3%. Range of sweep is variable.

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Operating Instructions: Before plugging the oscilloscope into a wall receptacle, set the controls as follows:

(a) Power switch at off

(b) Intensity fully counter clockwise

(c) Vertical centering in the center of range

(d) Horizontal centering in the center of range

(e) Vertical at 0.2

(f) Sweep times 1

Plug line cord into a standard ac wall receptacle (nominally 118 V). Turn power on. Do not advance the Intensity Control.Allow the scope to warm up for approximately two minutes, then turn the Intensity Control until the beam is visible on the screen.

PROCEDURE:

I. Set the signal generator to a frequency of 1000 cycles per second. Connect the output from the generator to the vertical input of the oscilloscope. Establish a steady trace of this input signal on the scope. Adjust (play with) all of the scope and signal generator controls until you become familiar with the function of each. The purpose for such "playing" is to allow the student to become so familiar with the oscilloscope that it becomes an aid (tool) in making measurements in other experiments and not as a formidable obstacle. Note: If the vertical gain is set too low, it may not be possible to obtain a steady trace.

II. Measurements of Voltage: Consider the circuit in Fig. 4(a). The signal generator is used to produce a 1000 hertz sine wave. The AC voltmeter and the leads to the vertical input of the oscilloscope are connected across the generator's output. By adjusting the Horizontal Sweep time/cm and trigger, a steady trace of the sine wave may be displayed on the screen. The trace represents a plot of voltage vs. time, where the vertical deflection of the trace about the line of symmetry CD is proportional to the magnitude of the voltage at any instant of time.

To determine the size of the voltage signal appearing at the output of terminals of the signal generator, an AC (Alternating Current) voltmeter is connected in parallel across these

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terminals (Fig. 4a). The AC voltmeter is designed to read the dc "effective value" of the voltage.

This effective value is also known as the "Root Mean Square value" (RMS) value of the voltage.

The peak or maximum voltage seen on the scope face (Fig. 4b) is V m

volts and is represented by the distance from the symmetry line CD to the maximum deflection. The relationship between the magnitude of the peak voltage displayed on the scope and the effective or RMS voltage (V

RMS

) read on the AC voltmeter is

V

RMS

= 0.707 V m

(for a sine or cosine wave).

Thus

Agreement is expected between the voltage reading of the multimeter and that of the oscilloscope. For a symmetric wave (sine or cosine) the value of V m may be taken as 1/2 the peak to peak signal V pp

The variable sensitivity control a signal may be used to adjust the display to fill a concenient range of the scope face. In this position, the trace is no longer calibrated so that you can not just read the size of the signal by counting the number of divisions and multiplying by the scale factor.

However, you can figure out what the new calibration is an use it as long as the variable control remains unchanged.

Caution: The mathematical prescription given for RMS signals is valid only for sinusoidal signals. The meter will not indicate the correct voltage when used to measure non-sinusoidal signals.

III. Frequency Measurements: When the horizontal sweep voltage is applied, voltage measurements can still be taken from the vertical deflection. Moreover, the signal is displayed as a function of time. If the time base (i.e. sweep) is calibrated, such measurements as pulse duration or signal period can be made. Frequenciescan then be determined as reciprocal of the periods.

Set the oscillator to 1000 Hz. Display the signal on the CRO and measure the period of the oscillations. Use the horizontal distance between two points such as C to D in Fig. 4b.

Set the horizontal gain so that only one complete wave form is displayed.

Then reset the horizontal until 5 waves are seen. Keep the time base control in a calibrated position. Measure the distance (and hence time) for 5 complete cycles and calculate the frequency from this measurement. Compare you result with the value determined above.

Repeat your measurements for other frequencies of 150 Hz, 5 kHz, 50 kHz as set on the signal generator.

IV. Lissajous Figures: When sine-wave signals of different frequencies are input to the horizontal and vertical amplifiers a stationary pattern is formed on the CRT when the ratio of the two

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frequencies is an intergral fraction such as 1/2, 2/3, 4/3, 1/5, etc. These stationary patterns are known as Lissajous figures and can be used for comparison measurement of frequencies.

Use two oscillators to generate some simple Lissajous figures like those shown in Fig. 5.

You will find it difficult to maintain the Lissajous figures in a fixed configuration because the two oscillators are not phase and frequency locked. Their frequencies and phase drift slowly causing the two different signals to change slightly with respect to each other.

V. Testing what you have learned: Your instructor will provide you with a small oscillator circuit.

Examine the input to the circuit and output of the circuit using your oscilloscope. Measure such quantities as the voltage and frequence of the signals. Specify if they are sinusoidal or of some other wave character. If square wave, measure the frequency of the wave. Also, for square waves, measure the on time (when the voltage is high) and off time (when it is low).

Result:-

Study is completed

Precautions:

1.Operate cro carefully

2.Take all reading carefully

3.Use correct power supply

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Experiment no 12

Objective:- To study the different waveforms, to measure peak and rms voltages and the frequency of A.C.

Apparatus Require:- A C.R.O and a signal generator.

Theory :- Cathode ray oscilloscope is one of the most useful electronic equipment, which gives a visual representation of electrical quantities, such as voltage and current waveforms in an electrical circuit. It utilizes the properties of cathode rays of being deflected by an electric and magnetic fields and of producing scintillations on a fluorescent screen. Since the inertia of cathode rays is very small, they are able to follow the alterations of very high frequency fields and thus electron beam serves as a practically inertia less pointer. When a varying potential difference is established across two plates between which the beam is passing, it is deflected and moves in accordance with the variation of potential difference. When this electron beam impinges upon a fluorescent screen, a bright luminous spot is produced there which shows and follows faithfully the variation of potential difference.

When an AC voltage is applied to Y-plates, the spot of light moves on the screen vertically up and down in straight line. This line does not reveal the nature of applied voltage waveform. Thus to obtain the actual waveform, a time-base circuit is necessary.

A time-base circuit is a circuit which generates a saw-tooth waveform. It causes the spot to move in the horizontal and vertical direction linearly with time. When the vertical motion of the spot produced by the Y-plates due to alternating voltage, is superimposed over the horizontal sweep produced by X-plates, the actual waveform is traced on the screen.

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Precautions: Operate cro carefully

Take all reading carefully

Use correct power supply

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Experiment no 13

Object: Study and use of C.T. & P.T

Material require: C.T. and P.T.

Theory: . current transformer (CT)

In electrical engineering, a current transformer (CT) is used for measurement of electric currents. Current transformers, together with voltage transformers (VT) (potential

transformers (PT)), are known as instrument transformers. When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the.

Like any other transformer, a current transformer has a primary winding, a magnetic core, and a secondary winding. The alternating current flowing in the primary produces a magnetic field in the core, which then induces a current in the secondary winding circuit. A primary objective of current transformer design is to ensure that the primary and secondary circuits are efficiently coupled, so that the secondary current bears an accurate relationship to the primary current.

The most common design of CT consists of a length of wire wrapped many times around a silicon steel ring passed over the circuit being measured. The CT's primary circuit therefore consists of a single 'turn' of conductor, with a secondary of many hundreds of turns. The primary winding may be a permanent part of the current transformer, with a heavy copper bar to carry current through the magnetic core. Window-type current transformers are also common, which can have circuit cables run through the middle of an opening in the core to provide a single-turn primary winding.

When conductors passing through a CT are not centered in the circular (or oval) opening, slight inaccuracies may occur.

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Shapes and sizes can vary depending on the end user or switchgear manufacturer. Typical examples of low voltage single ratio metering current transformers are either ring type or plastic moulded case. High-voltage current transformers are mounted on porcelain bushings to insulate them from ground. Some CT configurations slip around the bushing of a high-voltage transformer or circuit breaker, which automatically centers the conductor inside the CT window.

The primary circuit is largely unaffected by the insertion of the CT. The rated secondary current is commonly standardized at 1 or 5 amperes. For example, a 4000:5 CT would provide an output current of 5 amperes when the primary was passing 4000 amperes. The secondary winding can be single ratio or multi ratio, with five taps being common for multi ratio CTs. The load, or burden, of the CT should be of low resistance. If the voltage time integral area is higher than the core's design rating, the core goes into saturation towards the end of each cycle, distorting the waveform and affecting accuracy

Use Current transformers are used extensively for measuring current and monitoring the operation of the power grid. Along with voltage leads, revenue-grade CTs drive the electrical utility's watt-hour meter on virtually every building with three-phase service and single-phase services greater than 200 amp.

The CT is typically described by its current ratio from primary to secondary. Often, multiple CTs are installed as a "stack" for various uses. For example, protection devices and revenue metering may use separate CTs to provide isolation between metering and protection circuits, and allows current transformers with different characteristics (accuracy, overload performance) to be used for the different purposes.

Potential transformer

The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism), and, second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.

An ideal transformer

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An ideal transformer is shown in the adjacent figure. Current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron, so that most of the magnetic flux passes through both the primary and secondary coils.

Induction law

The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that: where V s

is the instantaneous voltage, N s is the number of turns in the secondary coil and Φ is the magnetic flux through one turn of the coil. If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is the product of the magnetic flux density B and the area A through which it cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the excitation of the primary. Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer,

[29]

the instantaneous voltage across the primary winding equals

Taking the ratio of the two equations for V s

and V p

gives the basic equation

[30]

for stepping up or stepping down the voltage

N

p

/N s

is known as the turns ratio, and is the primary functional characteristic of any transformer.

In the case of step-up transformers, this may sometimes be stated as the reciprocal, N s

/N p

. Turns

ratio is commonly expressed as an irreducible fraction or ratio: for example, a transformer with primary and secondary windings of, respectively, 100 and 150 turns is said to have a turns ratio of 2:3 rather than 0.667 or 100:150.

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Ideal power equation

The ideal transformer as a circuit element

If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all the incoming energy is transformed from the primary circuit to the magnetic field and into the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power: giving the ideal transformer equation

Transformers normally have high efficiency, so this formula is a reasonable approximation.

If the voltage is increased, then the current is decreased by the same factor. The impedance in one circuit is transformed by the square of the turns ratio.

[29]

For example, if an impedance Z s

is attached across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of (N p

/N s

)

2

Z

s

. This relationship is reciprocal, so that the impedance Z circuit appears to the secondary to be (N s

/N p

)

2

Z

p

. p

of the primary

Detailed operation

The simplified description above neglects several practical factors, in particular the primary current required to establish a magnetic field in the core, and the contribution to the field due to current in the secondary circuit.

Models of an ideal transformer typically assume a core of negligible reluctance with two windings of zero resistance. When a voltage is applied to the primary winding, a small current flows, driving flux around the magnetic circuit of the core.

[31]

The current required to create the flux is termed the magnetizing current; since the ideal core has been assumed to have near-zero reluctance, the magnetizing current is negligible, although still required to create the magnetic field.

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The changing magnetic field induces an electromotive force (EMF) across each winding.

[32]

Since the ideal windings have no impedance, they have no associated voltage drop, and so the voltages V

P

and V

S

measured at the terminals of the transformer, are equal to the corresponding

EMFs. The primary EMF, acting as it does in opposition to the primary voltage, is sometimes termed the "back EMF".

[33]

This is due to Lenz's law which states that the induction of EMF would always be such that it will oppose development of any such change in magnetic field.

Practical considerations

Leakage flux

Leakage flux of a transformer

Leakage inductance

The ideal transformer model assumes that all flux generated by the primary winding links all the turns of every winding, including itself. In practice, some flux traverses paths that take it outside the windings.[34] Such flux is termed leakage flux, and results in leakage inductance in series with the mutually coupled transformer windings Leakage results in energy being alternately stored in and discharged from the magnetic fields with each cycle of the power supply. It is not directly a power loss (see "Stray losses" below), but results in inferior voltage regulation, causing the secondary voltage to fail to be directly proportional to the primary, particularly under heavy load

Transformers are therefore normally designed to have very low leakage inductance.

However, in some applications, leakage can be a desirable property, and long magnetic paths, air gaps, or magnetic bypass shunts may be deliberately introduced to a transformer's design to limit the short-circuit current it will supply. Leaky transformers may be used to supply loads that exhibit negative resistance, such as electric arcs, mercury vapor lamps, and neon signs; or for safely handling loads that become periodically short-circuited such as electric arc welders.

Air gaps are also used to keep a transformer from saturating, especially audio-frequency transformers in circuits that have a direct current flowing through the windings.

Leakage inductance is also helpful when transformers are operated in parallel. It can be shown that if the "per-unit" inductance of two transformers is the same (a typical value is 5%), they will automatically split power "correctly" (e.g. 500 kVA unit in parallel with 1,000 kVA unit, the larger one will carry twice the current).

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Transformer universal EMF equation

If the flux in the core is purely sinusoidal, the relationship for either winding between its rms

voltage E

rms

of the winding , and the supply frequency f, number of turns N, core cross-sectional area a and peak magnetic flux density B is given by the universal EMF equation:

[31]

If the flux does not contain even harmonics the following equation can be used for half-cycle

average voltage E avg

of any waveshape:

The time-derivative term in Faraday's Law shows that the flux in the core is the integral with respect to time of the applied voltage.

[36]

Hypothetically an ideal transformer would work with direct-current excitation, with the core flux increasing linearly with time.

[37]

In practice, the flux would rise to the point where magnetic saturation of the core occurs, causing a huge increase in the magnetizing current and overheating the transformer. All practical transformers must therefore operate with alternating (or pulsed) current.

[37]

The EMF of a transformer at a given flux density increases with frequency.

[31]

By operating at higher frequencies, transformers can be physically more compact because a given core is able to transfer more power without reaching saturation and fewer turns are needed to achieve the same impedance. However, properties such as core loss and conductor skin effect also increase with frequency. Aircraft and military equipment employ 400 Hz power supplies which reduce core and winding weight.

[38]

Conversely, frequencies used for some railway electrification systems were much lower (e.g. 16.7 Hz and 25 Hz) than normal utility frequencies (50

– 60 Hz) for historical reasons concerned mainly with the limitations of early electric traction motors. As such, the transformers used to step down the high over-head line voltages (e.g. 15 kV) are much heavier for the same power rating than those designed only for the higher frequencies.

Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetizing current; at lower frequency, the magnetizing current will increase.

Operation of a transformer at other than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. For example, transformers may need to be equipped with "volts per hertz" over-excitation relays to protect the transformer from overvoltage at higher than rated frequency.

One example of state-of-the-art design is those transformers used for electric multiple unit high speed trains, particularly those required to operate across the borders of countries using different standards of electrification. The position of such transformers is restricted to being hung below the passenger compartment. They have to function at different frequencies (down to 16.7 Hz) and voltages (up to 25 kV) whilst handling the enhanced power requirements needed for operating the trains at high speed.

Knowledge of natural frequencies of transformer windings is of importance for the determination of the transient response of the windings to impulse and switching surge voltages.

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Energy losses

An ideal transformer would have no energy losses, and would be 100% efficient. In practical transformers energy is dissipated in the windings, core, and surrounding structures. Larger transformers are generally more efficient, and those rated for electricity distribution usually perform better than 98%.

[39]

Experimental transformers using superconducting windings achieve efficiencies of 99.85%.

[40]

The increase in efficiency can save considerable energy, and hence money, in a large heavilyloaded transformer; the trade-off is in the additional initial and running cost of the superconducting design.

Losses in transformers (excluding associated circuitry) vary with load current, and may be expressed as "no-load" or "full-load" loss. Winding resistance dominates load losses, whereas hysteresis and eddy currents losses contribute to over 99% of the no-load loss. The no-load loss can be significant, so that even an idle transformer constitutes a drain on the electrical supply and a running cost; designing transformers for lower loss requires a larger core, good-quality silicon steel, or even amorphous steel, for the core, and thicker wire, increasing initial cost, so that there is a trade-off between initial cost and running cost. (Also see energy efficient transformer)

Transformer losses are divided into losses in the windings, termed copper loss, and those in the magnetic circuit, termed iron loss. Losses in the transformer arise from:

Winding resistance

Current flowing through the windings causes resistive heating of the conductors. At higher frequencies, skin effect and proximity effect create additional winding resistance and losses.

Hysteresis losses

Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. For a given core material, the loss is proportional to the frequency, and is a function of the peak flux density to which it is subjected.

[41]

Eddy currents

Ferromagnetic materials are also good conductors, and a core made from such a material also constitutes a single short-circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and inverse square of the material thickness.

[41]

Eddy current losses can be reduced by making the core of a stack of plates electrically insulated from each other, rather than a solid block; all transformers operating at low frequencies use laminated or similar cores.

Magnetostriction

Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and contract slightly with each cycle of the magnetic field, an effect known as magnetostriction. This produces the buzzing sound commonly associated with transformers,

[30]

and can cause losses due to frictional heating.

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Mechanical losses

In addition to magnetostriction, the alternating magnetic field causes fluctuating forces between the primary and secondary windings. These incite vibrations within nearby metalwork, adding to the buzzing noise, and consuming a small amount of power.

Stray losses

Leakage inductance is by itself largely lossless, since energy supplied to its magnetic fields is returned to the supply with the next half-cycle. However, any leakage flux that intercepts nearby conductive materials such as the transformer's support structure will give rise to eddy currents and be converted to heat.

[43]

There are also radiative losses due to the oscillating magnetic field, but these are usually sma

Dot convention

It is common in transformer schematic symbols for there to be a dot at the end of each coil within a transformer, particularly for transformers with multiple primary and secondary windings. The dots indicate the direction of each winding relative to the others. Voltages at the dot end of each winding are in phase; current flowing into the dot end of a primary coil will result in current flowing out of the dot end of a secondary coil.

Equivalent circuit

The physical limitations of the practical transformer may be brought together as an equivalent circuit model (shown below) built around an ideal lossless transformer. Power loss in the windings is current-dependent and is represented as in-series resistances R p

and R s

. Flux leakage results in a fraction of the applied voltage dropped without contributing to the mutual coupling, and thus can be modeled as reactances of each leakage inductance X p

and X s

in series with the perfectly coupled region.

Iron losses are caused mostly by hysteresis and eddy current effects in the core, and are proportional to the square of the core flux for operation at a given frequency. Since the core flux is proportional to the applied voltage, the iron loss can be represented by a resistance R

C

in parallel with the ideal transformer.

A core with finite permeability requires a magnetizing current I m

to maintain the mutual flux in the core. The magnetizing current is in phase with the flux; saturation effects cause the relationship between the two to be non-linear, but for simplicity this effect tends to be ignored in most circuit equivalents.. With a sinusoidal supply, the core flux lags the induced EMF by 90° and this effect can be modeled as a magnetizing reactance (reactance of an effective inductance) X m

in parallel with the core loss component. R c

and X m

are sometimes together termed the magnetizing branch of the model. If the secondary winding is made open-circuit, the current I

0

taken by the magnetizing branch represents the transformer's no-load current.

The secondary impedance R s

and X s

is frequently moved (or "referred") to the primary side after multiplying the components by the impedance scaling factor (N p

/N s

)

2

.

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Transformer equivalent circuit, with secondary impedances referred to the primary side

The resulting model is sometimes termed the "exact equivalent circuit", though it retains a number of approximations, such as an assumption of linearity. Analysis may be simplified by moving the magnetizing branch to the left of the primary impedance, an implicit assumption that the magnetizing current is low, and then summing primary and referred secondary impedances, resulting in so-called equivalent impedance.

The parameters of equivalent circuit of a transformer can be calculated from the results of two transformer tests: open-circuit test and short-circuit test.

Result : Study is completed

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Experiment no 14

Objective:

Range Extension using shunt

Instruments Required

Sl no

1

Instruments

Voltmeter 0-3V MC

0-300V MC

Quantity

1

1

2

3

4

Ammeter 0-10mA MC

0-1A MC

Standard Resistance 0.1Ω

Rheostat 370Ω, 1.1A

600Ω. 1A

1

1

1

1

1

1 5 Decade resistance box

Theory

Range Extension using shunt

The moving

–coil ammeter has a coil wound with very fine wire. It can carry only few milli- amperes safely to give full- scale deflection. For measuring high currents, a low resistance is connected in parallel with the instrument. The low resistance connected in parallel with the instrument is called a shunt. Figure1 shows a shunt resistance R s connected in parallel with the ammeter of resistance R m.

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Calculation of shunt resistance:

Let I = current to be measured

I m

= meter current

I s

= shunt current

To calculate the value of shunt resistance the following points should be kept in mind

1.

2.

The meter current I m should never exceed the current required to produce full scale deflection.

Since the arrangement shown in figure is simply a parallel circuit

Current to be measured = instrument current + shunt current

I = I m

+ I s

(1)

And voltage across the meter = voltage across the shunt

That is I m

R m =

I s

R s

(2)

Equations (1) and (2) are used to calculate R s

Result

Studied the range extension of moving coil and moving iron instruments and to

measure high values of current and voltage using low range meters.

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Experiment no 15

Objective: Range Extension using Multiplier

Instruments Required

Sl no

1

Instruments

Voltmeter 0-3V MC

0-300V MC

Quantity

1

1

2

3

4

Ammeter 0-10mA MC

0-1A MC

Standard Resistance 0.1Ω

Rheostat 370Ω, 1.1A

600Ω. 1A

Decade resistance box

1

1

1

1

1

1 5

Theory: Range Extension using Multiplier

For measuring high voltage, a high resistance is connected in series with the instrument to limit the current in the coil to a safe value. This value of current should never exceed the current required to produce the full- scale deflection. The high resistance connected in series with the instrument is called a multiplier. A multiplier is made of magnanin or other suitable alloy having a negligible coefficient of resistance. It therefore also serves as swamping resistor for the instrument coil.

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Calculation of multiplier

Let V = voltage to be measured, V

I = circuit current to give full scale deflection, A

I m

= meter current, A

R m

= meter resistance, Ω

R s

= resistance of multiplier, Ω

Since the same current flow through the instrument and multiplier

I = I m

By ohm‟s law,

Total circuit voltage = total circuit current X total circuit resistance

V= I m

(R m

+ R s

)

Circuit Diagram

For extension of voltage

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For extension of current

Procedure

voltage measurement

Connections are done as shown in fig. using a multimeter note voltmeter resistance. Adjust the decade box resistance in such a way that the resistance of 0-300V voltmeter and the series combination of 0-3V voltmeter and decade box equal. Then by varying the potential divider various readings are taken.

Current measurement

Connections are done as shown in figure. Keeping the rheostat at maximum position supply is switched on. By varying the rheostat different readings are taken.

Tabulation

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For extension of voltage

Sl.No

Voltmeter reading v (0-3V)

Standard voltmeter reading

(0-300V)

(R.R)

Extended voltage range (A.R)=

v

3

300

% Error

=

R

.

R

R

.

R

A

.

R

For extension of current

Sl. no

Ammeter reading I (0-

10mA)

Standard ammeter reading R.R (0-1A)

Extended current range(A.R)

=

10

I

1

10

3

% Error

=

R

.

R

R

.

R

A

.

R

Calculation

Voltmeter reading v (0-

3V) =………….V

Standard voltmeter reading (0-

300V (R.R) =………….V

Extended voltage range (A.R)=

v

300

3

% Error =

R

.

R

R

.

R

A

.

R

Ammeter reading I (0-

10mA)=…………….A

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Standard ammeter reading R.R (0-

1A) =………..A

Extended current range(A.R) =

10

I

1

10

3

% Error =

R

.

R

R

.

R

A

.

R

Result

Studied the range extension of moving coil and moving iron instruments and to

measure high values of current and voltage using low range meters.

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Experiment no-16

Object: Study of Displays

–Cold cathode displays, Fluorescent displays, Light emitting

diodes, Liquid crystal diodes, Alpha-numeric display Recorders

– Analog Recorder, Graphic

Recorder, Strip Chart Recorder

Theory:

A vacuum fluorescent display (VFD) is a display device used commonly on consumerelectronics equipment such as video cassette recorders, car radios, and microwave ovens.

Invented in Japan in 1967, the displays became common on calculators and other consumer electronics devices.

[1]

Unlike liquid crystal displays, a VFD emits a very bright light with high contrast and can support display elements of various colors. VFDs can display seven-segment numerals, multi-segment alpha-numeric characters or can be made in a dot-matrix to display different alphanumeric characters and symbols. In practice, there is little limit to the shape of the image that can be displayed: it depends solely on the shape of phosphor on the anode(s).

Hundreds of millions of units are made yearly.

[2]

Design

The device consists of a hot cathode (filaments), anodes (phosphor) and grids encased in a glass envelope under a high vacuum condition. The cathode is made up of fine tungsten wires, coated by alkaline earth metal oxides, which emit electrons when heated by an electric current. These electrons are controlled and diffused by the grids, which are made up of thin metal. If electrons impinge on the phosphor-coated plates, they fluoresce, emitting light. Unlike the orange-glowing cathodes of traditional vacuum tubes, VFD cathodes are efficient emitters at much lower temperatures, and are therefore essentially invisible.

[3]

The principle of operation is identical to that of a vacuum tube triode. Electrons can only reach

(and "illuminate") a given plate element if both the grid and the plate are at a positive potential with respect to the cathode. This allows the displays to be organized as multiplexed displays where the multiple grids and plates form a matrix, minimizing the number of signal pins required.

In the example of the VCR display shown to the right, the grids are arranged so that only one digit is illuminated at a time. All of the similar plates in all of the digits (for example, all of the lower-left plates in all of the digits) are connected in parallel. One by one, the microprocessor driving the display enables a digit by placing a positive voltage on that digit's grid and then placing a positive voltage on the appropriate plates. Electrons flow through that digit's grid and strike those plates that are at a positive potential. The microprocessor cycles through illuminating the digits in this way at a rate high enough to create the illusion of all digits glowing at once via persistence of vision. Several amateurs have experimented with the possibilities of using VFDs as triodes.

[4][5]

The extra indicators (in our example, "VCR", "Hi-Fi", "STEREO", "SAP", etc.) are arranged as if they were segments of an additional digit or two or extra segments of existing digits and are

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scanned using the same multiplexed strategy as the real digits. Some of these extra indicators may use a phosphor that emits a different colour of light, for example, orange.

Use

Besides brightness, VFDs have the advantages of being rugged, inexpensive, and easily configured to display a wide variety of customized messages, and unlike LCDs, VFDs are not limited by the response time of rearranging liquid crystals and are thus able to function normally in cold, even sub-zero, temperatures, making them ideal for outdoor devices in cold climates.

Early on, the main disadvantage of such displays was their use of significantly more power (0.2 watts) than a simple LCD. This was considered a significant drawback for battery-operated equipment like calculators, so VFDs ended up being used mainly in equipment powered by an AC supply or heavy-duty

A light-emitting diode (LED)is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting. Introduced as a practical electronic component in 1962, early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet and infrared wavelengths, with very high brightness.

When a light-emitting diode is forward biased (switched on), electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. An LED is often small in area (less than

1 mm

2

), and integrated optical components may be used to shape its radiation pattern. LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved robustness, smaller size, faster switching, and greater durability and reliability. LEDs powerful enough for room lighting are relatively expensive and require more precise current and heat management than compact fluorescent lamp sources of comparable output.

Light-emitting diodes are used in applications as diverse as replacements for aviation lighting, automotive lighting (particularly brake lamps, turn signals and indicators) as well as in traffic signals. The compact size, the possibility of narrow bandwidth, switching speed, and extreme reliability of LEDs has allowed new text and video displays and sensors to be developed, while their high switching rates are also useful in advanced communications technology. Infrared LEDs are also used in the remote control units of many commercial products including televisions, DVD players, and other domestic appliances.

A light-emitting diodeis a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting. Introduced as a practical electronic component in 1962,

[2]

early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet and infrared wavelengths, with very high brightness.

When a light-emitting diode is forward biased (switched on), electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. An LED is often small in area (less than

1 mm

2

), and integrated optical components may be used to shape its radiation pattern. LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved robustness, smaller size, faster switching, and greater durability and reliability. LEDs powerful enough for room lighting are relatively expensive and require more precise current and heat management than compact fluorescent lamp sources of comparable output.

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Light-emitting diodes are used in applications as diverse as replacements for aviation lighting, automotive lighting (particularly brake lamps, turn signals and indicators) as well as in traffic signals. The compact size, the possibility of narrow bandwidth, switching speed, and extreme reliability of LEDs has allowed new text and video displays and sensors to be developed, while their high switching rates are also useful in advanced communications technology. Infrared LEDs are also used in the remote control units of many commercial products including televisions, DVD players, and other domestic appliances.

Common circuit diagram symbols

Parts of an LED

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A liquid crystal display (LCD) is a thin, flat electronic visual display that uses the light modulating properties of liquid crystals (LCs). LCs do not emit light directly.

They are used in a wide range of applications, including computer monitors, television, instrument panels, aircraft cockpit displays, signage, etc. They are common in consumer devices such as video players, gaming devices, clocks, watches, calculators, and telephones. LCDs have displaced cathode ray tube (CRT) displays in most applications. They are usually more compact, lightweight, portable, less expensive, more reliable, and easier on the eyes.

[citation needed]

They are available in a wider range of screen sizes than CRT and plasma displays, and since they do not use phosphors, they cannot suffer image burn-in.

LCDs are more energy efficient and offer safer disposal than CRTs. Its low electrical power consumption enables it to be used in battery-powered electronic equipment. It is an electronicallymodulated optical device made up of any number of pixels filled with liquid crystals and arrayed in front of a light source (backlight) or reflector to produce images in colour or monochrome. The earliest discovery leading to the development of LCD technology, the discovery of liquid crystals, dates from 1888.

[1]

By 2008, worldwide sales of televisions with LCD screens had surpassed the sale of CRT units.

Reflective twisted nematic liquid crystal display

Strip Chart Recorder,

Strip Chart Recorders

Strip chart recorders consist of a roll or strip of paper that is passed linearly beneath one or more pens. As the signal changes the pens deflect producing the resultant chart. Strip chart recorders are well suited for recording of continuous processes. Sections of the paper can be torn off and archived for future reference. Strip chart recorders are commonly used in laboratory as well as process measurement applications.

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Strip Chart Recorders chart recorder is an instrument used to record various process and electrical signals. The most traditional chart recorders record data on paper. The paper is passed under a pen and the pen is deflected in proportion to the signal. The result is a graph or chart of the data. Chart recorders are available in single or multichannel styles (single or multipen) and in various configurations. Many of today's chart recorders can also record information in a digital format for download to a computer.v

Result: study is completed

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Experiment no 17

Objective. Digital Instruments

– Digital Voltmeter, Digital Frequency Meter, Digital Panel Meter,

Digital Storage Oscilloscope

Theory: Digital Voltmeter.

A voltmeter is an instrument used for measuring the electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to digital converter.

Voltmeters are made in a wide range of styles. Instruments permanently mounted in a panel are used to monitor generators or other fixed apparatus. Portable instruments, usually equipped to also measure current and resistance in the form of a multimeter, are standard test instruments used in electrical and electronics work. Any measurement that can be converted to a voltage can be displayed on a meter that is suitably calibrated; for example, pressure, temperature, flow or level in a chemical process plant.

General purpose analog voltmeters may have an accuracy of a few percent of full scale, and are used with voltages from a fraction of a volt to several thousand volts. Digital meters can be made with high accuracy, typically better than 1%. Specially calibrated test instruments have higher accuracies, with laboratory instruments capable of measuring to accuracies of a few parts per million. Meters using amplifiers can measure tiny voltages of microvolts or less.

Part of the problem of making an accurate voltmeter is that of calibration to check its accuracy. In laboratories, the Weston Cell is used as a standard voltage for precision work. Precision voltage references are available based on electronic circuits.

Digital Instruments

Digital Frequency Meter,

A frequency counter is an electronic instrument, or component of one, that is used for measuring frequency. Frequency is defined as the number of events of a particular sort occurring in a set period of time. Frequency counters usually measure the number of oscillations or pulses per second in a repetitive electronic signal.

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Digital Frequency Meter

Operating principle

Most frequency counters work by using a [counter] which accumulates the number of events occurring within a specific period of time. After a preset period (1 second, for example), the value in the counter is transferred to a display and the counter is reset to zero. If the event being measured repeats itself with sufficient stability and the frequency is considerably lower than that of the clock oscillator being used, the resolution of the measurement can be greatly improved by measuring the time required for an entire number of cycles, rather than counting the number of entire cycles observed for a pre-set duration (often referred to as the reciprocal technique). The internal oscillator which provides the time signals is called the timebase, and must be calibrated very accurately.

If the thing to be counted is already in electronic form, simple interfacing to the instrument is all that is required. More complex signals may need some conditioning to make them suitable for counting. Most general purpose frequency counters will include some form of amplifier, filtering and shaping circuitry at the input. DSP technology, sensitivity control and hysteresis are other techniques to improve performance. Other types of periodic events that are not inherently electronic in nature will need to be converted using some form of transducer. For example, a mechanical event could be arranged to interrupt a light beam, and the counter made to count the resulting pulses.

Frequency counters designed for radio frequencies (RF) are also common and operate on the same principles as lower frequency counters. Often, they have more range before they overflow.

For very high (microwave) frequencies, many designs use a high-speed prescaler to bring the signal frequency down to a point where normal digital circuitry can operate. The displays on such instruments take this into account so they still display the correct value. Microwave frequency counters can currently measure frequencies up to almost 100 GHz. Above these frequencies the signal to be measured is combined in a mixer with the signal from a local oscillator, producing a signal at the difference frequency, which is low enough to be measured directly.

Digital Panel Meters Digital Panel Meters are used to perform digital processing and display of voltage, current, pulse signal and others. They can also compare input values to set values, transfer data and perform other functions. OMRON Digital Panel Meters feature a high tech

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backlight LCD display that gives excellent read-out of values in dual color, thus provides intuitive feedback of value .

Digital Panel Meter

Digital Storage Oscilloscope

An oscilloscope (also known as a scope, CRO, DSO or, an O-scope) is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a twodimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time, (horizontal or 'x' axis). Although an oscilloscope displays voltage on its vertical axis, any other quantity that can be converted to a voltage can be displayed as well. In most instances, oscilloscopes show events that repeat with either no change, or change slowly.

Oscilloscopes are commonly used to observe the exact wave shape of an electrical signal. In addition to the amplitude of the signal, an oscilloscope can show distortion, the time between two events (such as pulse width, period, or rise time) and relative timing of two related signals.

[1]

Oscilloscopes are used in the sciences, medicine, engineering, and telecommunications industry.

General-purpose instruments are used for maintenance of electronic equipment and laboratory work. Special-purpose oscilloscopes may be used for such purposes as analyzing an automotive ignition system, or to display the waveform of the heartbeat as an electrocardiogram.

Originally all oscilloscopes used cathode ray tubes as their display element and linear amplifiers for signal processing, (commonly referred to as CROs) however, modern oscilloscopes have LCD or LED screens, fast analog-to-digital converters and digital signal processors. Although not as commonplace, some oscilloscopes used storage CRTs to display single events for a limited time.

Oscilloscope peripheral modules for general purpose laptop or desktop personal computers use the computer's display, allowing them to be used as test instruments

.

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Digital Storage Oscilloscope

Basic Oscilloscope

Description

Display and general external appearance

The basic oscilloscope, as shown in the illustration, is typically divided into four sections: the display, vertical controls, horizontal controls and trigger controls. The display is usually a CRT or

LCD panel which is laid out with both horizontal and vertical reference lines referred to as the graticule. In addition to the screen, most display sections are equipped with three basic controls, a focus knob, an intensity knob and a beam finder button.

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The vertical section controls the amplitude of the displayed signal. This section carries a Voltsper-Division (Volts/Div) selector knob, an AC/DC/Ground selector switch and the vertical

(primary) input for the instrument. Additionally, this section is typically equipped with the vertical beam position knob.

The horizontal section controls the time base or “sweep” of the instrument. The primary control is the Seconds-per-Division (Sec/Div) selector switch. Also included is a horizontal input for plotting dual X-Y axis signals. The horizontal beam position knob is generally located in this section.

The trigger section controls the start event of the sweep. The trigger can be set to automatically restart after each sweep or it can be configured to respond to an internal or external event. The principal controls of this section will be the source and coupling selector switches. An external trigger input (EXT Input) and level adjustment will also be included.

In addition to the basic instrument, most oscilloscopes are supplied with a probe as shown. The probe will connect to any input on the instrument and typically has a resistor of ten times the oscilloscope's input impedance. This results in a .1 (-10X) attenuation factor, but helps to isolate the capacitive load presented by the probe cable from the signal being measured. Some probes have a switch allowing the operator to bypass the resistor when appropriate.

[1]

Result: Study is completed

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