1.Electrical Instrumentation lab manual for diploma electrical engineering 3rd sem as per GTU

PRACTICAL 1

AIM: - TEST THE MEDIUM RESISTANCE USING WHEATSTONE   BRIDGE.

v Specific Objective:-

        Student will be able to find out resistance of the above bridge.

v Apparatus:-

1.      Adtran’s trainer kit

2.      C.R.O

3.      Patch cords

4.      Digital multimeter

v Theory:-

    A very important device used in the measurement of medium resistances is the Wheatstone bridge. A whetstone bridge has been in use longer than almost any electrical measuring instrument. It is still an accurate and reliable instrument and is extensively used in industry. The Wheatstone bridge is an instrument for making comparison measurements and operates upon a null calibration principle. This means the indication is independent of calibration of the null indicating instrument or any of its characteristics. For this reason, very high degrees of accuracy can be achieved using Wheatstone bridge. Accuracy of 0.1% is quite common with a Wheatstone bridge as opposed to accuracies of 3% to 5% with an ordinary ohmmeter for measurement of medium resistances.

     Fig shows the basic circuit of a Wheatstone bridge/ it has four resistive arms, consisting of resistances P,Q,R, and S together with a source of emf (a  battery ) and a null detector, usually a galvanometer G or other sensitive current meter. The current through the galvanometer depends on the potential difference between points c and d. the bridge is said to be balanced when there is no 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 ‘b’; or, by referring to the other battery terminal, when the voltage from point ‘d’ to point ‘c’ equals the voltage from point ‘b’ to point ‘c’.

                 For bridge balance, we can write:

                                          I₁P = I₂R…………………………………………. (1)

 For the galvanometer current to be zero, the following conditions also exist:

                                          I₁ = I₃ =E/(P+Q)

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

Where

        Combining Eqns. (1), (2), and (3) and simplifying, we obtain:

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

                                        QR = PS …………………………………………   (5)

Eqn.(5) is the well known expression for the balance of Wheatstone bridge. If three of the resistances are known, the fourth may be determined fro eqn. (5) and we obtain:

                               R = S P/Q…………………………... (6)

 Where R is the unknown resistance S is called the ‘standard arm’ of the bridge and P and Q are called the ‘ratio arm’.

v Procedure :-

1.      Connect the required supply and switch ON the unit. See that the supply LED glows.

2.      Observed the sine wave at the respective terminals.

3.      Now connects the C.R.O between the ground and the terminals marked ‘detector’.

4.      Connect the unknown resistance to the terminal marked S.

5.      Select one multiplier arm by connecting link.

6.      Vary P for minimum position.

7.      Similarly vary R  For minimum position.

8.      If the selection of the multiplier is correct the balance point can be observed on the C.R.O.

9.      Substitution the same in the formula and the value of S.

v Observation Table:-

SR.NO	P	Q	S (Multiplier arm)	R = S P/Q

v Calculation :-

v Conclusion :-

FOR CIRCUIT DIAGRAM......

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1.practical for Energy Conservation Technique for Diploma electrical engineering.

AIM: STUDY OF POWER FACTOR IMPROVEMENT.

Objective: after studying this experiment, it will be possible to;

            (1) Identify the disadvantage of low power factor.

            (2) Identify the causes of low power factor

            (3) Mechanism of power factor correction improvement

Significance:-Machines and devices with inductive property forms the considerable part of energy considerable part of energy consumption system. Due to the inherent property of inductor the current always lags behinds voltage with reference to time and thus the total amount of electrical energy produced is divided into two sectionsL1) Real power : the main function desired from the device (2) Reactive Power: the power necessary for the functioning of the device. The ratio of such division is governed by the important parameter named as power factor and is a versatile master key for unlocking the many of our power systems problems.

Theory:

            Technically, the power factor is defined as cosine of angle between voltage vector and current vector of a.c. circuits. It is represented diagrammatically as shown in figure-1. It can also be derived from other relations as shown in figure-2 and figure-3.

Disadvantages of low power factor: As mentioned previously, majority of inductive devices present in the system results into low power factoe which leads to one or more of the following disadvantages:

1.      Large KVA rating of equipment

2.      Greater conductor size in transmission, distribution and consumer systems.

3.      Large copper loss

4.      Poor voltage regulation

5.      Reduction handling capacity of the system.

Thus low power factor is an objectionable feature in the supply system as well as form economical point of view.

Causes of low power factor:  Normally the power factor of the load on the supply system is lower than 0.8 lag. Following are the causes of low power factor.

1.      Induction motor works at a power factor which is extremely small on light load (0.2 to 0.3) and rises to 0.8 to 0.9 at full load.

2.      Arc lamps, electric discharge lamps and industrial heat furnaces operate at low lagging power factor.

3.      The load on the power system is varying: being high during morning and evening and low at other time. During low load periods, supply voltage is increased which increases the magnetization current. This results in the decreased power factor.

Power factor improvement:  So, to eliminate the causes and improve the power factor, we desperately require to consider the property of the elements which is compensative in nature. Capacitor is one of each element in which the current leads the applied voltage and this property can be utilized to improve the power factor. The same thing is illustrated diagrammatically in figure-4.

            As shown in the figure, due to the capacitive effect the charging current flows ahead of voltage vector and resultant current drawn from supply is brought to II and from ф₁ to ф_2. Thus improving the power factor.

Elements utilized in power factor improvement: The following equipment can be utilized for power factor improvement.

1.      Static capacitors

2.      Synchronous condensers

3.      Phase advancers

Calculation of power factor correction: With the of power factor triangle, the power factor improvement can be calculated in terms of leading KVAR supplied by power factor correction improvement.

As shown in figure-5,

KVAR₁=KVAR₂=KVAR_c

Therefore,

KVAR_c = KVAR₁ + KVAR₂  

Where KVAR_c = Leading KVAR supplied by power factor correction improvement. If we divide the main triangle into two subordinate right angle triangles, we can have the following relationship of KVAR from the basic rules:

1.      KVAR₁/KW = tan ф₁ i.e. KVAR₁ = KW tanф₁

2.      KVAR₂/KW = tanф₂ i.e. KVAR₂ = KW tanф₂  

Utilizing these results for equation (a) we get,

KVAR_c  = KW tanф₁ - KW tanф₂

Therefore,

KVAR_c = KW(tanф₁ -  tanф₂ )

This is an important relation to find out the leading KVAR_c supplied by power factor correction improvement.

As power factor improvement is intended for reducing maximum demand and also decreasing the tariff rate on KWH, it also incurs the capital the cost of p.f. improving equipment in terms of rate interest and depreciation. The net saving is also affected by all such factors. Therefore,

The value to which the power factor should be improved so as to have maximum net annual saving is known as the MOST ECONOMICAL POWER FACTOR.

Importance of power factor improvement:

It is desired below for both consumers and generating stations.

(i)    Consumers: A consumer has to pay less electricity charges for his maximum demand in KVA plus the units consumed.

(ii)  Generating Stations: Number of units supplied by it depends upon the power factor. Greater the p.f. of generating stations, higher the KWH it delivers to the system. This is according to the formula, KW = KVAcosф

Suppose if we consider the following parameters for power factor improvement, P- Peak load in KW taken by consumer at p.f. cosф, rate is x Rs. Per KVA maximum demand per annum, cosф₂ is improved p.f. due to p.f. improving equipment, y Rs. Is expenditure incurred on p.f. improving equipment per KVAR per annum then,

Most economical p.f.

 cosф₂ = square root of (1-(y/x)2).

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2.practical for Energy Conservation Technique for Diploma electrical engineering.

PRACTICAL -2

AIM: STUDY OF ENERGY EFFICIENT MOTORS.

 OBJECTIVES: after studying this experiment, it will be possible to;

(I)   Identify various electrical, mechanical, magnetic & design parameters responsible for energy efficient motors.

(II) Categorization of various losses occurring during the operation of motors.

(III) Identify various measures to improve the efficiency.

(IV) Advantage of energy efficient motors.

SIGNIFICANCE: Statistical data shows that the electrical motors consume more than half (50%) of the total electricity produced and (2) more than 75% of the electrical consumption in industry. Further it is major equipment responsible for low power factor in the system. So if various parameters and design factors are considered in appropriate conditions, the motor performance improves which result into saving of electrical and mechanical energy. This is the concept of energy efficient motors.

THEORY:

Electrical motor is a device which converts the electrical energy into mechanical energy. This conversion when considered in relative aspects defines the efficiency as follows:

Efficiency η = mechanical input/electrical input

                     = output/input

                     = output/(output + losses)

                     = (input – losses)/input

As there are various types of motors

(1)   Operating on a.c./d.c. supply

(2)   Working on 1-phase/3-phase

(3)   Working on the principle of dynamically induced e.m.f., rotating magnetic field

(4)   Magnetic locking, reluctance, hysteresis & some other.

There are various types of motors

(1)   Stator/field system

(2)   Rotor/armature

As a result of interaction of electric and magnetic energy within this part mechanical energy produced which is utilized for various purpose. As the mechanical energy is not directly converted from electrical energy but takes the path through magnetic energy, the various losses occurring between as well as in electrical and mechanical form. The losses are recognized as under:

(1)   Rotor copper losses

(2)   Stator copper losses

(3)   Core losses(i) Hysteresis losses (ii) eddy current losses

(4)   Windage and friction losses

(5)   Stray load losses-which is partly electrical and mechanical

Further, the motor is designed considering one of the following parameters which also affect the motors performance considerably.

(1)   Voltage

(2)   Frequency

(3)   Voltage unbalance

(4)   Load

(5)   Output

(6)   Speed

(7)   Sleep

When we consider the duty of the motor it classifies it into three classes.

(1)   Continuous duty

(2)   Intermittent duty

(3)   Short duty

Thus the performance of motor depends on all such instant mixture of selected parameters set which defines its losses and ultimately the efficiencu of energy conversion. This is the base for developing energy efficient motors. When we develop the energy efficient motors, we also consider the factors such as:

CHARACTERISTICS:

(1)   Starting characteristics

(2)   Running characteristics

(3)   No load characteristics

(4)   Torque/speed characteristics

(5)   Torque/slip characteristics

The following design parameters are also considered.

DESIGN PARAMETERS:

1.      Physical parameters: Size of machine, length & diameter of machine, weight of machine, cross sectional area of conductors, width of slots. Resistivity, conductivity of materials, air gap etc.

2.      Electrical parameters: Voltage rating, frequency, current density, di-electric strength of materials, resistance and inductance of material, various capacitive effects, etc.

3.      Magnetic parameters: Flux density, mmf, magnetic saturation limit, permeability of material, armature reaction etc.

4.      Thermal parameters: Cooling path, thermal insulation, air flow etc.

5.      Mechanical parameters: Speed, torque, slip, etc.

Thus all such conditions are necessary for developing energy efficient motors, when considered appropriately. It thus results into the measures of improving the efficiency of motors.

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