Manuals

Basic Electrical EngineeringBE-104

S.NO. EXPERIMENT NAME DATESIGNATURE

&REMARKS

1 Measurement of power & powerfactor in a single phase AC circuitusing three Ammeter Method

2 Measurement of active & reactivepower in single phase AC circuit

3 Measurement of impedance ofR-L, R-C,R-L-C & study ofresonance phenomena

4 Study of constructional feature s ofa D.C. M/C.

5 Perform load test on a single phasetransformer

6 Study of transformer name plateRating & Determination of ratio

7 Open circuit & Short circuit teston 1- Transformer

8 Measurement of power in 3 phaseA.C. circuit by two wattmetermethod.

9 Study of three Point & Four pointstarter

MeasurementOf

Power & Power factor in a Single Phase ACCircuit using ThreeAmmeter Method

OBJECT:-

To measure power factor in a single –phase A.C. circuit using Three

Ammeters.

APPARATUS REQUIRED:-

Voltmeter 0 -300V, MI; ammeters 10A, 5A, 5A, MI; single –phase

inductive variable load, rheostat 100 ,5A; variac 230V, 10A.

THEORY:-

The circuit to be used for measurement of power in an A.C. circuit

using three ammeter is shown in fig 1. We know in a D.C. circuit

the power is given by the product of voltage and current, whereas ,in

A.C. circuit it is given by the product of voltage, current and power

factor. For this reason, it is not possible to find power in an A.C. circuit

simply from the readings of a voltmeter and ammeter. In A.C. circuits

power is normally measured by wattmeter. However, this method

demonstrates that the power in a single –phase A.C. circuits can be

measured by using three ammeters. From the circuits shown in fig 1.we

can write.

Power consumed by load =P =VI3 cos ………………… (1.1)

Where I3 is current through load and V is the voltage across load. The

phasor diagram of this circuit can be drawn by taking the supply

voltage V as the reference phasor diagram is shown in fig.2

From the phasor diagram we can write I1

2 =I22+I3

2+2I2I3Cos ………………….. (1.2)

Power factor, Cos = I1

2-I22-I3

2/2I2I3………………………….. (1.3) I2 = V/R [R is a known resistance]……………… (1.4)Now from eq. 1.1 I3Cos = P/VPut this value in eq. 1.2 I1

2= I22+I3

2+2I2I3Cos I1

2= I22+I3

2+2I2P/V

2I2 P/V= I12-I2

2-I32

or P= (I12-I2

2-I32)V/2I2

From eq. (1.4) we put the value of I2 So P = (I12-I22-I32) (V.R)/(2V) P = (I12-I22-I32) (R/2)………………….

Eqs.1.3 and 1.5 show that we can find the power and power factor in an a.c.circuits by using 3- single phase ammeters, instead of a wattmeter.

v

I3

I2

I1

PROCEDURE:-

The stepwise procedure for conducting this experiment is given below:

1. Make the connections as per the fig.

2. Keep the rheostat at its maximum value.

3. keep the variac at its mini9mum position

4. Switch on the supply.

5. Increase the voltage applied using variac slowly, so that the reading of

voltmeter and ammeter, A1 are appreciable.

6. Decrease the resistance R (rheostat) so that ammeter A2 gives suitable

reading.

7. Take down the readings of voltmeter and three ammeters.

8. Change the position of rheostat and repeat step 7 a number of times.

OBSERVATION:-

Record your observation as shown in table

Table observation and calculations for 3-Ammeters method.

Observations CalculationS.No.V Volts I1 Amp I2 Amp I3 Amp P Cos

1.2.3.4.

CALCULATION:-

For each set of observation calculate the power consumed (Eq 1.5) and the

power factor (Eq 1.3). Next take the average of all the set of calculation for

cos i.e., power factor; and P i.e., power consumed in the load.

RESULT:-

The power factor of the circuit and the power consumed in circuit should be

recorded here.

PRECAUTIONS:-

Following precautions should be taken care of while performing this

experiment.

1. All connections should be tight.

2. The zero setting of all the meters should be checked before connecting

them in the circuit.

3. The current through ammeter should never be allowed to exceed the

current rating of rheostat and load used.

MeasurementOf

Active & Reactive powerin

Single Phase AC circuit

Object:-

Measurement of active and reactive power in single phase A.C. Circuit.

Apparatus Required:-

S.No. Name of

equipments

Quantity Specification

range

Type

1. Wattmeter 01 2.5A,125V Dynamo

2. Ammeter 01 0-0.5 Amp. A.C.

3. Voltmeter 01 0-300V A.C.

4. Rheostat 01 500 ohms -

5. Inductance(choke) 01 - -

6. Auto-transformer 01 0-270V A.C.

7. Connecting wires 10-12 - -

Theory:-

Wattmeter has two coils, one is called “current coil” and other is

“pressure coil”. The current coil carries the load current and pressure coil

carries a current proportional to and in phase with supply voltage. The

deflection of wattmeter depends upon the currents in the two coil and upon

the P.F. of the circuit.

In the wattmeter the current coil are arranged for different ratings i.e.

2.5,5 Amp.etc. and similarly voltage coils are rated for 125 V, 250V,

500Vetc. While doing the experiment the proper range should be selected

according to the load voltage and current.

The reading should be multiplied by a factor called the “Multiplying

Factor”.

Multiplying factor of wattmeter = Current range * Voltage range

Full scale reading of wattmeter

Active power:-

It is the power, which is actually dissipated in the circuit

Resistance.

P = I2 R = VI COS watts

Reactive Power:-

It is the power developed in the inductive reactance ofthe circuit.

Circuit Diagram:-

Q = I2XL

From diagram:-

It is clearSin = XL/ZOr,ZSin =XLQ = I2XL = I*I*XLQ = I*I*ZSinQ = V*I*Sin

Q= V*I*Sin Volt Ampere Reactive (VAR)

Power Triangle:- VI COS

Impedance Triangle:- R XL

Z

Z = (R2) + (XL2)

VI

VISIN

Procedure:-

(1) Connect the circuit according to the circuit diagram.

(2) For different value of supply voltage takes the various observations.

(3) Take atleast 3 sets or reading.

Observation Table:-

Multiplying factor of wattmeter = ……............

S.NO.

V I Wattmeterreading

Actual powerWattmeterReading*M.F.

P.F.=Power/(V*I) ActivePower P=V*I*Cos

ReactivePower(Q)=V*I*Sin

1.2.3.45.

Calculation: -

Report: -

Precautions: -Following precautions should be taken care of while performing this

experiment.

1. All connections should be tight.

2. The zero setting of all the meters should be checked before connecting

them in the circuit.

3. The current through ammeter should never be allowed to exceed the

current rating of rheostat and load used.

4. Do not increase the current beyond the rated value of wattmeter.

5. The wattmeter should be connected properly in the circuit.

MeasurementOf

Impedance ofR-L, R-C,R-L-C & study of

resonance phenomena

Object :-

Measurement of impedance of R-L, R-C & R-L-C series circuit. Study ofResonance phenomenon.

Apparatus Required :-

S.no. Name of Equipment Quantity Range Type1 Voltmeter 2 0-15-30-75V A.C.2 Voltmeter 2 0-150-300V A.C.3 Ammeter 1 0-0.5-1A A.C.4 Resistance 1 --5 Inductance 1 --6 Capacitance 1 --7 Auto-Transformer 1 0-270V A.C.8 Connecting Wires 12-15 --

Theory :-

(A) R-L Series Circuit :-It consists of a resistance of R -ohms & inductance of L- henry connected in series.

Vs = R.M.S. Value of supply voltage

Vr = R.M.S. Value of resistance voltage drop = I R

VL = R.M.S. Value of Inductance voltage drop = I XL

I = R.M.S. Value of current

In R-L Series circuit

Vr = I R and VL = I XL

VS = (Vr2 + VL

2 ) = (I R)2 + (I XL) 2 = I (R)2 + (XL) 2

I = V / R2 + XL 2 = V / Z

Where,

Z = R2 + XL 2 is the impedance of the circuit

XL = 2 f L is the inductive reactance of the circuit

L = XL/ (2 f)

XL = VL / I

(B) Series R – C Circuit :- It consist of R – Ohm and C – Farads connected in

series with source.

Vs = R.M.S. Value of supply voltage

Vr = R.M.S. Value of resistance voltage drop = I R

VC = R.M.S. Value of capacitance voltage drop = I XC

I = R.M.S. Value of current

In R-C Series circuit

Vr = I R and VC = I XC

VS = (Vr2 + VC

2 ) = (I R)2 + (I XC) 2 = I (R)2 + (XC) 2

I = V / R2 + XC2 = V / Z

Where,

Z = R2 + XC 2 is the impedance of the circuit

XC = 1 / 2 f C is the capacitance reactance of the circuit

C = 1 / 2 f XC

XC = VC / I

(C) R-L-C Series circuit :-It consists Resistance of R –ohms, Inductance of L- henry and Capacitance of C -

Farads connected in series.

Vs = R.M.S. Value of supply voltage

Vr = R.M.S. Value of resistance voltage drop = I R

VC = R.M.S. Value of capacitance voltage drop = I XC

VL = R.M.S. Value of Inductance voltage drop = I XL

I = R.M.S. Value of current

In R - L - C Series circuit

Vr = I R VC = I XC VL = I XL

VS = Vr2 + (VL - VC)2

= (I R)2 + (I XL - I XC) 2

= I R2 + (XL - XC) 2

Where,

Z = R2 + (XL - XC) 2 is the impedance of the circuit

XL = 2 f L is the inductive reactance of the circuit

L = XL/ (2 f)

XL = VL / I

XC = 1 / 2 f C is the capacitance reactance of the circuit

C = 1 / 2 f XC

XC = VC / I

Vr = I R

R = Vr /I

Resonance: -Resonance in R-L-C series circuit:

When capacitive reactance XC is equal to the inductive reactance XL then the circuit is said

to be in resonance. The current will maximum, power factor is unity and lie in to phase

with the supply voltage.

VS = Vr2 + (VL - VC)2= (I R)2 + (I XL - I XC) 2 = I R2 + (XL

- XC) 2

At resonance , XL = XC Then I = V / R (Max)

Now, XL = WC XC= 1 / WC

WL= 1 / WC , 2 f L= 1/2 f C

Fr = 1/2 L C

Fr = Resonance Frequency

Procedure :-

(1) Connect the circuit diagram connecting R-L, R-C & R-L-C Series as shown in the

circuit diagram.

(2) The Auto-Transformer to zero position and switch on supply.

(3) Adjust the Auto-Transformer till a suitable voltage is applied.

(4) Take the reading from the voltage for VR ,VL , VC & VS respectively.

(5) Note the reading of ammeter.

(6) Repeat step (3) Varying the supply voltage and record the reading in observation

table.

Observation Table :-

(A) For R-L Series Circuit :-

Voltmeter ReadingS.No. Vr in

VoltVL inVolt

VS inVolt

AmmeterReading (A)

in Amp I

CircuitImpedance

Z = V / I

CircuitResistanceR = Vr / I

CircuitReactanceXL = VL/I

CircuitInductanceL = XL/2 f

1.2.3.4.5.

(B) For Series R – C Circuit :-

Voltmeter ReadingS.No. Vr in

VoltVC inVolt

VS inVolt

AmmeterReading (A) in

Amp I

CircuitImpedance

Z = V / I

CircuitResistanceR = Vr / I

CircuitReactanceXC= VC/I

CircuitInductanceC = 1/2 fXC

1.2.3.4.5.

(C)For R-L-C Series circuit :-

Voltmeter ReadingS.No. Vr in

VoltVC inVolt

VL inVolt

VS inVolt

AmmeterReading (A)

in Amp I

CircuitImpedance

Z = V / I

CircuitResistanceR = Vr / I

CircuitReactanc

eXC= VC/I

CircuitReactanceXL = VL/I

1.2.3.4.5.

Calculation :-Calculation the various quantities as follows :(1) R = Vr / I

(2) XL = VL/I & L = XL/ (2 f)

(3) XC= VC/I & C = 1/ 2 f XC

Result:-

(1)Resistance (R) = ------------------

(2)Capacitance Reactance = -------------------

(3)Inductive Reactance = --------------------

(4)Inductor (L) = -------------------- Henry

(5)Capacitor (C) = -------------------- F

Precaution :-

(1) Make tight connection .

(2) Do not touch any live wire.

(3) Remove parallax error while taking reading from various

instruments

Study of constructionalfeatures of a D.C. Machine

Object:-

Study of constructional features of D.C. machines.

Apparatus Required:-

D.C. Machines model.

Theory:-

A d.c. machine is an Electro-Mechanical energy conversion device .

It can convert mechanical power into d.c. electrical power and is known

as a d.c. generator. On the other hand, when it converts d.c. electrical

power into mechanical power it is known as d.c. motor.

Contructional Details:-

There are two main parts of a d.c. machine:-

(A) Field System: -

(i) Electromagnetic Poles

(ii) Yoke

(iii) Field Winding

(B) Armature: -

(i) Armature Core

(ii) Armature Winding

(iii) Commutator

(1) Magnetic Frame or Yoke :-

The outer cylindrical frame to which main poles and inter poles are

fixed and by means of which the machine is fixed to the foundation is

called the Yoke. It serves two purposes:

(i) It provides mechanical protection to the inner parts of the

machine.

(ii) It provides a low reluctance path for the magnetic flux.

The yoke is made of cast iron for smaller machines and larger

machines; it is made up of cast steel.

(2) Pole core and Pole shoes:-

The pole core and pole shoes are fixed to the magnetic frame or yoke

by bolts. They serve the following purpose:

(i) They support the field or exciting coils.

(ii) They spread out the magnetic flux over the armature

periphery more uniformly.

(iii) Since pole shoes have large X-section , the reluctance of

magnetic path is reduced.

Usually, the pole core and pole shoes are made of thin cast steel.

(3) Field or Exciting coils:-

Anamelled copper wire is used for the construction of field or exciting

coils. The coils are wound on the former and then placed around the

pole core. When direct current is passed through the field winding, it

magnetizes the poles which produce the require flux. The field coils of

all the poles are connected in series in such a way that when current

flows through them, the adjacent poles attain opposite polarity.

(4) Armature core:-

It is cylindrical in shape and keyed to the rotating shaft. At the outer

periphery slots are cut, which accommodate the armature winding. The

armature core serves the following purpose:

(i) It houses the conductors in the slots.

(ii) It provides an easy path for magnetic flux.

Since armature is a rotating part of the machine, reversal of flux takes

place in the core, hence hysterisis losses are produced . To minimize

these losses silicon steel material is used for its construction. The

rotating armature cuts across the magnetic field which induces an e.m.f.

in it. The e.m.f circulates eddy currents which results in eddy current

losses in it. To reduce these losses armature core is laminated , in other

word we can say that about 0.3 to 0.5 mm thick stampings are used for

its construction. Each lamination or stamping is insulated from the

outer by varnish layer.

(5) Armature Winding:- The insulated conductors housed in the armature

slots are suitably connected. This is known as armature winding. The

armature winding is the heart of d.c. machine. It is a place where

conversion of power takes place i.e. in case of generator, mechanical

power is converted into electrical power and in case of motor, electrical

power is converted into mechanical power. On the basis of connections,

there are two types of armature winding names as:-

(a) Lap Winding (b) Wave Winding.

(5) Commutator:-

It is the most important part of d.c. machine and serves the following

purposes:-

(i) It connects the rotating armature conductors to the stationary

external circuit through brushes.

(ii) It convert the alternating current induced in the armature

conductor into unidirectional current in the external load circuit

in generator action whereas, it converts the alternating torque

into unidirectional torque produced in the armature motor action.

The commutator is of cylindrical shape and is made up of wedge-

shaped hard drawn copper segments. The segments are insulated from

each other by a thin sheet of mice. The segments are held together by

means of 2 V-shaped rings that fit into the V-grooves cut into the

segments. Each armature coil is connected to the commutator segment

through riser.

(6) Brushes:-

The brushes are pressed upon the commutator and from the

connecting link between the armature winding and the external

circuit. They are usually made of high grade carbon because carbon is

conducting material and the same time in powdered form provides

lubricating effect on the commutator surface. The brushes are held in

particular position around the commutator by brush holders.

(7) End housings:-

End housings are attached to the ends of the main frame and support

bearings. The front housing supports the bearing and the brush

assemblies whereas the rear housing usually supports the bearing

only.

(8) Bearings:-

The ball or roller bearings are fitted in the end housings. The function

of the bearings is to reduce friction between the rotating and

stationary parts of the machine. Mostly high carbon steel is used for

the construction of bearings as it is very hard material.

(9) Shaft :-

The shaft is made of mild steel with a maximum breaking strength.

The shaft is used to transfer mechanical power from or to the

machine. The rotating parts like armature core, commutator, cooling

fan etc. are keyed to the shaft.

Perform load testOn

a single phase transformer

Object: -

To perform load test on a single-phase transformer & to determine the

following:

(a)- Efficiency at different loads & to plot graph between efficiency Vs

load currents.

(b)- Regulation of the transformer & to plot graph between regulation

Vs load currents.

Apparatus Required:-

S.No. Name of equipments Range Quantity Type

1. voltmeter 0-150V 1 A.C.

2. Ammeter 0-10 A 1 A.C.

3. Wattmeter 250V,2.5A 1 A.C.

4. Lamp bank load 250V,1Kw 1 -

5. 1- Transformer 230\115V 1 -

6. 1- Variac 0-260V 1 -

Theory: -

Wattmeter has two coils ,one is called “current coil” & other is

called as “pressure coil”. The current coil carries the load current & the

pressure coil carries a current proportional to and in phase with supply

voltage. The deflection of wattmeter depends upon the currents in the

two coils and upon the P.F. of circuit.

In the wattmeter the current coils are arranged for different ratings i.e.

2.5,5 Amp etc. and similarly voltage coils are rated for 125V, 250V,

and 500V etc. While doing the experiment the proper range should be

selected according to the load voltage & current.

The reading should be multiplied by a factor called the “Multiplying

factor”.

Multiplying factor of wattmeter = Current range * Voltage range

Full scale reading of wattmeter

Performance of the transformer can be determined as follows from the

observation of load test-

“Efficiency of the transformer can be determined as ratio of the power

output to the power input”.

Let power input to the transformer = W1

Power output to the transformer = VI

Thus the efficiency of particular load = VI\W1 *100 %

Efficiency of transformer will be maximum if Iron losses = Copper

losses.

Regulation of transformer determined as “ The change in secondary

terminal voltage from no load to full load with respect to no load

voltage is called voltage regulation of the transformer”.

Let E2 = Secondary terminal voltage at no load (Bulb off)

V2 = Secondary terminal voltage at full load

Then Voltage regulation = (E2-V2) \ E2 * 100

Circuit Diagram:-

Procedure:-

1. Connect the diagram as shown in fig.

2. Ensure that there is no load on the secondary winding of the

transformer.

3. Switch on the A.C. supply & record no load voltage across the

secondary winding.

4. Adjust approximately 10% of full load current in the secondary

by switching on certain lamp bank load. Record the reading of

the entire meter.

5. Reduce the load on the transformer by switching off the bulbs in

the lamp bank load.

6. Switch off the A.C. supply.

Observation table:-

Multiplying factor: - _______________

S.No. Input

V1

V2 E2 I2 V2I2 % %Regulation Load

1.

2.

3.

4.

5.

Calculation:-

Calculate the efficiency using the formula

= (Output power \ Input power) *100 = (V2I2\W1)*100 %

The Voltage Regulation

%Voltage Regulation = (E2-V2)\E2 *100 %

Report:-

The efficiency of the transformer on full load =

The regulation of the transformer =

Precaution:-

1)-Connection should be tight.

2)-Load on the transformer should nit increase beyond its capacity.

Study of transformer name plate

Rating &

Determination of ratio

OBECT:1. Study & construction of single phase transformer.

2. Name plate rating of single phase transformer

3. Determination of transformation ration.

APPARATUS REQUIRED:

S.NO. Name of equipment Quantity Range Type1. 1- Transformer 1 230/115V,1KVA Shell type2. Auto Transformer 1 0-270,10A Variac type3. Voltmeter 1 0-300 V Moving iron4. Voltmeter 1 0-150 V Moving iron

THEORY:

Study & construction of single phase transformer: The main elements of a

transformer are two copper coils & laminated silicon steel core.A transformer is a

static device or a machine that transforms electrical energy from one circuit to

another electrical circuit through the medium magnetic flux. And without a change

in frequency. The electrical circuit which receive energy from the supply mains is

called primary winding and the other circuit which ,which delivers electrical energy

to the load ,is called secondary winding .Theoretically it may seem that

transformers may be built to handle any voltage or current. But in reality there are

limits to both the voltage & current.

The name plate rating of a power transformer : The name plate rating of a

power transformer usually contains

Volt –ampere rating of transformer in KVA ……….

Voltage ratio or turn ratio in V1/V2 ……....

Frequency of 1- or 3- ……….

Equivalent impedance of a transformers in % ……….

A typical name plate of a 1- transformer is as follows:

230 Volts/115Volts, 50 Hz, 1KVA, Shell type,10 Amp.

Here 1 KVA is the rated output at output terminals.230/115means when 230V. is

the applied to the primary ,the secondary voltage on full load at specified power

factor is 115volts.The ratio of V1& V2 is not exactly equal to N1/N2,because of

voltage drop in primary & secondary. Rated primary & secondary current can be

calculated from the rated KVA and corresponding rated voltage thus

Rated (Full load ) primary current = KVA /V1 = 1000/230 = 4.35 Amps

Rated (Full load ) secondary current = KVA /V2 = 1000/115 = 4.35 Amps

Rated frequency is the frequency for which the transformer is designed to operate.

TRANSFORMATION RATIO:

The turn ratio of the single phase transformer can be found by measuring the

primary & secondary voltage. Let V1 &V2 is the primary and secondary voltage at

on load.

1/K = V1/ V2 = N1/N2 = I2/I1 = Turn Ratio

Induced E.M.F. in primary winding, E1 =4.44f N1 Volts

Induced E.M.F. in secondary winding, E2 =4.44f N2 Volts

For ideal transformer E1 = V1 and E2 = V2

Hence, Transformation Ratio K = V2/ V1 = N2/ N1 = I1/ I2

PROCEDURE:

1. Connect the circuit as per figure & set up auto transformer to zero position.

2. Switch on A .C. supply and adjust the Auto transformer till a suitable voltage.

3. Record voltage, V1 across the primary and V2 across the secondary winding.

4. Vary the Auto transformer and repeat above step,take at least 3 readings.

5. switch off the supply.

OBSERVATION:

S.NO. Primary Voltage V1 Secondary Voltage V2 K = V2/ V1

1.2.3.

CIRCUIT DIAGRAM:

RESULT:

The transformation ratio of given transformer is ……..

PRECAUTION:1. Connection should be tight.2. Do not touch on live wire.3. Load on the transfer should not increase beyond its capacity.

Open circuit

&

Short circuit test on 1-

Transformer

OBECT:

1. To calculate the complete parameter of the equipment of 1- transformer.

2. To determine iron & copper losses.

3. To calculate efficiency & voltage regulation at 1/4, 1/3, 1/2, 3/4 full load

and 1.25times full load at 0.8 P.F. lagging.

4. To plot the efficiency curve vs load.

APPARATUS REQUIRED:

S.NO. Name of equipment Quantity Range Type1. 1- Transformer 1 230/115V,1KVA Shell type2. Auto Transformer 1 0-270,10A Variac type3. Voltmeter 1 0-150 V Moving iron4. Ammeter 1 0-0.5A Moving iron5. Ammeter 1 0-0.10A Moving iron6. Wattmeter 1 2.5/0.5A,125/250/500V Dynamometer7. Connecting leads 10-12 ……. …….

THEORY:

These two test on transformer help to determine-

1. The parameters of equipments circuit of 1- transformer.

2. The voltage regulation of 1- transformer.

3. The efficiency of 1- transformer.

OPEN CIRCUIT TEST OR NO LOAD TEST:

In this test voltmeter, Ammeter & Wattmeter are connected on low voltage side of

transformer.The high voltage is left open circuited.The rated voltage applied to the

primary.The ammeter reads no load current, or the exciting I0.Since I0 is quite small

(2 to 6% of rated current) the primary leakage impedance drop is almost negligible

and for all practical purpose the applied voltage V1,is equal to induced E.M.F

V1.The input power (iron loss) is given by wattmeter reading,consist of core loss

and ohmic loss.Since the exciting current is very small, the ohmic losses during

open circuit test is negligible as compared to normal core loss.

CIRCUIT DIAGRAM: -

CALCULATION:

Applied rated voltage on low voltage side = V1

Exciting Current or no load current = I0

Wattmeter reading, Wo/ Iron loss, PC = VOIOCos o

No load power factor, Cos o = PC/ VOIO

Working component , IW = IO Cos o

Magnetizing component, I = IO Sin o

Core loss Resistance, RC = PcI2W = V1 /IW =V1/I0 Cos 0

Magnetising reactance, X = V1 /I = V1 /Io Sin o

Thus open circuit test gives the following information:

1. Core loss at rated voltage & frequency.

2. The shunt branch parameter of equivalent circuit i.e., X & RC.

SHORT CIRCUIT TEST:

The low voltage side of the transformer of the transformer is short circuited &

instrument are placed on H.V. side. Apply the low voltage on H.V. side & with the

help of autotransformer go on increasing the applied voltage till the rated current

starts flowing in the short circuited winding(L.V. side).The primary voltage 10% to

12% of its rated value is sufficient to circulate the rated current in short circuited

winding. Since the core flux induces the voltage, which is 1% to 6% of its rated

value hence core loss can be neglected. The wattmeter records only the ohmic loss

is both, the primary & secondary winding.

CALCULATION:

Vsc, Isc & Psc are the voltmeter ammeter & wattmeter reading

ZSC = VSC /ISC

RSC = PSC/I2SC

XSC = Z2SC – R2

SC

Thus the short circuit test gives the following information

1. Ohmic loss at rated current and frequency.

2. Equivalent resistance and leakage reactance and leakage impedance.

Load x P.F.

The efficiency at any load, = X 100 %

Load x P.F.+ Wo+ Io2Ro

PROCEDURE FOE OPEN CIRCUIT TEST:

1. Connect the circuit diagram as shown in figure and set up the autotransformer at

zero position .

2. Adjust the supply voltage with the help of autotransformer to 230 volts with

secondary winding terminal open.

3. Record the ammeter, voltmeter ,wattmeter reading.

4. Vary the supply voltage with the help of the auto transformer and enter the reading

in observation table.

OBSERVATION TABLE FOR OPEN CIRCUIT TEST

S.NO. Primary VoltageVoltmeter Reading

Input CurrentAmmeter Reading

Input power in wattsWattmeter reading

1.2.3.

PROCEDURE FOE SHORT CIRCUIT TEST:

1. Connect the circuit diagram as shown in figure and set up the autotransformer at zero

position .

2. Adjust the supply voltage with the help of autotransformer (keep in mind that 10-12%

of rated voltage is sufficiency) with secondary winding terminal short circuited and

circulate full rated current in short circuited winding.

3. Record the ammeter, voltmeter ,wattmeter reading.

4. Vary the supply voltage with the help of the auto transformer and enter the reading in

observation table.

5. Three readings adjust at 50% ,86.6% & 100% rated full load current.

OBSERVATION TABLE FOR SHORT CIRCUIT TEST

S.NO. Primary VoltageVoltmeter Reading

Input CurrentAmmeter Reading

Input power in wattsWattmeter reading

1.2.3.

RESULT:

PRECAUTION:

1. In open circuit test, the H.V. side should be open circuited(left side).

2. In open circuit test, low voltage should be apply to the H.V. side & it should be

increased gradually to circulate the rated current in H.V. side.

3. Connection should be tight.

4. Do not touch on livewire.

Measurement of power

in 3 phase

A.C. circuit by two wattmetersmethod.

OBJECT:

1. To Measure the active reactive power in 3 circuit.

2. To Measure the power factor.

APPARATUS REQUIRED:

1. 3-phase Auto transformer 20 Amp. 440v 50 Hz.

2. Wattmeter dynamometer type 2 No. 250v, 5A

3. Ammeter moving Iron type :1 no(10A)

4. Voltmeter Moving Iron type 1 No.(600V)

5. 3 Load or 3 induction motor (415V, 5H.P.)

6. connective leads.

THEORY:

Two wattmeter method can be employed to measure power in a 3- phase,3 wire star

or delta connected balance or unbalanced load. In this method, the current coils of

the wattmeters are connected in any two lines say R and Y and potential coil of

each wattmeters is joined across the same line and third line i.e. B.

Then the sum of the power measured by two wattmeters W1 and W2 is equal to the

power absorbed By the 3 load

Total power P = 3VLILCOS = (W1+W2 watts)* M.F.

Power factor COS = (W1+W2) *M.F.

3 VLIL

= P/ 3 VLIL

And reactive power of load= Q= 3(W1+W2)* M.F.

CIRCUIT DIAGRAM:

PROCEDURE:

1. Connect the Voltmeter, Ammeter and Wattcmeters to the load

through 3 Autotransformer as shown fig and set up the

Autotransformer to Zero position.

2. Switch on the 3 A.C. supply and adjust the autotransformer till a

suitable voltage. Note down the readings of wattcmeters,

voltmeter& ammeter

3. Vary the voltage by Autotransformer and note down the Various

readings.

4. Now after the observation switch off and disconnect all the

Equipment or remove the lead wire.

OBSERVATION TABLE

Multiplying factor of the wattmeter is…………….

Wattmeter Readingin watt

S.NO. VoltmeterReadingsV in volts

AmmeterReadingsI in Amp.

W1 W2

Total powerP=(W1+W2)* M.F.

Total reactivepowerP =

3(W1+W2)*M.F,

Power factorCos =

(W1+W2)M.F3 VLIL

12345

CALCULATION:

Total power = (W1+W2) *Multiplying factor

tan = 3 (W2-W1)

W1+W2

= tan-1 3 (W2-W1)

W1+W2

Power Factor = cos = ( W1+W2) M.F.

3 VLIL

Reactive power = 3(W1-W2)* M.F. , IR =IY = IB for Balance Load

RESULT:

The power measured in the circuit and there corresponding power factors in

observation table.

PRECAUTION & SOURCES OF ERROR:

1. Proper currents and voltage range must be selected before putting the

instruments in the circuit.

2. If any Wattmeter reads backward, reverse its pressure coil connection and

the reading as negative.

3. As the supply voltage Fluctuates it is not possible to observe the readings

correctly.

Study ofThree Point

&Four point starter

OBJECT:

Study of 3 point and 4 point starter.

THEORY:

When a motor is stationary, there is no back E.M.F. and the armature behaves as a

low resistance circuit. If the motor is switched on across the supply, it draws a

heavy current from the mains. This may result

1. In damage to the armature winding and insulation due to overheating.

2. In detrimental sparking at commutator.

3. In large dips in the supply voltage.

4. In high starting torque and a very rapid acceleration with possibility of

damage to the rotating parts of the D.C. machine and thus load connected

to the shaft.

To limit the starting current, a starter is used which reduces the voltage

applied to the armature during the starting period.

THREE-POINT STARTER:

The wiring diagram of a three-point starter is shown. The starting resistance is

connected between the contact studs 2& 7, the various tapings on the resistance

being connected to intermediate studs. The connection to the starter resistance is

made through a movable contact fitted to a handle H capable of sliding over the

studs. The H is fitted with a spring, which tends to restore the handle to stud 1 i.e.

off position. The field of the motor is supplied through the starter terminal marked

F as shown in the diagram. One of the supply lines is connected to the L- terminals

of the starter and the other to the armature and field of the motor. As soon as the

handle is pulled to stud 2 against its spring tension, the armature as well as the field

of the motor gets the electrical supply and armature starts rotating. At this stage the

total starting resistance between studs 2& 7 is in the armature circuit. When the

motor has picked up speed, the handle is moved to stud 3, cutting off the resistance

between studs 2&3. The contact between the handle and the stud is such that the

electrical connection does not break in shifting it from one step to another. Slowly

the handle is moved to stud 7 where it is held in position by the magnetic force of

attraction, a soft iron keeper . The force of attraction is provided by an

electromagnet having a winding placed in series with the field circuit. The

electromagnet is called the holding magnet or no volt release or sometimes low

voltage release and performs

Following functions:-

(1) In the event of power failure, the electromagnet NC is no longer energized

and the spring tension pulls back the handle to ‘off’ position. If during

supply interruptions the starter handle fails to return to ‘off’ position, the

motor will be damaged when the power is restored as these would be no

additional resistance in the armature circuit. Thus the whole purpose of the

starter will be defected. The holding coil in conjunction with the spring

takes care of this hazard automatically.

(2) If accidentally the shunt field circuit opens, the starter handle is released

and it comes to ‘off’ position providing a protection against the tendency of

motor to run away in such a case.

(3) This arrangement prevents the possibility of the starter handle being left in

advertently on any one of the intermediate steps. This is the starting

resistance cannot be left in the circuit permanently.

The total current drawn by the motor passed through the winding of a small

electromagnet OC known as overload release. This magnet in the event of overload

attracts a soft iron armature P that in turn short circuits the terminals X and Y of the

no volt coil through contact ‘C’. The distance between the magnet and P is usually

adjustable and is so adjusted that P gets attracted only when current drawn by the

motor also flowing through the overload release coil exceeds a preset value.

Therefore when the motor is overloaded, this electromagnetic short circuit the

holding coil terminals and the starter handle swings back to “off” position,

automatically switching off the motor. Since it prevents continuous operation of the

motor on overload, it is known as overload release.

FOUR POINT STARTER:

In a three point starter, the field current of the motor flows through the holding

coils. When the resistance is inserted in a field circuit to increase the speed beyond

a certain limit, the holding coil current may reduce to such an extend that the

electromagnetic pull of the holding coil is no longer sufficient to overcome the

handle spring tension. In such a case, the starter is not suitable for wide range of

speed control by field weakening method is desired. For such application a four-

point starter is used.

Therefore 4 point starter has a holding coil connected across the d.c. supply in

series with the starting resistance instead of being connected in series with the field

winding.

As in a 3- point starter the holding coil functions as no volt release. An overload

release is also provided in the 4 point starter.

Occasionally a 4 point starter is equipped with additional field weakening resistors

for speed control.