Date of conduction:- - · PDF fileDate of conduction:- ... The short circuit characteristic is a plot between armature current and field excitation with a symmetrical short ... A DC

NAME OF LABORATORY: MACHINE – 2 LAB SUBJECT CODE: EX-503 NAME OF DEPARTMENT : ELECTRICAL DEPARTMENT Date of conduction:- Date of submission:- Submitted by other members:- 1. 2.

3. 4.

5. 6.

7. 8.

Group no:- Signature Name of faculty incharge: Name of Technical Assistant:

NAME OF LABORATORY: MACHINE – 2 LAB SUBJECT CODE: EX-503 NAME OF DEPARTMENT : ELECTRICAL DEPARTMENT

LIST OF EXPERIMENT

S.No. NAME OF EXPERIMENT 1. OPEN CIRCUIT AND SHORT CIRCUIT TEST ON A THREE PHASE ALTERNATOR 2. V CURVES OF SYNCHRONOUS MOTOR

3. SPEED CONTROL OF DC SHUNT MOTOR 4. PERFORMANCE CHARACTERISTICS OF DC GENERATORS


OPEN CIRCUIT AND SHORT CIRCUIT TEST ON A THREE PHASE ALTERNATOR

AIM To conduct open circuit and short circuit tests on a three phase alternator and predetermine the regulation curve by emf method at half load and full load. APPARATUS REQUIRED Voltmeter - 0-300V, MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no. - 0-10V, PMMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .. . . . . . . . . . . . 1 no. Ammeter - 0-10A, MI . . . . . . . . . . . . . . … . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no. - 0-2A, PMMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no. - 0-5A, PMMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no. Rheostat - 300, 1.7A . . . . ... … . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no. - 1000, 1.2A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no. - 50, 5A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no. PRINCIPLE As the load on the alternator is varied the terminal voltage also varies. This is due to 1. Voltage drop due to armature resistance IR. 2. Voltage drop due to armature reactance IXL. 3. Voltage due to armature reaction effect. The voltage regulation of a synchronous generator is defined as the rise in voltage at the terminals when the load is reduced from full load rated value to zero, speed and field current remaining constant

100Re%

V

VEg

Where E - Generated emf V - Terminal voltage For small machines the regulation may be found by direct loading. For large machines the voltage regulation is predetermined by using indirect methods like emf method, mmf method, Potier and ASA methods All these methods require open circuit characteristics and short circuit characteristics. The open circuit characteristics [also called open circuit saturation curve or magnetization curve] are a plot of no load terminal voltage versus field excitation with the machine running at rated speed. Under these conditions the induced voltage is directly proportional to the flux. The shape of curve is therefore a typical B-H curve or magnetization curve. The short circuit characteristic is a plot between armature current and field excitation with a symmetrical short circuit applied across the terminals. Under these conditions current in the armature winding.


Line voltage .......................................LV

............................3 L

PHVV

.................................dcRa Effective Value .................6.1 dcRR aa From Graph ......................../ scocs IVZ

...............22 ass RZX Sample Calculation

21

220 sincos sa IXVIRVE

‘-ve’ for leading ‘+ve’ for lagging

100Re% 0

V

VEg

Wholly depends on the internal impedance consisting of synchronous reactance Xs and the winding resistance Ra. Now Ra being small compared to Xs the pf under short circuit condition is zero power factor lagging and therefore the armature reaction mmf is almost wholly demagnetizing.

NAME OF LABORATORY: MACHINE – 2 LAB SUBJECT CODE: EX-503 NAME OF DEPARTMENT : ELECTRICAL DEPARTMENT The short circuit characteristic is a straight line. From O.C.C & S.C.C the synchronous impedance is evaluated as follows. For any value of excitation or field current If , if VOC is the open circuit voltage & ISC is the short circuit current, then synchronous impedance Zs=VOC/ISC. The value of Zs is calculated for the unsaturated region. For the computation of regulation, it is convenient to take Zs at such a value of excitation which give rise to Vph [normal voltage per phase]on open circuit. The armature resistance is measured using ammeter-voltmeter method. Under working conditions the effective value of Ra is increased due to skin effect and temperature effect. The effective value of Ra is generally taken as 1.6 times the d.c value.

Synchronous reactance per phase 22aas RZX Per Phase

220 sincos sa IXVIRVE

where +ve sign for lagging power factor and -ve for leading. Now percentage regulation for each case is computed as

100Re% 0

V

VEg

PROCEDURE O.C test Connections are made as shown in the connection diagram. Switches S3 and S2 are kept in the open position. The motor field rheostat Rh1 is kept in minimum position and the alternator field rheostat Rh2 in the maximum position. Supply is switched on by closing switch S1. The dc motor is started using the 3-point starter. The motor field rheostat Rh1 is varied till the speed becomes equal to the rated speed. Switch S2 is closed. Rh2 is varied in steps and the field current and voltmeter reading are noted down. The experiment is repeated for different values of field current till the voltmeter reading shows 120% of the rated voltage of the alternator. Rheostat Rh2 is brought back to the maximum resistance position. S.C test Switch S3 is closed and rheostat Rh2 is varied till the ammeter reading in the alternator (A2) reads the rated current of the machine. The corresponding value of field current is noted down. Armature resistance is found by voltmeter-ammeter method. The regulation is then determined at various power factors for half and full loads and the regulation curve is plotted. RESULT The open circuit and short circuit test was conducted on the given 3-_ alternator and the regulation curves for half load & full load are plotted.



V CURVES OF SYNCHRONOUS MOTOR

AIM To conduct a no-load test on the given Synchronous motor and to draw its V curves. NAME PLATE DETAILS

APPARATUS REQUIRED

NAME OF LABORATORY: MACHINE – 2 LAB SUBJECT CODE: EX-503 NAME OF DEPARTMENT : ELECTRICAL DEPARTMENT PRECAUTIONS 1) There should not be any load on the motor. 2) Initially the field current should be adjusted to rated value. 3) The direction of the rotation of the rotor should be in proper direction only. 4) If Ia value is increased more than rated value, then it should be brought to rated value by adjusting the field current. 5) The I/P voltage should be kept constant throughout the experiment. 6) After completion of the experiment only 3-phase supply should be disconnected first and then DC supply. THEORY The variation of field current effects the power factor at which the synchro nous motor operates. For a syn motor, the armature current phasor is given by Ia=V-E where V is the applied voltage .From the above equation it is clear that the magnitude and phase angle of phasor Ia depends upon the value of DC excitation. When the syn. Motor is operated at constant load with variable field excitation , it is observed that: a) When the excitation is low, the armature current is lag in nature & the magnitude is comparatively high. b) If the excitation is gradually increased, the magnitude of Ia is gradually decreasing and the angle of lag is

gradually reduced. c) At one particular excitation, the magnitude of Ia corresponding to that load in minimum and vector will be in

phase with V vector. d) If the excitation is further increased, the magnitude of Ia again gradually increased and Ia,vector goes to leading

state and the angle of load is also gradually increased. PROCEDURE 1) Give all connections as per the circuit diagram. 2) Switch-ON the supply and apply the rated voltage by using D.O.L starter keeping SPST switch open. 3) Now field supply is given to the field by closing SPST switch. At this position, the rotor will be pulled into

synchronism. 4) By varying the field current If , Note down the values of armature currents. 5) Switch-Off the supply. GRAPH A graph is drawn b/w a) Exciting current (If) verses armature current (Ia) : V curve. b) Exciting current (If) verses power factor (cosΦ) : curve taking If on X-axis and Ia & cosΦ on y-axis.


OBSERVATIONS


MODEL GRAPH

RESULT


SPEED CONTROL OF DC SHUNT MOTOR

AIM To obtain speed control of DC shunt motor by a. Varying armature voltage with field current constant. b. Varying field current with armature voltage constant APPARATUS REQUIRED

PRECAUTIONS 1. Field Rheostat should be kept in the minimum resistance position at the time of starting and

stopping the motor. 2. Armature Rheostat should be kept in the maximum resistance position at the time of starting and

stopping the motor. TABULAR COLUMN

(I) ARMATURE VOLTAGE CONTROL


(II) FIELD CONTROL

PROCEDURE 1) Make the connections as per circuit dia. 2) Set up the field and armature rheostat to their maximum value. 3) Switch on the D.C. Supply start the motor with the help of starter. Adjust the field current to rated value. 4) Note the speed with the help of tachometer, the voltage across armature and the field current. 5) Change the speed by varying the rheostat in the armature circuit. Note the speed and armature voltage, the field

current remaining constant. 6) Repeat steps 4. and 5. above, for different field currents. 7) Plot speed V/s armature voltage 8) Keep the rheostat in the armature circuit to some fixed value. Note the voltage across armature. Note the field

current and speed. 9) Vary the field current. (i) Armature Control: 1. Field current is fixed to various values and for each fixed value, by varying the armature rheostat,

speed is noted for various voltages across the armature. (ii)Field Control: 1. Armature voltage is fixed to various values and for each fixed value, by adjusting the field rheostat,

speed is noted for various field currents. 2. Bringing field rheostat to minimum position and armature rheostat to maximum position DPST switch

is opened. MODEL GRAPHS

NAME OF LABORATORY: MACHINE – 2 LAB SUBJECT CODE: EX-503 NAME OF DEPARTMENT : ELECTRICAL DEPARTMENT CIRCUIT DIAGRAM

RESULT Thus the speed control of DC Shunt Motor is obtained using Armature and Field control methods.


PERFORMANCE CHARACTERISTICS OF DC GENERATORS

OBJECTIVES • To obtain the no-load magnetization characteristics. • To obtain the external characteristics of DC shunt and compound generators. EQUIPMENTS • Three digital multimeters from the stockroom. • One tachometer from the stockroom. • Rheostat with 125 Ω and 1.69 Amp rating. (Use a Built-In Rheostat) • 3 Adjustable load resistor bank. • A bench mounted motor-generator set. REFERENCES • Vincent Del Toro, Basic Electric Machines , pp. 303-320, Prentice-Hall, 1990. • A. Fitzgerald, C. Kingsley Jr., and S. Umans, Electric Machinery , pp. 390-414, 5th Edition, McGraw-Hill, 1990. 11

BACKGROUND A DC generator, whose schematic is shown in figure 3.1, is an electrical machine which converts the mechanical energy of a prime mover (e.g. DC motor, AC induction motor or a turbine) into direct electrical energy. The generator shown in figure 3.1 is self exciting. It uses the voltage aE generated by the machine to establish the field current fI , which in turn gives rise to the magnetic-field flux Φ. When the armature winding rotates in this magnetic field so as to cut the flux, the voltage aE is induced in the armature. This voltage is commonly referred to as the armature electromotive force or EMF. The induced EMF is proportional to the rate of cutting the flux and is given by

na

pZEa 60

(3.1)

NAME OF LABORATORY: MACHINE – 2 LAB SUBJECT CODE: EX-503 NAME OF DEPARTMENT : ELECTRICAL DEPARTMENT Where Φ = flux in webers (3.2) n = armature speed in rpm (3.3) Z = total number of armature conductors (3.4) p = number of poles (3.5) a = number of parallel paths (3.6)

(3.7)

The magnetic field necessary for generator action may be provided by (a) permanent magnets, (b) electromagnets receiving their exciting current from an external source, and (c) electromagnets being excited from the current obtained from the generator itself (like that shown in figure 3.1). The use of permanent magnets is confined to very small generators. The electromagnetic excitations listed in (b) and (c) above give rise to generators having somewhat different types of characteristics. In the case of a compound generator, the series and shunt fields may be connected so as to aid each other, i.e. the fluxes set up by each will add up.

Figure 3.2: Magnetization characteristic (a) and internal and external characteristics of a DC shunt generator (b). An increase in the total flux will generate a greater EMF. Such a connection is known as cumulative. If, however, the shunt and series winding are so connected that the flux setup by one opposes the other, then the induced EMF will be smaller. This type of connection is called differential. MAGNETIZATION CHARACTERISTICS The typical magnetization curve for a shunt DC generator is shown in figure 3.2a. The generated voltage Ea is related to the field winding current If . This generator generates a voltage Ea even in the absence of a current If The small voltage at zero excitation is due to residual magnetism in the pole material. Thus a self excited shunt generator is self exciting provided that an external voltage of the proper polarity is momentarily applied to the field winding to create the residual magnetic field at the time the generator is put into service for the first time. The magnetization curve rises very steeply while the magnetic circuit is unsaturated. As the magnetic circuit saturates the curve flattens out. There is a critical field resistance Rc that allows a self excited shunt generator to be self exciting. In order to buildup voltage in the generator, the total resistance in the field must be less than the critical resistance. The critical resistance Rc, for the rated speed of the machine, can be determined from the magnetization curve. To do this, a tangent line is drawn to the magnetization curve starting from the origin. The slope of the tangent line represents the critical field resistance Rc.


OBSERVABLE CHARACTERISTICS OF A SHUMT GENERATOR The voltage induced in the armature of a shunt generator is due to the armature wires cutting the magnetic field established by the field current. The induced voltage Ea , and hence the terminal voltage Vt , would be constant if other factors did not affect them. But the armature current Ia affects the terminal voltage Vt in two manners. The armature current distorts the magnetic field thus reducing the terminal voltage Vt . This effect is called armature reaction . In addition to the above there is the ohmic voltage drop Ia Ra , the product of the armature current Ia passing through the armature resistance Ra. The graph of terminal voltage Vt versus load current IL is called the “Ex-ternal Characteristic” as shown in figure 3.2b. It is directly measurable by observing the terminal voltage Vt for di_erent load currents IL . As is obvious from figure 3.1, the load current IL and the armature current Ia di_er by the field current If , which can also be measured. The armature resistance Ra is a measurable quantity. As a consequence the ohmic voltage drop Ia Ra , which is a straight line, can be added to the external characteristic for the calculation of the internal generated EMF of the machine, which is shown plotted as the internal characteristic in figure 3.2b. The drop in voltage of the internal characteristic as the load current IL (and hence armature current Ia ) is increased is due to armature reaction. MAGNETIZATION CHARACTERISTICS PROCEDURE 1. Record the name-plate data of the DC generator. 2. Connect the circuit as shown in figure 3.3. 3. Run the generator at rated speed (1730 rpm) and no load.


4. Connect the 240 volts DC source to the potentiometer to generate the field current If . 5. Record the voltage generated when if is zero. 6. Adjust the field current If (300mA scale, DC) only in an ascending direction and in approximate 20 mA increments,

then record the generated voltage Va (DC). Repeat until the generated voltage is (almost) at 220 volts. (The rated value is 240 volts.)

Note: The maximum field current is about 230 mA. If the reading goes over the desired value, do not turn the

potentiometer back as the DC generator will follow another hysteresis loop pattern. 7. After reaching the maximum voltage generated, decrease the field current If in the same manner in 20 mA

increments until 0 mA is reached. At each If , measure and record the voltage Va. Note: Again, if the reading goes under the desired value, do not turn the potentiometer backup, just record the values

of If and Va. REPORT 1. Plot the curves between the generated voltage Va and field current If both for ascending and descending currents. 2. Obtain the mean magnetization curve by using the above curve. 3. Compute the value of the critical resistance Rc. CHARACTERISTICS OF A SHUNT GENERATOR PROCEDURE 1. Measure the load rack resistor values for 8 different combinations.

Suggestions: Leave the first switch half way down and measure the resistor. Value, then all the way down and measure the resistor value. While the first switch is closed all the way, put the second switch half way down, measure the resistor value then all the way down and measure again. Do the same thing for the other switches. This way, there should be 8 resistor load values available, ranging from 500 down to about 50. Each two- stage switch represents 500 when closed half way and 250 when closed all the way.

2. Complete the circuit as shown in figure 3.4. 3. Run the generator at no load and rated speed (1730rpm). Note: If the measured voltage Vt is only 6 or 7 volts, simply reverse the connection of the field winding. The expected

voltage generated at no load should be around 220 volts. 4. Connect the loading rack to the DC shunt generator. With each load resistor value-change, record the field current If

(on 300 mA range, DC), the generated voltage Vt (DC), generator speed N and load current IL (on 10Arange, DC).

NAME OF LABORATORY: MACHINE – 2 LAB SUBJECT CODE: EX-503 NAME OF DEPARTMENT : ELECTRICAL DEPARTMENT 5. Measure the resistance of the armature winding Ra at the end of the experiment by inserting the probes a cross the

generator armature connector with all other wires disconnected.

REPORT 1. Plot the external curve of the terminal voltage Vt against load current IL. 2. On the same graph draw the voltage drop line IaRa against the load current IL. 3. Obtain the internal curve using the curves above. CHARACTERISTICS OF A COMPOUND GENERATOR PROCEDURE 1. Connect the circuit as shown in figure3.5. 2. Run the generator at no load and rated speed. Reverse the shunt field connection if the generated voltage is

substantially below the rated voltage of about 210 volts. 3. Connect the load rack to the circuit and with each load resistor value measure the motor speed N, terminal voltage

Vt (DC), load current IL (10Ascale, DC) and field current If (300mAscale,DC). 4. Reverse the series field connection and repeat the last part.

REPORT From the above data, plot the external characteristics for a compound generator. DISCUSSION QUESTION 1. Obtain the mean magnetization curve at 125% of rated speed. 2. Explain why the total resistance of the field circuit must be less than its critical resistance in a DC shunt generator. 3. Explain why an internal characteristic of a shunt generator is not a flat curve.

Date of conduction:- - · PDF fileDate of conduction:- ... The short circuit characteristic is a plot between armature current and field excitation with a symmetrical short ... A DC

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