mc lab cycle 2

8/11/2019 mc lab cycle 2

1/17

Experiment-7

NO LOAD AND BLOCKED ROTOR TEST ON 3 SQUIRREL CAGE

INDUCTION MOTOR

AIM

1. To conduct no load and blocked rotor test on 3 squirrel cage induction motor and hence

determine the equivalent circuit.2. To predetermine the performance characteristics.

3. To predetermine the line current, power factor, torque, mechanical power developed and efficiency

for a specified slip.4. To predetermine the maximum output power, maximum input power, maximum torque and

maximum power factor from the circle diagram.

5. To predetermine the torque-slip characteristics.

THEORY

The no load test is performed todetermine the no load current Io, no load

power factor Coso, winding andfrictional losses, no load core loss, no load

input and no load resistance Ro andreactance Xo. Since the motor is running

at no load the power factor would be less

than 0.5. Hence total input will be equal to

difference of the two readings of thewattmeter. At no load power, input is

equal to the core loss, copper loss and friction and winding loss.

The blocked rotor test is performed to determine the short circuited current Isc with normal voltage

applied to the stator, short circuit power factor, total equivalent resistance and reactance of motor as

referred to stator. Starting with zero voltage across stator, the applied voltage is gradually increased in

steps till the load current flows into the stator. The readings of voltmeter and ammeter are noted


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PROCEDURE

NO LOAD TEST

Connections are made as shown in the figure. Supply is switched on with autotransformer in theminimum position and motor on no load. The supply voltage is gradually varied to get the rated voltage.All the meter readings are noted. Autotransformer is brought back to the initial position and supply is

switched off.

BLOCKED ROTOR TEST

Connections are made as shown in the figure. Supply is switched on with autotransformer in the

minimum position and motor kept blocked. The supply voltage is gradually varied till the rated current

flows. All the meter readings are noted. Autotransformer is brought to the initial position and supply is

switched off.

Resistance of the stator winding is measured using ammeter-voltmeter method.

TABULAR COLUMN

Test V(v) I(A) P(W)

No load test

Blocked rotor test

Stator resistance measurement

V(v) I(A) R = V/I()

RESULT

1) Conducted the no load and blocked rotor test on 3 squirrel cage induction motor anddetermined the equivalent circuit

2)

Determined the performance characteristics3) Determined line current, pf, torque, mechanical power developed and pf for a specified slip4) Determined the maximum output, maximum input, maximum pf, maximum torque from circle

diagram

5) Predetermined torque slip characteristics


4/17

Experiment-8

STARTING TORQUE AND PULL OUT TORQUE OF A 3 SLIP RING

INDUCTION MOTOR

AIM

1. To determine the starting torque for different known values of external resistance.

2. Plot the variation of starting torque with external resistance.

3. To determine the pullout torque at different supply voltages.

THEORY

Starting torque is the torque developed by the motor at starting. It increases with the value of external

resistance.

Pullout torque is the maximum value of torque that the induction motor can handle. If the giventorque is more than the pull out torque then the machine will fail to start. It is also known as breakdown

torque

CALCULATION

External resistance R = Vr/Ir

Starting Torque Ts = (S1-S2) g R

PROCEDURE

To find Pullout Torque

1. Connections are made as shown in the figure.

2. Keep the auto transformer at minimum position and switch on the supply.

3. Adjust the autotransformer till 25% of the rated voltage is obtained.4. Load the machine gradually till the motor stops rotating.

5. Note down the spring balance readings.

6. Release the load and repeat the same for voltages up to 50%

To find Starting Torque

1. Connections are made as shown in the figure

2.

All the three rheostats were kept at the minimum position and the rotor is kept blocked.3.

A small voltage is applied to the stator so that a current of 5A flows through the rotor.

4. Now the load is released gradually till the rotor starts rotating.

5. The spring balance readings along with other meter readings are noted.

6. A small external resistance is added in the 3 rotor phases.7. The 3 stator voltage is increased to make the current in all the rotor phases equal to 5A.

8. The 3 rotor current can be adjusted by adjusting the individual resistances.

9. The experiment is repeated for different external resistances.


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CIRCUIT DIAGRAM

STARTING TORQUE

PULL OUT TORQUE

1l B1l

C1l

A2l

B2l

C2l

Stator


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TABULAR COLUMN:

PULL OUT TORQUE

%V I(A) S1(kg) S2(kg)S=S1-S2

(kg)

T=sgr

(Nm)

STARTING TORQUE

Vs(V) Is(A) Ws(W) Vr(V) Ir(A) S1(kg) S2(kg)S=S1-S2

(kg)

T=(S1-S2)

XgXr

(Nm)

R01() R2'()

Exter

Resist

r(

RESULT

The staring torque for different known values of external resistances was determined and the variationof the same with respect to the external resistances was plotted. Also determined the pullout torque at

different supply voltages.


7/17

Experiment-9

PERFOMANCE OF INDUCTION MACHINE AS MOTOR AND GENERATOR

AIM

1.

Determine the performance characteristics of the induction machine when working as both motorand generator

2. To find the hysteresis power and hysteresis torque

THEORY

When a balanced 3 supply is given to an induction motor it starts running at a speed slightly lower

than the synchronous speed. If the motor is made to run at a speed higher than the synchronous speed in

the same direction by means of a prime mover connected to the motor then the induction machine runs asa generator delivering electrical power to the mains while it is absorbing mechanical power from the

prime mover. It also absorbs energy lagging reactive power from the mains to set up its own magnetic

field. In the stable operating region the power delivered is directly proportional to slip in both the cases.

The hysteresis torque is determined from the intercept corresponding to zero slip in the output Vs slipcharacteristics

CALCULATION

1.As Motor

Let N be the speed of the motor

W be the watt meter reading

Input PowerP

i= W

P0= Vdc X Idc

= X 100

Now % Slip is given by

%S =

X 100

Power Factor is given by

P.F =

2.As Generator

Let N be the speed of the motor

W be the wattmeter reading


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Output Power

P0 = W

Input Power

Pi= Vdc X Idc

= X 100

Now % Slip is given by

%S =

X 100

Power Factor is given by

P.F =

3.

Hysteresis Power and TorqueThe hysteresis power can be found from the graph of Output Vs Slip as

Hysteresis power= (Y intercept motor + Y intercept generator)/2

Corresponding Torque =

CIRCUIT DIAGRAM


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TABULAR COLUMN

MachineSpeed,

N(rpm)

Wattmeter

Reading

(W)

VDC(V)

IDC(V)

VL(V)

IL(A)

P0(W)

Pi(W) %

Slip

(S%)P.F

AsMotor

AsGenerator

PROCEDURE:

1. The connections are made as shown in the figure.2. Switches S1 and S3are closed and S2 is kept open.

3. The DC machine field is kept at maximum position.

4. The induction machine is started using Y- starter

5.

Field rheostat is adjusted to obtain the condition for parallel operation on the DC side, i.e. bothvoltmeter readings should be equal in magnitude and polarity.

6. Now switch S2is closed

7. By varying the field rheostat different speeds are obtained and at each speed the correspondingmeter readings are noted down.

8. If the speed is less than the synchronous speed the machine runs as a motor and if the speed is

greater than the synchronous speed the machine runs as a generator.

RESULT :

The induction machine was operated as a motor as well as a generator and the graphs are plotted. The

hysteresis power and torques are found out.


10/17

Experiment-10

V AND INVERTED V CURVE OF A SYNCHRONOUS MOTOR

AIM:

To plot the V Curves and the inverted V Curves at no load, 1/4th

full load, 1/2th

full load and full load

THEORY:

A V-Curve shows the relationship between the armature current and the field current. There are two

values in pair, one of them leading the terminal voltage V and the other lagging behind it. Because of the

shape of the lower portions of the curve they are usually called V Curves of the Synchronous motor.

Theoretically all the curves at different loads will have same line in which the minimum point occurs,

however practically as the machine is loaded the V curves shifts towards right.

The Inverted V Curve shows the relation between power factor and the field current. At starting powerfactor improves and reaches a maximum value but then it falls. The maximum point on the inverted curve

is corresponding to the minimum point of V Curve. If the Synchronous motor at full load is operating at a

power factor 1, then removal of the shaft load causes the motor to operate at a leading power factor.

Compounding curves are obtained for particular power factor by connecting the points on the VCurves at different load for same power factor. These curves show the manner in which the field current

should be varied in order to maintain constant power factor under changing loads.

PROCEDURE:

1. Connections are made as shown in figure

2. Supply is switched on and the machine is started at no load using DOL starter.

3. The field is excited using the DC supply when the machine reaches the rated speed and variousmeter readings are taken

4. The experiment is repeated on no load for increasing values of field current till rated value

5.

After this the load is adjusted to 1/4thfull load and the experiment is repeated by keeping the loadconstant and for different values of field current

6. The same procedure is repeated for half full load and full load.

7. The load on the motor is reduced to minimum and supply is switched off.


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CIRCUIT DIAGRAM:

TABULAR COLUMN:

Load VL(V) IL(A) IF(A) W(W) Cos

NoLoad

1/4 load

1/2 Load

Full Load

RESULT: V and inverted V Curves of the given synchronous induction motor at no load, 1/4th

full load,

full load and full load were plotted.


12/17

Experiment-11

VOLTAGE REGULATION OF ALTERNATOR BY EMF, MMF AND POTIER METHOD

AIM:

1. To conduct O.C and S.C test on a 3 alternator and plot the O.C Characteristics and S.CCharacteristics. Predetermine the voltage regulation at UPF, 0.8 pf lag, 0.5 pf lead and zpf for full

load and half load by emf and mmf methods.2. Verify the above results for half load, UPF condition by direct loading.3. Plot the ZPF full load characteristics by connecting a 3 balanced inductance loading.

Predetermine the voltage regulation by Potier method for full load at UPF, 0.8 pf lead and lag

THEORY:

The open circuit characteristics of the alternator relate the terminal voltage on O.C at normal speed to

the amount of field excitation. The OCC is a measure of the saturation in magnetic circuit. The SCC

relates the armature reaction to the field excitation. The field excitation has to exceed the AC equivalent ofDC motor excitation by an amount necessary to produce small flux that generates emf to circulate SC

Current through resistance and leakage reactance. If this flux is small, SCC is linear which is designed up

to full load.

The ZPF is obtained by loading the alternator with pure inductor. Such a test requires little storage of

power.

The regulation of the synchronous generator is the rise in the terminal voltage of an isolated machinewhen full load at a given pf is removed from the machine while the field excitation and the speed

remaining constant. It depends on the generator resistance, leakage reactance armature reaction and the pf

of load. The regulation may be obtained by

Synchronous Impedance Method (EMF Method)

Ampere turn method (MMF Method)

Zero power factor or Potier method.

% regulation =

X 100

Where E0= No load voltage and V = Terminal Voltage

CALCULATION

For direct loading

% regulation =

X 100

EMF Method

From the SC Characteristics we get the values of ISC and IF. From the OC Characteristics corresponding

value of VOC is also noted.

Zregulation=

when IFis constant

XS=

where RSis the stator winding resistance


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E0=

-ve for leading pf and +ve for lagging power factor

% regulation =

X 100

MMF Method

Field Current is given by

IF= [IF12+ IF2

2+ 2IF1IF2Cos(90+)]

1/2

Where

IF1 = IFcorresponding to the VOC considered

IF2 = IF Corresponding to half load from graph

= PF

% regulation = X 100

Potier Method

From the graph we will get the value of IF2=BE, IaXS = DE

At 0.8 pf lag E1= _______

IF1= ______

IF= [IF12+ IF2

2+ 2IF1IF2Cos(90+)]

1/2

E0=

% regulation =

X 100

CIRCUIT DIAGRAM:


14/17

TABULAR COLOMN:

ZPF

Test

IF(A) VL(V)

PROCEDURE:

OC

Test

IF(A) V0(V)

SC

Test

IF(A) ISC=I0(A)

DirctLoading IF(A) I0 (A) VL(V) VOC(V)


15/17

OC Test: Start the DC motor (prime mover) with all necessary precautions and adjust the speed to rated

speed of alternator. Vary the field current of alternator in steps upto the rated speed current and note down

the corresponding alternator armature voltage (OC Voltage), keeping the speed constant through out.

SC Test: Keep the field current of alternator as zero and short circuit the armature of alternator. Carefully

increase the filed current for a fixed value of rated armature current (Say Half the value) keeping the speedconstant at rated speed. Note down the field current and the armature current making the field current to

zero and short circuit is removed. Calculate the Voltage regulation.

Verification by load test: Adjust the speed current of alternator for rated voltage. Then connect balanced

loading rheostat. Adjust load for half rated current. Take readings. Remove load and observe voltage rise

at some speed and excitation. Calculate Voltage regulation and compare results.

Zero power factor characteristics: Connect the three phase balanced choke coil to the armature (initially

keep at maximum position). Maintain speed constantly at rated value. Adjust the field current so that the

alternator line current is equal to the rated value. Note down the corresponding terminal voltage and field

current. Repeat the procedure by reducing the choke coil impedance in steps. Each time the alternator linecurrent is maintained equal to rated value (by adjusting Zf) and terminal voltage and field current are

noted. Plot the Zero Power factor characteristics and draw Potier triangle and calculate Potier reactance

and voltage regulation.

RESULT:

The OCC, SCC and ZPFC of the given alternator were plotted. Percentage Voltage regulation is

predetermined by EMF, MMF, and Potier Method and also verified by direct method.

Experiment-12


16/17

LOAD TEST ON 3 SQUIRREL CAGE INDUCTION MOTOR IN STAR ANDDELTA MODES

AIM:

1.

Obtain and compare the performance characteristics in star and delta modes.

2. Obtain the percentage loading up to which star mode of operation is efficient.

3.

Find the saving in KVAR under star mode with 50%loading

THEORY:

Star-Delta starter is used in case of motors which are built to run normally with delta connected starter

winding. It consists of a two way switch which connects the motor in star for starting and then on delta fornormal Winding. When starter is connected, the applied voltage over each motor phase is reduced by a

factor of 1/and hence torque becomes 1/3 of that which would have been developed by the motor ifwhere connected in delta. The line current is reduced to 1/3. Hence during starting when the motor is star

connected it takes 1/3 as much as starting current and develops 1/3 as much torque as would have been

developed when it is connected directly in delta.

CALCULATIONS:

Torque T = (S1-S2) X g X r

Output Power P0 =

Input power Pi = W1 + W2

% Efficiency % =

X 100

Slip, %S =

X 100

Power Factor P.F =

Output VAR KVAR = I/P X Tan [Cos-1

]

PROCEDURE

1. Connections are made as shown in figure

2. Connected a TPDT Switch and kept the brake drum under no load condition.

3. Started the motor with winding connected in star.

4.

No Load readings are taken.5. Load was increased in steps up to 40% of rated load and all the readings are noted again

6. The load is then released and the switch is now changed from Star to Delta connection

Take the no load readings and then load is increased in steps and various other readings are noted.The load was released completely and then switched off the motor.

CIRCUIT DIAGRAM:


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TABULAR COLOUMN:

peed

Nrpm)Voltage(V)

Current

(A)

Wattmeter

Reading (W)Spring

P0(W) Pi(W) T(Nm) %Slip

(S%)P.F KV

W1 W2 S1 S2

RESULT:

The load test on Squirrel Cage Induction motor under star and delta mode was performed. The

Efficiency of the motor is higher for delta compared to star mode. The performance characteristics of

machine where obtained along with the VAR Vs output as well as the Torque Slip Characteristics. Saving

in KVAR under star mode with 50%loading was found out.

mc lab cycle 2

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