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ELECTRICAL ENGINEERING LABORATORY

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING Page 1

EX.NO. 1A

LOAD TEST ON DC SHUNT MOTOR

OBJECTIVETo conduct the Load test on DC shunt motor and draw the characteristic curves.

APPARATUS REQUIRED

S.No Equipments Type Range Quantity

1. Voltmeter MC (0-300)V 1

2. Ammeter MC (0-15)A 1

3. Tachometer Digital - 1

4. Rheostat Wire wound 360 /1.1A 1

5. Connecting Lead - - -

PROCEDURE

1. Note down the name plate details of the motors.

2. Connections are made as per the circuit diagram shown in figure.

3. Supply is given by closing the DPST switch.

4. Using the DC three point starter, start the motors.

5. The field rheostat is adjusted to run the motor at rated speed.

6. At no load condition, the input voltage, current and speed are noted and enter the value in

the table.

7. Increase the load current in the steps of1/4th, 1/2th, 3/4th, full load and one 1/4th of the

load.

8. Note down the corresponding ammeter, voltmeter, spring balance reading and speed and

enter the values in the table.

9. Then the load is gradually decreased and field rheostat is brought to the minimum

resistance position and the supply is switched off.

10. The torque, input power, output power and percentage efficiency are calculated by using

the using the above formula and enter the values in the table.

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FORMULA USED

1. Torque (T) = (S1-S2)*R*9.81 in Nm

S1, S2 spring balance reading in Kg

R Radius of the brake drum in M

2. Input power (Pi) = VL IL in watts

VL Line voltage in volts.

IL Line current in amps.

3. Output power(Po) = 2NT/60 in watts

N - Speed in rpm

T - Torque in rpm

4. Efficiency () = (output power/input power)*100 in %

TABULATION

S.No

Load

Current

IL

inAmps

Load

Voltage

VL

in volts

Input

power

Pi in

watts

ILVL

Speed

N in

rpm

Spring balance

reading in Kg Torque

T

in Nm

Output

Power

Po

in watts

Efficiency

in %S1 S2 S1~S2

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GRAPH

The performance characteristics curves are drawn as shown in figure

Electrical characteristics

1.Output Power Vs Line Current

2.Output power Vs Speed

3.Output power Vs Torque

4.Output power Vs Efficiency

Mechanical characteristics

1. Torque Vs Speed

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MODEL CALCULATION

RESULT

Thus the load test on DC shunt motor was conducted and the characteristic curves were

drawn.

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EX.NO.1 B

LOAD TEST ON DC SERIES MOTOR AIM

To conduct the Load test on DC series motor and draw the characteristic curves.

APPARATUS REQUIRED


1. Voltmeter MC (0-15)A 1

2. Ammeter MC (0-300)V 1



Theory

As the name implies the field coils, consisting of few turns of thick wire, are connected in series

the armature, as illustrated in figure. The cross sectional area of the wire used for field coils has to be

fairly large to carry the armature current, but owing to the higher current, the number of turns of wire in

them need not be large. In D.C series motor,

Armature Current, Ia = Series field current, Ise

= Line current, IL = I

Back emf developed, Eb = V I(Ra+Rse)

Power drawn from supply mains = VI

Mechanical Power Developed

Pm = Power input Losses in armature and field

= VI - I2

(Ra + Re) = I [V- I (Ra+Rse)] = EbI

= VIa I2

aRa= Ia (V - IaRa) = EbI

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PRECAUTION

The DC series motor should always be started with some load. Otherwise the speed will be

enormous and fuse will blown out.

PROCEDURE


2. Connections are made as per the circuit diagram.

3. Ensure that 1/4th of the load are applied to the brake drum.

4. Supply is given by closing the DPST switch and the motor is started.

5. At 1/4th load condition, Note down the ammeter, voltmeter, spring balance reading and

speed and enter the values in the table.

6. Then increase the load current in the steps of1/2th

, 3/4th

rated value and one 125% of the

rated value.

7. Note down the corresponding ammeter, voltmeter, spring balance reading and speed and


8. The torque, input power, output power and percentage efficiency are calculated by using

the using the above formula and tabulated in observation table.

FORMULA USED

1. Torque (T) = (S1- S2)*R*9.81 in Nm

Where, S1, S2 Spring Balance Reading in Kg

R Radius of the brake drum in m

2. Input power (Pi) = VLIL in watts

VL Line voltage in volts.

IL Line current in amps.

3. Output power(Po) = 2NT/60 in watts

N Speed in rpm

T Torque in Nm

4. Efficiency () = (Output Power/ Input Power)*100 in %

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TABULATION

S.No

Load

Current

IL

inAmps

Load

Voltage

VL

in volts

Input

power

Pi in

watts

ILVL

Speed

N in

rpm

Spring balance

reading in Kg Torque

T

in Nm

Output

Power

Po

in watts

Efficiency

in %

S1 S2 S1~S2

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GRAPH

The performance characteristics curves are drawn as shown in figure

a) Electrical characteristics

1. Output power Vs Line current

2. Output power Vs Speed

3. Output power Vs Torque

4. Output power Vs Efficiency

b) Mechanical characteristics

1. Torque Vs Speed

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MODEL CALCULATION

RESULT

Thus the load test on DC series motor was conducted and the characteristic curves were drawn.

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EX.NO.2A

OPEN CIRCUIT CHARACTERISTICS AND LOAD CHARACTERISTICS OF DC SHUNT GENERATOR

AIM

To obtain open circuit characteristics and load characteristics of DC shunt generator.

APPARATUS REQUIRED

S. No Description Range Quantity

1. Voltmeter (0-300V)MC 1

2. Ammeter (0-2A)MC 1

3. Ammeter (0-10A)MC 1

4. Rheostat 400/1.4A 2

5. Tachometer - 1

6. Connecting wires - As per requirement

FORMULAE

Eg = V + Ia Ra (Volts)

Ia = IL + If(Amps)

Eg : Generated emf in Volts

V : Terminal Voltage in Volts

Ia : Armature Current in Amps

IL : Line Current in Amps

If : Field Current in Amps

Ra : Armature Resistance in Ohms

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PROCEDURE

I Open Circuit Characteristics

1. Connections are made as shown in the circuit diagram

2. The motor field rheostat should be in minimum resistance position and the generator field

rheostat should be in maximum resistance position while switching ON and switching OFF

the supply side DPST switch.

3. Ensure that the DPST switch on the load side is open.

4. Switch ON the supply DPST switch.

5. Using the 3- point starter the DC motor is started and it is brought to rated speed by

adjusting the motor field rheostat.

6. Keeping the DPST switch on the load side open, the generated voltage Eg and field current

Ifof generator is noted down by varying the generator field rheostat.

7. The above step is repeated till 125 % of rated voltage is reached.

8. Plot the graph for reading taken as shown in model graph.

II Load Characteristics

Connections are made as shown in the circuit diagram

1. The motor field rheostat should be in minimum resistance position and the generator field

rheostat should be in maximum resistance position while switching ON and switching OFF

the supply side DPST switch.

2.

Ensure that the DPST switch on the load side is open.3. Switch ON the supply DPST switch

4. The generator is brought to its rated voltage by varying the generator field rheostat.

5. The DPST switch on the load side is closed, and the load is varied for convenient steps of

load current up to 120 % of its rated capacity and the voltmeter V L and ammeter Ia

readings are observed. On each loading the speed should be maintained at rated speed.

6. Plot the graph for reading taken as shown in model graph.

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TABULAR COLUMN: Open Circuit Characteristics

S.No.

Field Current

If (Amps)

Armature Voltage

Eo (Volts)

TABULAR COLUMN: Load Circuit Characteristics

S.No.

Field

Current

If (Amps)

Load

Current

IL (Amps)

Terminal

Voltage

(V) Volts

Ia = IL + If

(Amps) Eg =V + Ia Ra

(Volts)

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PROCEDURE: Determination of Armature Resistance



3. Readings of Ammeter and Voltmeter are noted.

4. Armature resistance in Ohms is calculated as Ra = (Vx1.5) /I

CIRCUIT DIAGRAM: DETERMINATION OF ARMATURE RESISTANCE

TABULATION TO DETERMINATION OF ARMATURE RESISTANCE

S.No. Voltage

V (Volts)

Current

I (Amps)

Armature Resistance

Ra (Ohms)

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MODEL GRAPH: Open Circuit Characteristics

MODEL GRAPH: Load Characteristics

RESULT:

Thus the open circuit characteristics and load characteristics of DC shunt generator are

obtained.

VL,

E

(Volts)

If, IL (Amps)

V Vs IL

E Vs IL

Eo

If

Critical Resistance = Eo / IfOhmsEo

(V

olts)

If (Amps)

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EX.NO. 2 B

OPEN CIRCUIT CHARACTERISTICS AND LOAD CHARACTERISTICS OF DC SERIES GENERATOR

AIM

To conduct OCC and load characteristics of DC series Generator & obtain the

characteristics

APPARATUS REQUIRED

S.No Equipment Type Range Quantity

1 Ammeter MC (0-1)A 1

2 Ammeter MC (0-10)A 1

3 Voltmeter MC (0-300) V 1

4 Rheostat Wire wound 175 / 1.1 A 1

5 Rheostat Wire wound 50 /5 A 1

6 Loading Rheostat Resistive 5 KW 1

7 Tacho Meter Digital - 1

PRECAUTION

At the time of starting

1. Motor field rheostat should be kept at minimum position

2. Generator field rheostat should be kept at max position

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TABULATION

(i)OCC Circuit

S.No Field CurrentOpen Circuit Voltage

(to) Volts

(ii) Load Characteristics

S.No

Load Current

IL=IF=Ia Amps

Terminal Voltage (VL)

Volts

IL(Ra +Rse) VoltsGenerated emf

Eg=VL+IL (Ra +Rse)

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PROCEDURE

(i)OCC

1. Close the DPST switch and start the DC shunt motor

2. Increase field regulating resistance and bring the speed of the M/R & Generator (1500

rpm)

3. Switch On the load to increase the current through the series field winding look for any

change in speed has taken place

4. If so bring back the speed for rated speed

5. Note down the Ammeter, Voltmeter reading and enter in the tabulation

6. Repeat the steps above previous step till the generator reach the rated current

(ii) Load Characteristics

1. Close the DPST switch & start the DC shunt motor

2. Increase the regulative resistance & bring the speed of the M/R & genset equal for the

rated speed

3. Switch on some load to increase the load current and tabulate the reading

4. Switch on some load to increase the load current greater than the previous and tabulation

the readings.

5. Likewise take readings up to 125% of the rated current.

6. Switch off the supply and return the apparatus safely.

MODEL CALCULATION

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MODEL GRAPH: Open Circuit Characteristics

MODEL GRAPH: Load Characteristics

RESULT

Thus the open circuit characteristics and load characteristics of DC series

generator are obtained.

Eo

If

Critical Resistance = Eo / IfOhmsEo

(V

olts)

If (Amps)

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EX.NO.3

SPEED CONTROL OF DC SHUNT MOTOROBJECTIVE

To control the speed of the given DC shunt motor by field and armature control methods

and also to draw their characteristic curves.

APPARATUS REQUIRED

S.No Apparatus Type Range Quantity





5. Rheostat Wire wound 50/5A 1

6. Rheostat Wire wound 360/1.1A 1

7. Tachometer Digital - -

8. Connecting leads - - -

THEORY

The different ranges of speeds are required for different applications. A single motor can be used

for various work in different speeds. Smooth speed control is possible in DC shunt motor. The

speed of a DC motor can be expressed by the equation,

N = K(V-IaRas) /

Neglecting the small voltage drops IaRa, the speed is directly proportional to the voltage across

armature and inversely proportional to the flux or field current. Hence, the speed of the DC motor

is controlled by either voltage or field current.

Filed current (or) flux control method

In field current method, the speed of the DC motor is inversely proportional to the flux per

pole. When armature voltage is kept constant, the change of field current can change the flux per

pole of a DC motor. The field current can be changed with the help of shunt field rheostat. As the

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field resistance is increased, the current flowing through the field is decreased. So, the speed is

increased. In this method, the speed can be varied above the rated speed.

Armature (or) voltage control method

This method is used when the speed required is below the rated the rated speed. The

voltage across the armature is varied by inserting a variable resistance in series with the armature

circuit. As the armature resistance is increased, the voltage across the armature is decreased. So

the armature speed is decreased.

PRECAUTIONS

1. The motor field rheostat should be kept at minimum resistance position at the time of

starting.

2. The motor armature rheostat be kept at minimum resistance position at the time of

starting.

3. The motor should be in no load condition throughout the experiment

4. The motor should run in anticlockwise direction.

PROCEDURE

Field control method :



3.

Using the three point DC starter, start the motor.4. The armature rheostat is adjusted to run the motor at rated speed.

5. Keep the armature voltage constant at rated value.

6. Increase the speed in steps of 50rpm above the rated speed and note down the

corresponding field current value in table.

Armature control method:

1. Connections are made as per figure.

2.

Using the three-point starter the motor is stared to run.3. The armature rheostat is adjusted to run the motor at rated speed by means of applying

the rated voltage.

4. Keep the field current constant at rated speed.

5. Decrease the speed in steps of 50 rpm below the rated speed and note down the

corresponding field current value in table.

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TABULATION:

FIELD CONTROL METHOD

ARMATURE CONTROL METHOD

S.No

Armature voltage, Va=

Speed (N) rpm Field current (If) Amps

S.No

Field current, If=

Speed (N) rpm Armature voltage, Va (Volts)

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MODEL GRAPH:

The graphs are drawn for the following

1. Armature voltage (Va) Vs Speed (N)

2. Field current (If) Vs Speed (N)

RESULT

Thus an experiment to control the speed of a DC shunt motor was conducted and its

Characteristics curves were drawn.

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EX.NO.4

LOAD TEST ON SINGLE PHASE TRANSFORMER

AIM

To conduct the load test on the given single phase transformer and determine its

performance.

APPARATUS REQUIRED


1 Voltmeter MI

(0-300) V

(0-150) V

1

2 Ammeter MI

(0-10) A

(0-5)A

1

3 Wattmeter LPF 300 V / 10 A,UPF 1

4 Autotransformer 1 Phase (0-270) V / 10 A 1

5 Transformer 1 Phase Step Down 230 V / 110 V 1 KVA 1

PRECAUTIONS

1. At the time of stating, the DPST switch on secondary side should be kept in open

condition.

2. At the time of starting, there should be no lode on the loading rheostat.

FORMULA

1.Output power = Vs Is cos in watts

Where

Vs Secondary voltage in volts

Is secondary current in Amps

cos - power factor

2.Input power = wattmeter reading in watts

3.Efficiency = (output power/ input power)* 100%

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4.Voltage regulation=( Vno load - Vload ) / Vno load

THEORY

Transformer is a static device, which is used to convert AC electricity from one

voltage to another without any change in frequency. Transformer works only with AC

and this is one of the reasons why mains electricity is AC. Step-up transformers

increase voltage, Step-down transformers decrease voltage. Most power supplies use a

step-down transformer to reduce the dangerously high mains voltage (230V) to a safer

low voltage.

PROCEDURE

1. Note down the name plate detail of the machine

2. Connection are made as per the circuit diagram

3. Supply is given by closing the DPST switch on the primary side and the secondary is

kept open

4. Adjust the auto transformer to energise the transformer with rated primary voltage

5. At no load condition, the input voltage, current and power are noted and enter the

values in the table

6. Close the secondary winding and increase the load in steps of1 amps and note down

the corresponding ammeter, voltmeter and wattmeter readings in both the primary

and secondary sides and enter the values in the table7. Then the load is gradually decreased and the auto transformer is brought to the

minimum voltage position and the supply is switched off

8. The output power, efficiency and regulation are calculated by using the formulae and


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TABULATION

Primary

voltage

VP

(Volts)

Primary

current

IP

(Amps)

Secondary

voltage

VS(Volts)

Secondary

current

IS(Amps)

Wattmeter

reading

Wi(watts)

Input

power Pi

(watts)

Output

power

Po(watts)

Efficiency

(%)

Regulation

R (%)

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MODEL GRAPH

The graph is drawn between Output power Vs efficiency

MODEL CALCULATION

RESULT

Thus the load test on the given single phase transformer was conducted and performance

characteristics were drawn.

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EX.NO.5

OPEN CIRCUIT AND SHORT CIRCUIT TEST ON SINGLE PHASE TRANSFORMER

AIM

(I) To conduct O.C. and S.C. test on single phase transformer

(II) To find the equivalent circuit parameters

(III)To predetermine the performance of single phase transformer

APPARATUS REQUIRED


1 Voltmeter MI (0-300) V 1


3 Ammeter MI (0-10) A 1


5 Wattmeter LPF 300 V / 10 A 1

6 Wattmeter UPF 150V / 10 A 1


8 Transformer 1 Phase Step Down 230 V / 110 V 1 KVA 1

9 Connecting leads - - -

PRECAUTIONS

1. At the time of stating, the transformer should be in the minimum voltage position.

2. For O.C test, the HV side of the transformer is in open circuit condition.

3. For S.C test, the LV side of the transformer is short circuited

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PROCEDURE FOR O.C TEST

1. Note down the name plate details of the machine.

2. Connections are made as per circuit diagram shown in the Figure

3. Supply is given by closing the DPST Switch

4. Vary the Autotransformer such that the voltmeter reads the rated LV side of the

transformer.

5. Note down the corresponding reading of the ammeter, voltmeter and wattmeter and

enter the value in Table

PROCEDURE FOR S.C TEST

1. Connections are given as per circuit diagram shown in the Figure

2. Supply is given by closing the DPST Switch

3. Vary the Autotransformer such that the ammeter reads the rated HV side of the

transformer.

4. Note down the corresponding reading of the ammeter, voltmeter and wattmeter and

enter the value in Table.

FORMULAE USED

O.C TEST

1. No load power factor, COS 0 =[Woc / (Vocx1oc)]

WhereW oc No Load Losses

V oc No Load Voltage

I oc No Load Current

2. Magnetizing component, Im = Ioc x sin 0

3. Working component, Iw = Ioc x cos 0

4. No load resistance, R0 = Voc / Iw in ohms

5. No load reactance, X0 = Voc / Im in ohms

6. Primary winding resistance, R1 = Wsc / Isc2

in ohms

Where

Wsc Short circuit power in watts

Isc short circuit current in amps

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7. Primary winding impedance, Z1 = Wsc / Isc2

in ohms

Where

Vsc Short circuit voltage in volts

8. Primary winding reactance, X1 =

9. Secondary Winding Resistance R2= Wsc / Isc2

in ohms

10.Secondary Winding Impedance, Z2=

11.Secondary Reactance, X2 =

12.Transformer Ratio (K) V2/V1

13.Equivalent Secondary Resistance referred to primary side, R2 = R2/K2 in ohms

14.Equivalent Secondary Reactance referred to primary side, X2 = X2/K2 in ohms

EFFICIENCY

15. Iron loss, =

Where,

16.Copper loss

Where,

17.Total loss,

18.Output power,

Where

19. Input power,

20.Efficiency

REGULATION

21.Percentage regulation,=

22.Total resistance as referred to primary side, Ro1 = in ohms

23.

Total reactance referred to primary side, X01 = in ohms

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OPEN CIRCUIT TEST:

Voltage Voc

(volts)

Current Ioc

(Amps)

No load lossesWoc

(Watts)

SHORT CIRCUIT TEST:

Current Isc

(Amps)

Voltage Vsc

(volts)

No load lossesWsc

(Watts)

TO CALCULATE THE EFFICIENCY:

S.NoLoad

Fraction X

Iron Loss

Wi

(Watts)

Copper

Loss

X2Wsc

(Watts)

Total Loss

W (Watts)

Output Power

P0

(Watts)

Input

Power Pi

(Watts)

Efficiency

%

UPFPF=

0.8UPF

PF=

0.8UPF

PF=

0.8

1 0.25

2 0.5

3 0.75

4 1.00

5 1.25

6 1.5

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TO CALCULATE THE REGULATION:

Power factor cos 125% Load

X=1.25

100% Load

X=1

75% Load

X=0.75

Lagging Leading Lagging Leading Lagging Leading

0.2

0.4

0.6

0.8

1

MODEL GRAPH

The graph is drawn

1. Output power Vs efficiency

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2. Power factor Vs regulation

MODEL CALCULATION:

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EQUIVALENT CIRCUIT OF SINGLE PHASE TRANSFORMER

RESULT

(I) Open circuit and short circuit tests were conducted on single phase transformer.

(II) The equivalent circuit parameters are calculated.

(III)Efficiency & voltage regulation performance are estimated.

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EX.NO.6

REGULATION OF A THREE PHASE ALTERNATOR BY EMF AND MMF METHODS

AIM

To predetermine the voltage regulation of the given three phase alternator by (1) EMF and

(2) MMF methods and draw its characteristics curves.

APPARATUS REQUIRED

S.No Equipments Name Type Range Qty

1. Ammeter MI (0-10)A 1

2. Ammeter MC (0-2) A 1

3. Voltmeter MI (0-600) V 1

4. Rheostat Wire wound 400 / 1.1A 2

5. Rheostat Wire wound 50 / 5A 2

6. Tacho Meter Digital - 1

7. Connecting wires - - Few

PRECAUTION

y Motor field rheostat in minimum position

y Output voltage setting in the potential divider in minimum position.

PROCEDURE: OPEN CIRCUIT CHARACTERISTICS


2. Close the DPST switch

3. Start the DC motor using 3 point starter.

4. Increase the field regulating rheostat of the motor and drive the alternator at its rated

speed

5. Note down the Ammeter and Voltmeter Readings

6. Close the SPST switch

7. Increase the output voltage of the potential divider and hence the current through the

alternator field winding little greater than the previous.

8. Look for any change in speed has taken place. If so bring back the speed to earlier value


10.Repeat the step 7,8 and 9 till rated voltage is applied to the alternator field winding.

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CIRCUIT DIAGRAM

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PROCEDURE: SHORT CIRCUIT CHARACTERISTICS

1. Close the DPST switch

2. Start the DC motor using 3 point starter.

3. Increase the field regulating rheostat of the motor and drive the alternator at its rated

speed

4. Close the TPST switch


6. Close the SPST switch

7. Increase the current through the alternator field winding to increase the short circuit

current greater than the previous value

8. Look for any change in speed has taken place. If so bring back the speed to earlier value


10.Repeat the step 7,8 and 9 till rated current

FORMULA

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TABULATION: Open circuit test

S.NO OPEN CIRCUIT LINE

VOLTAGE VL (VOLTS)

FIELD CURRENT IF

(AMPS)

OPEN CIRCUIT PHASE

VOLTAGE VPH =VL/

(VOLTS)

Short circuit test

S.NO SHORT CIRCUIT CURRENT

(AMPS)

FIELD CURRENT IF (AMPS)

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TO FIND PERCENTAGE REGULATION (EMF METHOD)

S.NO POWER FACTOR LAGGNG LEADING

TO FIND PERCENTAGE REGULATION (MMF METHOD)

s.no Power factor Laggng leading

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CIRCUIT DIAGRAM: DETERMINATION OF STATORWINDING RESISTANCE

PROCEDURE: DETERMINATION OF STATORWINDING RESISTANCE



3. Readings of Ammeter and Voltmeter are noted.

4. Stator winding resistance in Ohms is calculated as Ra = (Vx1.5) /I

TABULAR COLUMN: DETERMINATION OF STATORWINDING RESISTANCE

S.NO. VOLTAGE

V (VOLTS)

CURRENT

I (AMPS)

STATOR WINDING

RESISTANCE

RA (OHMS)

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MODEL CALCULATION

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MODEL GRAPH

RESULT

The regulation of 3 phase alternator by EMF and MMF methods were determined and its

characteristics curve was drawn.

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EX.NO.7

V CURVE & INVERTED V CURVE OF SYNCHRONOUS MOTOR

AIM

To plot V Curve & Inverted V curve of given Synchronous Motor

APPARATUS REQUIRED

S.No Equipments Name Type Range Qty


2. Ammeter MC (0-5) A 1



5. Wattmeter UPF 6 A, 250 V 2

6. Rheostat Wire wound 50 / 5A 2

7. Tacho Meter Digital - 1

8. Connecting wires - - -

PROCEDURE

1. Connection are given as per the circuit diagram

2. Supply is given to stator of Motor by adjusting 3 phase autotransformer and it is made to

run as induction motor

3. Now close DPST switch & DC supply is given to rotor circuit and made to run as

Synchronous Motor

4. DC supply is increased step by step by varying potential divider in the rotor circuit and

corresponding readings are tabulated

5. Graph is drawn for IF and Ia, IF & PF.

6. Switch off the supply and return the apparatus safely.

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CIRCUIT DIAGRAM

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TABULATION

MODEL CALCULATION

S.NO. TERMINAL

VOLTAGE VL

IN VOLTS

LOAD

CURRENT IL

IN AMPS

FIELD

CURRENT IF

IN AMPS

WATTMETER READING IN

WATTS

POWER

FACTOR PF=

Pi/(3 VLIL)W1 W2 Pi=W1+W2

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MODEL GRAPH

Load

Current

In amps

Field current in amps

Power

Factor

Field current in amps

RESULT

Thus the V Curve & Inverted V curve characteristics of Synchronous Motor are obtained.

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EX.NO.8

LOAD TEST ON THREE PHASE SQUIRREL CAGE INDUCTION MOTOR

AIM

To conduct load test on three phase squirrel cage induction motor and draw its characteristics

curves

APPARATUS REQUIRED




3 Wattmeter UPF 250 V / 6 A 2

4 Tachometer Digital - 1


FORMULAE USED

1. Torque T = (S1=S2)9.81R in N-m

S1 S2 Spring Balance Reading in KG

R - Radius of Brake Drum in m

2. Output power = 2NT/60 Watts

3. Input power = Wattmeter Reading in Watts.

4. % Efficiency = Out Power x 100

Input Power

5. % of Slip = NS Nr x 100

NS

6. Power Factor (Cos ) = Input Power / VI

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THEORY

The Induction motor is a single phase and three phase AC motor and is the most

widely used machine. These motors are probably the simplest and most rugged of all

electric motors. The supply is connected to the stator and the rotor power is received

by induction caused by the stator rotating flux, hence the motor obtains its name

induction motor. Its characteristic features are:

1. Simple and rugged construction.

2. Low cost and minimum maintenance.

3. High reliability and sufficiently high efficiency.

4. Needs no extra starting motor and need not be synchronized

PROCEDURE

1. Note down the name plate detail of the machine


3. Supply is given by closing the TPST switch

4. Using star- delta starter, start the motor

5. At no load condition, the input voltage, current and power are noted and enter the

values in the table

6. Then increase the load gradually in steps and tabulate the different readings

7. Then the load is gradually decreased and the supply is switched off

8.

The torque, output power, efficiency, slip and power factor are calculated by usingthe formulae and enter the values in the table.

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MODEL GRAPHS


1. Efficiency Vs Po 2. Line current Vs Po 3. Speed N Vs Po

4. Power factor Vs Po 5. Torque Vs Po


Speed Vs Torque

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MODEL CALCULATIONS

RESULT

Thus the load test on three phase squirrel cage induction motor was conducted and

characteristic curves were drawn.

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EX.NO.9

SPEED CONTROL OF THREE PHASE SLIP RING INDUCTION MOTOR BY STATOR VOLTAGE CONTROL

AIM

To obtain speed control characteristic of three phase slip ring induction motor by stator

voltage control.

APPARATUS REQUIRED


1. Voltmeter MI (0-600)V 1



4. Auto transformer 3 (0-470)V 1


THEORY

Stator voltage control is cheapest and easiest method but rarely used, because a large

voltage is required for a relatively small change in speed. This large change in voltage will result in

a large change in the flux density thereby seriously disturbing the magnetic conditions of the

applied voltage.

PROCEDURE

1. Close the TPST Switch

2. Increase the output voltage of variac / Auto Transformer to rated voltage of motor

3. Note down the readings of Speed, Voltage, Current and enter then in tabulation

4. Decrease the voltage less than the previous value and measure the speed, Voltage,

Current and enter then in tabulation as a 2nd

reading

5. Repeat the step 4 for 4 to 5 readings and enter in tabulation.

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CIRCUIT DIAGRAM

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TABULATION

No load condition

S.NO. STATOR VOLTAGE IN VOLTS SPEED IN RPM

Load condition

S.NO. STATOR VOLTAGE IN VOLTS SPEED IN RPM

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MODEL GRAPH

SPEED

IN

RPM

STATOR VOLTAGE IN VOLTS

RESULT

Thus the speed control characteristic of three phase slip ring induction motor by

stator voltage control was obtained.

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EX.NO.10

LOAD TEST ON SINGLE PHASE INDUCTION MOTOR

AIM

To conduct the load test on the given single phase induction motor and draw its characteristic

curves.

APPARATUS REQUIRED




3 Wattmeter UPF 250 V / 6 A 1


5 Tachometer Digital - 1


FORMULAE USED

1. Torque T = (S1=S2)9.81R in N-m

S1 S2 Spring Balance Reading in KG

R - Radius of Brake Drum in m

2. Output power = 2NT/60 Watts

3. Input power = Wattmeter Reading in Watts.

4. % Efficiency = Out Power x 100

Input Power

5. % of Slip = NS Nr x 100

NS

6. Power Factor (Cos )= Input Power / VI

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THEORY

A single phase induction motor is similar to a three phase squirrel cage induction motor

in physical appearance. The rotor of single phase squirrel cage induction motor is essentially the

same as that employed in three phase induction motors and needs no further description. There is

uniform air-gap between stator and rotor but no electrical Connection between them. Except for

shaded pole types, the stator core is also very similar. A single phase motor can be wound for any

even number of poles two, four and six being most common. A single phase induction motor is

similar to a three phase squirrel cage induction motor.

PROCEDURE

1. Note down the name plate of the motor


3. Keep the various I zero position before the II PST switch is Switched ON

4. The II PST is closed and the Autotransformer is adjusted for rated voltage

5. At no load the speed current voltage and power are noted

6. Switch off the supply after reducing the load and draw the graph

MODEL CALCULATION

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MODEL GRAPHS


1. Efficiency Vs Po 2. Line current Vs Po 3. Speed N Vs Po

4. Power factor Vs Po 5. Torque Vs Po


Speed Vs Torque

RESULT

Thus the load test on single phase induction motor was conducted and characteristic

curves were drawn.

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EX.NO.11

STUDY OF AC STARTERS AND DC STARTERS

AIM

To study about the ac starters and dc starters.

Necessity of Starter

The necessity of starter is not only to start the motor but also for the following functions

y Reduction of the heavy starting current

y Over load and no voltage protection

THREE POINT STARTERS

The component used and the internal wiring for a three point starter are shown. Three

terminals L, Z, and A are available in the starter circuit for connecting to the motor. The starting

resistance Rs provided with tapping and each tapping is connected to a brass stud. The handle of

the starter, H is fixed in such a way to move over the brass studs. Two protective devices namely

over load release and no voltage coil provided to protect the motor during over and during failure

of supply.

To start the motor, the starter handle, full resistance is connected in series with the armature

and the armature circuit of the motor is closed through the starting resistance and over load

release coil. Field circuit of motor is also closed through the no voltage coil. Then the handle is

moved over the studs against the spring force offered by a spring Sp mounted on the handle. As

handle movers, the staring resistance is gradually cut out from the motor circuit.

A soft iron pieces is attached to the handle. The no voltage coil, NVC consists of an electro

magnet energized by the field current. When the handle reaches the ON position, the NVC attracts

the soft iron piece and holds the handle firmly.

Whenever there is a failure of supply, the NVC de-energies and releases the handle. The

handle position returns to off position due to the spring tension. If this arrangement is provided,

then when the power supply is restored, the armature alone will be connected to the supply and

the current through the armature will be high and it will damage the armature winding. Thus the

armature is protected against failure of supply by NVC.

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The over load release also has an electromagnet and the line current energizes it. When the

motor is overloaded, the iron strip P is attracted to the contacts (c and c) due to the

electromagnetic force produced by the overload release coil and the contacts c and c are bridged.

Thus in this case NVC is de-energized and the handle comes to off position thus the motor is

protected against overloading.

We can see that under normal running of the motor the starting resistance when the handle

touches the first stud it also touches the brass arc through which full voltage is supplied to the

field coil.

Three point starter

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Disadvantage

This three point starter is not suitable when we have to control the speed of the motor by

connecting a variable resistance in series with the field winding. When the speed, the no voltage

coil will be de-energized and handle will return the off position. Due to this disadvantage, four

point starters is widely used for starting shunt and compound motors.

FOUR POINT STARTER

The basic difference between three point and four starters is the connection ofNVC. In three

point, NVC is in series with the field winding while in four point starter NVC is connected

independently across the supply through the fourth terminal called N in addition to the L, F

and A.

Hence any change in the field current does not affect the performance of the NVC. Thus it is

ensured that NVC always produce a force which is enough to hold the handle in Run position,

against forces of the spring, under all the operating conditions. Such a current is adjusted through

NVC with the help of fixed resistance R connected in series with the NVc using fourth point N as

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shown

FOUR POINT STARTER

Disadvantages:

The only limitation of the four point starter is , it does not provide high speed protection to

the motor. If under running condition, field gets opened, the field current reduces to zero. But

there is some residual flux present and NJ

E1

the motor tries to run with dangerously high speed.

This is called high speeding action of the motion. in three point starter as NVC is in series with the

field, under such field failure, NVC releases handle to the OFF position. But in four point starter

NVC is connected directly across the supply and its current is maintained irrespective of the

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current through the field winding, hence it always maintains handle in the RUN position, as long as

supply is there. And thus it does not protect the motor from field failure condition which result

into the high speeding of the motor.

TWO POINT STARTER:-

Three point and four point starters are used for d.c. shunt motors. In case of series motors,

field and armature are inserted and hence starting resistance is inserted in series with the field and

armature. Such a starter used to limit the star4ting current in case of dc series motor is called two

point starters. The basic construction of two point starter is similar to that of three point starter

the fact that is has only two terminal namely line (L) and field F. The terminal is one end of the

series combination of field and the armature winding.

The action of the starter is similar to that of three point starter. The handle of the starter is

in OFF position. When it is moved to on, motor gets the supply and the entire starting resistance is

in series with the armature and field. It limits the starting current. The current through no volt coil

energizes it and when handle reaches to RUN position, the no volt coil holds the handle by

attracting the soft iron piece on the handle. Hence the no volt coil is also called hold on coil.

The main problem in case of dc series motor is it over speeding action when the load is

less. This can be prevented using two point starters. The no volt coil is designed in such a way that

it holds the handle in RUN positions only when it carries sufficient current, for which motor can

run safely. If there is loss of load then current drawn by the motor decreases, due to which no volt

coil losses its required magnetism and releases the handle. Under spring force, handle comes back

to OFF position, protecting the motor from over speeding. Similarly if there is any supply problem

such that voltage decreases suddenly conditions.

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Two point starter

The overload condition can be prevented using overload magnet increases. This energizes the

magnet up to such an extent that it attracts the lever below it. When lever is lifted upwards, the

triangular piece attached to it touches the two pints, which are the two ends of no volt coil. Thus

no volt coil gets shorted, losing its magnetism and releasing the handle back to OFF position. This

protects the motor from overloading conditions.

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NECESSITY FOR STARTER

At starting when full voltage is connected across the stator terminals of an induction

motor, large current is drawn by the windings. This is because, at starting (i.e before the rotor

starts rotating) the induction motor behaves as a short circuited transformer. This induced emf of

the rotor will circulate a very large current through its windings, due to short. The primary will

draw very large current nearly 7 times of the rated current from the supply main to balance the

rotor ampere turns. This current will however be gradually decreasing as the motor will pick up

speed.

Hence if induction motors are started direct-online heavy current is drawn by the motor,

such as heavy starting current of short duration may not cause harm to the motor since the

construction of induction motors are rugged. Moreover, it takes time for intolerable temperature

rise to endanger the insulation of the motor windings. But this heavy in high of current will cause

a large voltage drop in the lines leading to the motor. Other motors and equipment connected to

the supply lines will receive reduced voltage.

In industrial installation, however, it a number of large motors are started direct on-line,

the voltage drop will be very high and may be really objectionable for the other types of loads

connected to the system. The amount of voltage dro0p will no only dependent on the size of the

motor but also on factors like the capacity of the power supply system, the size and length of the

line leading to the motors, etc.

Types of starters available for induction motors are:

1. Full voltage direct online starting

a. DOL starter

2. Reduced voltage starting Stator control

a. Star-Delta starter

b. Auto transformer starter

3. Rotor resistance starter Rotor control

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FULL VOLTAGE DIRECT ONLINE STARTING:

D.O.L STARTER:

It is recommended that large three phase squirrel-case induction motors be started with

reduced voltage applied across the stator terminals at starting. But small motors up to 5HP ratings

may however be started Direct ON-Line (DOL)

Direct-on-line method of starting of induction motors applicable up to a rating of5 HP is shown in

fig1. In the circuit in addition to fuses, thermal motor windings against overload.

Derivation for starting current and torque in case of DOL starters

Rotor input kTTNS !T2 (1)

Rotor copper loss vS rotor input

@ kTSRI v!22

23

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@ SIT /2

2w 9if R20is the same

Now 12 II w

@ SIT /2

1w (or) SIKT /

2

1!

At starting moment, S = 1

@2

ststIKT ! where Ist = Starting current

If,

If = normal full load current and

Sf = Full load slip

THE STARTERS FOR SQUIRREL CAGE INDUCTION MOTOR

REDUCED VOLTAGE STARTING:

STATOR RESISTANCE (OR) PRIMARY RESISTANCE STARTER:

Their purpose is to drop some voltage and hence reduce the voltage applied across the

motor terminals. In this way, the initial current drawn by the motor is reduced. However, it

should be noted that whereas current varies directly as the voltage, the torque varies as square of

applied voltage.

[Note: When applied voltage is reduced, the rotating flux J is reduced which in turn

decreases rotor e.m.f and hence rotor current I2. Starting torque which depends both on J and I2

suffers on two counts when impressed voltage is reduced]

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For example if voltage applied across motor terminals is reduced by 50%, starting current

is reduced by 50%, but torque is reduced to 25% of the full-voltage value.

Then,

fff SKIT /2

!

@ ff

st

f

stS

T

I

T

T.

2

!

When motor is direct-switched on to normal voltage, then starting current is the short

circuit current Isc.

@ fff

sc

f

stSaS

T

I

T

T..

2

2

!

!

Where fsc IIa /!

Suppose in a case, ,7 fsc II !

Sf = 4% = 0.04,

Then,

96.104.072

!v!f

st

T

T

@ Starting torque = 1.96 x Full load torque.

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Hence, even if current is greater than full load current the starting torque is only 1.96

times full-load torque.

If applied voltages/phase can be reduced by fraction x, then

scstxII ! and scst TxT

2!

f

f

sc

f

f

st

f

stS

I

xIS

I

I

T

T..

22

!

!

f

f

scS

I

xIx .

2

2

!

f

f

stSax

T

T..

22!

f

f

stSax

T

T..

22!

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It is obvious that the ratio of starting torque of full-load torque is x2

of that obtained with

direct switching (or) across the line starting. This method is useful for the smooth starting of small

machines only.

(ii). PRIMARY REACTANCE STARTER:

The working principle of the primary reactance starter is same as that of primary

resistance starter except that voltage drop occurs across the reactor, so that i/p voltage applied to

the stator of induction motor reduces.

AUTO TRANSFORMER STARTERS

An auto transformer consists of a n auto transformer and a switch as shown in fig.

When the switch S is put on start position, a reduced voltage is applied across the motor

terminals. When the motor picks up speed, say to 80 percent of its normal speed, the

switch is put to RUN position. Then the quato-transformer is cut out of the circuit and full

rated voltage gets applied across the motor terminals.

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The switch making these changes from start to run may be air break (for small motors)

or may be oil-increased (for large motors) to reduce sparking.

Derivation for autotransformer starter:

When full voltage is applied without suing autotransformers say starter, then current

taken by the motor is 5 times the full load current. If V is pre line voltage then

voltage/phase across the motor is 3/V .

@Z

VISI fsc

3!! Where Z is starter impedance /phase

In the case of autotransformer, if a tapping of transformation ratio k is used, then phase

voltage across motor is 3/kV

@ Motor starting currentZ

kV

Z

kVI33

2 !!

The current taken from supply (or) by auto transformer is2

21 kkII !!

scif TkS

2.! if magnetizing current of the transformer is ignored phase is

reduced only k times, the direct switching current ,1k3 the current taken by the line is

reduced to k2

times.

The torque is proportional to square of the voltage, we get, (i) with direct =switching,

;3/ 21

VT w with auto transformer,

22

3/kVT w

2

2

1

2

3/

3/

V

kV

T

T!@

(or)

scTkT

2

2 !

@ Torque with quto transformer starter is, = k2 x Torque with direct switching.

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AmpereIIPLD

3!

Relation between starting and full-load torque:

It is seen that voltage across the motor phase on direct switching is 3/V and starting

current isscst

II ! . With autotransformer starter, voltage across the motor phase is 3/kV

andscst

kII !

Now,

12 !w SITstst and

..

2

f

f

st

f

stS

I

I

T

T

!@

(Or)

..

2

2

f

f

sc

f

stS

I

Ik

T

T

!

..922

f

f

stSk

T

T!@

scst kII !@

From Fig. it is seen than for star connection of windings, phase current.

P

LYpZ

VII

1

3v!! Ampere

when ILY is the line current when windings are connected and Zp is the windings impedance per

phase.

For delta connection of windings

p

pZ

VI !

and

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The ratio of line currents drawn in star and delta connection is therefore,

p

p

L

LY

ZV

ZV

I

I

/3

3/ v!

(

3

1!

LD

LY

I

I

STAR DELTA STARTER:

In this method, the stator phase windings are first corrected in star and full voltage is

connected across its free terminals. As the motor pickup speed, the windings are disconnected

through a switch and they are reconnected in delta across the supply terminals. The current drawn

by the motor from the lines is reduced to 1/3 as compared to the current it would have drawn if

connected in delta.

PROOF:

Thus, by connecting the motor windings, first in star and then in delta, the line current

drawn by the motor at starting is reduced to one third as compared to starting with the

windings delta connected.

(! ILI

I

LD

LY

3

1

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Reduced Torque due to star connection:

In induction motor, torque developed is proportional to the square of applied voltage. As

the phase voltage is reduced to 3/1 times that in star-connection, the starting torquewill be reduced to one third. To get full torque in the motor, it must be switched over to

delta connection.

In making connections for star-delta starting, care should be taken such that sequency of

supply connections to the winding terminals does not change wile changing form star-

connection to delta-connection. Otherwise the motor will start rotating in opposite

direction, when connections are changed from star to delta.

Star-delta starters are available for manual operation using push-button control. An

automatic star-delta starters uses time-delay relays (TDR) through which star to delta

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connections take place automatically with some pre-fixed time fixed keeping in view the

starting time of the motor.

Derivation:

Relation between starting and full load torque:

Ist per phase ScI

3

1!

per phase.

Where ISc is the current /phase which ( -connected motor would have taken if switched on

to supply directly (however line current at start 3/1!of Line ISC).

Now,

2

ststIT w 1!s

@f

f

st

st

stS

I

I

T

T.

2

!

f

f

scS

II .1.3

2

!

fSa .3

1 2!

f

f

stSa

T

T.

3

1 2!@

Here, Ist and ISC represent phase values.

It is clear that star-Delta switch is equivalent to a n auto-transformer of ratio 3/1 (or)

58% approximately.

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THE STARTERS FOR SLIP RING INDUCTION MOTORS:

ROTOR RESISTANCE STARTERS:

The easiest method of starting wound rotor (slip-ring) induction motors is to

connect some extra resistance in the rotor circuit as shown in fig. Connection of extra resistance

in the rotor circuit decreases the starting current and at the same time increases the starting

torque. As the motor starts rotating the extra resistance is gradually cut out. When the motor

attains rated speed the resistance is fully cut out.

When the motor attains rated speed the resistance is fully cut out and the slip ring

terminals are short circuited. The motor now operates on its own characteristics which give rise to

maximum torque at a low slip.

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