MA’DIN POLYTECHNIC MACHINES LAB 437.pdf · PDF fileMA’DIN POLYTECHNIC COLLEGE...

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Transcript of MA’DIN POLYTECHNIC MACHINES LAB 437.pdf · PDF fileMA’DIN POLYTECHNIC COLLEGE...

  • MADIN POLYTECHNIC

    COLLEGE

    MALAPPURAM

    DEPARTMENT OF ELECTRICAL

    ENGINEERING

    AC MACHINE LAB(437)

    SEMESTER 6

  • CONTENTS 1. Load test on single phase transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

    2. Load test on 3 squirrel cage induction motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

    3. O.C.C of dc shunt generator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

    4. Load test on dc shunt generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 20

    5. Load test on dc series motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

    6. Measurement of coupling coefficient of transformer coils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

    7. O.C and S.C tests on single phase transformer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

    8. Three phase power measurement by two wattmeter method. . . . . . . . . . . . . . . . . . . . . . . . . . .42

    9. Calibration of single phase energy meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

    10. Resistance measurement using Wheatstones bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

    11. Resistance measurement using Kelvins double bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

    12. V-I Characteristics of incandescent lamp and linear resistance . . . . . . . . . . . . . . . . . . . . . . . . 58

    13. Open circuit and short circuit test on three phase alternator . . . . . . . . . . . . . . . . . . . . . . . . . . 60

    14. No load and blocked rotor tests on 3 slip ring induction motor. . . . . . . . . . . . . . . . . . . . .

  • CONNECTION DIAGRAM

    0-10A MI 600V,10A,upf

    R A M L

    10A

    C V

    V

    S1 S2

    AC

    0-600V R

    MI

    50

    Hz

    Y IM

    3

    -

    10A Y

    40

    0V

    B

    B

    C V

    10A M L

    D.O.L 600V,10A,upf

    STARTER

    MACHINE DETAILS

    Voltage V -

    Current I -

    Power -

    Connection -

    Speed(rpm) -

    Phase -

    120f Synchronous speed = P

  • Experiment 1

    LOAD TEST ON 3 SQUIRREL CAGE INDUCTION MOTOR

    AIM

    To conduct load test on the given 3- squirrel cage induction motor and plot the performance characteristics.

    APPARATUS REQUIRED

    1. Voltmeter 0-600V MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

    2. Ammeter 0-10A MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.

    3. Wattmeter 600V,10A, upf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 nos.

    4. Tachometer To measure speed

    THEORY A squirrel cage induction motor essentially consists of a stator and a rotor. The stator is a hollow cylindrical structure with slots on the inner periphery and carries a three phase winding. The

    winding can be connected in star or delta and is connected across a 3- supply.

    The rotor is also a cylindrical structure with slots on the outer periphery. The slots carry

    thick Al or Cu bars. These bars are short circuited at both ends by means of end rings.

    When a 3- supply is given to a 3- winding displaced by 120 in space, a magnetic field

    of constant magnitude but rotating at synchronous speed is produced. This flux links with the

    stationary rotor, thus inducing an emf in it. As the rotor circuit is closed, a current flows

    through it. The direction of the induced current is such as to oppose the cause producing it.

    The cause is the relative motion between the stator magnetic field and the rotor. So the rotor

    starts rotating in the same direction as the stator magnetic field and tries to catch up with it.

    But practically it is never able to do so. Because if it does so, there would be no relative

    motion, no emf and hence no torque. 9

  • OBSERVATIONS Sl.NO

    V(VOLTS)

    I(AMPS) W1(WATTS)

    W2 (WATTS)

    S1(KG) S2(KG) N(rpm) T

    (Nm)

    Output (watts)

    Input (watts)

    slip (%)

    pf

    Efficiency

    %slip = Ns

    N

    100 Ns Where,

    Ns - Synchronous speed = 120

    f

    P N - rotor speed f - frequency P - No. of poles of the machine

    An induction motor can never operate at s=0. It always operates between s=0 and s=1(starting). The

    performance characteristics are plots of efficiency, torque, speed, slip, pf and line cur-rent versus output.

    Current and torque increases with increase in output. The induction motor is essentially a constant speed motor. However speed reduces gradually with increase in output and slip increases gradually with increase in output. The pf is low at low loads and increases with increase in output.

    The efficiency increases with increase in output, reaches a peak value and then gradually drops with further increase in output.

    PROCEDURE The load on the motor is completely removed by loosening the brake drum. The motor is to be always

    started and stopped at no load, The supply is switched on and the motor is started using a Direct On

    Line Starter (DOL Starter). The readings of the voltmeter, ammeter, wattmeters and spring balance are noted down. The speed

    is measured using a tachometer. The load is then increased in steps, each time noting down all the

    above readings. The experiment is repeated for different values of load currents till the rated current of the machine is reached.

    During the experiment, the machine may get heated up. It is cooled by pouring some water into

    the brake drum.

    At low loads,(when pf < 0.5) one of the wattmeters read negative, in such cases, the supply is

    switched off and the connections to the M and L terminals of the wattmeter are interchanged. The meter now reads positive, but it is to be recorded as negative.

    The load on the machine is removed completely and the supply is switched off. The readings are tabulated and the performance characteristics are plotted.

    Sample Calculation (set no ) .

    Voltage (V) = . . . . . . . . .

  • Current (I) = . . . . .

    . .

    . .

    Wattmeter Reading 1 (W1) =

    . . .

    . . . . . .

    Wattmeter Reading 2 (W2) =

    . . .

    . . . . . .

    Spring balance Readings S1 =

    . . .

    . . . . . . S2 = . . . . . . . . .

    Speed (N) = . . . . . .

    . .

    .

    Torque (T) = (S1 S2)Rg = . . . . . . Where,

    R = Radius of brake drum = . . . . . . . . .

    g = 9.8 m/s2

    Synchronous speed = 120 f =

    (Ns N )

    P

    slip(%) =

    100 = . . . . . . . . . Ns

    Input power = (W1 + W2) = . . . . . . . . .

    (W1 + W2) power

    factor(cos ) = 3V I

    Output power= 2NT

    60

    Efficiency = Output

    Input = . . . . . . . . .

    Performance Characteristics

    Slip pf T N Efficiency

    (%)

    (Nm

    )

    (rp

    m)

    (%)

    Speed

    pf

    Torque

    Slip

    Output (watts)

  • RESULT Conducted load test on the given 3- squirrel cage induction motor and plotted the performance

    characteristics.

    VIVA QUESTIONS

    1. How are the meter ratings selected for this experiment? 2. Why does one of the wattmeters read -ve at starting? 3. What is slip in an induction motor?

    4. What are the two types of 3- induction motors and what is the difference between the two?

    5. What is the value of slip at starting? 6. What are the advantages and disadvantages of squirrel cage induction motor? 7. What is the condition for maximum torque in an induction motor? 8. What are the different losses in an induction motor? 9. Give some applications of 3- squirrel cage induction motor? 10. Explain a typical Torque-slip characteristic.

    11. What is the effect of increased rotor resistance on the performance of an in-duction machine?

  • CIRCUIT DIAGRAM - OCC & SCC

    15A L F A

    0-10A MI

    +

    A

    Rh1 A1

    R V

    0-300V S3

    300 MI

    220V 1.7A M GS R F

    1

    DC B N Y Y

    A

    2 B

    F

    1 F

    2

    15A F2

    -

    S1

    + 2A Rh2 D.C motor alternator

    V - V - V

    1000 1.2A

    I- I-

    H.P - KV A-

    + A -

    rpm- rpm-

    - S

    2 2A Conn.-Star

    0-2A MC

    MEASUREMENT OF ARMATURE RESISTANCE

    5A 0-5A MC

    + A -

    +

    45 5A R

    20V

    +

    V DC 0-10V -

    MC

    5A N

    -

    OBSERVATIONS AND CALCULATIONS

    O.C TEST S.C TEST

    If I

    a VOC

    If

    Measurement of Ra

    V I R

    a