Laboratory induction motors

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    College of Engineering and Science

    Experiment-1

    Characteristics of a Two-Winding Power Transformer

    SAJAN PRAKASH SHRESTHA

    WALTER ESTEBAN GIL

    ELEN 489-003

    January 17th

    , 2013

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    Introduction:

    The purpose of this lab is to observe, analyze and collect the data from measurement of a two-winding-

    transformer when rated at various load voltages, at no load as the input voltage is varied, and with

    secondary terminal shorted. During the experiment, we will observe hysteresis loop and inrush current

    in order to approximate the characteristics of the magnetic core. In the first part of the experiment, wewill adjust the load to see how the average power decreases due to core losses with higher loads. Then,

    we are performing open-circuit tests and short-circuit tests to get the real values and calculate practical

    power transformer characteristics. We will also observe how the exciting current will behave at time

    when transformer is activated. During last section, we will compare the inrush current to the steady-

    state exciting current drawn by the unloaded transformer.

    Theory:

    In this experiment, we are analyzing how the data collected from the practical transformer differ withthe ideal transformer. We used the BIM Meter instrument to record the measurement of primary and

    secondary terminal of the transformer. We will be measuring voltages, currents, and power for the

    primary and secondary windings. Then, we analyze the collected data to see how close our actual

    transformer behaves to ideal transformer. In order to have shunt branch and leakage flux, we are

    performing open circuit and short circuit test.

    Figure 1. Practical Power Transfer

    In order to find, Rc and Xm we have to perform the open circuit test.

    Then, using following equation, we can calculate Rc,

    . (Eq. 1)

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    For, the calculation of Xm, we must find reactive power using apparent power value of circuit.

    Then, equation to calculate reactive and apparent power as follows,

    (Eq.2)

    .(Eq.3)

    For, reactive power:

    (Eq. 4)

    Then, we can solve for Xm using the following formula.

    (Eq. 5)

    Hence, to calculate the value of both Rp and Xm , we must have to use short circuit test. In short circuit

    test, we need to assume the whole system as combined circuit in which both primary and secondary are

    joined together. Then, finally the combined schematic of transformer become as follows

    Fig.2: combined schematic of transformer during short circuit test.

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    In order to calculate, Rp and Rs we use following formula,

    . (Eq. 6)

    (Eq. 7)

    Using the Eq.4, we can calculate the reactive power using short circuit test values for the secondary

    load.

    (Eq. 8)

    . (Eq. 9)

    To solve both the Xps and Rp, we need to solve the turn ratio of primary and secondary windings of

    transformer. This can be found from the voltage rating of primary and secondary side of transformer.

    . (Eq. 10)

    Then, using the Eq.6 and 8, we can solve values for Xps and Rp using turns ratio of transformer. We use

    following formula, to get these component;

    .(Eq. 11)

    . (EQ 12)

    There is alternative way to solve for these Xps and Rp value using average power, reactive power and

    short circuit current. The equations are as follows;

    .... (Eq. 13)

    .. (Eq. 14)

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    Next, we need to solve for the percentage impedance of transformer using short circuit values and

    transformer rating values. Using the following equation we can get percentage impedance of

    transformer.

    ... (Eq. 15)

    . (Eq. 16)

    .. (Eq. 17)

    Then, we have to calculate the loss of real transformer compared to the practical transformer which can

    be done comparing the average power of primary to average power of secondary. This can be calculated

    by using following equation

    .. (Eq. 18)

    In order to find efficiency and voltage regulation of transformer, we can use following formula

    . (Eq. 19)

    (Eq. 20)

    .. (Eq.21)

    (Eq. 22)

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    List of Equipment:

    1. Digital Power Meter-Manufacturer: Basic Measuring Instruments (BMI)

    -Model no. 3030A Power Profiler

    Fig.3: BMI 3030A Power Profiler

    2. Oscilloscope- Manufacturer: Tektronix- Model no. 2430A 9

    Fig.4: Tektronix 2430A Oscilloscope

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    3. Current Probe Amplifier:-Manufacturer: Tektronix

    -Model: TCP A300

    Fig.5: Tektronix 2430A Oscilloscope

    4.

    Current Probe:

    - Manufacturer: Tektronix- Model no.TCP303- Power supply (single phase 120V source @60Hz)

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    5. Lamps:- Bank of lamps rated 230V-250V at 100W (per lamp)

    Fig. 6: Lamp Bank

    6. Single Phase Power Transformer:- Manufacturer : Westinghouse- 178/220V- 2.6% impedance- 5K

    Fig.7: Power Transformer

    7. Induction Regulator

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    Experiment Setup:

    For the first part of the experimental set up, the final circuit diagram is shown as follows. In this

    section, load and voltage will be altered during the test using the lamp Bank and induction

    regulator. During the setup, the induction voltage V1 is set to be 120 V single-phase source and

    R1 is the lamp bank.

    Fig. 8: First Experimental Setup

    All necessary connection to create system as a circuit diagram should be made from power

    distribution panel to load bank, bench station, transformer, and main distribution panel.

    Since, we have different section in experiment while doing with load, short circuit test and

    open circuit test, there will be slight change in circuit in later section.

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    Procedure:

    First of all, lamp bank is used to load the transformer in appropriate steps of 5/4, 1, 3/4, 1/2,

    and no load. Be sure to keep the output voltage of the transformer at rated value for each load step by

    adjusting the induction regulator. Open circuit test is carried out with no load leaving secondary of

    transformer open. Digital power meter is used to measure the voltage and current probe is used to

    measure current going across the primary and secondary coil. In every step, voltage, current and

    average power is recorded for calculation.

    In the next step, we reduced the input voltage to zero by adjusting the induction regulator and then

    short circuit the secondary. Carefully, the transformer voltage is increased until the input current is

    approximately 5/4, 1, , , and of the rated value. Similarly, the digital power meter and current

    probe are setup correctly as earlier step to get the recoding values.

    In final step, we used the oscilloscope/current probe to capture the transformer inrush current at ratedvoltage with no load under the instruction of professor. The inrush current and steady state exciting

    current drawn by the unloaded transformer are compared. . The spectrum analyzer is used to measure

    the amplitude of the harmonic components of the exciting current. An RC integrating network/phase

    shifting network (1 M resistor in series with a 0.47F capacitor) across the input terminals of the

    transformer is added. This setup can be seen below in figure 9. Connect a voltage probe to the vertical

    channel (Y) between the resistor and capacitor; connect a current probe to the horizontal channel (X) to

    measure the exciting current. Finally, the B-H characteristics of the core as the input voltage of the

    transformer is observed in varied with the induction regulator.

    Fig.9: Experimental Setup

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    Data:

    Primary winding Secondary winding

    Load Voltage (V) Current (I) Power(W) Voltage(V) Current(I) Power(W)

    1.25 183.1 35.6 6500 220.2 28.6 6299

    1 182.4 28.6 5200 219.7 23 5030

    0.75 182.1 21.2 3835 220.4 17 3730

    0.5 180.9 14.66 2624 220 11.52 2537

    0.25 180 7.631 1322 220 5.747 1264

    Open Circuit Test: (primary winding)

    Open circuit (Primary winding )

    Voltage (V) Current ( I) Power(W)

    178.9 2.0 45.6

    Short Circuit Test:

    Short Circuit Test (Primary winding)

    Load Voltage(V) Current(I) Power(W)

    1.25 5.6 35 154

    1 4.5 28.1 99.3

    0.75 3.4 21.1 56.3

    0.5 2.3 14 25.2

    0.25 1.2 7.2 6.3

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    Calculation:

    = 354.88VAR

    = 45.6W 0W = 45.6W

    Total loss with no load

    = 1322W 1264W = 58W

    Total losses with at load

    = 2624W 2537W = 87W

    Total losses with at 1/2 load

    = 3835W 3730W = 105W

    Total losses with at 3/4 load

    = 5200W 5030W = 170W

    Total losses with at 1 load

    = 6500W 6299W = 201W

    Total losses with at 5/4 load

    = 96.91%

    percent efficiency at 1 PF for 5/4 load

    = 96.73%

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    percent efficiency at 1 PF for 1 load

    = 97.26%

    percent efficiency at 1 PF for 3/4 load

    = 96.68%

    percent efficiency at 1 PF for 1/2 load

    = 95.61%

    percent efficiency at 1 PF for 1/4 load

    voltage regulation at 1 PF for 5/4 load

    voltage regulation at 1 PF for 1 load

    voltage regulation at 1 PF for load

    voltage regulation at 1 PF for 1/2 load

    voltage regulation at 1 PF for 1/4 load

    Actual Rated Load Impendence

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    Theoretical Rated Load Impendence

    Percent Impedance of the Transformer

    Caclulation of the Exciting Current in Frequency Domain

    Converting from Frequency Domain to Time Domain

    Graphs:

    1.

    Fig.no.10: Graph of input voltage Vs input current using SC test data.

    0

    5

    10

    15

    20

    25

    30

    35

    40

    0 1 2 3 4 5 6

    Inputcurrent(I)

    Input Voltage(V)

    SC Test data(primary)

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    2.

    Fig.no.11: Graph of efficiency of transformer Vs Power output (PF unity).

    3.

    Fig.no.12: Graph of Exciting current Vs time.

    0

    1000

    2000

    3000

    4000

    5000

    6000

    7000

    95.5 96 96.5 97 97.5

    Outputpower(W)

    Efficiency(%)

    -2.5

    -2

    -1.5

    -1

    -0.5

    0

    0.5

    1

    1.5

    2

    2.5

    0 0.005 0.01 0.015 0.02 0.025

    Excitingcurrent(A)

    Time(s)

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    Analysis:

    The first data values that were recorded were the measurements that were taken during the different

    rated load circuit testing. The data values used in the first calculation was collected during measurement

    of open circuit testing. These values were used to calculate Rc and Xm required in practical transformer

    circuit. The value of Rc and Xm came out to be and respectively using eq.1 for Rcvalue and for Xm value we used eq.2 to eq.5. We calculated the ratio of transformer winding with

    respect to current ratio and voltage ratio from data using eq.10 in theory section. The voltage ratio was

    determined to be 0.813 and current ratio was 1.2297. We know that this calculation always keeps the

    primary winding attribute on the top of the ratio.

    The third set of data values that were recorded were the measurements that were taken during the

    short circuit testing. Using the data values from short circuit test, we calculated the value of Rp and Xps

    in power transformer circuit using the eq.13 and eq.14 of theory section. After this, we calculated the

    total power loss of transformer with varying rated load , , , 1, and 5/4 at secondary. The values of

    power loss we got were 58W, 87W, 105W, 170W, and 201W respectively. Following the power loss of

    transformer, we calculated the efficiency at respective rated load and voltage regulation. The

    efficiencies were 96.91%, 96.73%, 97.26%, 96.68% and 95.61% of efficiency of transformer. The voltage

    regulation we calculated were 0.61%, 1.11%, 1.79%, 1.96%, and 2.34% at 1/4, , , 1 and 5/4 rated

    load. After, these value were calculated, we calculated the impedance, base impedance and percentage

    impedance of transformer using eq. 13, 14 and 15 in theory section. The value for the short circuit

    impedance was .16 and the base impendence was 6.4. The percent impendence calculated was 2.5%.

    We got that for the 0.8 power factor calculations the average power was divided by the power factor to

    get the complex power, but the results were still the same.

    Discussion:

    The first part of the experiment was carried out to calculate the parameter values of practical

    transformer. Analyzing these values, we can see that we get fairly close with the values of the ideal

    transformer. In the next section we measured voltages and currents in order to perform turn ratio to

    the transformer and verify nameplate data. The next calculation was carried out to find the percent

    impedance of the transformer. Analyzing the information of data, what we got was the voltage drop of

    the transformer at full load due to the leakage reactance and resistance of the coils. There was slight

    loss in voltage at the primary at full load that corresponds to the percent impendence. Using the data,

    we were able to get different graphs.

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    Conclusion:

    The various data collected during experiment helped to analyze and compare the actual power

    transformer with ideal transformer, mathematically and by inspection. Even though, the actual

    transformer is slightly off in data with ideal transformer, still we can consider the transformer for

    practical use. The differences were so small that circuit diagram still can be used as ideal model. Smalldifference in data value of actual transformer seen and it could be human error or error in collecting

    data as well as could be instrumental error. Finally, this experiment showed us the relatively ideal model

    of transformer mathematically which allows us to predict the losses, efficiency, voltage regulation, and

    required input power.