Lab 2: Transformers

ECE330 - Introduction to Power Engineering

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1 Introduction

In this lab the performance and operating characteristics of a single-phase transformer will be measured and compared with theoretical predictions based on the measured equivalent circuit model parameters that are found from the open-circuit and short-circuit tests. The operation of a 3-Phase Transformer in a Delta-Wye configuration will also be investigated. The four circuits to be completed in the laboratory are as follows:

• Circuit 1: Single-phase transformer - Short-circuit Test
• Circuit 2: Single-phase transformer - Open-circuit Test
• Circuit 3: Single-phase Transformer - Load Test
• Circuit 4: Delta-Wye Three-phase Transformer - Load Test

1.1 Pre-lab

• Familiarize yourself with the lab procedures and requirements by reading through the lab manual.

• Familiarize yourself with the Safety Rules as well as the Equipment and Software used in the second Laboratory by viewing the information on the laboratory webpage below:

  https://sites.google.com/ualberta.ca/ece330-introtopowerengineering/home

  • Note the equipment datasheets can be located at the bottom each page.

  • To access the webpage, you need to be logged-in with your CCID.

  • If you cannot access the webpage contact: terheide@ualberta.ca

• Re-watch the General Lab Safety Video .

• Review Transformer theory.

• Have at least, the ECE330 - Lab 2 - Sign-off sheet printed off before coming to the lab.

• Review the ECE330 - Lab 2 - Results Sheet that is used to collect your results and make plots that are to be handed in for your post-lab.

1.1.1 Pre-lab Questions

The information for questions 1-7 can be found either in the Lab 2 manual, the Background section, or on the webpage and datasheets mentioned above. Answer the questions on a separate piece of paper to be handed-in at the beginning of your schedule lab session (Show all of your work). Make sure to clearly put your name, student ID number, CCID and your lab section at the top of the page.

1. What are the Specified Ratings for: Primary Voltage, Secondary Voltage, Power and Line Current for one of the transformers (Model 8348-40) used in the Three-phase Transformer Bank? Explain in your own words why the Secondary Voltage has 2 ratings separated by a slash (/).

2. What are the 2 test circuits used in the lab to find the equivalent circuit model parameters? Describe how these tests are performed and which test circuit gives you which parameters.

3. Describe in your own words what Voltage Regulation is.

4. Most transformer cores are constructed using thin strips of silicon steel that are laminated, explain in your own words why they are constructed this way.

5. There are 2 types of loss that are typically associated with the core of a transformer. Explain in your own words what these two losses are.

6. Three-phase transformers are used extensively to increase the voltage on power lines to transmit power long distances. Explain in your own words why this is done.

7. Three-phase transformers can be constructed simply by using 3 identical single-phase transformers. What are the 3 main advantages to constructing a purposely build Three-phase Transformer as opposed to using 3 single-phase transformers.

1.2 Equipment Required

List of Equipment used for this Laboratory Experiment.

• Power Supply – Model 8525-20
• Data Acquisition and Control Interface (DACI) – Model 9063
• Three-phase Transformer Bank – Model 8348-40
• Resistance Load – Model 8311-00
• Inductive Load – Model 8321-00
• Capacitive Load – Model 8331-00
• Safety banana Leads – 3 Colors, 3 Lengths
• Computer running LVDAC-EMS software
• Accessories – (USB cable, Barrel Power Cable)

1.3 Background

• Transformer Basics
• Transformer Construction
• Transformer Loading
• Transformer Voltage Regulation
• Three-phase Transformers

2 Experimental Procedure

2.1 Single-phase Transformers

2.1.1 Circuit Models

2.1.1.1 Complete Transformer Equivalent Circuit

In Figure 1. below is a common equivalent circuit that is used for practical single-phase two-winding transformers. The model below try’s to capture all of the non-ideal quanities of the a real transformer and differs from the ideal transformer in the following ways.

1. The wire that makes up the transformers winding’s have resistance.

2. The iron used to make up the core of the transformer has a finite permeability.

3. The magnetic flux is not entirely contained within the transformers core.

4. There are real and reactive power losses in the transformers core.

Figure 1. Complete Transformer Equivalent Circuit

• N₁, N₂ - Are the number of turns in the primary and secondary winding respectively.

• a - Is the turns ratio.

\[a = \frac{N_1}{N_2} \approxeq \frac{V_1}{V_2} (\text{during open circuit test}) \approxeq \frac{I_2}{I_1} \text{(during short circuit test)}\]

• V₁, V₂ - Voltage of the transformers primary and secondary terminals, respectively.

• I₁, I₂ - Current going into the transformers primary and secondary terminals, respectively.

• I’₂ - Affect of the transformers secondary winding current on the primary winding. \[I'_2 = \frac{I_2}{a}\]

• I_m - The magnetizing current.

• R_C - The resistance representing the real power loss that happens within a transformers core.

• X_m - The inductance representing the reactive power loss that happens within a transformers core.

• R₁, R₂ - The resistance due to the copper wire that makes up the primary and secondary winding’s, respectively.

• X₁, X₂ - Not all of the magnetic flux is transferred from the primary to secondary winding, it is also known as leakage flux and represented by a series inductance for each winding.

2.1.1.2 Laboratory Simplifed Transformer Equivalent Circuit

For the laboratory we will consider a slightly simplified equivalent circuit as shown below which will make it easier to identify the transformers parameters using only the two tests that are carried out during this laboratory.

Figure 2. Laboratory Simplified Transformer Equivalent Circuit

To simply the circuit model the winding resistances of R₁ and R₂ have been combined using the following formula. The equivalent resistance and reactance is placed for convienence to simply calculation.

• Req - Equivalent total series resistance of the transformers winding’s which is apparent on the primary side.

\[R_{eq} = R_1+(\frac{N_1}{N_2})^2R_2\]

To further simply the circuit model the leakage inductance’s of X₁ and X₂ have also been combined in a similar manner.

• Xeq - Equivalent total series inductance of the transformers winding’s which is apparent on the primary side.

\[X_{eq} = X_1+(\frac{N_1}{N_2})^2X_2\]

2.1.2 Short-circuit Test

In the short-circuit test a low voltage is applied to the transformer primary winding while the secondary winding is shorted. If we consider the effect of a short circuit applied to the secondary winding in Figure 2, the voltage V₂ will be zero. Since the impedance R_c // X_m >> R_eq + X_eq the circuit of Figure 2 can be re-drawn as shown in Figure 3. It should be obvious that only a small applied voltage is required to reach rated current.

Figure 3. Equivalent Circuit (Shorted Secondary)

Measuring the input current, input power and power factor, we can again calculate the parameters of the equivalent circuit using the following formulas:

\[P_{1_{\text{SC}}\ } = I_{1_{\text{SC}}}^{2}\ R_{eq} \Longrightarrow R_{eq} = \frac{P_{1_{\text{SC}}}}{I_{1_{\text{SC}}}^{2}}\]

\[Q_{_{\text{SC}}} = I_{1_{\text{SC}}}^{2}X_{eq} \Longrightarrow X_{eq} = \frac{Q_{1_{\text{SC}}}}{I_{1_{\text{SC}}}^{2}}\]

The values for R_eq and X_eq obtained from these formulas can be used with R_c and X_m obtained from the open-circuit test to provide the parameters needed for the single-phase equivalent circuit shown in Figure 2. The circuit can then be used to calculate the theoretical performance of the transformer.

2.1.2.1 Circuit Setup

1. Connect the circuit shown in Circuit 1.

   1. Use the variable ac supply connected from a phase to neutral

   2. Use E1 and I1 to measure the input voltage and current to the transformer.

   3. Use I2 to short the secondary windings of the transformer between terminals 4 and 5.

Circuit 1. Short-circuit Test

2. Warning: While making these measurements as only a small voltage is required to obtain a large and destructive current due to the secondary being short-circuited. You will only need ~5% variac setting to reach the maximum current of the transformer. Please do not excessively exceed the current rating (1.2 Amps) of the Transformer.

2.1.2.2 Instrument Setup

3. Setup the Metering Instrument as follows:

Table 1. Metering Setup (Short-circuit Test)

Meter	Description	Type	Input/ Function	Mode
M1	Transformer Primary Voltage	Voltage	E1	AC
M2	Transformer Primary Current	Current	I1	AC
M3	Input PF	Power Factor	PF (E1,I1)	True
M4	Secondary Short-circuit Current	Current	I2	AC
M5	Parameter R1	Impedance	RXZ (EI, I1)	R
M6	Parameter X1	Impedance	RXZ (E1, I1)	X

2.1.2.3 Results

4. When you think your circuit and instrumentation is setup correctly get an instructor or TA to verify it before you apply power.

5. Make sure the variac is at 0%

6. Apply power by using the main power switch, L1, L2 and L3 should light up to indicate power.

7. Make sure that you have hit continuous refresh on the meter display and that it’s refreshing.

8. While adjusting the variac always keep an eye on the value of I2. You should never allow the value to increase above 1.5 Amp. Again, a small change in the variac setting will result in a large change in current. Resettable fuses protect the primary and secondary windings of each transformer against overcurrents. Fuse status lamps on the module front panel turn on when the resettable fuses open.

9. Using the Data Table to record the 6 meters you setup in Table 1 slowly increase the variac until you get an I2 of Approximately 0.25Amps.

10. Continue recording results for approximate values of I2 for 0.5A, 0.75A, 1.0A and 1.25A.

11. Once you have captured all five I2 set-points in ascending order, export the Data Table to a .csv file. Save the file somewhere you will have access to after the Laboratory.

12. Get an instructor or TA to view your .csv file and sign off on your results sheet.

13. Return the variac to 0%, and turn-off the main power supply.

2.1.3 Open-circuit Test

In the transformer open-circuit test, a voltage is applied to the primary winding with the secondary winding open-circuit. Considering Figure 2, if the secondary is open circuit then I₂ must be zero. Figure 2 can then be re-drawn as shown in Figure 4.

Figure 4. Equivalent Circuit (Open Secondary)

If we consider Figure 4, we can see that the primary current I₁ will equal the magnetizing branch current I_m. Since the only power losses are core losses, we can calculate R_m and X_m from the input voltage, input power and power factor, as follows:

\[P_{1_{\text{OC}}} = \frac{V_{1_{\text{OC}}}^{2}}{R_{C}} \Longrightarrow R_{C} = \frac{V_{1_{\text{OC}}}^{2}}{P_{1_{\text{OC}}}}\]

\[Q_{1_{\text{OC}}} = \frac{V_{1_{\text{OC}}}^{2}}{X_{m}} \Longrightarrow X_{m} = \frac{V_{1_{\text{OC}}}^{2}}{Q_{1_{\text{OC}}}}\]

X_m and R_c are non-linear circuit elements and will vary with the applied voltage (and in a real transformer, with load). To simplify analysis, X_m and R_C are assumed to be constant. This assumption of constant parameters is generally acceptable over the normal operating range of applied voltages (V_nominal ±10%).

2.1.3.1 Circuit Setup

1. Connect the circuit shown in Circuit 2 by changing the following from Circuit 1.

   1. Adjust the input voltage to use a line-to-line input voltage instead of a phase-to-neutral voltage from the variable ac supply.

   2. Disconnect I2 from the output of the secondary and connect the voltage meter E2 across transformer terminals 4 and 5 instead.

Circuit 2. Open-circuit Test

2.1.3.2 Instrument Setup

2. Setup the Metering Instrument as shown in Table 2 by making the following changes:

   3. Change M4 to measure the voltage E2.

   4. Change M5 and M6 to measure parallel connected impedances instead of series connected.

Table 2. Metering Setup (Open-circuit Test)

Meter	Description	Type	Input/ Function	Mode
M1	Transformer Primary Voltage	Voltage	E1	AC
M2	Transformer Primary Current	Current	I1	AC
M3	Input PF	Power Factor	PF (E1,I1)	True
M4	Secondary Open-circuit Voltage	Voltage	E2	AC
M5	Parameter Rm	Impedance	RXZ// (EI, I1)	R
M6	Parameter Xm	Impedance	RXZ// (E1, I1)	X

2.1.3.3 Results

3. When you think your circuit and instrumentation is setup correctly get an instructor or TA to verify it before you apply power.

4. Apply power by using the main power switch, L1, L2 and L3 should light up to indicate power.

5. Using the Data Table to record the 6 meters you setup in Table 2 slowly increase the variac until you get an I1 of Approximately 0.01 Amps.

6. Continue recording results for values of I1 from 0.01 Amps to the maximum variac setting increasing by 0.01 Amps each step. Make sure to record results for the maximum setting regardless if it falls on the 0.01 increment.

7. Once you have captured all I1 set-points in ascending order, export the Data Table to a .csv file. Save the file somewhere you will have access to after the Laboratory.

8. Get an instructor or TA to view your .csv file and sign off on your results sheet.

9. Open the Oscilloscope Instrument and view E1, I1 and E2 on channels 1, 2 and 3 respectively.

10. Adjust the variac from 0% through to 100% while observing the waveforms on the scope. Make a note of what you observe to answer question 1 in the post-lab report.

11. Return the variac to 0%, and turn-off the main power supply.

2.1.4 Load Test

During a transformer load test the primary winding is connected to the supply voltage and various load levels are applied to the secondary.

The actual transformer on-load efficiency (η) can be determined from experimental readings and is defined as:

\[\eta = \frac{P_{\text{out}}}{P_{\text{in}}} \times 100\%\]

The input and output power are defined as:

\[P_{\text{out}} = V_{2,rms}\ I_{2,rms}\ \cos\theta_{2}\] \[P_{\text{in}} = P_{\text{out}} + P_{\text{fe}} + P_{\text{cu}}\]

Where the iron or core losses are:

\[P_{\text{fe}} = \frac{V_{1}^{2}}{R_{C}}\]

and the copper or resistive losses are:

\[P_{\text{cu}} = I_{2}^{'2}\ R_{eq} = \frac{I_{2}^{2}\ R_{eq}}{a^{2}}\]

The theoretical maximum transformer efficiency occurs when the fixed losses (independent of the current drawn) equal the variable losses (dependent upon the current drawn):

\[P_{\text{fe}} = P_{\text{cu}}\]

“Voltage Regulation” (VR) is a measure of the voltage droop for the transformer under load (i.e., the change in the secondary voltage from “no-load” conditions to “full-load” conditions). The transformer voltage regulation expressed as a percentage of the on load secondary voltage is defined as

\[VR = \frac{V_{2_{\text{OC}}} - V_{2_{\text{Load}}}}{V_{2_{\text{Load}}}} \times 100\%\]

2.1.4.1 Circuit Setup

1. Connect the circuit shown in Circuit 3 by changing the following from Circuit 2.

   1. Add two Resistive Load banks connected in parallel to load the secondary of the transformer.

   2. Add I2 to measure the secondary load current.

   3. Make sure you connect the voltage meters E1 and E2 directly to the transformer terminals.

Circuit 3. Load-test Circuit

2.1.4.2 Instrument Setup

2. Setup the Metering Instrument as shown in Table 3:

Table 3. Metering Setup (Load Tests)

Meter	Description	Type	Input/ Function	Mode
M1	Transformer Primary Voltage	Voltage	E1	AC
M2	Transformer Primary Current	Current	I1	AC
M3	Input PF	Power Factor	PF (E1,I1)	True
M4	Input Power	Power	PQS1 (E1, I1)	P
M5	Input Apparent Power	Power	PQS1 (E1, I1)	S
M7	Transformer Secondary Voltage	Voltage	E2	AC
M8	Transformer Secondary Current	Current	I2	AC
M9	Output PF	Power Factor	PF (E2,I2)	True
M10	Output Power	Power	PQS2 (E2, I2)	P
M11	Output Apparent Power	Power	PQS2 (E2, I2)	S
M13	Transformer Efficiency	Efficiency	P2 / P1

2.1.4.3 Results

3. When you think your circuit and instrumentation is setup correctly get an instructor or TA to verify it before you apply power.

4. Start with all load switches off.

5. Apply power by using the main power switch, L1, L2 and L3 should light up to indicate power.

6. Increase the variac slowly watching the meters to verify that your circuit is operating correctly. Current should remain fairly small (< 0.1 Amp). Once you verify your measurements look okay, increase the variac to 100%.

7. Using the Data Table to record the 11 meters you setup in Table 3. First, record the No-load state, then continue going through the load states in Table 4 recording the Results for each step.

Table 4. Switching States (Load Test)

.	|	Load bank 1	|	|	Load bank 2	|
.	1200 Ω	600 Ω	300 Ω	1200 Ω	600 Ω	300 Ω
No-load	0	0	0	0	0	0
600 Ω	0	0	0	0	1	0
300 Ω	0	0	0	0	0	1
200 Ω	0	0	0	0	1	1
150 Ω	0	1	0	0	1	1
120 Ω	1	1	0	1	1	1
100 Ω	1	0	1	1	1	1
85.7 Ω	1	1	1	1	1	1

8. Once you have captured all load steps in the above order, export the Data Table to a .csv file. Save the file somewhere you will have access to after the Laboratory.

9. Return the variac to 0%, and turn-off the main power supply.

10. Get an instructor or TA to review your three .csv files (one for each load type) and get them to sign off on your results sheet.

2.2 Three-phase Transformers

Three-phase supplies are used for electrical power generation, transmission, distribution as well as for most industrial uses. Three-phase supplies are used mainly as there is a cost benefit on transporting large quantities of electrical power when compared to single phase.

A three-phase transformer can be constructed by either connecting three single-phase transformers to form a three-phase transformer bank, which is what we will be doing, or using one three-phase transformer which consists of three pairs of single phase windings mounted on to a single laminated core.

The advantages of building a single three phase transformer is that for the same power rating it will be smaller, cheaper and lighter than three individual single phase transformers connected together because the copper and iron core are used more effectively. The methods of connecting the primary and secondary windings are the same, whether using just one three-phase transformer or three separate single-phase transformers.

The primary windings or the secondary windings of a three-phase transformer can commonly be connected as either a Delta (Mesh) or a Wye (Star). These connections are each shown below in two equivalent drawings:

Figure 5. Wye and delta transformer connections

Using the wye and delta connections above for either the primary winding or the secondary winding you can obtain the four common three-phase transformer connections as shown below in the table.

Table 5. Three-phase Transformer Connections Compared

Config	Voltage	Current	Phase	Wires
Wye-wye	1:1	1:1	0°	4:4
Delta-delta	1:1	1:1	0°	3:3
Wye-delta	√3:1	1:√3	30° lag	4:3
Delta-wye	1:√3	√3:1	30° lead	3:4

• Config - Describes what winding configuration is used for both the primary and secondary windings (Pri-sec).

• Voltage - Describes the resultant line-to-line voltage ratio of primary to secondary assuming that the turns ratio of the windings is 1 (Pri:Sec).

• Current - Describes the resultant line current ratio of primary to secondary assuming that the turns ratio of the windings is 1 (Pri:Sec).

• Phase - Describes the resultant phase difference between the secondary line-to-line voltage relative to the primary line-to-line voltage.

• Wires - Describes the number of wires that are available to be used at both the primary and secondary sides of the transformer (Pri:Sec). When a Wye configuration is used you have the star-point (N for Neutral) where an additional wire may or may not be attached.

2.2.1 Delta-Wye Transformer

2.2.1.1 Circuit Setup

11. Connect the circuit shown in Circuit 4.

    1. Using the variable three-phase supply and connecting the primary windings of the THREE-PHASE TRANSFORMER BANK in a delta configuration. (CAUTION! Note the orientation of the dot’s of each winding.) Use the ammeter ‘I4’ to measure the primary line current and ‘I3’ to measure the current going through the winding with the red terminals. Use ‘E4’ to measure the primary line-to-line voltage.

    2. Connect the secondary windings of the THREE-PHASE TRANSFORMER BANK in a Wye configuration. (CAUTION! Note that the low voltage (120V) tap output of the secondary has to be used) Use voltmeter ‘E3’ to measure the secondary phase voltage. Setup the meters ‘E1’, ‘I1’, ‘E2’ and ‘I2’ to make measurements on the output of the transformer using a 2-wattmeter configuration.

    3. Use the RESISTIVE LOAD connected in a Wye to load the three-phase transformer. Make sure the resistors are all off (no-load/open-circuit) on all 3 banks to begin the experiment.

Circuit 4. Three-phase Transformer Circuit (Delta-Wye)

2.2.1.2 Instrument Setup

12. Setup the Metering Instrument as shown in Table 4:

Table 6. Metering Setup (Three-phase Transformer Circuit)

Meter	Description	Type	Input/ Function	Mode
M1	Output Voltage (Red-Black)	Voltage	E1	AC
M2	Output Voltage (Blue-Black)	Voltage	E2	AC
M3	Output Phase Voltage (Red)	Voltage	E3	AC
M4	Input Voltage (Red-Black)	Voltage	E4	AC
M5	Voltage Phase Shift (Output->Input windings)	Phase Shift	PS(E3,E4)
M6	Voltage Phase Shift (Output->Input Line-line)	Phase Shift	PS(E1,E4)
M7	Output Current (Red)	Current	I1	AC
M8	Output Current (Blue)	Current	I2	AC
M9	Primary Current (Red winding)	Current	I3	AC
M10	Input Current (Red)	Current	I4	AC
M11	Output Current (Black)	Current	I1+I2	AC
M12	Output Power (1 wattmeter)	Power	PQS1(E1,I1)3~	P
M13	Output Power (2 wattmeter)	Power	PQS1+PQS2	P
M14	Output Power Factor	PF	PF(EI1,EI2)	True
M15	Input Power	Power	PQS4(E4,I4)3~	P
M16	Input Reactive Power	Power	PQS4(E4,I4)3~	Q
M17	Input Apparent Power	Power	PQS4(E4,I4)3~	S
M18	Input Power Factor	PF	PF(E4,I4)3~	True

2.2.1.3 Results

13. When you think your circuit and instrumentation is setup correctly get an instructor or TA to verify it before you apply power.

14. Start with all load switches off.

15. Apply power by using the main power switch, L1, L2 and L3 should light up to indicate power.

16. Increase the variac slowly watching the meters to verify that your circuit is operating correctly. Current should remain fairly small (< 0.1 Amp). Once you verify your measurements look okay, increase the variac to 100%.

17. Using the Data Table to record all 18 meters you setup in Table 6. First, record the No-load state, then continue going through the load states in Table 7 recording the Results for each step. For each load step make sure that all 3 resistors in the RESISTOR LOAD bank are configured for the same resistance so the system is balanced.

Table 7. Load States (Three-phase Transformer)

1200 Ω	600 Ω	300 Ω	Total Resistance
0	0	0	Open
1	0	0	1200 Ω
0	1	0	600 Ω
1	1	0	400 Ω
0	0	1	300 Ω
1	0	1	240 Ω
0	1	1	200 Ω
1	1	1	171 Ω

18. Once you have captured all load steps in the above order, export the Data Table to a .csv file. Save the file somewhere you will have access to after the Laboratory.

19. Return the variac to 0%, and turn-off the main power supply.

20. Get an instructor or TA to review your .csv files and get them to sign off on your results sheet.

2.3 Clean Up

1. Verify your results with an instructor or TA to show that you have completed everything.

2. Cleanup your station, everything should be returned to where you got it from.

3. Once everything is completed and tidy get a signature on your Results page before you leave.

3 Post-lab

Lab Reports are due approximately 1 week after you attend your lab section, check eClass for the exact time. All reports need to be submitted to the appropriate link as a single pdf on eClass. You only have to hand-in one copy per group. Please have your pages in a single pdf file in the following order:

• Use a scanned/picture copy of the ECE330 - Lab 2 - Sign-off sheet as your cover sheet. Make sure your names, student ID’s, CCID’s and lab section are visible in the table at the top of the page. You need to obtain LI/TA signatures for completing each circuit during the Laboratory exercise.

• A pdf of the completed ‘Results’ and ‘Graphs’ worksheets from the ECE330 - Lab 2 - Results Sheet .

• The completed sample calculations (showing all of your work).

• The answers to the post lab questions.

3.1 Sample Calculations

The following sample calculations must be handed in with your results. Use the methods that are discussed in the lab manual. (Show all your work):

1. Use the measured data in the short-circuit test at I₂ = 0.75 Amp to calculate R_eq and X_eq.

2. Use the measured data in the open-circuit test at I₁ = 0.05 Amp to calculate R_c and X_m.

3. For the full-load (100 Ω) resistive case, use the calculated R_eq and R_c from sample calculations 1 and 2 to calculate the iron and copper losses.

4. For the over-load (85.7 Ω) resistive case, calculate the actual VR and η using the necessary measured data.

3.2 Questions

Answer the following questions to hand in with your lab report.

1. What happens to the input current waveform in the open-circuit test as the transformer primary voltage increases? Explain.

2. Is the current ratio equal to the turn’s ratio? Explain. Use your results to back up your answer.

3. Looking at the Saturation Curve from the results spreadsheet.

   1. Explain what is occurring in the transformer when you increase the primary voltage.

   2. What causes this to occur?

   3. Does the saturation affect the voltage ratio between the primary and secondary windings?

4. In an ideal transformer the power available in the secondary winding will be the same as the power in the primary winding, ideal transformers are constant wattage devices and do not change the power only the voltage to current ratio. Does the power in the primary windings approximately equal the power in the secondary windings for every value of load resistance? Explain.

5. A transformer typically has very low impedance, what effect does this have on load regulation and short circuit current?

6. The graphs on the spreadsheet compare your measured results against results predicted by the equivalent circuit model. How does the equivalent circuit model do in predicting the Voltage Regulation, Transformer Losses and Efficiency? Explain.