Single Phase Transformers

1. DC Test

1. Turn on the low-voltage DC power supply available on the bench.
2. Set its voltage output to 0 V, and set the current limit to 0.8 A.
3. Double-check the circuit connections, then connect the power supply output across the primary side windings (IN1 and IN2). Leave the secondary side windings (OUT1 and OUT2) open.
4. Turn on the supply and slightly increase the voltage until the current limit is reached. Note that the supply might already be current-limited when the supply is turned on. Do not increase the current limit.
5. Record the voltage and current readings from the power supply display.
6. Set the voltage back to 0 V and disconnect the supply.
7. Adjust the current limit to 4 A, then connect the supply output across the secondary side windings (OUT1 and OUT2). Leave the primary side windings (IN1 and IN2) open.
8. Turn on the supply and slightly increase the voltage until the current limit is reached. Note that the supply might already be current-limited when the supply is turned on. Do not increase the current limit.
9. Record the voltage and current readings from the power supply display. For this transformer, the input voltage is 3.5 V and the current is 0.8 A.
10. Set the voltage back to 0 V, turn the supply off, and disconnect it.
11. Measure the resistance across the primary windings with a multi-meter.
12. Measure the resistance across the secondary windings with a multi-meter.
13. It is common to have the higher voltage side resistance to be higher than the lower voltage side resistance due to the fact that power on both sides is ideally equal, and higher voltage means lower current and thus lower resistance. The DC test and measured resistance on the multi-meter should match closely.

2. Open-circuit Test

1. Make sure the three-phase source is off.
2. Connect the circuit for the open-circuit test (Fig. 1). Use a digital power meter.
3. Make sure the VARIAC is at 0%.
4. Double-check that the circuit connections are as expected from Fig. 1, and then turn on the three-phase source.
5. Slowly adjust the VARIAC knob until the voltage reading on the digital power meter reaches 24 V.
6. Record the voltage, current, real power, and power factor of the power meter.
7. Set the VARIAC back to 0%, turn off the three-phase source, and disconnect the VARIAC output.
8. In the open-circuit or no-load test, the magnetizing reactance (X_m) and core loss resistance (R_C) are found from the current (I_OC), voltage (V_OC), and power (P_OC) measurements as follows:
   R_C= V_OC²/P_OC (1)
   and X_m= V_OC²/Q_OC (2)
   where Q_OC²=(V_OCI_OC)² - P_OC² (3)

Figure 1: DC test schematic. Please click here to view a larger version of this figure.

3. Short-circuit Test

1. Make sure the three-phase source is off.
2. Connect the circuit for the short-circuit test (Fig. 2). Make sure IN1 and IN2 are connected to the VARIAC output.
3. Make sure the VARIAC is at 0%.
4. Calculate the rated input current of the transformer. This is found by dividing the VA rating by the voltage rating on the input side. For example, if the input is 115 V and the VA rating is 100 VA, the input current rating is 100/115= 0.87 A.
5. Check the circuit, and then turn on the three-phase source.
6. Slowly and carefully adjust the VARIAC knob until the current reading on the digital power meter reaches rated input current.
7. Record the voltage, current, real power, and power factor on the power meter.
8. Set the VARIAC back to 0%, turn off the disconnect switch, and disconnect the VARIAC output. Keep the VARIAC three-phase cable connected.
9. Remove the short circuit placed across the transformer secondary.
10. In the short-circuit test, the leakage reactance (X₁+X₂'=X_eq) and wire resistance (R₁+R₂'=R_eq) of both windings are found from the current (I_SC), voltage (V_SC), and power (P_SC) measurements as follows:
    R_eq=P_SC/I_SC² (4)
    and X_eq= Q_SC/I_SC² (5)
    where Q_SC²=(V_SC I_SC)² - P_SC² (6)
11. X₁ is assumed to be equal to X₂', while R₁ and R₂' can be used from the DC test (or at least one of them). If the DC test is not performed, it is common to assume that R₁ and R₂' are equal.

Figure 2: Short-circuit test schematic. Please click here to view a larger version of this figure.

4. Load Test

Load tests show how the current and voltage values correlate between the input and output sides of the transformer where ideally, V₁/V₂ = I₂/I₁ = N₁/N₂ = a where N is the number of turns, subscripts 1 and 2 are for the primary and secondary sides, respectively, and a is the turns ratio. The impedance on the secondary side reflected to the primary side is R'=a²R or X'=a²X.

1. Make sure the three-phase source is off.
2. Connect the circuit for the load test (Fig. 3). Make sure IN1 and IN2 are connected to the VARIAC output.
3. Make sure the VARIAC is at 0%.
4. Connect an oscilloscope differential voltage probe across the primary with a 1/200 setting. Adjust the probe measurement for 0 V offset with an appropriate scaling factor.
5. Connect an oscilloscope current probe to measure the load current. Adjust the probe measurement for 0 mV offset with a 1X scaling factor for a 100 mV/A setting.
6. Check the circuit, and then turn on the three-phase disconnect switch.
7. Slowly adjust the VARIAC knob until V_P reads 115 V.
8. Record the voltage, current, real power, and power factor of both digital power meters.
9. Capture the oscilloscope screen with at least three cycles shown.
10. Turn off the three-phase source and set the VARIAC at 0%.
11. Replace the 100 Ω resistor with three 100 Ω resistors in parallel.
12. Turn on the three-phase source and slowly adjust the VARIAC knob until V_Preads 115 V.
13. Record the two digital power meter readings only (no oscilloscope screen capture).
14. Set the VARIAC back to 0%, turn off the disconnect switch, and disconnect the setup.

Figure 3: Load test schematic. Please click here to view a larger version of this figure.