DC Motors

1. DC Tests

1. With the low-power DC power supply limited to 0.8 A, connect the supply terminals to the DC machine armature.
2. Record the supply's DC voltage and current readings.
3. Estimate the resistance of each winding.
4. Repeat for the other windings, shunt field and series field, one at a time.
5. Turn off and disconnect the low-power DC power supply.
6. Set the built-in field rheostat to maximum resistance and measure its resistance.
7. Set the series field rheostat (external) to the maximum resistance and measure its resistance.

2. Prime-Mover Setup and Residual Magnetism

The prime-mover in this experiment is the synchronous machine, which operates as a motor that spins the DC generator rotor (armature).

1. Make sure the three-phase disconnect switch, synchronous motor switch, and DC motor switch are all off.
2. Check that the VARIAC is at 0%.
3. Wire the VARIAC to the three-phase outlet, and connect the setup shown in Fig. 5.
4. Check that the "Start/Run" switch is in the "Start" position.
5. Turn on the three-phase disconnect switch.
6. Turn on the high-voltage DC power supply.
7. Make sure all connections are clear from the supply terminals.
8. Press the "V/I DIS" button on the supply to display the voltage and current operating points. Adjust the voltage knob to 125 V.
   1. Do not press the start button.
9. Press the "Start" button on the DC power supply panel.
10. Slowly increase the VARIAC output until V_AC1 reads 120 V.
11. When the synchronous motor reaches a steady-state speed, flip the Start/Run switch into the Run position.
12. Measure and record the rotational speed using the strobe light and record V_A.
13. Turn off the DC power supply and return the VARIAC to 0%.
14. Reset the "Start/Run" switch to "Start."
15. Turn off the three-phase disconnect switch.

Figure 5: A schematic of how to setup the prime-mover. Please click here to view a larger version of this figure.

3. DC Shunt Generator Characterization

1. On the DC generator side, connect the shunt field in parallel with the armature field as shown in Fig. 6.
2. Use the built-in rheostat for R_Fsh(ext), and use the multi-meter as an ammeter to measure I_Fsh.
3. Keep "S₁" open for a no-load test.
4. Keep "R_Fsh(ext)" at maximum resistance.
5. Turn on the three-phase disconnect switch.
6. Press the "Start" button on the DC power supply panel.
7. Slowly increase the VARIAC output until V_AC1 reads 120 V.
8. When the synchronous motor reaches a steady-state speed, flip the "Start/Run" switch into the "Run" position.
9. Measure the shaft speed using the strobe-light technique described elsewhere.
10. Record V_A at this no-load condition on the DC generator side.
11. Reduce R_Fsh(ext) until the voltage generated at V_A is around 150 V.
12. After that point, reduce "R_Fsh(ext)" in five almost-equal steps until the minimum resistance is reached.
    1. For each step, measure V_A and I_Fsh.
13. Leave R_Fsh(ext) at its minimum value.
14. Turn off the DC power supply.
15. Reduce the VARIAC output to 0%.
16. Move the ammeter from measuring I_Fsh to measure I_A.
17. Restart the setup as described earlier.
18. Set R_L to 300 Ω, and turn on "S₁". Measure V_A and I_A.
19. Turn off "S₁," set R_L to 200 Ω, then turn on "S₁." Measure V_A, and I_A.
20. Turn off "S₁," set R_L to 100 Ω, then turn on "S₁." Measure V_A, and I_A.
21. Turn off the DC power supply and set the VARIAC output to 0%.
22. Keep the synchronous generator side of the setup intact.
23. Disconnect the DC generator connections.
24. Reset the "Start/Run" switch to "Start."
25. Turn off the three-phase disconnect switch.

Figure 6: A schematic of the shunt DC generator setup. Please click here to view a larger version of this figure.

4. DC Series Generator Characterization

1. On the DC generator side, connect the series field in series with the armature field as shown in Fig 7.
   1. Use the external rheostat for R_Fse(ext).
   2. Use the built-in rheostat as R_L and have it at maximum resistance.
   3. Keep "S₁" open for a no-load test.
   4. Keep R_Fse(ext) at maximum resistance.
2. Turn on the three-phase disconnect switch.
3. Press the "Start" button on the DC power supply panel.
4. Slowly increase the VARIAC output until V_AC1 reads 120 V.
5. When the synchronous motor reaches a steady-state speed, flip the "Start/Run" switch into the "Run" position.
   1. Measure V_A at this no-load condition on the DC generator side.
6. Turn on "S₁" and reduce R_Fse(ext) as needed to see non-zero V_A.
7. Vary R_L in five almost-equal steps until its 50% setting is reached, set to 300 Ω, and turn on "S₁." Measure the speed, V_A, and I_A.
   1. Turn off "S₁," set R_L to 200 Ω, then turn on "S₁." Measure the speed, V_A, and I_A.
   2. Turn off "S₁," set R_L to 100 Ω, then turn on "S₁." Measure the speed, V_A, and I_A.
8. Turn off the DC power supply.
   1. Set the VARIAC output to 0%.
   2. Keep the synchronous generator side of the setup intact.
   3. Disconnect the DC generator connections.
   4. Reset the "Start/Run" switch to "Start."
9. Turn off the three-phase disconnect switch.
10. Disassemble all wires and meters.

Figure 7: A schematic of the series DC generator setup. Please click here to view a larger version of this figure.

<