AC Induction Motor Characterization

1. DC Test

Note that a squirrel-cage induction machine has only stator terminals accessible.

1. Turn on the low-power DC power supply and limit the current to 1.8 A.
2. Turn the supply off.
3. Connect the supply terminals across two of the induction motor terminals (labeled A, B, and C).
4. Turn the supply on and record the output voltage and current.
5. Repeat for the two other phase combinations.
   1. Note that the measured resistance is for two phases in series, thus the per-phase resistance is half the measurement.

2. No-Load Test

Test the induction machine with no load to find the per-phase magnetizing branch parameters X_m and R_C. For this test, make sure the load dynamometer has all its terminals disconnected, where it is generating no power and supporting no load.

1. Make sure the three-phase source is off.
2. Check that the VARIAC is at 0% and then wire the VARIAC to the three-phase outlet, and connect the setup (Fig. 1).
3. Double-check that the circuit connections are as shown in Fig. 1, and then turn on the three-phase source.
4. Quickly increase the VARIAC output until each of the digital power meters reads around 208 V.
5. Record the power, voltage, and current readings from both meters.
6. Measure the speed using the strobe light (tune the strobe light to a reasonable speed), and label the measurement as ω_o.
7. Record the torque reading in N·m or lb·ft, and label the measurement as T_o just in case the torque transducer or torque-measuring apparatus is not well calibrated. This is the no-load torque.
8. Set the VARIAC back to 0% then turn off the three-phase source. Leave the rest of the circuit intact.

Figure 1: Electrical setup for no-load test. Please click here to view a larger version of this figure.

3. Locked-Rotor Test

Test the induction machine with a locked rotor in a manner similar to the short-circuit test of a transformer. Use this test to find the per-phase series resistances and leakage inductances. For this test, make sure the load dynamometer has all its terminals disconnected.

1. Make sure the three-phase source is off.
2. Check that the VARIAC is at 0%.
3. Lock the rotor on the dynamometer side using a mechanical clamp or a zero-torque setting, if the dynamometer is digitally controlled.
4. Note that the setup is still similar to that of Fig. 1, except with a locked rotor.
5. Double-check that the circuit connections are as shown in Fig. 2.
6. Turn on the three-phase source and the induction machine switch.
7. Slowly and carefully increase the VARIAC until rated current is reached on one or both of the digital power meters.
8. Record the power, voltage, and current readings from both meters.
9. Set the VARIAC back to 0% then turn off the three-phase source. Leave the rest of the circuit intact.

Figure 2: Setup for load test. 

4. Load Test

Use this test to trace the linear torque-speed characteristic of the induction machine. For this test, use the dynamometer with a shunt-field as a generator (more on this operating condition is given later in the DC machines video, but the armature is the generator output port).

1. Make sure the three-phase source and induction machine switch are off.
2. Check that the VARIAC is at 0%.
3. Remove the locking clamp from the rotor shaft.
4. Connect the circuit (Fig. 2). Use R_L=300Ω but keep S_D off.
5. Do not use the series field.
6. Check the circuit, then turn on the three-phase source and the induction machine switch.
7. Quickly increase the VARIAC output until each of the digital power meters reads around 208 V.
8. Record the power, voltage, and current readings from both meters.
9. Measure the speed and label it as ω₁. To measure the speed, adjust the "Coarse" frequency knob on the strobe light until the shaft looks almost stationary, and then fine-tune the frequency setting using the "Fine" knob.
10. Record the torque reading and label it as T₁.
11. Note that this operating point (ω₁, T₁) is not the same as no load, because the field winding is also acting as a load in parallel with the armature. As S_D is turned later and R_L is decreased, the load is increased since the load current increases as R_L decreases.
12. Turn on S_D. Measure the speed and label it as ω₂.
13. Record the torque reading and label it as T₂.
14. Turn off S_D. Change R_L to 200 Ω, then turn on S_D.
15. Measure the speed and label it as ω₃.
16. Record the torque reading and label it as T₃.
17. Turn on S_D. Change R_L to 100 Ω. Turn on S_D.
18. Measure the speed and label it as ω₄.
19. Record the torque reading and label it as T₄.
20. Set the VARIAC to 0%, turn off the three-phase source, and disassemble the circuit.