The rotor speed is denoted as Nor and the phenomenon that the rotor is continuously falling behind the stator rotating miff is called slip. The mathematical expression is shown as below [pica] (Equation 3) 3. APPARATUS Three-phase induction motor 1 unit DC generator Power Converter Power Analyzer Tachometer Three-phase variable resistive load bank Three-phase load switch 4. PROCEDURE 4. 1 Nameplate data and preliminaries 4. 1. 1 Record the nameplate (or rated) data of the motor. 4. 1. 2 Measure the resistance per phase (RI ) of the stator winding using power analyzer. 4. 2 No-Load Test 4. 2. Make sure that the shaft of the induction motor is decoupled from the shaft of the DC generator. 4. 2. 2 Connect the three-phase stator winding of the induction motor in delta configuration. Connect the three-phase variable AC supply to the stator winding of the motor. Also connect the power analyzer to he circuit to measure the input voltage, current and power. The whole configuration is shown as figure 2. 4. 2. 3 Set the dial of the three-phase variable AC supply to zero position. Switch on the power supply and gradually increase the voltage to the rated value for delta connection.
Record the line current, line voltage and two power reading WI and WWW. Also record the speed of the motor using a tachometer. 42. 4 Reduce the variable voltage to zero and switch off the power supply. [pica Figure 2: No-Load Test 4. 3 Blocked-Rotor Test Loosen the screws on the base plate of the induction motor and move it 3. 1 as close as possible to the DC generator. Lock the shaft of the induction motor with the mechanism so that it cannot rotate. Tighten the screws to make the motor unmovable. 4. 3. Connect the power analyzer to measure the line current.
Make sure that the dial of the three-phase AC supply source is at zero position. Switch on the power supply and gradually increase the variable AC voltage until the line current of the motor reaches the rated value. 4. 3. 3 Record the line current, line voltage and two power reading WI and WWW. 4. 3. 4 Reduce the variable voltage to zero and switch off the power supply. . 4 Motor performance under load connections (Load Test) 4. 4. 1 Loosen the screws on the base of the induction motor. Remove the locking mechanism and couple the shaft of the induction motor to the shaft of the DC generator by inserting the plastic coupler.
Tighten the screws to prevent vibration. 4. 4. 2 Connect the iv fixed DC supply to generator. Connect only two phases of the three-phase variable load resistor in parallel. Set the load resistor to the maximum resistance position and the load switch S at “off’ position. The whole configuration is shown as figure 3. 44. 3 Switch on the DC supply and adjust the field current to 0. AAA. Switch on the three- phase AC power supply and gradually increase the variable AC voltage to the rated value. Measure the line voltage, line current, power WI, WWW, the motor speed and the shaft torque. 4. 4 Turn to the switch S. Increase the line current of the induction motor up to the rated value in step of about 0. AAA by reduce the load resistance. Record the corresponding line voltage, line current, power reading, shaft torque and the speed of the motor. 44. 5 Increase the load resistance to the maximum position and turn off the switch S and DC supply. Reduce the variable AC voltage to zero value and switch off the AC power supply. [pica] Figure 3: Connections of DC generator for the load test of Induction motor 4. 5 No-Coda speed control by varying supply frequency 4. 5. Loosen the screws on the induction motor base plate. Move the induction motor so that it can rotate freely. Tighten the screws on the induction motor base plate. 45. 2 Connect the induction motor to the power converter. Then connect the input of the power converter to a single-phase supply socket. The whole configuration of the circuit is shown as figure 4. 4. 5. Switch the single- phase supply and set the out put frequency of the converter to Coho. Record the corresponding line voltage, frequency and the motor speed for the converter output frequency from Coho to GHz in steps of Coho. 4. 5. Switch off all the power supply and disconnect the circuit. [pica Figure 4: Speed control by varying supply frequency 5. RESULTS 5. 1 data on the nameplate Since it is a three-phase squirrel-cage induction motor, it has four poles. So the data on the nameplate, the measured RI and the corresponding synchronous speed of the motor is listed in table 1. IP (k) If (Haze) Nor(rev/min) Pip ([email protected]) I RI (Q) INS (rev/min) 1 11. 5 150 220-240 16. 4-6. 3 11415 10. 77 15. 0 1. 5 1 500 | 5. 0 | 380-420 13. 7-3. 6 11415 | 1500 1 11. 8 | 1800 160 1440-480 13. 7-3. 6 11710 10. 78 Table 1: data on the nameplate 5. Determine the values of the equivalent circuit parameters of the induction motor 5. 2. 1 Blocked-Rotor Test Since the rotor is blocked, the slip s=l, and the shunt path of the equivalent circuit is ignored because most of the current flow through the rotor circuit. So the equivalent circuit is simplified as figure 5. Then perform the calculation using he data recorded in table 2, we could get the results of RE’, X 1, XX’ in table 4. (The details of the calculation could be referred in Appendix A) [pica Figure 5: equivalent circuit for Blocked-rotor test IVR(V) 1 153. 8 Ill (A) | 6. 33 BIRR(A) 13. 5 III (w) 1310 Table 2: data recorded for block-rotor test 5. 2. 2 No-Load Test The slip s is almost zero because the rotor rotates freely under the no-load test which means that the rotor speed is more or less the same as the synchronous speed. Thus the RE’/s is very large, so the rotor circuit is ignore in the calculation. And the equivalent circuit is simplified as figure 6. We could easily get Arc and XML by using the data in table 3. The results are listed in Table 4. (The details of the calculation could be referred in Appendix A) [pica Figure 6: The equivalent circuit for No-Load Test IVAN(V) 1 1220 Table 3.
IR (Q) XML (Q) 199. 02 ILL(A) | 3. 65 NIL(A) 12. 10 1-296 14. 26 1468 data recorded for no-load test XIX (Q) 15. 00 | 5. 73 11215 Table 4: The result of parameters of the equivalent circuit Table 4 shows that the calculated RE’ is more or less equal to RI. Arc is much great than RI and RE. What’s more, XML is great than XSL and XX’, which are expected. So the results are reasonable. 5. 3 relationships between the parameters under the load condition Figure 6 shows the relationship between shaft torque and slip.
It’s easy to tell that the shaft torque is nearly linearly proportional to the slip. What’s more, the torque is nearly zero under no load condition. The reason is that the hypothetical rotor resistance plays a dominant role in this slip region. As the slips decrease, the hypothetical resistance is decreasing, thus the current in the rotor is decreasing, and eventually cause the toque decreases. Figure 7 shows that the efficiency of odor first increases to reach a maximum value with the increasing of the output power.
Then it decreases when the output power increases further more. Figure 8 tells us that the power factor (if) is linearly proportional to the output power, i. E. The power factor increases with the increasing of output power. (All the raw data is kept in Appendix C for reference) Figure 6: torque vs… Slip Figure 7: efficiency vs… Output power [pica][pica] Figure 8: if vs… Output power Figure 9: speed vs… Frequency 5. 4 The relationship between the no-load speed and the frequency Figure 9 tells s that the no-load speed is linearly proportional to the frequency.
Since this experiment is under no-load condition, the slip is nearly zero, which means that the rotor speed is almost equal to the synchronous speed. But the synchronous speed is linearly dependent on the frequency for the same motor. Thus the rotor speed under no-load condition is linearly dependent on the frequency. 6. DISCUSSION 6. 1 Why the no-load input current and power of the motor are considerably higher than those of a transformer? The no-load input current is 58. 0% of the rated current and the no-load power is nearly 1 1. % of the rated input power for the motor.
Because the rotor rotates freely under no-load condition, and the speed is quite higher (about 1 radar/sec). So the motor needs extra power to support the rotor rotation except the copper loss and the coil loss. Thus the motor needs considerably higher input power compared to that of the transformer. As the motor under the rated voltage, higher no-load input current is required for the higher input power. 6. 2 How can the starting current be reduced to an acceptable value in practice? The approximate starting current of the motor is 27. A at the rated voltage, which is 432% of the rated current.
Such high current is not acceptable in practice, because it results in an excessive line voltage drop which, in turn, may affect the operation of other machines operating on the same power supply. Equation 4 shows that the starting current is proportional to the starting voltage but inverse proportional to the rotor resistance. So the starting current could be reduced either by supplying a lower voltage or increasing the rotor resistance or both. However, the drawbacks are that both adjustments would decrease the tarring torque. So the methods would only employ for the applications that do not require higher starting torque. . 3 The accuracy of the calculated parameters The theoretical results, the recorded results and the corresponding errors are list in the table 5. (The detail calculation is listed in Appendix B) It is clear that the calculated values are quite accurate comparing with the recorded values. So the parameters values got from the no-load test and the blocked-rotor test are quite accurate and acceptable. There are two possible error sources. First one is the measurement error. The error occurs during the measurement because of the incorrect reading.
Secondly, the errors are generated during the motor operation, such as high friction force loss of the rotation and the loss of the motor vibration. I I Rated voltage I Rated frequency II-nine current I Power factor I Output power Developed torque Efficiency Calculated value 220 I(W) I(Nm) 16. 04 11418 179. 75% I I Recorded value 1220 | 6. 30 | 0. 77 1420 4. 13% NINA 10 10. 14% I I Error in I I percentage Table 5 7. CONCLUSION After the experiments, we have learned how to determine the equivalent circuit ramset’s of the three-phase squirrel-cage induction motors through no- load test and blocked-rotor test.
Meanwhile, we obtain several performances of the induction motor: Firstly, the shaft torque is linearly proportional to the slip during the certain region, the power factor increases with the increasing of output power and the efficiency has a maximum value according to the output power. Secondly, we found that the no-load speed is linearly dependent on the frequency because of the zero slip under no-load condition. Thus we could control the rotor speed by setting the appropriate frequency. . REFERENCE  B. S Guru, H. R. Highlight.