ALTERNATING CURRENT MACHINES – I(Practical:)
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9. Perform the operation of auto-transformer and determining its transformation ratio.
Perform the operation of auto-transformer and determining its transformation ratio.
9.1 Sketch the circuit diagram.
9.2 Collect tools, equipment & materials required.
9.3 Connect the equipment according to the circuit diagram.
9.4 Energize the circuit and tabulate the meter readings.
9.5 Calculate transformation ratio, conducted power & transformed power.
Auto Transformer Starting
Objectives
To start the induction motor using 3-phase auto-transformer.
Overview
In this method, an auto-transformer is placed in series with the motor during staring. The transformer action reduces the voltage applied to the motor terminals. Because of the transformer action, the line current is less than the motor current. For a given starting current on the line side, the motor terminal voltage can be higher than for other reduced voltage start methods. Thus, this method gives the highest motor torque per line ampere.
The auto- transformer is used to reduce the phase voltage to the fraction x of normal value. The advantage of the method lies in the fact that the voltage is reduced by transformation, not by dropping the excess in impedance, so that the line current and power input are reduced in comparison with stator resistance starting.
Neglecting magnetizing current and losses in the auto-transformer, the current on the output side is Is in the fraction of the turns, and on the input side will consequently be xIs = x2Ix in all the turns of each phase. Thus for a starting torque of x2 of that obtainable by direct switching, only x2 of the short-circuit current is taken from the line. Alternatively, for the same line current, the starting torque with the auto-transformer is 1/x times as great as that obtainable with the stator impedance startor.
The system of connections shown provides voltage steps during starting, the auto-transformer being cut completely out of service after the start has been completed. Since it is only in use for short periods, the current density in the windings may be increased considerably above that normal for power transformers. At the same time, since the starting torque depends on the square of the current, the impedance of the transformer should be kept small. The auto-connection is here of particular advantage.
Steps for Connection:
1. Connect CH1 of the oscilloscope to L21.
2. Connect CH2 and CH3 with the motor shaft.
3. Choose the red colour cable button.
4. Connect L21 to the one of the terminal of the ammeter.
5. Connect U1 to the other terminal of the ammeter.
6. Choose the green colour cable button Connect L22 to V1.
7. Connect w1 to one of the terminal of the voltmeter.
8. Choose the blue colour cable button Connect L32 to W1.
9. Connect w2 to the other terminal of the voltmeter
10. Choose the black colour cable button.
11. Connect U2 and V2
12. Connect V2 and W2
13. Click the check button and if the connection is correct, click OK.
14. If the connection is wrong, follow the error message displayed and correct the connection.
15. Select 1 on the two way switch on the left hand side of the panel.
16. Switch ON the MCB and click the start button.
17. Switch ON the second MCB.
18. Select star or delta by clicking the three way switch in the star delta starter
19. Note the readings of ammeter and voltmeter.
20. Select CH1, CH2 and CH3 button on Oscilloscope for instantaneous current , speed and torque reading