ALTERNATING CURRENT MACHINES – I(Practical:)
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12. Start a 3-phase induction motor by Star-delta starter using PLC.
Start a 3-phase induction motor by Star-delta starter using PLC.
12.1 Take a 3-phase squirrel cage induction motor.
A 3 phase squirrel cage induction motor is a type of three phase induction motor which functions based on the principle of electromagnetism. It is called a ‘squirrel cage’ motor because the rotor inside of it – known as a ‘squirrel cage rotor’ – looks like a squirrel cage.
Squirrel Cage Induction Motor Construction
A squirrel cage induction motor consists of the following parts:
- Stator
- Rotor
- Fan
- Bearings
Stator
It consists of a 3 phase winding with a core and metal housing. Windings are such placed that they are electrically and mechanically 120o apart from in space. The winding is mounted on the laminated iron core to provide low reluctance path for generated flux by AC currents.
Rotor
It is the part of the motor which will be in a rotation to give mechanical output for a given amount of electrical energy. The rated output of the motor is mentioned on the nameplate in horsepower. It consists of a shaft, short-circuited copper/aluminum bars, and a core.
The rotor core is laminated to avoid power loss from eddy currents and hysteresis. Conductors are skewed to prevent cogging during starting operation and gives better transformation ratio between stator and rotor.
Fan
A fan is attached to the back side of the rotor to provide heat exchange, and hence it maintains the temperature of the motor under a limit.
Bearings
Bearings are provided as the base for rotor motion, and the bearings keep the smooth rotation of the motor.
Application of Squirrel Cage Induction Motor
Squirrel cage induction motors are commonly used in many industrial applications. They are particularly suited for applications where the motor must maintain a constant speed, be self-starting, or there is a desire for low maintenance.
These motors are commonly used in:
- Centrifugal pumps
- Industrial drives (e.g. to run conveyor belts)
- Large blowers and fans
- Machine tools
- Lathes and other turning equipment
12.2 Collect the equipment & tools required.
12.3 Draw the working diagram to perform the experiment.
12.4 Connect the starter with the motor as per diagram.
12.5 Push the start button to start the motor automatically first in star and then in delta
position.
12.6 Measure the speed of the motor with tachometer.
12.7 Calculate slip of the motor.
Star-Delta Starting
Objectives
To start the induction motor by applying Star-Delta starter.
Overview
This is a starting method that reduces the starting current and starting torque. At starting the Induction motor is star connected after it reached the approximate operational speed it is switch to delta. The motor must be delta connected during a normal run, in order to be able to use this starting method.
This starting method only works when the application is light loaded during the start. If the motor is too heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position.
The induction motor must be built to run normally with a mesh-connected stator winding. At starting, the winding is connected temporarily in star. The phase voltage is thus reduced to 1/√3 = 0.58 of normal, and the motor behaves as if the auto-transformer were employed with a ratio x = 0.58. The starting current per phase is Is = 0.58Ix , the line current is (0.58)2Ix =0.33Ix , the starting torque is one-third of short-circuit value.
The method is cheap and effective, so long as the starting torque is not required to exceed about 50 percent of full load torque. It can therefore be used for machine-tools, pumps, motor generators etc. The method is unsuitable for motors at voltages exceeding 3000 V. because of the excessive number of stator turns needed for delta running. Where induction motors are required to run for considerable periods on small loads, a star-delta switch permits the machine to be star-connected during these periods, with reduction of magnetizing current and increase in efficiency.
Steps for Connection:
1. Connect CH1 of the oscilloscope to L11.
2. Connect CH2 and CH3 with the motor shaft.
3. Choose the red colour cable button.
4. Connect L11 to the one of the terminal of the ammeter.
5. Connect l1 to the other terminal of the ammeter.
6. Choose the green colour cable button Connect L12 to l2.
7. Choose the blue colour cable button Connect L31 to l3.
8. Choose the black colour cable button.
9. Connect w1 to one of the terminal of the voltmeter.
10. Connect w2 to the other terminal of the voltmeter.
11. Connect u1 of the star delta starter to u1 of the motor terminals.
12. Connect v1 of the star delta starter to v1 of the motor terminals.
13. Connect w1 of the star delta starter to w1 of the motor terminals.
14. Connect u2 of the star delta starter to u2 of the motor terminals.
15. Connect v2 of the star delta starter to v2 of the motor terminals.
16. Connect w2 of the star delta starter to w2 of the motor terminals.
17. Click the check button and if the connection is correct, click OK.
18. If the connection is wrong, follow the error message displayed and correct the connection.
19. Select 1 on the two way switch on the left hand side of the panel.
20. Switch ON the MCB and click the start button.
21. Switch ON the second MCB.
22. Select star or delta by clicking the three way switch in the star delta starter
23. Note the readings of ammeter and voltmeter.
24. Select CH1, CH2 and CH3 button on Oscilloscope for instantaneous current , speed and torque reading
For video tutorial click the link below