1. SOFT START OF 3 PHASE INDUCTION MOTOR BY USING 2 NUMBERS BACK TO BACK SCRS IN EACH PHASE
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2. Contents Project overview Block diagram Power supply Induction Motor
Soft start of IM
Comparator
Opto-isolator
Schematic & Working of the project
Advantages
Applications
Future scope
Conclusion

3. Project overview
These Solid State Switches are phase controlled in a similar manner to a light dimmer, in that they are turned on for a part of each cycle.
The average voltage is controlled by varying the conduction angle of the switches.
Increasing the conduction angle will increase the average output voltage.
Controlling the average output voltage by means of solid state switches has a number of advantages.
One of the major advantages being the vast improvement in efficiency relative to the primary resistance starter, due to its low on state voltage of the solid state switches.

4. Contd..
Typically, the power dissipation in the starter, during start, will be less than 1% of the power dissipated in a primary resistance starter during start.
Another major advantage of the solid state starter is that the average voltage can be easily altered to suit the required starting conditions.
By variation of the conduction angle, the output voltage can be increased or reduced, and this can be achieved automatically by the control electronics.

5. Block diagram

6. Power supply

7. Contd..
The 230V AC supply is first stepped down to 12V AC using a step down transformer.
This is then converted to DC using bridge rectifier.
The AC ripples is filtered out by using a capacitor and given to the input pin of voltage regulator 7812.
At output pin of this regulator we get a constant 12V DC which is used for different ICs in this project.

8. Three Phase Induction Motor
It consists of the fixed stator or frame, a three-phase winding supplied from the three-phase mains and a turning rotor
There is no electrical connection between the stator and the rotor
Stator and rotor are made of highly magnetizable core sheet providing low eddy current and hysteresis losses

9. Stator & Rotor
The stator winding consists of three individual windings which overlap one another and are offset by an electrical angle of 120°
Synchronous Speed (Ns)= 60*f/p
Ns= Synchronous Speed/ minute;
F= frequency;
P=Pole pair Number;
Rotor in induction machines with squirrel-cage rotors consists of a slotted cylindrical rotor core sheet package with aluminum bars which are joined at the front by rings to form a closed cage

10. Soft starter
A soft starter is another form of reduced voltage starter for A.C. induction motors.
The soft starter is similar to a primary resistance or primary reactance starter in that it is in series with the supply to the motor.
The current into the starter equals the current out.
The soft starter employs solid state devices to control the current flow and therefore the voltage applied to the motor.
Soft starters are connected in series with the line voltage applied to the motor, or can be connected inside the delta loop of a delta connected motor, controlling the voltage applied to each winding.

11. Contd..
Voltage control is achieved by means of solid state A.C. switches in series with one or more phases. These switches comprise either:
a. Triac b. diode & SCR c. Two SCRs

12. SCR
A Silicon Controlled Rectifier (or Semiconductor Controlled Rectifier) is a four layer solid state device that controls current flow
An SCR can be seen as a conventional rectifier controlled by a gate signal
It is a 4-layered 3-terminal device
When the gate to cathode voltage exceeds a certain threshold, the device turns 'on' and conducts current

13. Comparator op amps & comparators look very similar
But a comparator gives a logic output indicating the relative potentials on its two inputs
An op amp amplifies the differential voltage between its two inputs – and is designed always to be used in closed-loop applications

14. Contd..
Potential dividers are connected to the inverting and non inverting inputs of the op-amp to give some voltage at these terminals.
Supply voltage is given to +V and –V is connected to ground.
The output of this comparator will be logic high (i.e., supply voltage) if the non-inverting terminal input is greater than the inverting terminal input of the comparator.
If the inverting terminal input is greater than the non-inverting terminal input then the output of the comparator will be logic low (i.e., gnd).

15. Optocoupler
Opto coupler is a 6 pin IC. It is a combination of 1 LED and a diac. Pin 5 is not generally used and when light falls on the diac then it switches ON the diac.

16. Contd..
When logic zero is given as input to the LED then the light doesn’t fall on diac so the diac is off means the current does not flow through the diac.
When logic 1 is given as input to the LED then light emitted by LED falls on diac so it starts conducting i.e., now there will be current flowing through the diac.

17. Contd..
The operation of a SCR can be understood in terms of a pair of tightly coupled Bipolar Junction Transistors
SCR has three states:
Reverse blocking mode, forward blocking mode, and forward conducting mode

18. Working of project
To start the motor with low current, two SCR’S are connected back to base in each phase.
They are triggered slowly initially and gradually the triggering pulse is increased, so that motor gets high current after a time delay when windings are in the position of withstanding the high current.
To trigger the gates of SCRS the op-amps are used.
Op-amp is configured to get a level voltage that will initially be high and gradually fall to zero.