soft starter for 3 phase induction motor using pic microcontroller

This project is about soft starter for 3 phase induction motor using pic microcontroller. In this project, we have designed a soft starter for three phase induction motor using PIC16F877A microcontroller. I have already posted a project on soft starter for single phase induction motor using pic microcontroller. Soft starter is a electronics device which used for smooth start of three phase induction motor.  Every induction motor takes high current at the start which is more than the rated capacity of three phase induction motor.

This high current is also knows as inrush current.  This inrush current may also damage the motor winding’s permanently. So we need a device which can control the starting high current of three phase induction motor.  Smooth starter or soft starters are usually used to control this inrush current of the motor when motor starts to run.   So Now lets start with the introduction of soft starter and see how smooth starter works? And How I designed the soft starter for three phase induction motor using pic16f877a microcontroller and at the end of article I will show you the simulation of soft starter in proteus. If you don’t know how to use proteus software, you may like to check my tutorials on proteus.

Introduction to soft starter for three phase induction motor

When we initially start a 3 phase induction motor, it draws a enormous amount of current which is also known as inrush current.  3 phase induction motor also reaches it full speed in minimum time or instantly.  Due to instantly reaching full speed, it causes mechanical vibrations to motor and it also causes electrical stress on motor windings which may damage the motor windings. So to avoid this effect, we use soft starter or 3 phase smooth starter for induction motor.  To avoid this instantly speed increasing effect of three phase induction motor, we need to control the speed of induction motor at the start sch that the motor speed should increase gradually only.

  This gradual increase in speed of three phase induction motor also forces the current to increase gradually. So this gradual increase in speed and current  saves the motor from mechanical jerks and avoid the chances of motor windings get burned. Now the question is how to achieve this gradual increase in motor speed using pic microcontroller. So now lets see how I achieved it using pic16f877a microcontroller.  You may also like to check my other induction motor related projects:

soft starter for three phase induction motor using pic microcontroller

In this project, I have used controlled voltage through thyristors to control the voltage provided to  three phase induction motor. At the start, thyristors firing angle is triggered with full delay and after that I started decreasing thyrsitors firing angle and it will keep decreasing gradually. Finally thyristor firing angle will become zero and full voltage will appear across all the three phase of induction motor. In other words, at the start minimum voltage will be applied to 3 phase induction motor and than it will start increasing. After sometime full voltage will appear across motor windings. It will help the three phase induction motor to start smoothly without any inrush current. So initially motor starts slowly and then reaches its full rated speed gradually.  This is how easy it is to implement smooth starter for three phase induction motor.

Circuit diagram of soft starter for three phase induction motor

Circuit diagram of smoother start for three phase induction motor using pic microcontroller is shown below.  Its consists of three zero crossing detection circuits for each phase of three phase power supply and PIC microcontroller which is used to provided firing angle to thyristors through opto isolator.soft starter for three phase induction using pic microcontroller

If you don’t know how to control firing angle of thyristor, you can check these article:

To check the complete working of this power electronics project, check the simulation given below.

16 thoughts on “soft starter for 3 phase induction motor using pic microcontroller”

  1. Hi!
    This is an excellent post. This is exactly what I have been desperately looking for for a while. I have seen a bunch of circuitry with thyristors or triacs but never a circuitry to drive them.
    Still, I have some (silly) questions.

    1) What is the ref of the transformers you used? Particularly, what is the output AC voltage to reach the near-5DCV at the LM7805 input (after the bridge of Graetz )?

    2) If I got it right, you need three potentiometers (1 for each phase) to set the triacs firing angle. I suppose also that you must “turn” them all at once? How does the 3-phase motor react otherwise?

    3) You never mention the power of the motor that your system was designed for. I want to apply your circuitry to start a 11kW motor. Are there specific recommendations for that?

    Thank you again for this marvelous document.

  2. So Interesting I like it . I would like to make one by myself . Please help me with a soft starter circuit diagram to drive a 1kw 380/400vac induction motor .

  3. why transistors are connected like that to moc3021, how the transistor connected to moc3021 will switch on as it is in float state, i think it should be like darlington pair

  4. Hello sir
    plz can i get the design and the whole circuit of a soft starter of single phase induction??
    i really want to realize it. TKS


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