Soft Starter – Principle and Working

A soft starter is any device which controls the acceleration of an electric motor by means of controlling the applied voltage.

Now let us have a brief recall of the need for having a starter for any motor.

An Induction motor has the ability to self start owing to the interaction between the rotating magnetic field flux and the rotor winding flux, causing a high rotor current as torque is increased. As a result the stator draws high current and by the time the motor reaches to full speed, a large amount of current (greater than the rated current) is drawn and this can cause heating up of the motor, eventually damaging it. To prevent this, motor starters are needed.

Motor starting can be in 3 ways

  • Applying full load voltage at intervals of time: Direct On Line Starting
  • Applying reduced voltage gradually : Star Delta Starter and Soft starter
  • Applying part winding starting: Autotransformer starter
Defining Soft Starting

Now let us shift our particular attention to soft starting.

In technical terms, a soft starter is any device which reduces the torque applied to the electric motor. It generally consists of solid state devices like thyristors to control the application of supply voltage to the motor. The starter works on the fact that the torque is proportional to the square of the starting current, which in turn is proportional to the applied voltage. Thus the torque and the current can be adjusted by reducing the voltage at the time of starting the motor.

There can be two types of control using soft starter:

Open Control: A start voltage is applied with time, irrespective of the current drawn or the speed of the motor. For each phase two SCRs are connected back to back and the SCRs are conducted initially at a delay of 180 degrees during the respective half wave cycles (for which each SCR conducts). This delay is reduced gradually with time until the applied voltage ramps up to the full supply voltage. This is also known as Time Voltage Ramp System. This method is not relevant as it doesn’t actually control the motor acceleration.

Closed Loop Control: Any of the motor output characteristics like the current drawn or the speed is monitored and the starting voltage is modified accordingly to get the required response.  The current in each phase is monitored and if it exceeds a certain set point, the time voltage ramp is halted.

Thus basic principle of soft starter is by controlling the conduction angle of the SCRs the application of supply voltage can be controlled.

2 Components of a basic soft starter
  • Power switches like SCRs which need to be phase controlled such that they are applied for each part of the cycle. For a 3 phase motor, two SCRs are connected back to back for each phase. The switching devices need to be rated at least three times more than the line voltage.
  • Control Logic using PID controllers or Microcontrollers or any other logic to control the application of gate voltage to the SCR, i.e. to control the firing angle of SCRs in order to make the SCR conduct at the required part of the supply voltage cycle.
Working Example of Electronic Soft Start System for 3 phase induction motor

The system consists of the following components.

  • Two back to back SCRs for each phase, i.e. 6 SCRs in total.
  • Control Logic circuitry in form of two comparators- LM324 and LM339 to produce the level and the ramp voltage and an opto-isolator to control the application of gate voltage to the each SCR in each phase.

A power supply circuitry to provide the required dc supply voltage.

Block Diagram showing Electronic Soft Start System for 3 phase Induction Motor
Block Diagram showing Electronic Soft Start System for 3 phase Induction Motor by Edgefx Kits

The level voltage is generated using the comparator LM324 whose inverting terminal is fed using a fixed voltage source and the non inverting terminal is fed through a capacitor connected to the collector of an NPN transistor. The charging and discharging of the capacitor causes the output of the comparator to change accordingly and the voltage level to change from high to low. This output level voltage is applied to the non inverting terminal of another comparator LM339 whose inverting terminal is fed using a ramp voltage. This ramp voltage is produced using another comparator LM339 which compares the pulsating DC voltage applied at its inverting terminal to the pure DC voltage at its non inverting terminal and generates a zero voltage reference signal which is converted to a ramp signal by the charging and discharging of a electrolyte capacitor.

The 3rd comparator LM339 produces a High pulse width signal for every high level voltage, which decreases gradually as the level voltage reduces. This signal is inverted and applied to the Opto isolator, which provides gate pulses to the SCRs. As voltage level falls, the pulse width of the Opto isolator increases and more the pulse width, lesser is the delay and gradually the SCR is triggered without any delay. Thus by controlling the duration between the pulses or delay between applications of pulses, the firing angle of SCR is controlled and the application of supply current is controlled, thus controlling the motor output torque.

The whole process is actually an open loop control system where the time of application of gate triggering pulses to each SCR is controlled based on the how earlier the ramp voltage decreases from the level voltage.

Advantages of Soft Start

Now that we have learnt about how an electronic soft start system works, let us recollect few reasons why it is preferred over other methods.

  • Improved Efficiency: The efficiency of soft starter system using solid state switches is more owing to the low on state voltage.
  • Controlled startup: The starting current can be controlled smoothly by easily altering the starting voltage and this ensures smooth starting of the motor without any jerks.
  • Controlled acceleration: Motor acceleration is controlled smoothly.
  • Low Cost and size: This is ensured with the use of solid state switches.


  1. Amaan mansuri says:

    Can u send me connection diagram of this circuit..I don’t know about the connection .Can u help me in my project ..? My project is soft start single phase induction motor…Plz.

    1. Hi Amaan mansuri
      once you purchase the project from our website then only we provide you with all documents, circuit diagram, source code, and AV tutorials
      please go through this link
      please visit our domestic website

      For more details please contact to Sathish on +91 8885507011 or you can email us on

  2. Nagarajan says:

    Hi Mr.Tarun Agarwal

    Do you have any Project experience or Idea about to control the Turbo Alternator(2MW) Output Voltage by using followings:-
    1.Automatic Voltage Corrector unit (AVR)
    2.Compounding Excitation Transformer

    If having any ideas or development, Can you pls inbox me.


  3. Abdulrahman says:

    Hi every one,
    Can we assume that soft starters will eventually lead to power saving if installed on air conditioning system?
    Power consumption in hot climate areas like Saudi Arabia is a major concern to everyone and I thought soft starter can help in power saving. Appreciate your feedback.

      1. sai rahul says:

        sir i need the documentation for my major project . topic is soft starting of dc motor using scr module via firing circuit (electronic starter) thank you in advance sir

        1. Hi Sairahul,
          As per the telephonic conversation,already you have been discussed with our technical team and also you are ready to come for our office within few days for knowing the more information regarding the project.

  4. Irfan khan says:

    soft starter is used only for smooth starting of induction motor . there is no any intermediate speed control like VVFD (variable voltage & variable frequency control) drive. It solve the purpose to reduce the starting current & voltage fluctuation . its compact unit therefore all the electronics component like SCR & other electronics remains inside the starter. The problem of heat dissipation is still with unit therefore cooling is must for that. this is very use full where the sooth torque is require (jerk free starting) like conveyor belts, crane operation, lift operation , elevators.

    1. Hi Irfan Khna,
      Please send this requirements to our Official mail id:
      So,that it would be easily guided you by the support of our technical team.
      For further details please contact Niranjan on +91-9959178000.

    2. Anil Waghmore says:

      Dear Sir,
      As i have experience with some soft starters, i found that the starting current of the starter is more than star-delta starter. Can you please guide me why it is so? i will be happy if i will get reply on it. for ex. If star delta starter is taking 33% starting current of the RLA, then for the same motor softstarter is taking 45 to 50 % dtarting current on it. Please explain.


      1. Hi Anil,
        We Suggest you to please visit our website once:
        And also,
        As we have gone through your Comment,so we suggest you that,
        Please send your specifications and queries on our official mail id :
        So,that our technical team will get back to you regarding your queries.
        For more details please contact Mr.Tarun Agarwal on +91-9908208883.

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