Basics of Soft Starter, Working Principle


 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.