What is a soft starter? What does a soft starter do?

A soft starter is a device that is used to provide a soft start to the motor by gradually ramping up the voltage. During the starting of a three-phase motor, it would have a very high starting torque and also has a high starting current. A soft starter can protect the AC electric motor from damages that could happen by the sudden influxes of power by limiting the large inrush of the current during the motor start-up, a soft starter is also called as reduced voltage soft starters. So basically a soft starter would protect the motor and the connected equipment from damage by controlling the terminal voltage.

What are the functions of a soft starter?

  • Step-less starting
  • Protection against short circuit
  • Protection against overload
  • Reduction of current peaks
  • Avoid the voltage fluctuations during starting
  • Maintenance-free switching

How does a soft starter work?

A soft starter is an electronic device that can control the power of the engine at the starting as well as its braking. The operation of a soft starter is based on a static component like a thyristor. A soft starter would adjust the voltage applied to the motor during starting, the current drawn by the motor, and the torque produced by the motor can be reduced and controlled without changing the frequency. The motor voltage is adjusted by soft-starters by using back to back thyristor or traic in each AC supply line to the motor. During the start-up, the thyristors are actuated, so that their turn-on is successfully delayed less for each AC half cycle. The average AC voltage to the motor will be ramped up due to the delayed switching until the motor reaches its voltage level.

What is the difference between VFD and a soft starter?

A soft starter would control the initial inrush of the current that occurs during the motor start-up and after that, it would provide a gentle ramp-up to full speed. A soft starter is only used during the motor start-up. A VFD would control the motor speed by changing the frequency of the power and it can control the motor speed at any time during the motor operation.

How can we do a soft starter connection?

We can connect a soft starter in two-ways they are in-line and inside delta. Inline is the most common method to do a soft starter connection while inside delta connection is only done for very few soft starters.

Inline connection

This type of connection is very simple and is used commonly, in this type all three phases are connected in series with the overload relay. The devices that are selected to do the inline connection must cope with the rated motor current.

Inside delta connection

This type of connection is usually done very less, by this type of connection we could place the soft-starter in the delta-circuit, and because of that, it can replace the existing Y/D starter. In this connection when the soft-starter is inside the delta, it will only be exposed to sixty percent of the inline current. So we can downsize the device to get a cost-effective solution.

Where should we use a soft starter?

• It can be used for fast starting loads, with a low starting torque
• It can be used with drives that start in an unloaded state
• Applications which can be started with star-delta combination
• It can be used in applications that use another voltage reducing start process

What are the advantages of a soft starter?

  • Soft starters are a very economical choice for applications that require speed and torque control only during the motor start-up.
  • It doesn’t use much space, it would take less space when compared to a VFD
  • No need to worry about harmonics
  • It would reduce voltage drops and most of the problems associated with it
  • Efficiency will improve
  • The motor acceleration is controlled smoothly

What are the applications of a soft starter?

  • Soft switching of motor starters in conveyor and transport belts
  • Three-phase resistive and inductive loads
  • Soft starting of the pump reduces the load on the entire installation
  • Escalators
  • Compressors and coolers
  • Assembly lines
  • Ventilators
  • Transport systems
  • Air conditioning system
  • Centrifuges


post-graduate in Electronics & communication.

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