What is Soft Starter in Electrical Machines? – Diagram, Working | New Topic [2024]

In this note, we are going to learn about a topic called β€œWhat is Soft Starter in Electrical Machines?β€œ, also about it circuit diagram, working principle, advantages, disadvantages, and applications as well. Welcome to Poly Notes Hub, a leading destination for engineering notes for diploma and degree engineering students.

Author Name: Arun Paul.

What is Soft Starter in Electrical Machines?

A soft starter is an electrical device that works with AC electric motors to minimize the load and torque during startup. It does this by gradually increasing the starting voltage provided to the motor until it reaches its maximum functioning speed. This technique reduces mechanical stress on the motor and its associated drive systems, increasing their life and lowering the chance of damage.

what is soft starter in electrical machines - poly notes hub

Key Functions of Soft Starter –

  1. It reduces the first surge, eliminating potential motor damage and decreasing strain on the electrical supply system.
  2. A soft starter allows for a smooth transition to full speed by gradually increasing the voltage.
  3. Soft starters also help to manage torque upon startup.
  4. Soft Starters reduce mechanical loads on belts, gears, chains, and other drive train components, resulting in longer equipment life and reduced maintenance.
  5. Soft Starters can help with energy savings by optimizing the motor’s start-up procedure and lowering peak demand charges from utilities.

Soft Starter Diagram

soft starter diagram - poly notes hub

Soft Starter Working

The thyristors are coupled to this soft starting circuit’s three-phase power supply lines. These thyristors modify their conduction angle to regulate the voltage supplied to the motor. When the motor first starts, a low voltage is applied and then gradually increased. This gradual increase in voltage lowers mechanical stress and restricts inrush current. The bypass contactor engages when the motor reaches its maximum speed, shorting out the thyristors to reduce power losses and connecting the motor straight to the power source.

The control circuit, which includes sensors, continuously checks the motor’s voltage and current and adjusts in real-time to ensure a smooth start. The control circuit adjusts the firing angles of the thyristors to give controlled and linear acceleration. This technique ensures that the motor accelerates gently, which reduces mechanical wear and extends the life of both the motor and the driven equipment. During normal operation, the soft starter stays in standby mode, ready to handle subsequent startup cycles, improving overall system performance and dependability.


Here listed some advantages of this device –

  1. It is not costly.
  2. Operation is very smooth.
  3. Reduce load on the power supply system.
  4. It is optimizes the starting process of the electrical machine.
  5. It is also prevent mechanical shocks and damages.


Here we listed some disadvantages of this device –

  1. Easily generate heat.
  2. This heat will impact the operation of this system.


These are the applications of this device –

  • Used in Pumps, Fans, Mixers, Crushers and Grinders as well.
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