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How to Control the Compressor Motor in Refrigeration Using Soft Starter?

Controlling a Three-Phase Compressor Motor in Refrigeration using Soft Starter

In refrigeration systems, compressors play a vital role in maintaining the desired temperature levels. The compressor motor, being a crucial component, requires an efficient and reliable method of control. One such method is the utilization of a soft starter.

A soft starter is an electronic device that gradually increases the voltage supplied to the motor, reducing the startup current and ensuring a smooth and controlled acceleration. This article aims to explain the concept of soft starters and provide a step-by-step guide on how to control the compressor motor in refrigeration using a soft starter.

Soft Starters

A soft starter is a solid-state device that is installed between the power supply and the motor. Its primary function is to regulate the initial current surge during motor startup. Traditional direct-on-line (DOL) starters deliver full voltage to the motor from the beginning, causing high inrush current and mechanical stress on the motor and connected equipment.

In contrast, a soft starter employs a voltage ramp-up technique, gradually increasing the voltage over a specific time period. This controlled acceleration not only reduces the mechanical stress but also enhances energy efficiency and extends the lifespan of the motor.

What is Soft Starter?

Components Needed

  • 3-Phase Motor
  • MCCB – 3-P, 400V
  • MCB – 2-P, 100-230V
  • Soft Starter
  • ON OFF Push Button
  • Indicators and Pilot Lights
  • 230V Single Phase AC Supply
  • 400V – Three Phase AC Supply
  • Wires and Cables

Wiring & Control Diagram

The following wiring, power and control diagram shows how to control a 400V three-phase compressor motor in refrigeration using soft starter.

Click image to enlarge

How to Control the Compressor Motor in Refrigeration using Soft Starter

As shown, the output terminals of the contactor and OL relay (T1, T2 & T3) are connected to the common terminals of the soft starter (L1, L2 & L3). The single-phase line (L1 & L2 or Neutral) is directly connected to the A1 and A2 terminals of the soft starter. Lastly, the output terminals of the soft starter (T1, T2, and T3) are connected to the compressor motor.

Additional pilot lights and pushbuttons are connected to the contactor through a single-phase supply, with phase and neutral wires, as illustrated in the wiring diagram above.

Controlling the Compressor Motor using Soft Starter

Step 1: Understand the Soft Starter Requirements

Before implementing a soft starter in a refrigeration system, it is crucial to consider the following factors:

  • Voltage and current ratings: Ensure that the soft starter is compatible with the voltage and current ratings of the compressor motor.
  • Enclosure type: Choose an appropriate enclosure type based on the installation environment (e.g., IP20 for indoor use).
  • Cooling requirements: Evaluate the cooling requirements of the soft starter to ensure optimal performance and prevent overheating.

Step 2: Mount and Connect the Soft Starter:

Mount the soft starter in a suitable location, close to the compressor motor. Follow the manufacturer’s instructions regarding mounting options and ensure proper ventilation to prevent overheating. Connect the soft starter to the contactor, relay and compressor motor according to the given wiring diagram as above.

Step 3: Configure Soft Starter Parameters:

Access the soft starter’s control panel or interface to configure the necessary parameters for compressor motor control. Typical parameters include:

  1. Time selector for acceleration ramp when starting the motor ranges from 1 to 20 seconds, which allows us to adjust the acceleration ramp for the motor.
  2. Time selector for deceleration ramp when starting the motor ranging from 0 to 20 seconds
  3. Selector for percentage of initial voltage of the acceleration ramp and final voltage of the motor deceleration ramp (where you can choose the percentage of voltage (from 40 to 70) to be applied to the load from the beginning of the acceleration ramp to the end of the deceleration ramps.

The above selectors provide the following options

  • Starting voltage: Set the initial voltage level from which the soft starter will gradually ramp up during motor startup.
  • Ramp-up time: Adjust the duration over which the soft starter will increase the voltage to reach the full voltage level.
  • Current limits: Specify the maximum allowable current to protect the motor and prevent overload conditions.
  • Motor protection: Enable and configure protection features such as overload protection, overtemperature protection, and phase loss protection.

Refer to the soft starter’s user manual for specific instructions on parameter configuration.

Step 4: Ensure Safety Precautions

Prioritize safety when working with electrical components. Always follow safety guidelines, including wearing appropriate personal protective equipment (PPE) and adhering to electrical safety procedures. Ensure that the power supply is disconnected before initiating any electrical connections or modifications.

Step 5: Test and Monitor:

Once the soft starter is properly installed and configured, conduct thorough testing and monitoring to ensure its effectiveness. Observe the startup process, paying attention to the motor’s performance, current draw, and any abnormal behavior. Use appropriate monitoring equipment to measure and record the voltage, current, and temperature during operation.


Controlling the compressor motor in refrigeration systems using a soft starter offers significant benefits in terms of reduced inrush current, improved energy efficiency, and enhanced motor protection. By following the step-by-step guide outlined in this article, you can effectively implement a soft starter, ensuring a smooth and controlled startup process for the compressor motor. Remember to prioritize safety and consult the manufacturer’s guidelines for the specific soft starter model being utilized.

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