Traditional Star-Delta Starters and Direct-On-Line Starters are direct starters. They generate starting currents as high as 6 to 8 times the rated current at the moment of startup, severely impacting components such as the compressor and pump in industrial chillers, shortening their service life.
 
Furthermore, excessive starting currents can cause power grid fluctuations, impacting the stable operation of other equipment. To reduce these impacts, modern chillers often use soft starters and VFDs. What are these? How should you choose one?

What is a motor soft starter?

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A soft starter is installed in the control circuit of the compressor or pump motor in a chiller system to control the motor’s starting current and voltage. It controls torque by adjusting the rate of rise of the motor’s supply voltage. Initially, the soft starter reduces the output voltage to allow the motor to start slowly.
 
Within the set startup time, the output voltage gradually increases to accelerate the motor. When the motor reaches full speed, the soft starter is bypassed by a contactor, allowing the motor to run at full voltage. This gradual startup process results in a smoother starting current.

What is a VFD?

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A VFD (Variable Frequency Drive) adjusts the motor’s speed and torque by varying the frequency and voltage of the power supply. During startup, the VFD starts at a lower frequency and then gradually increases the frequency to its rated operating point to avoid impacting the equipment and the power grid.
 
During chiller operation, the VFD automatically adjusts the supply frequency and motor speed based on cooling demand, thus saving energy. During system shutdown, the VFD gradually reduces the frequency over a set deceleration time, smoothly decelerating the motor until it stops.

Motor Soft Starter vs. VFD in Chiller

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The following are the main differences between soft starters and VFDs:

Speed Regulation

Compared to direct-on-line starting, a chiller equipped with a soft starter has a smoother starting current, resulting in less frequent equipment tripping and less severe power grid impact. However, it cannot adjust the voltage during operation. This means that after starting, the motor can only operate at its rated frequency and speed and cannot be adjusted. If necessary, a VFD with both soft start and speed regulation can be selected.

Energy Efficiency

Since soft starters cannot adjust motor speed based on cooling demand and can only reduce the peak starting current, they do not offer significant energy savings. When the chiller is partially loaded, a VFD can slow the motor speed by reducing the power supply frequency. Our customers typically report energy savings of 30% to 60% with VFD chillers.

Control Accuracy

VFDs have built-in PID control. After setting the target temperature, the control system uses sensors to measure the outlet water temperature and outputs signals to control the frequency of components such as the compressor and pump. This enables precise temperature control and flow rate regulation. Soft starters only control starting current and lack these functions.

Cost

VFDs are more complex and have more features, resulting in a higher cost. Soft starters are suitable for applications with limited budgets or limited feature requirements.

Communication

VFDs support multiple communication protocols. Using protocols such as Modbus and Profibus, VFDs can be connected to remote control systems for intelligent control and remote monitoring. Soft starters, on the other hand, generally lack communication capabilities.