Soft-Starters in practice.

As I explain in the previous article direct online starting of large AC motors can cost some difficulties for the motor and equipment. Uncontrolled starting of several motors at the same time may cost a trip of overload protection or an Undervoltage relay. One of the best and the most reliable methods are to reduce voltage soft starters. Silicon Controlled Rectifier is made from a silicon semiconductor material which has a third terminal used for voltage control purpose. Silicon has a great withstand for high power and temperature.

How are build soft starters?

A soft starter is built from three pairs of CSR’s – each pair for one phase of power. Applied gradually for each voltage phase. They are limiting the voltage provided to the motor, which works on the current which is also reduced proportionally. Reduce Voltage by 50% reduce torque by 75%. After the motor is started, soft started is bypassed and the motor is connected directly to the line. Soft starters are also responsible for reducing harmonics in the system by 99%.

The full control system is built from 4 main blocks. Power supply with thyristors built-in. Analog interface to monitor power circuit, the micro-controlled circuit which process control to firing circuit which generates a pulse to control SCR’s. Process of control looks like at animation below:

Soft start is controlled by the direct start/stop wiring pushbuttons or controlled by PLC can receive signals by ethernet connections. There is also a third solution, to control soft starter by ethernet cable in IoT standard, however personally I didn’t meet this solution onboard yet. Depends on needs, the motor can be started by a program or simple solution just by a start button. Of course, the use of PLC increases the cost of installation, but if e-motor needs to be started and stopped frequently this would be a perfect solution.

Conclusion about soft starters

The biggest advantages in comparison to the delta star method are the efficiency and fully controlled start-up. The motor is accelerating in a controlled way, jumps of current are not an issue anymore. The price of soft starters is also reasonable to advantages which the user receives.

VFD controllers are not a type of soft start device, the purpose is completely different, however, they obtain the function of soft started in a different way. This is the reason we people mix these two solutions. But more about this in the next article!

[1] Soft Starting of Large Induction Motors at Constant Current With Minimized Starting Torque Pulsations. Gurkam enginobuz, Isik Cadrici, Muammar Ermis and Cuneyt Barlak


  • Mateusz Białas

    Researcher of Autonomous and Remotely Controlled ships. Since 2014 in Maritime Industry. Experience gained at Ro-Ro, Ferry, Container, and Heavy Lifts, design of Navy Ships.

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